Frankly there are times where trying to be reasonable and courteous in the
discussion of matters of diving doesn't seem to do any good. Well. That is it
doesn't seem to do my blood pressure any good... or so it feels.
This is where I let off steam on subjects I feel strongly about and some I
don't. I will leave you to decide which is which.
PS: They are in no particular order and there will be some overlap between
sections but I'll try not to repeat myself too much (unless I really want
Most of the physical constants I quote come from the CRC Handbook of Chemistry
and Physics 77th Edition (1997) aka The Old Rubber Book.
I will use SI derived units except where I believe I need an old one to give
the right feel. So, sometimes I will use tons, knots or mph but normally
meters, seconds and bar. I will assume that atmospheric pressure is 1 bar
although the barometer on my wall tells me that right now it is 0.993 bar. I
will assume that 10 meters of sea water adds another 1 bar although I know it
is really salinity dependant and should be 9.81*1.027 meters per bar. Please
don't give me grief about psi, feet or ATA as me laughing at you may cause
I live on the south coast of England and diving is supposed to be fun. I may
personally dive outside what is known as recreational diving but it is
still amateur and diving for my own enjoyment, so many divers seem to forget
this. Once I have booked and paid for the trip then nothing will change the
price so I can dive completely on whim. OK so if nobody is forcing me into the
water why should I be cold?
Down to my dry suit. I have a dry suit to keep me warm. Getting cold sucks.
Getting cold also triggers those little receptors in your brain that say 'Bad
News'. Their function is to train you not to place yourself at risk of freezing
so places you got cold get black balled. This, I think, is why a good
percentage of UK divers have given up within three years. Their wetsuits allow
the aversion therapy effect of cold water to extracted its due.
Seals. OK so what do we want from a dry suit? Well first
and foremost it must be dry. Don't assume this is always true. The seals on a
suit are a common problem. If you are of the more chubby build with smooth
rounded flesh (bioprene) you can use either latex seals or neoprene seals. In
fact neoprene, being an insulator is nicer on your neck as it helps keep things
warmer. However if you are a scrawny runt like me who can see the ligaments in
his wrists and neck standing out like wires you need a latex seal that snaps
round you like an elastic band. My current rig has a neoprene collar round the
latex seal. It seems a nice idea but I can't admit that I notice any
I used to use cheap Boots talc on my latex seals for everything but now only
use it for storage. Avoid the baby talcs and any additives as they are usually
oils and latex hates oils. Do inspect your seals regularly as putting a thumb
through a perished seal as you put the suit on sucks. Now I have some barrier
cream and it stops the 'hanged man' scar back at work on Mondays. I've also had
very happy dives on Vet Lube (dirt cheap) or the quick and simple to obtain
'Gloop' from the dive shop. These are especially good when you come to disrobe
as a dab on a finger slipped under the seal and wiped round makes things so
easy to slide off.
One thing you will need is to do is stretch new latex seals. If they're the
right size they will be horribly tight for the first few hours and while the
advice is often to trim them down a bit that just hastens the day when the go
all sloppy on you. My preference is to stick something in that measures the
same circumference as my wrists and neck and let the latex get used to what I
want then cut it. That seems to work for me.
Dumps. The dump lets the expanding gas out of the suit
as you ascend and allows you to control the total buoyancy. There are two sorts
of dump. The one we used to use and the one we use now. They are called the
cuff dump, simulating a sloppy wrist seal, and the auto, or shoulder, dump. The
cuff dump is a one way valve letting gas out so if you raise your arm the air
runs up it and bubbles out. Drop you arm below your body and it is trapped. If
you have been using a cuff dump for years you might as well carry on but if you
are starting out from new there is no choice. Go for the Auto.
So what is so auto about an auto dump? An auto dump is just a one way
valve outwards for air with a wind up spring so you can shut it off when
hanging about on the surface waiting for the boat to pick you up. When, on the
way down, the gas in your suit compresses and you feel the suit beginning to
squeeze a bit you push the injector to add some more and when you ascend the
gas in the suit expands and pushes its way out. Easy if your weighting is right
as auto equates to no think, it just happens.
However then somebody says 'You should use your suit for buoyancy'. So you have
to put more gas in and the auto dump correctly lets it out again. So you are
told to wind up the spring a bit. This has the effect of making the suit run a
constant amount of over pressure so your buoyancy goes totally to pot as you
ascend. Using you suit for buoyancy is a useful drill to practice as it
converts ARGH! My BCD has failed! I'm going to die! into Oh Bother.
Put some more air in the suit. I wonder what it is going to cost to fix this
BCD. BUT you don't have to do it all the time. I'll come back to this in a
later section I expect but for now I'll leave it that I open the dump right up
on my predive checks and close it when I surface après dive so I can
blow the suit up like the Michelin man for maximum warmth in case I'm last in
the queue to be picked up.
Undersuits. A dry suit doesn't keep you warm. Yes the
original ones were made of thick neoprene but it didn't work. They compressed
as you descended and it was the whole sorry wet suit story all over again. You
have to wear something underneath. Because it is dry inside that is something
that is fluffy and traps air and does not let it circulate. Air is a fantastic
insulator but it must be still air to work. To stop your fluffy undersuit
getting compressed into a very flat and unfluffy thing you have to top up the
air in the suit as you go deeper and it compresses but that's it.
Now once you have the idea of an undersuit you can start to improve it by
adding features. The first is to coat the fibres that trap the air in a
hydrophobic (water hating) film so it is very hard to saturate it if the suit
leaks. This can make a big difference if you tear a hole in things. Secondly
add a hydrophilic (water loving) layer next to the skin to wick away sweat and
any minor seepage so you don't get cold wet patches.
Now out of the water an undersuit feels slightly strange. The arms are too
short because your wrists must stick out to go through the suit cuffs. Also the
neck feels funny as you can't have a collar or it would get into the neck seal.
It's not really quite as warm as you might expect as it is the wrong shape but
it is the right shape to dive. When you try and wear a nice warm jumper to add
to things it all bunches up under the neck seal and lifts it off your neck and
you get this nasty trickling sensation. I've seen this done...
To the suit I added booties. Socks are never enough to keep your feet warm and
cold feet bring on the classic divers calf cramp that they taught you to pull
on the end of your fin to relieve in beginner classes. I used to have a real DM
demonstration quality cramp release before I got my booties. Now it has
withered away like all those other skills I don't use.
Oh and beware of fluffy linings stuck in suits. They are the very dickens to
launder unless you want to push the whole thing in the washing machine in which
case it leaps about the kitchen like a thing possessed if you fail to stop the
Inflators. Not much to say about these. This is where
you put the air in to stop the suit shrink wrapping you as you descend and the
air within it compresses. I'm told it can be handy to have the nipple on the
wing and the nipple on the suit the same but I never bothered. I feel slightly
squeamish at the idea that if one of my buoyancy sources has failed that I am
going to disconnect the working one? No if the drysuit fails I'm out on the
wing and vice versa. Nothing that works is going to get unplugged. After all I
only need to add gas to the suit as I descend. If I have lost my suit inflation
am I really going to carry on down? That was my backup buoyancy.
You might want to revise this doing complex cave penetrations with serious
ascents and descents on the route as out might be down but if you're reading me
for advice I'm guessing you're not ready for that yet.
There is another point but it might be easier to fix. All my Apeks suit hoses
are equipped with a big chunky pull to release sleeve. This is good news when
you get an inflator jam open fault as you can just grab and pull. If I were to
swap to a wing/BCD style nipple there I fear I would have to swap to the much
smaller pull to release sleeve they use. I don't like that idea as a drysuit
runaway is a serious problem calling for a very fast response. A big chunky
release would not fit onto either my BCD or either of my wings, there just
isn't room so the more I have thought about it the more I think I'm going to
stick to the standard fittings.
I have three different rigs I dive with my drysuit hence the fill hose can be
coming from three different directions so I have one of the Apeks 350°
swivelable ones so I don't have to slack it off and reseat it every time I
Zips. Front entry zips look the business so my new suit
has one but what else do you take a buddy along for? My old shoulder to
shoulder type was never a problem and it was not a consideration in selecting
the new suit just an option.
The only important thing about zips seems to be to use bee's wax on them. I've
tried some spray stuff (dire) and some sort of foamy stuff with a brush on the
top of the pot (even worse). Wax is a fraction of the price too. Do it
regularly and then some more. I have got to the point of keeping a stick of
bees-wax in the bottom of one of my suit pockets so it's always there when I
unload when dekitting. A quick swipe takes seconds and the zip runs like magic.
I didn't always do this and I paid the penalty.
Pockets are a good idea. When you upgrade from a simple
BCD to a more technical rig the big thing you miss is pockets so having some
put on your dry suit helps. If you RIB dive you don't want them on the front
but on the sides or you stuff gets squashed as you clamber back over the side.
Big is good so beware of the documentation style flat ones. Having a
ring you can bolt snap your car keys too helps avoid the problem of how far you
have to dekit in the car park before you can get a place to put things
I carry the spare torch, spare spare torch (a tiny one to read the depth timer
and tables), tables, spare blob with the spool, yellow blob and a spare mask in
my pockets and it's getting a bit crammed.
Storage. I've had a problem with mice chewing at a suit
that I left on the floor. It wasn't a very big hole but I got very wet. Hang
the suit up by the boots with the zip open. You can buy a hanger to do this or
make one. This means air can circulate and it can dry out properly. Don't ask
me how to store it in the 'off' season because once you have a dry suit there
isn't an off season. If you store it damp and rolled up and when you come to
use it it has fuzzy bits growing on it that's your problem not mine.
Material. There have been three choices for what to make
your drysuit out of over time so I will deal with them in order.
Uncompressed Neoprene. This was the first
material and it has largely gone out of favour now because it's junk. Neoprene
is an insulator so the suit provided additional warmth when shallow but, like
your wet suit, it compressed at depth and provided less and less insulation.
Annoyingly this was compression you could not deal with by adding air to the
suit so it didn't gain from the idea of being dry.
Membrane or Trilaminate. This gave up the idea
of the suit material insulating and focused on being waterproof and neutral.
All your warmth came from the undersuit that you wore. I have two of
Compressed Neoprene or Crushed or the high
density stuff some people use. This is an attempt to recover some of the
insulating properties of the neoprene suit but by pre-compressing the bubbles
to lessen the changes caused by depth on it. I can see the point but I'm not
convinced enough to make it a reason to choose. As far as I can see it is just
Neoprene pretending to be Membrane. If you are worried about floods the little
extra insulation compressed neoprene gives you can easily be multiplied by
buying the right undersuit. Neoprene suits tend to have neoprene seals and
Membrane suits latex by default but you can swap things round. If you have a
suit that fits it doesn't really matter much. The real problem is that you
don't use foamy stuff like neoprene for waterproof seals anywhere else so why
in a dry suit? Add what salt water does as it dries out in the surface pores of
the suit and they soon look very tacky unless you rinse them in fresh water
each and every time you dive them.
Fit. Your drysuit needs to fit. This is why the
companies offer made-to-measure services and actively promote them. However you
need to be aware of two points on the fit of a suit.
Firstly you must not compress you undersuit too much. You are paying for all
that nice insulating air trapping fibres so don't squeeze all the air out to
put the suit on and then expect to stay warm. It may have looked quite the dive
god in the shop and disguised the tummy a bit but this is not a fashion
accessory, it is here to keep you warm.
Secondly you need to be able to move about in it. Technical divers worry about
getting a hand to their tank valves and that can be hard enough without a suit
on and impossible in a restrictive suit. Also not being able to put your own
fins on because you can't bend over far enough will the cause of endless
amusement to your mates but the day a fin strap pops off in the water you are
faced with a more serious problem.
Using a dry suit
A quick rundown on why and how we use drysuits.
Dry suit. A bag to keep your temperature sensitive bits
in that stays dry on the inside so you can wear some nice warm clothing.
Problem. Nice warm clothing is only nice and warm
because it is nice and fluffy ie: full of air. As you descend the air in it
compresses and it becomes progressively less fluffy and hence less warm.
Solution. Add air to the suit as you descend to keep it
at roughly the same volume, fluffiness and warmth.
Snag. As you re-ascend you need to progressively dump
the excess air in the suit or it expands and you leave the realms of fluffy and
become a balloon leading to a very rapid and rather unhealthy rush for the
If you are diving a single cylinder you can probably just get away with putting
a bit too much air in your suit at the start and not putting any in your BCD
because your tank only changes in weight by about 3Kgms between the start of
the dive and the end. Whether you choose to do this routinely or not is up to
you but it is a good trick to have in the mental toolbox as I've had the
inflator of a BCD come off in my hand and if all you do is sigh and switch to
buoyancy by suit it's just a trip to the shop not a serious problem.
My personal take is to put the right amount of air in the suit to keep it
comfortable and do the actual control on the BCD. Getting excess air out of a
suit does require encouraging it to head for the dump and the bit in your boot,
the other side of your macho strap on knife, will take its time especially if
you are holding a nice, stylish horizontal position in the water.
Weighting. This is key with a dry suit. Get the weight
right and the auto dump really is auto - you just inject air to keep the suit
comfortable on the descent and the ascent looks after itself.
My method for this is to ascend and do a pretend stop at about one meter depth
and keep handing off the lumps of lead until I can't do it comfortably anymore
because the BCD is empty and the suit has to be encouraged to squeeze a bit.
Then work out the weight of the gas left in the tank and add that to the weight
belt so I can do the same on virtually empty. Be sure that you are coming into
the stop from below so you are hitting the real case of what air you can get
out of the suit ascending not what gets squeezed out on the surface talking to
the guys taking the lead from you.
Also beware. Your nice new undersuit is at its fluffiest so you are going to
have to revise down the amount of weight you need over the first couple of
hours wearing it. This change can easily run to several kilos. I pool dived a
new undersuit and got it right and ten minutes later dropped two kilos, another
quarter of an hour and another two went and more. I thought it was stable but I
wore it for a couple of days boat handling for other divers and when I finally
sea dived it I was still overweight.
Lubrication. Your drysuit needs looking after. The zip,
well the metal zips, need lubricating. Not on the tiny metal teeth that
actually engage and do the seal that nobody sees but on the outside where the
zip 'closer' slides along. Taking a thing like that and putting it in the sea
is looking for trouble so look after it. The conventional wisdom is sticks of
bee's-wax but there are several commercial products ready to take your money.
Personally I like the bee's-wax as, since lubricating my front entry suit
regularly makes it so much easier, so a stick of wax in a pocket means it gets
a quick wipe over after I do it up.
The other thing that needs lubricating is you. Sliding a dry hand into, or
worse out of, a dry seal is hard and stresses the seal. For latex you can use
talc as it is a super lubricant. Avoid the 'scented' or such versions as the
oils involved are not good for latex but get the cheap 'baby's bottom' version.
Alternatively both neoprene and latex work well with water based 'personal'
lubricants. KY gel if you're posh, vet lube if you remember to buy it in
advance with commercial products like 'Gloop' coming about half way on price.
I find a squirt of lube on a finger that is then run under the seals helps no
end when you're struggling to get out of the suit after a couple of hours in
Drysuit diving isn't a black art, it isn't even hard. The main trick, as with
all buoyancy work, is to correct early based on how fast you are moving not how
close you are to your target. That way a descent rounds out nicely with a deep
breath to bring you to a halt and as you exhale you are left just hanging above
the wreck. The only time I actually fiddle with my autodump is when I hit the
surface where I shut it right up or the waves massage all the air out and
leave you rather unfluffy and chilly.
One word of warning however. Every dry suit newbie thinks their dump isn't
working fast enough and you see long discussions and some quite fanciful
solutions being presented. Why do they feel this? Because they are trying to
dump too much gas too late. Now with a BCD you can get away with dumping gas in
a last minute panic when you realise you are going too fast to stop easily but
in a dry suit the air is all spread out keeping you warm. You adjust buoyancy
to set the speed of an ascent/descent not the final destination. So, when you
see the depth of your safety stop coming up you slow it down and slow it down
so you virtually just ease into it with no overshoot. If you try and zoom up
to it and stop dead you are going to be disappointed.
Floods. Nothing is watertight forever and, although if
you're careful leaks just result in a slightly damp dive occasionally some
individual is going to have a catastrophic flood. So there are two
consequences. The first, obviously, is that you don't stay so warm. Yes a good
undersuit will still do its best to keep the water still but you are going to
lose heat more rapidly and if you are committed to a lot of deco stops this
may be a problem. Secondly you are losing a lot of nice buoyant air and
replacing it with neutral water so you effectively become 'heavier'.
I did a simple test in the pool using my Otter 'Commercial 300' undersuit,
Weezle overboots and a big pile of weights. I adjusted myself for neutral trim
in the shallow end with no scuba kit other than a mask, fins and a pocketed
weight belt. Then I unzipped the suit, let it fill up and then unloaded
weights onto the side until I was back to neutral. The difference between
zipped up and dry and unzipped and soaked was 7kgms. This means that after a
total flood I would need 7Kgm of extra buoyancy in the BCD/wing to get back to
neutral to ascend. This is the sort of number you need to factor into your
buoyancy plans. Try it but remember to bring a big tough plastic sack to take
the undersuit home in.
I like waistcoat style BCDs. I know that's not a very hip thing to say in tecky
circles but for diving a single cylinder rig they are tried and tested gear,
they work and, what's more, they have pockets. I still have the Scubapro Club
jacket I bought way back when I started diving and it still works just as well
now as then so it goes on holiday with me and does training with club
beginners. However there are a few things I see people doing that I think are
Have you ever watched a diver stop fining and suddenly slump to feet down? I
see it quite often and it normally correlates to first stage behind the
shoulders tank position. I find that a steel tank certainly, and normally
ali too, needs to be set so the valve and first stage are behind your head so
the tank is well up your back for a diver to naturally hang horizontally in the
water. Secondly I do like a string operated pull to dump on the left shoulder.
I'm not quite so happy about pull to dump on the inflator hose as one
once came off in my hands - dumping the contents of the jacket. Clips that
separate the shoulder from the chest section are good too as you can easily get
out of the whole thing in the water to pass it off to a RIB crew.
However let's just consider the physics of the whole BCD/Wing controversy and
see if we can shed any light:
What is a Wing? It's the one o the right. Well normally it is a single
inflatable bag, either shaped like a horseshoe or it is an oval donut shape.
The idea is to provide a reasonably large buoyancy compensating bag with the
bulk of its volume either side of your cylinder or cylinders and connected
across the top and, if possible, across the bottom. A wing may be sold with a
harness and a means of attaching a cylinder or cylinders. Also, to reassure the
pedants that I haven't lost the plot, a Wing is a BCD (Bouyancy Control
What is a waistcoat style BCD? Well it does more than a Wing because it almost
invariably designed to attach itself to the diver and attach the (normally
single) cylinder. The key trick is that its inflatable sections are within not
only the back but also the sides and the fronts of the shoulders. Where the
wing put everything behind you the... Heck I'm going to call it a BCD, the BCD
lets the gas go round you.
So what happens? Well in the water air moves strongly to the highest point so
if you're trimmed out nicely horizontal on both the buoyancy is behind you
back, well above your back in truth. This is why both wings and BCDs have those
nice pull the string dumps behind your shoulder and behind your hips. Wringing
air out of the inflator hose needs a good pull up, behind your head, to provide
an uphill all the way out path. Remember that neither of them are tightly
inflated balloons so air will only escape if it can go up. To me this says that
under water they are pretty much identical which matches my experience.
However when you turn to the head up position to ascend and to hold yourself on
the surface the differences start to show. On the wing the gas is constrained
to stay pretty much in the same place and just rise to the top of the wing
while on the BCD a good deal of the 'top' is at the front. Why is a BCD
designed that way? Well the idea is to mimic the design of a life jacket and
ensure your head and most importantly your face is out of the water pretty much
regardless of how you rig it. It's not that a Wing tries to drown you just that
this is not a priority in it's design and you can rig it (cylinders low, wing
very high) so it tends to want to be on top.
A Wing is just designed with different priorities in mind. They usually have
more volume to give more lift and are often very wide so they can accommodate a
twinset between the two sides without the tanks stopping the bags fully
inflating. Blowing a BCD up until the over pressure dump squeaks is common but
few people would find a wing comfortable like that.
My preference is to use a Wing on the twinset or the rebreather and a BCD on
a single. I have them and that works for me. Of course there are several people
that that tell me they can make a wing that is equally good on a single or a
twin but after exhaustive research I discover that what they really mean is
equally bad. There are just too many tradeoffs to make that one work. Of course
there are some people who insist that they must have kit that is the
same under all configurations. Since I dive lots of different
configurations I find that firstly rather amusing but then, when I consider how
they will cope with a fault, rather worrying.
So he pins me in a corner "Right. What's all the hassle about twinsets? Two
tanks? I have two tanks and I use them for two dives. Didn't I see you do two
dives on one fill? What's the difference?"
The twinset is about redundancy. In diving things break and, sadly, if you're
not able to supply a solution to the problem you drown. The problem is that in
some situations the surface, in terms of an emergency ascent, is not available
to you. If you are in a wreck or a cave, if you have significant deco racked up
or if you are just too deep you must do something to manage faults.
So what is the simplest fault? It is common too. It is a regulator freeflow. We
all practiced this in the pool before an instructor ever took us into wild
water. You can get a few breaths from the gas rushing past your reg to help you
on your way up but you get up and out at that level.
The best solution is to turn off the cylinder. Let the reg warm up a bit as
most freeflows are icing somewhere in the system and turn it on again and carry
on. If you're on a single that needs a buddy and they might not be instantly
available so a pony might help. However the twins thinking is to split your gas
into two equal halves and carry it in two tanks. If you equip each tank with
its own regulator set if one goes wrong you can breathe from the other.
In its simplest form that is all a twinset is. Two completely independent
cylinders. So you ensure that there is always a good reserve of gas in both
sides you adopt a gas management plan that involves swapping between the two at
intervals so that at whatever moment a failure strikes there is always a way
However, although independent twins are used in some circumstances (some solo
divers like them), there are some things we can do to improve on them.
Joining the two tanks below the valves with a manifold which itself contains
a valve allows you to dispense with the side to side swapping and ensures that
you use your gas evenly. The downside is that you need to react to a freeflow
promptly as it can empty both your tanks but the trick with the manifold valve
is to only just open it so it can equalise the tanks as you breathe but not
keep up with a freeflow. This keeps the loss from the good side down and if
your problem drill starts with close the manifold valve the remaining
half a turn whatever has happened cannot cost you more than half your
Normally closing the pillar valve feeding the offending regulator set solves
the problem and you can open the manifold and have access to all your gas
through the good regulator. The manifold valve only really becomes into its own
when a pillar valve goes - a most unlikely event but who wants that said at
their inquest? However since the tanks are behind you accurate diagnosis is not
easy so the manifold gets shut first and only opened when the problem is
On the usual two dive boat trip the twinset does involve you taking the dive
two gas along on dive one but that is not a bad thing. Despite much discussion
we tend to still dive the deepest dive first and make it the significant dive
of the day with the second as an optional extra. This means we not only take
the gas along as spare but we remove much of the gas limits from the first dive
and can split the gas maybe 60/40% if we want. After all who minds cutting
short a scenic drift over a bunch of weedy boulders because you
wasted the gas diving the deep wreck?
The key trick with the twinset is practising the shutdown drill that enables
you to get payback from the valves. Your hands need to know the drill and it
needs to be fast. I practice the sequence of close manifold, close primary reg
breathing it dry, swap to secondary (on the necklace), close secondary and open
primary (simultaneously is easy on my inverted twins) breathe the secondary dry
and swap back to the primary, open the secondary and finally open the manifold
valve. I can't see any reason why I'd ever need more than a small part of that
drill but the hands know where the valves are.
As a final note let's just discuss the manifold. Some people don't have the
picture in their minds. The manifold joins the two cylinders below the pillar
valves. If the manifold is open the two tanks are joined and both pillar valves
draw from the common volume. Either pillar valve can be shut to close off a
failing regulator and you can still breathe all the remaining gas on the other
If however the fault is more systematic, if the pillar or the valve itself has
failed, then that side is bound to leak away all its gas so shutting the
manifold valve saves half the supply. Continue to breathe the side that is
losing gas until it runs out so at least you get some of it and then move over
to the remaining side. If you are diving rule of thirds (bring one third of you
total gas back home) and if you are slick with your valves you should retain
enough gas to finish the dive without resorting to your buddy.
You may go an entire diving lifetime and never have a fault that needs you to
close the manifold but it is still worth practising. Certainly I have had
regulators freeflow, but if you insist on quarry diving in February you must
expect that occasionally, but I've never had a pillar or valve fault. However
they do happen and it's always nice not to die.
UK divers tend to use Delayed Surface Marker Buoys.
They suit the way we dive. The problems we face aren't uncommon. They will get
a mention again and again in this page: Tide, Visibility and Cold. The DSMB
addresses the problem that due to the vis being only a couple of meters on a
good day finding your way back to the shot line to ascend back to your boat
might be a good trick but it's a bad plan. As having a string up to the surface
makes a controlled ascent much easier and, as slow ascents are good for your
life expectancy and general health, take the string with you.
I deploy the DSMB from the bottom or from the wreck and if I can tie the reel
down I prefer too. Then I have a rate I turn the handle at to give me the 9 or
10 meter per minute, or slower, ascent rate that I like.
I use the Buddy DSMBs although any self sealing one would do for what I want.
My primary blob is a self inflator with a small gas bottle on it. When in full
tecky mode (all the toys) I have that blob and reel hanging behind my right hip
and a non-self inflator with a 20 meter spool of line in my pocket.
Add a bright yellow blob with a piston clip to run up an existing string as a
HELP indicator normally to imply "Please send my
drop tank down". (I don't carry it if there isn't a tank available.)
If you are using blobs there are a few things you need to be clear about:
Firstly: Blobs are expendable! If the reel jams
or some problem develops where the blob starts pulling you towards the surface
you need to be able to just let it go without undoing any clips or such. A blob
being pulled out of your hands is annoying but being pulled up is a route to a
helicopter ride. Never clip a reel to yourself. Just rely on your fingers to
hold it. Clips don't let go under tension. You can hardly miss this big
red/orange thing bobbing about on the surface when you get back to the boat so
it won't stay lost.
Secondly: Even if you do have a self inflator practice
using it normally. Learn to blow it up with exhaust bubbles or the octo
or something. You should be able to get three fast breaths into it and if you
are reasonably deep as the gas expands this will blow it up tight by the time
it has got to the surface. Don't rely on the crack bottle. The number of
times they have failed on me (normally my fault) is horrible.
Thirdly: Watch your buddy when they deploy a blob. I
sometimes pop out my wreck reel and hitch it onto their handle so if we get a
jam he can let go and I can run out line. I have had a buddy hold
onto one of my fins as I sorted out a slightly inflated blob with a problem. He
had his other hand on the wreck and I greatly appreciated the assist.
Fourthly: Make sure the skipper knows what your blob
procedures will be. Some like one blob per diver if you're deco diving so they
can count you off the bottom and know you are on schedule. If you are reeling
as a pair warn them. If a good diver looks at his schedule and says he starts
his ascent after 28 minutes he will hit it accurately.
Ascending in darkness with 2 meters vis you really appreciate the DSMB. There
is no way you can tell how fast you are rising but keeping the string tight
tells you you are holding level. From the skipper's point of view he can stay
over the wreck but when the blobs appear he can count them and then follow them
as the tide starts to run as you do your stops. When you finally break surface
he is there watching for the heads to appear beside the markers.
Putting up a DSMB at depth
You want the reel held so the line and any tape loop is tight so it can't
tangle round your reg but you are ready to thumb down the release as it starts
You tip your head on one side if you have two exhaust valves on the reg like my
Apex50s so all the exhaled breath comes out of the top one.
You make sure the open end is well over the reg and give it three sharp breaths
then move it away in front of you so you can see it is clear. This means that
when the gas hits the top of the unreeling sausage and up becomes an imperative
it is all held out clear from you. Three breaths doesn't look big deep but by
the time it gets to the surface it is full.
Then thumb the ratchet and off it goes.
Keep a (gloved) finger on the reel so it doesn't overshoot and 'bird's nest'
and be prepared that if the reel jams you can give it one jerk to try and free
it then you must let go. (This has only happened to me twice, both times down
to knowingly reeling in badly and not bothering to run it out and do it up
As soon as it stops crank in the slack so you think you have stood it up and
'feel it' (you should feel the waves if there are any).
Then signal your buddy and start to ascend.
I know that half a turn every two seconds on my reel is roughly a 10m/minute
ascent rate so I stay a little negative all the way up. Again hand hold it so
if something does snag the line you can let go. I've never had this but I've
seen my buddy pulled to up a bit and then he let go and the reel vanished. He
shrugged and I deployed my DSMB and we carried on with the ascent. We recovered
his with the boat hook later, I think we just drifted through the shot line
but the tide was running strongly by then.
I carry a self inflate (bulky) as the device of choice on an 85m+ reel and have
a blow it up with your exhaust gas model with a 20m spool (no moving parts) in
a pocket. Most deep water charter skippers assume they are part of your kit and
drop you on slack water and then, when you've finished your wreck tour, you
blob off and do your stops as the tide begins to run and he just follows the
cluster of orange markers.
Technical divers use the code that running a yellow blob up the same string
means 'HELP! I'm in the poo' and we often carry a drop tank of gas to enable a
guy with a problem to finish their deco. I have a yellow with just a P-clip on
it to do that but I've never had to use it yet. Oh and I also have a drop tank
of 50% and the rig to run it down the line because without that you're wasting
your time putting up a yellow.
An additional note on colours.
A friend, who is a professional skipper who picks up divers and equipment
virtually every day in what ever conditions apply and who probably knows more
about practical diving than I ever will commented:
I would say the worst colour we have found to look for is white - a stealth
buoy, in other words a shotline with a Sofnolime tub tied to it, is not easy
Some SMBs are awful - the older buddy bi-coloured ones where the yellow half is
very yellow, not fluorescent, are hard to see sometimes. Any SMB shaped like a
liftbag is a bit of a problem as well as it can be discounted as a liftbag and
ignored if the skipper is having a bad time, or if you have drifted a long way
it can look like a lobster-pot buoy. The narrow Halcyon ones are not visible
and should only be used in a team when one person has put up a decent sized
(No credit as I edited it rather a lot and it's not fair to
apply somebody else's name to my version of their words.)
It wasn't quite what I expected but looking back at pictures I've taken from
dive boats I think I can see what's being said. The sea can look white, green,
black or even occasionally blue so you want a colour and an intensity
that contrasts. Bright, like white or yellow, isn't enough when the sky is
bright as the sea reflects it, dark doesn't work on a gloomy day but the sea is
never anything like orange or red.
I recognise three sorts of reels - others may differ.
A DSMB reel which has a ratchet which I can thumb down
to let it fly and then wind myself back up the line click by click and only
need one hand to hold the reel while the other busies itself taking care of
the other chores.
A Wreck or Cave reel that if it has a brake has a
friction brake on the spindle so I can drag it out and wind it up as I lay the
line I will retreat on. As cave line can be rope it might be a big reel.
A Spool which has no moving parts but is ultra compact
and, provided the line doesn't snag, has nothing to go wrong.
It is worth unreeling and rewinding a line before a deep dive. All my jams have
been down to a cack-handed reel up on one dive getting twisted up and then the
next deployment snagged. If you do a slack wind most of the ascent and then go
tight at some point the tight bit binds down into the pile of slack and is
almost bound to snag up.
It is also a good idea to have a handle that is an enclosed loop so your hand
traps the reel but can still be used (see picture). A handle that you must hold
commits that hand from when you launch the blob to when you pass it up to the
Weighting and Lift
There is some drivel talked about weighting. It is really very simple. You need
enough lead to get down and to stay down and no more. Lift gets the same stupid
treatment but is just as easy. You need enough lift to put you back on the
surface in an emergency.
OK lets put some numbers to that. The worst point for weight is doing the last
part of the ascent when all that gas you were carrying is gone. Now diving a
12L/232bar single cylinder set you carry 3.3Kgs of gas so you need enough lead
on your belt to hold a stop right down to empty. Now I do mean empty here. I
know you are a good little bunny and you will normally bring a reserve 50 or so
bar back to the boat but that is a reserve. It is no good saving your buddy
from certain death and then bending him because once you both started breathing
the reserve you couldn't hold the stop. That's not a reserve that's a
passenger. You can't use it.
The best way to do this is in the pool. Put on the full clobber with all your
thermals et. al. put your weights in the BCD pockets and dive the shallow end.
Fill the suit and BCD and then empty them to descend. This means that any
pockets of trapped air are filled just as they would be on the ascent. Now
start unloading your weights onto the pool floor and see what you can get down
too. Then note the SPG reading so you know how much gas you have.
Let's say you end up with 7.5Kgs of lead and 120bar of gas and you can still
exhale to go down. Less weight and you are pinned to the surface. Well in a 12L
232bar of air is 3.3Kgs weight so 3.3 divide by 232 multiply by 120 is 1.7Kgs
of gas weight. So now you know that in fresh (pool) water you need 7.5+1.7 is
9.2Kgs of lead. As you only have the old stuff in pounds multiply by 2.2 and
round it up to 20lbs. I confess I'm too stupid to keep swapping units so I put
all my weights in turn on the kitchen scales and stamped them with their weight
Good. Now we need to turn that into sea water weight. Sea water is 2.7% more
dense than fresh water so we need to increase your total weight by 2.7%. Wait
until everybody else is out. Get the bathroom scales. Switch them to kgs if
they are the fancy digital ones. Put the full kit on including your suit, tanks
and freshly calculated freshwater weight belt. Stand on the scales and note the
number then scurry off and change before anybody catches you playing dressing
up. It has been suggested that you only need to hold these items but very few
people can read a set of scales holding a tank in one hand, with the suit under
their arms and a BCD clutched in their teeth. If it reads 110Kgs you need
110*0.27 = 3Kgs of lead extra. So your seawater belt is 9.2+3=12.2Kgs. 12 will
Actually if you always assume you need 3Kgs more in the sea you won't be far
wrong and it's a good starting point if you don't fancy all the rigmarole with
scales et. al.
So where does this place us against the PADI Hold a normal breath with the
jacket deflated and you should float eyes level with the water? Well it's
just about the same. Eyes level places you about a kilo positive so when you
exhale and lose about 4 of your 5 litres of lung capacity you end up 3Kgs
negative and on the way down. It's a good trick but it only works with a single
and if you don't understand what hold a normal breath means or if you
just can't stop fining, and lots of people don't seem to be able too, you are
instantly wrongly weighted.
Right so now we have the lead right how much lift do you want? Well lift is not
a problem a beginner worries about as a BCD has enough for a single cylinder
rig but when you go to twins and stages it starts to be a worry.
Again the maths is simple. What is the worst case? It is that you start down
the shot line in full kit for a deep dive and, as you pause to see if your
buddy is following, you turn and your drysuit neck seal splits. You are
suddenly very cold, very wet and very negative as all that nice, warm,
insulating, buoyant air bubbles up past your ears. You need enough lift to put
you straight back on the surface.
So now, assuming you were correctly weighted before, you are negative by the
total weight of the gas you are carrying, say 6.6Kg in the twinset and 1.9Kgs
in each of your stages, plus the lift your drysuit provided and which you just
lost. Well I'll tell you the drysuit is 7Kgs and the gas about another
10Kgs then you want 3 or 4Kgs to float your head above water so until you start
to get into the realms of twin 18s all you need is a 22Kg wing. There are quite
a few 40 pound wings, 18Kgs, about and with these you are into ditching stages
territory and although you life is far more precious than your equipment you
know as well as I do you will hesitate to drop something with a three figure
price tag. If you are into diving twinsets stages will happen so plan for
them. Is 22Kgs enough? Well when you start carrying two 12L stages for 100
meter dives so much of your gas is helium you actually carry less mass.
Nobody ever needs a 100lb wing. Really.
Depth and Depth limits
How serious are those depth limits the certifying bodies stick on our cards?
Do we really need to take them as gospel or are they just guidelines?
The picture is Holland Road Baptist Church in
Hove. It was built in 1887 when spires were trendy so it has a spire. Look
at the cars, you all know what a car is like, and then look at the spire. This
is 20 meters. If you have a problem at the peak depth of an Open Water or Ocean
Diver qualification this is about how far you have to swim before you can
To picture my 60 meter Normoxic trimix qualification we need a block of flats.
About 15 stories. I won't bother with a picture as I'm sure you can find
(Sorry. That was a lie. I know you will have difficulty
finding a 15 story block of flats unless you live in a seriously high-rise
neighbourhood. Try counting a few.)
Please bear this in mind when we discuss ponies, twinsets and such. It is so
easy to get familiar with the numbers and forget what it looks like. Just
because I have 59.6 meters in my log doesn't make 20 meters any less daunting
than Holland Road's spire. So try it. Find something that is 20 meters tall
that you can stand on. Then look straight down. This helps keep a sense of
perspective to the idea that twenty meters 'isn't deep'. It's worth doing
It may not be trendy to say it in tecky circles but I quite like the old pony
bottle. OK it's not much gas so I'd only use one on a planned no-stop dive but
it has its place and can convert a disaster into an annoyance.
They come in for a fair amount of criticism because they are small but as a get
you out tool they do the job. The common feature of the too small
shouters is to postulate that you panic and your breathing rate goes through
the roof. Well if you haven't got a pony you will panic. If you have you have
the solution right to hand so you stuff it in your mouth and breathe
The three big problems with ponies are:
Firstly ponies that don't work. You didn't fill it, you
didn't check it, you didn't turn it on. If you have a pony you need to keep it
in a top quality state as when you need it you really need it. This means you
buy it a good reg, you service it, you test breathe it regularly and you top it
up again before the testing reduces the pressure much.
Secondly you remember that depth is the enemy of the
pony. The deeper you are the more gas you use so while a pony is masses for an
ascent and you can normally pop in a safety stop, if you stay at 35 meters you
will eat it up. As soon as you put the pony reg in your mouth you are on your
way up and out. If your buddy has waltzed off and got lost that is their fault.
You are not going to go and take a serious risk by looking for them. You are on
your last gas and you are going to get home alive now.
Thirdly you want it all accessible. You want to be able
to see the gauge. Saying I don't need to see it, I'm on my way up cuts
no ice with a nice 3 at 5 safety stop. You don't want the reg to go hard in
your mouth just because you didn't know how much you had when you could have
been in the boat. Conversely if it's all gone badly wrong already you might
just need that safety hang to pull you back from your day getting even
I bought a 3L/300bar tank with a TX40/DS1 reg set right back at the beginning.
The DS1 was a mistake but it's now doing suit inflate duties on an Argon bottle
so it didn't go to waste. That rig dived with me and my son for many years and,
although we never needed it, it was the insurance in case something broke. I
now use two 3L/232bar replacements and clip them onto the rebreather if I am
diving with a diver without spare gas.
A note on gas consumption.
People like to quote the "If you panic you might be breathing 100L/minute. How
long will your pony last then?" line.
100L/minute? I hooked up the pony to the big blending gauge and measured the
pressure, then I screwed in the Apex 50 regulator set, sat on the floor and
maxed out my lungs for 60 seconds. It was a good thing I was on the floor
because my head was spinning at the end of that but I am sure I could not have
got more gas out of the cylinder in the time. Once I was sure I was back to
breathing normally again it was back on the blending gauge and it worked out to
a few decimals under 100L/min. Since all the limiting factors depend on the
viscosity of gases and these increase with pressure this number must go
down at depth.
No, I don't think it was realistic but it was a test to draw a maximum line. I
tend to think that if you have a pony going OOG may be a nervy moment but
getting the pony reg in your mouth will settle things very quickly. It's if you
go OOG and you don't have a pony that you are really going to panic but then
your breathing rate will be 0L/min.
Oh and when you're planning a pony set up: I weighted mine, a Faber 3L/232
steel, with an Apeks regulator set on it and in the water butt it came out as
1.7Kgs so that is all the weight you get to remove and all the weight you need
to move about to stay in balance. In the simplest case take 2Kgs off your belt
on the same side that the pony is going. That's the empty weight by the way.
Since it only carries 800gms of gas it's not a problem.
Gases and Gas Switching
Air is a bad gas to dive. OK. Perhaps I'd better explain a bit. Nitrogen is
a bad gas to dive and air tends to have rather a lot of it. Now I'm not
talking about drills in the pool or a pootle about at 10 meters or less but for
most diving nitrogen is the pits. Nitrogen is biologically inert so all that
can happen is that it dissolves in the water and fats that make up most of your
body and when you ascend it will undissolve. If you stay well away from the no
decompression limits of your tables the amount of gas that dissolves is not
huge and should not cause you many problems but as soon as you get near to the
no stop limits you are in the position where you are as full of gas as a can of
coke. If you handle it gently it opens and tastes fizzy. If you let some seven
year old (mental age) shake it up before they pull the ring you will be
cleaning it off the ceiling.
These are the dreaded bends. They are named after the 'Grecian Bend' of society
ladies in the mid 1800s (:an affected carriage of the body, the upper part
being inclined forward) and seen in the workers who dug bridge foundations
and such in pressurised tunnels and caissons. They worked a shift under enough
pressure to keep the water out and then walked into an airlock and were
depressurised quickly and walked out. It used to hurt and they were often
folded up by the pain. However they were normally OK again in time for their
shift the next day and it was well paid work. Air dissolved in them and when
they came out it undissolved. If you were unlucky you got trapped bubbles and
it hurt. Probably little bits of you died but it felt a lot better when they
pumped up the pressure the next day.
See stuff on decompression diving for a fuller explanation.
So Nitrox, air but with the oxygen content boosted is better. Oxygen is
biologically active. A bubble of oxygen can spontaneously be absorbed as any
tissue, if its blood supply is blocked, will become oxygen deficient and the
bubble will redissolve to supply it. If a lot of the bubble is oxygen that
might be enough to let the bubble move and be removed. Even if you don't get as
far as bubbles more oxygen means less problems from nitrogen.
Oxygen has one big problem. It is poisonous. It is poisonous in two ways. Long
term breathing too much oxygen under pressure leaves your lungs raw and creaky
but far worse, in the short term too much oxygen blows your nervous system and
leads to unconsciousness and convulsions.
Now a convulsion to a diver, who needs to keep that reg in their mouth, is bad
news of the terminal kind. We have tables to work out our oxygen exposure and
the limits have been crept down over the years so now we tend to think that
1.4bar of oxygen is the max or perhaps 1.6 if you are pushing it. I follow the
guidelines that I run my rebreather at 1.3 because the tables say I can do a
three hour dive on that with margin. This limits the oxygen percentage we can
have by the depth of the dive so nitrogen creeps back in more and more.
Nitrogen has another nasty effect that is narcotic and an Anaesthetic. It not
only bends you but makes you drunk. People report silly things when under
narcosis (narked). "Oh poor fish. Do you need air? Here have my regulator. I
don't need it" at one extreme and claustrophobia, fear and general paralysis
of action at the other. Below 25 meters everybody is narked, below 45 meters
everybody is narked badly. Some people say they can become accustomed to the
effects but most proper tests reveal no such acclimatisation - they are just
learning to ignore it but their performance in dealing with a problem is no
better. Deep diving we need to get rid of the nitrogen, oxygen won't do so we
turn to helium.
Helium is not your friend. Yes helium can regain your brain from the blur of
nitrogen narcosis but helium is a fast, unforgiving gas to dive. Helium gasses
on fast and off fast and in large quantities. This means that decompression
stops, even very short ones are significant. Deviations from the plan are
significant. Safety margins are important. Accurate mixes are important. Helium
makes you pay for that clear head. If you get bent on helium you are seriously
bent. You don't mess about with helium.
However if you work out how much nitrogen you are breathing at depth as a
partial pressure and then work out what depth you would be diving at to get
that much nitrogen on air that gives you a feeling for the gain in clear
headedness you get. It's not accurate but it's about right. Think of it as
diving to 70m with you head at 30m on air - that's 43% helium for you.
A note on Helium and bends
The real problem with helium is that it is a 'fast' gas.
So what do I mean by fast?
On the Bühlmann model there are 16 'compartments' which represent
overlapping tissue types. It isn't that your spleen is a compartment four and
your leg is a compartment six just that all 16 overlap enough for all cases to
get represented. Also when one compartment feeds another it is just represented
by a slightly longer compartment. The system dates from Haldane and it's good
For nitrogen they have one set of 'half times' for each compartment
representing how long gases take to dissolve and release but for helium they
are about a third as long.
This implies that helium dissolves in your tissues at nearly three times the
rate so, in very approximate terms, a dive on helium is three times as long.
However, as your deco is three times as long too, it evens out and a helium
dive isn't all that much different from a Nitrox dive.
Until you miss stops. Then those ten minutes you missed on your 40 meter dive
is effectively half an hour missed on a dive with three times the bottom time
and your problem has moved up an order of magnitude.
Conversely when they haul your messed up carcass into a chamber all those
helium bubbles redissolve at three times the rate and you then are back to
doing the re-deco at 'normal' rates. Your untreated bend is much worse but, if
they get to it in time, the prognosis is much better than the same level of
bent on nitrogen.
One of the consequences of this is that your ascent needs to be spot on. The
deeper stops can have some compartments off gassing while others are still on
gassing so hitting those numbers is much more important. The fact that only a
two minute stop is called for doesn't make it trivial (think six minutes). I
much prefer a computer on helium precisely because in the deep ascent stage of
the dive it is recalculating 'on the fly'. If I were diving tables I would
select a far more conservative algorithm because I know I'm not going to hit my
numbers to the second and all my errors are multiplied by at least
PS: I have stated in the past that you gas on 'more' helium and I think I
mislead people. You gas on about the same total volume.
Computers and Tables
When you start diving you learn to use tables. This is a good thing as it means
that from day one you are planning your decompression. OK, not quite planned
decompression as planning not to do decompression but it's the same idea. Then
you rapidly discover that you get longer dive times if you switch to a computer
as it gives you credit for not staying at your planned maximum depth for the
entire dive and the tables tend to languish in the dive box unwanted and
unloved. Then, you migrate to more technical diving and you start cutting
tables with stops in on your PC and writing them up on your slate and following
them rigorously. At this point you start to wonder what use a computer is, and
in many circles it is elitist to deny owning one.
Let's be up front about it. I like computers. For no stop or simple one gas
diving a computer is good kit giving a plan on your wrist and, providing you
are watching it, it can keep you within no-stop limits when tables would be
screaming in terror and hiding under the bed. When it comes to more exotic
diving I bring out the more exotic computers and dive them.
I do not subscribe to the tables are best mantra even though for
serious decompression I have been taught at times to dive the plan on my slate.
However I'm convinced that tables are not the best way to dive a rebreather. As
I have said before I dive with a computer and a back-up computer and I got that
from a technical diving instructor and it made good sense. The crucial thing I
did learn from him was that you need to practice switching computers to bailout
mode, something I had never done before, because just thinking you know how to
do it won't cut it when the problem strikes.
The only table I carry is a little snippet with the depth/times I can bailout
from using my standard bailout mixes. Since I fill these when the tanks come
back from test they never change so they are very predictable. This is the
limit of my dive planning: give me the planned depth for the dive and I can
sling the appropriate cylinders into the car and I'm off. How near I go to that
max-bailout bottom time, or if I choose to overrun it, is a decision I make on
OK, that will make me evil and looking for a Darwin Award in some people's eyes
but I see it as a sensible way to dive CCR. Actually my 'standard bailout' for
a planned no (or small) stop dive is nothing external. I can breathe the dil
and ascend on my independent suit inflate bottle if things go wrong with the
loop. I am not convinced by people who clutter themselves up with large
external tanks for relatively minor dives. This just seems to negate all the
advantages of a rebreather and conflicts with what I was taught in Mod1.
This is not the place where I am going to rave about people who decry certain
makes of rebreather nor the attitude of some that all rebreathers are death
traps. What I want to do is put the case for rebreathers as I see it. Now I am
not bothered to justify the expense. Diving is a black hole for money so if you
want one I know you'll buy it, that is just the way it is and it's not my
fault. What I am interested in is what does the rebreather do well.
So we'll start with a quick recap - what is a rebreather? A rebreather takes
the exhaled gas that normally bubbles off in Open Circuit (OC) equipment and
goes to waste and recycles it. Naturally you need to replenish the oxygen in
that gas or the world will gently fade away. There are various methods of
controlling the mix you breathe that range from the simple oxygen only
rebreathers, through the more complex semi closed systems up to the high-tech
fully closed rebreathers with on-line oxygen measuring plus manual or fully
electronic oxygen injection.
Now having an Inspiration CCR and a fully closed Oxygen rebreather I tend to
view semi-closed systems as just drills I practice to manage faults but they
have their place in the diving world. However I'm not planning on commenting on
them in detail as I've never dived anything that primitive.
If you want to see my write-ups on my equipment with all the whinges look at my
Inspiration pages and the dreaded
Time for the sales pitch - what are the advantages?
Best mix for depth. Since you are blending the gas you
breathe as you go you can always get the highest oxygen which means the lowest
inert gas content so you have the shortest time to surface available. OK we
don't always go for the highest oxygen we tend to go for 1.3bar partial
pressure so we have some margin but that gives us 3 hours a dive on NOAA CNS
tables which would be a lot less if we went to, say, 1.4bar.
Ridiculously low gas consumption. When I moved the suit
inflation off the diluent tank and on to a separate 'Argon' bottle I moved into
the 30bar each side (oxygen and diluent) range for an hour plus of diving. This
means that I routinely put a helium mix in because I'm not going to need to
fill things very often. Remember this is 30bar on 3 Litre tanks. We are talking
less than five UK pound a fill here for a weekend's diving.
It's well lighter than the twinset. I'm getting to be
an old buffer and lugging the twins about was getting to be a pain. The
rebreather is not much less but it's significant. About the same as a 15 or
maybe a 15 and a pony.
It's safer. Hey? What? Haven't I heard about all those
accidents, fatal accidents, on rebreathers?
Haven't you heard about all those fatal accidents technical diving? The problem
is that a rebreather fault can be less obvious and the world just fades away
but if you are monitoring properly you catch the fault early and fix it. The
rebreather gives you something OC scuba can never give - time. If I look at my
oxygen read outs and wonder why I only have 1.2bar not the 1.3 it will take
several minutes for the level to fall to 0.4bar where the alarm sounds or
0.2bar that matches the air you are breathing now or 0.15bar where you start to
go wobbly. I have drilled driving the thing manually so I can control it. Even
if the displays are blank, ie: both my computers have failed
simultaneously, I can still run the thing as a semi closed rebreather and
stretch my diluent out to four times what it would be diving OC so my 3L tank
becomes a 12L and my 7L stage becomes a 28L twinset. Diving OC the one thing
you don't have is time. When a fault develops your next breath depends on you
Right back in our first scuba course we learnt to select a mask. You went
through the box and tried them until you found one that fitted. OK?
Right check the memory. You place the mask against your face. You breathe in
gently through your nose and if the mask sticks with just the lightest of
breath then it fits. That means it fits your face. Get the mask that fits your
face as a bad fitting mask becomes evil after an hour in the water. There can
be two masks from the same manufacturer, with the same name on them, to all
appearances identical and one fits and one doesn't.
Don't tighten a mask any more than you have too. That rapidly gets to be
uncomfortable and anyhow an over tight mask always leaks. You just want it
tight enough to stay snugly up against your face while you pop the regulator in
and out of your mouth. Remember the regulator changes the shape of your face so
taking your snorkel or similar with you when mask shopping helps.
Mask clearing is again something we learnt as beginners. The idea is that you
are going to blow air into the mask with your nose and that will cause it to
move forwards, stretching the strap slightly and bubble out all round. By
holding a finger on the top we just stop that bit moving forward, you don't
even have to press just stop it moving, and that only allows air to escape at
the bottom across your cheeks. If you tip your head back a bit that is where
the is water so it naturally goes out first. There is a complication with the
long masks like the Cressi Big Eyes, these don't really flex enough to clear
easily but the air bubbles out at the side. I have one but I just tip my head
so the water runs to the side then clear it there. Easy. ... but I do get some
You go to the shop and, starting at the cheapest work up until you find the
best possible fit in a colour you can stand. If it's a make nobody's ever heard
of don't worry.
Best fit? Take the mask and fold the strap forward so it is not involved in the
fitting. Place the mask on your face. Wiggle it. Does it press anywhere or is
it just gentle contact all round? Suck in very gently through your nose. Do you
feel it bunch up a bit? Again all round? No tight spots? Now just hold the
slight negative on your nose, let go and breath in and out through your mouth.
You are leak testing so does it leak and fall off?
This one feels good? Try it again with a mouthpiece in your mouth.
Still good? Keep hold of the mask and give the box to the shopkeeper and say
"I'll have this one please." Not this make. Not this model. This particular
piece of silicon and plastic.
Then when you wear it you can set the strap as loose as anything and it still
seals, it still fits and it doesn't leak. As I said at the beginning of this
section after an hour or so in the water an badly fitting mask becomes evil and
you will hate it.
Also all masks leak if you tighten them up. They are designed to seal to your
face and the strap just stops them drifting away with the tide.
If you buy a good fitting mask by mail order you did the wrong thing. You ought
to have bought a lottery ticket. It was your day.
If, like me you are in need of your glasses to do anything get lenses fitted
to a mask you have selected. Most shops are quite happy to take the mask you
select and send it off to be lensed. Get the stick in ones not the clip in ones
and if you need a distance and a reading prescription pay the extra for two
Why do you want to dump weight? You learnt this on rescue training. You dump a
diver's weight belt to pin them on the surface. What happens if you release
their weight belt at depth? Probably a lung over expansion injury and they
die. Read the accident statistics. A diver was non-responsive at depth so they
dumped his weights and killed him. Of course it doesn't say they killed him but
if the cause of death was a burst lung he must have been alive when they sent
him up so...
Right so now you've gathered that I'm not a fan of dumpable weight we can talk
about belts, harnesses and integration.
RIB diving. Here you want to be able to hand off your weights to the boat. The
problem is that when you become separated from your rig if the weights are
still with you either you or the dive set is suddenly heavy. This could be
either dangerous or expensive. Passing up the weights first will leave both
parts pretty buoyant. On a RIB you need accessible weight and you need to make
it clear to the crew what you are handing them.
For most other diving I prefer to attach it to me solidly. On the Rebreather I
swap between a pocketed belt and integrated pockets depending on what I'm
diving. On the twinset most of the weight is in the 300bar tanks and it's
always the belt on a single.
I have a harness, I won it shooting my mouth off on the letters page of a
magazine, I don't think I'd have bought one and I don't get on with it very
well. A harness, however, solves a serious problem for one type of diver. Well
one shape of diver actually. The shape with no hips.
I can pull a weight belt up above my pelvis, tighten it and snap the catch and
it is going to stay. If you have a 42 42 42 build it probably won't.
My weight belt is a pocketed thing. It is more comfortable than having raw
lumps of lead strapped round your tum and even more so with the shot pouches
that came with the harness in it. I confuse matters as I reverse the clip on
my belt so the weight goes on my strong arm handing it off. This has caused a
problem once and there is a weight belt somewhere off the Brothers Islands in
the Red Sea to prove it.
Dehydration is blamed for a lot of DCI hits when the diver was otherwise well
within the tables or the computer's plan for the dive. However staying well
topped up with water when you have some serious diving planned can give rise to
other problems. It is said that you can go in a wet suit although I'm far to
inhibited for that but wearing my lovely fluffy undersuit it would be just
daft. So what we need is a P-valve.
Start with Male Incontinence Sheaths. Every chemist stocks them because
it's a problem we might all face when we get old. Plumb this to a fitting that
connects through the suit with or without additional fittings so the waste
products are dumped overboard.
OK. The first time you use it it's a bit unnerving. You open the external
valve, you hesitate but there is no sudden cold feeling so no leaks and it
didn't run back in. Then go, hesitate again but there is no sudden warm feeling
so no leaks that way either. Sigh of relief.
There is some debate as to what sort of plumbing you need. Mine is the simple
sort as sold by O3 with collapsible pipe work and just a screw seal on the
outside. The alternative thinking says that you want a one way out valve to
stop the cold water getting back into your equipment and a second one way valve
to allow some air from your suit to enter the system to prevent a very
unpleasant case of squeeze.
But, just as you screw your face up and shudder, I need to point out that I've
taken my simple p-valve to 50meters and never been squeezed that I've noticed and I
don't have any one way valves. I suspect that we're dealing with nice sloppy
rubbery things that don't crease up like a squeezing drysuit so nothing is
uncomfortable. Well it works for me.
One thing you do need to do is run the scissors round if you are a hairy type.
Peeling the self adhesive stuff off can be a bit... tedious... otherwise. If
you see a guy in the changing rooms with his face to the wall hacking away with
his sea-snips somewhere inside his undersuit don't ask. The only problem I have
had with one is diving on a cold Stoney morning I stepped into the gents to pop
on the consumables and it was so cold I needed a size smaller than I was
carrying. Oh and always assemble the clip onto the catheter before putting it
on. Somehow it becomes ten times harder if you forget.
Update! Custom Divers now produce a thing called a 'Hee-wee'. I've got one. I'm
not describing it here in detail but Google will find it for you. You won't
believe how small the 'small' diaphragm is but once you have mastered putting
it on it is really comfortable and it is so easy to take off as there are no
adhesive bits. A long term burn-test was starting to feel a bit constricting
after about six hours which covers a day's gas diving in one hit. As a further
plus it is washable so you don't need a big box of the consumables.
There is some rubbish posted about 300bar cylinders on the internet and a lot
of it by people who really should know better. The first tale is that you can't
get it. Well I can get it free because the club compressor was replaced
and now does 300 and anywhere locally where I want to pay can do 300bar. Then
comes the fiction that you can't get Nitrox fills or at least you can't get
accurate Nitrox fills. More hogwash. Any blender with an ounce of sense and
more than two days experience can do 300bar Nitrox fills. You just aim a bit
low if you're partial pressure blending either getting the offset from your
tables or by experience. I'm lazy so I use my own blending software.
See here for the
IPAQ and Android versions
and here for the
Then they start wittering on about how you don't get as much as you think. Well
this is true if you just multiply it up but that would mean you are as stupid
as they are. My 10/300s hold as much as a set of 12/232s. OK it's 0.133% down
but if you just scale the pressures it should be over 7% up.
Then comes the cry of anguish that they are so heavy. We'll ignore the point
that everything is neutral in the water because steel on your back equates to
lead off your weight belt and climb the ladder back into the boat. My twin tens
displace virtually 4L of water less than my mates twin 12s so by the time we
count in the lead I must be nearly 4 kilos lighter. I don't know about you but
the ladder is the killer. Dragging stuff from the car into the shop for a fill
the extra 4 kilos of steel really don't hurt.
The real red rag to me is the guys who are six inches taller than me, half my
age, have never seen inside a cardiac ward who say "but it's too heavy for
me." Well give up diving wimps. Leave it to the girls and old men who can
While I'm on the subject of pressure...
...Can I complain about how divers have no clue about what 'pressure' means. We
talk glibly about 230bar or 300bar like they are trivial numbers. Hundreds of
bar are a serious business.
What is a bar? Well other than 14.5psi...
A bar is 100 kilopascals, a Pascal is one Newton per square meter,
a Newton is the amount of force required to accelerate a mass of one kilogram
at a rate of one metre per second per second, a kilogram is...
That didn't help did it?
Try one bar is roughly one kilogram of force per square cm.
That is about a 2Kg diving weight sitting on a postage stamp.
Let's start big.
The pressure in the breech of a big navel gun or, for that matter a field gun,
is about 3000 bar. Special guns are up to 4500 bar.
Just 0.07 bar was the pressure drop fuelling Hurricane Georges in September
1998. Georges did $5.1 billion worth of damage and killed 602 people.
A typical old steam railway locomotive ran at 15-20 bar. Those things always
impress me. Watch one pulling out of a station pulling grief knows how many
hundreds of tons of stock. And it's doing it all on one cylinder! Sure it has
two but they are arranged as double acting (they push and pull) and the
linkages are set in quadrature so they take it in turns to drive. Then the
linkages are a leverage step down so more distance and less force. All on 20
bar or less.
Am I making my point? Just because diving equipment manufacturers have tamed
this monster and packaged it in nice easy to use gadgets does not stop it being
a monster. Every now and then a tank gives up the game and bursts and hopefully
nobody is in the way. The
into a filled scuba tank is comparable to that in a hand grenade.
Training is good. Learning by experience is far more expensive. It is so much
better to have an instructor say "Well that isn't such a good idea. The last
guy that tried that died." A lot of diving procedures were developed in the
days of flaky, home made equipment and a lot of our procedures were tested
quite often by people who really needed them. They do work. Modern gear has
taken scuba from heroics to hobby. Kev and Sandra can do scuba, leaving little
Joaquin and Margarita in the crèche at the Club Chav resort hotel, and
have a killer time. Once you start doing it on your own, and I count diving
with a buddy with only your skill set as 'on your own' in this context, you
need to have already learned to save your own neck.
This goes through all levels of diving. If you want to dive trimix in your
rebreather find an instructor who has been there, got in the poo and dug
themselves out. If they think you should do an exercise again (and again) then
trust them. They have a reason. Somebody who fails you and makes you do a part
of the course again is to be treasured. They care.
Long Hoses and the Hogarthian Rig
The story, as I heard it, was that Florida Cave diving started with a serious
fatality rate and there was a significant chance that it was going to be
regulated either by the state or by the landowners. Several people put effort
into developing a diving protocol encompassing equipment, training, skills and
procedures to try and ensure that divers got back alive. The system involved a
scuba configuration named after its first populariser William 'Bill' Hogarth
Maine. Rather than call it the 'Bill rig' they settled for the 'Hog rig'.
Now in conventional beginner level scuba you know where the way out is. It is
straight up. All you need is a reasonable speed ascent and you have solved a
lot of the problems. In a cave this just isn't so. Above you is rock so you
need to know the way back. You need a torch and you need a line with clear
direction markers on it. Then, as a line in a silt-out may be invisible you
need procedures to get your hands back on a lost line so you can feel for the
Then what about an Out Of Air emergency? Firstly you need to have the
discipline not to swim your self into an OOA state and a rig that, if a fault
dumps your air, enables you to save what gas you can and as a final resort both
you and your buddy should be equipped to get out of a cave sharing air. To try
and make it possible to survive a failure the Hog rig is a twinset with a
sealable manifold joining the two halves and two hoses, one seven feet long
that wraps around you and is the one you normally breathe from and the other is
shorter and the regulator is held just below your chin on a necklace.
The long hose is the get out of the cave trick. Let us imagine the worst case.
You have blown something on your rig, you are suddenly OOA and the visibility
is now so churned up be the huge surge of bubbles that was the air that was
going to keep you alive for the rest of your dive that you can't even see your
gauges. The two buddies immediately converge. Since they both have a hand on the
line and they knew if they were leading or following this isn't too hard. Now
the OOA diver feels for the buddy's head and takes the regulator from his
mouth. This is the only one he can find fast. This is on the long hose that
runs behind your head so you virtually just 'nod' to release it. Breathe and
live. The buddy reaches up and puts his secondary from the necklace in his own
mouth. No discussion. Start for home.
The first idea in this system is that the OOA diver goes first as they exit. If
it is narrow and he stops the guy behind must stop so the hose never gets
pulled out of his mouth. This means it has to reach one whole diver's length
The second idea is that if you need air take the reg in my mouth. I may have
several regs on various deco mixes festooned about my person but only I know
which is which. You need air fast so go for the one reg that I promise is
switched on, is working and is the right mix for the depth. That is the one in
my mouth. This cuts through any complicate decision making. Take that reg.
I'm still not quite sure on this one. The numbers don't quite stack up the way
I'd like. I bought a separate suit inflation bottle when I started putting
trimix in the rebreather. Helium conducts heat six times better than air so it
is bad news pumping that into a suit that is supposed to keep you warm.
Suddenly the suit works a lot worse just when you are doing deeper for longer
and want it more.
However a lot of people said Just put air in the bottle. It makes no
Now certainly that returns you to the situation you had before and avoids
blowing your suit up with cold, expensive helium but is there an advantage with
going the extra step to Argon?
Well the numbers aren't so obvious. Argon's thermal conductivity is
17.9 mW/meter Kelvin while Air is 26.2 and Helium is a frightening 156.7 (at
1 bar/300K). Hence Argon only offers 1.46 times the insulation so I don't feel
a swoosh of warm as I push the injector. In fact I don't feel anything
different. However what I do notice is that after an hour in 5°C water my
lips can be too numb to talk properly but the rest of me isn't particularly
cold. It didn't used to be like that so I'll stick to the Argon when it's
I've read some reports where military divers were tank tested to see if Argon
suit inflate would maintain their core temperature longer and the results were
inconclusive. They didn't gain much time so we're not talking huge benefits.
Also you need to flush the air out of the suit a bit although the percentage
argon once you are deep, due to the compression of the gas, is automatically
Beware of welding Argon. Some is 100% but at Combro we use AP's Coogar which
has a big lump of carbon dioxide in it. This is not so good in a dry suit as it
reacts with any water there, eg perspiration, to produce a mild acid that
effects some people badly. Have you ever had the desperate desire to scratch
doing stops? This is the formula for just that.
I've actually used the word fail quite a lot in this document. This is
not because diving equipment fails a lot but because the consequences of a
failure can be so serious. A small problem with a valve that might be a minor
annoyance at home is desperate when it controls what you breathe.
This means that divers have to be trained and prepared to manage failures in
their equipment and need to look after their equipment to ensure that critical
failures are minimised.
Any dive equipment must be thoroughly appraised as to what might fail and how
do you cope with it. Quite a bit of this is covered in basic training and the
novice needs to follow the skills they are taught to minimise risk. However
when a diver progresses to more complex diving the buck stops with them.
It is your life and your life support equipment must be your own personal
concern. You may off load the configuration to trusted mentors and the
servicing to a trusted shop but you have to choose them.
One of the favourite tools for planning is counting "failure points". This is
an assessment of a rig or a change to a rig that weighs the benefits of an
addition against the new ways things can break. While this is one tool in your
mental tool-box it needs to be used selectively not just grabbed at as the
final argument that says something is a bad idea because it "adds a failure
point" when that failure can be insignificant, uncommon or easily managed and
the change is otherwise beneficial as a convenience or confidence
An example of this was my second SPG (pressure gauge) on the twinset. There are
straight forward procedures to manage with one and the addition of a high
pressure hose, rotating O-rings connecting the head and the pressure gauge
itself can all fail so you have added things to go wrong. However I had had a
pressure gauge jam on me and not indicate the gas correctly. It is my
preference to view myself as part of the kit and assume that I am one of the
weakest links in the chain. A second SPG warns me of several self induced
problems as well as mechanical failures and reduces task loading as I am not
required to remember the previous pressure reading to detect a jam.
Failure point counting has its place in assessing a rig but it is not the
be-all-and-end-all defining method.
On about our tenth dive, and our first away from the PADI fold, we hit a snag.
The sea state was up a bit when Adrian dived but when they surfaced it was
not what I would call diveable. Fortunately he was diving with an experienced
Buddy as I had trashed a regulator on the previous dive. Although they could
see the RIB they were diving from those in the RIB could not see them.|
Now this was Portland, and to this day I'd say that Portland is not the place
that you are going to have an incident just because you are not picked up
cleanly but they managed it.
Adrian was not a strong finner in those days and they were carried on the ebb
the tide out of the harbour mouth and further and further away. Diving inside
the harbour nobody had thought to carry a flag but when they saw a fishing boat
they waved at it and Adrian attempted to blow the whistle attached to his
jacket. He said some very rude things about that whistle later but his buddy
was a bit more forthright. It gurgled and it hissed but it never whistled. They
resorted to shouting and thankfully he heard/saw them, pulled over, listened to
their tale of woe and radioed the RIB.
Well the story may have ended happily but more by luck then anything else. So
now we carry surface detection aids and a dirty great whistle on the BCD
inflator blown by back gas. It's called a "Dive Alert" and the bumph says you
can hear it for a mile. Probably. The only time I have been fool enough to blow
it without ear defenders on I couldn't hear much for 10 minutes... It gets
I bought the Buddy-Blast version for the Inspiration as the Auto Air on the
wing has a funny Buddy fitting. If I ever have to use one in anger I'm putting
my ears under water as I push the button.
Oh a final note on other surface detection devices.
Flag: good. EPIRB: got one, hope I never have to try it out.
One item that often gets suggested is the old AOL CD as a reflector to attract
attention. It sounded good until I thought about it. No sunlight, nothing to
reflect. But if we have sunlight we have no mist so it is a clear day. If they
look my way they can see me. So they aren't seeing me because the sea is rough
and I only see them from the top of the wave. So any other detection aid like a
flag or a DSMB works. And they work in other cases too.
Marshalling is a very military concept. You write down all the parameters of
the divers (qualifications, next of kin, gas in and out, dive plan et. al.)
so it is available if required. However this raises the problem of what is
that information for? Are you recording or authorising?
Naturally if the divers are students and you are operating under a QA standard
either professionally or for a club you want those details to prove you are
adhering to standards so if an accident occurs it was an accident not your
negligence. However if you are just providing the boat it is the divers
themselves who must call to do or not do the dive. If it is my trip I do not
wish to be responsible for your accident and, in fact, if you ask me if you
should dive I will always say 'no'. I reason that if you are unsure enough to
ask that is reason enough not to dive. There will always be another day and
another dive so why risk a dive you are unsure of and almost certainly won't
This means that on my trips the only record is the skippers. I don't need to
know what gas you are carrying or what your plan is. Of course we will have
discussed the dive plan so we don't overstay the time but I do not approve your
plan. Well. Unless I'm marshalling for the club.
DIR is a rather holistic approach to diving embracing fitness, training,
equipment configuration, team work and attitude. It grew, I believe, out of the
danger that Florida cave diving would be virtually shut down due to the high
fatality rate so people devised an equipment and procedural plan to try to
ensure diving there would be much safer.
DIR is always identified with the manifolded twinset for gas redundancy, the
long hose on the primary regulator needed to get an out of air buddy out of a
narrow cave and the *take the regulator in my mouth* approach to gas sharing.
It goes well beyond that but you need a DIR trained diver to explain things
which I am not. If you are considering any form of technical diving DIR should
be on your reading list but not everybody wants to swallow the whole package.
It does tend to attract its zealots who endlessly debate the details with
fundamentalist glee which puts a lot of people off.
The best book on my shelf on the subject is "Doing it Right. The Fundamentals
of Better Diving" by Jarrod Jablonski ISBN 0-9713267-0-3. I bought it at a dive
show but I suspect you can find it in the usual places, maybe even the
OK that's a good start: so what don't I like about DIR?
Well, like so many things, it has engendered a whole mass of elitist hangers
on. People who feel big because they can claim they are *Doing it Right* not
because they want to do things well but because that means that other people
are doing it wrong. And they can say so. Loudly.
The attitude of many of the DIR minded instructors and divers I have met around
the country deserves nothing but praise, they are trying to promulgate a system
of diving that they see as enhancing safety and teamwork. However the internet,
that final bastion of the close minded bigot, brings me into contact with some
people I would prefer to avoid. Now most training systems like this would
condemn adherents with a name calling, insult shouting, "Do as we say or
%$&*^%$& off" mentality but sadly here, in the 1990s and early 2000s, it went
right to the top.
As I say DIR should be on your reading list if you are moving into technical
diving, GUE courses can help you acquire skills, Halcyon equipment is good if
rather overpriced but call me a stroke because I don't carry my kit in the
canonical positions handed down from on high and I will laugh in your
The guy who sent me hate-email when he read my web-page on my inverted twinset
got pretty much that. He did get asked some questions, the answers were on the
page he was complaining about, but he quickly gave up. He could use rude words
and make sweeping statements but all he knew about diving was what people
had told him and contradictions didn't seem to matter. Conversely maybe he
sobered up and realised what a load of slime he had been writing.
There is no one true way in diving. It is a changing, evolving sport. Use an
equipment set-up that suits you, your body and the type of diving you want to
do. If that is one set way as you are diving in a team and you want to be
interchangeable even in the water fair enough. If not and somebody shows you a
better way to do something adopt it. If your buddy can't cope with that but
can't give a good reason why it's wrong - get a better buddy. Nothing is cast
in stone. Nothing is forever except getting it wrong.
PS: DIR and rebreather users: Most of the rebreather world always remembers the
description that the Buddy Inspiration as "Should be sold with a shovel" from
the mouthpiece of DIR at the time. This does tend to mean that the letters DIR
are treated with contempt in these circles and that is a large percentage of
the technical diving fraternity. Perhaps it would be better to let the acronym
die and let GUE teach their style of diving without the acrimony that the name
DIR brings. Certainly I treat in your face internet DIR divers with the
contempt due to duckspeak while, for GUE trained friends, I have nothing but
Suit as buoyancy
"I was told to use the BCD on the surface and the suit for buoyancy in the
This is a good skill to have so learn it and practice it but when doing real
diving keep it for the backup when the BCD breaks. Set the autodump (you do
have an auto dump?) wide open and now ignore it. Add air to the suit to keep it
comfortable but not so there is a huge blob of air moving about. Trim out so
you naturally hang level (or feet actually higher because you bend your knees)
and you will be warm, comfortable and stream-lined so fining is easy. Use your
lungs and the BCD to control your buoyancy. If something goes wrong and you are
ascending too fast pull and hold the BCD dump and as soon as you stop rising
bang some air back in and trim for neutral hover regardless of depth. Once you
have your act back together resume your dive.
You will have moments like this in your first year and the BCD is a buoyancy
control device so it dumps fast. Get to know the pull dump at the bottom too.
You will not stop an accelerating ascent on the suit as the air is spread out
in a bag that is full of body and fluffy suit and can't get out fast
Let's think through the numbers:
You start the dive with a 12L/232bar cylinder. That is 3.3Ks of air. If you are
going to do a nice slow ascent having used most of your 50 bar reserve (perhaps
you have your buddy on your octo) you are neutral then so you started the dive
you were 3.3Kgs negative. 3.3Kgs of buoyancy is a 3.3 Litre bubble so look at
the milk in the fridge and imagine 3.3L loose in your suit. Is it at your
shoulders? your bottom? your feet? This is 3.3L over and above what it takes to
keep the suit comfortable. It is just flipping from one high point to the next.
3.3L is the top end of what is manageable but as soon as you go above that, a
15L/232bar is 4.1 and as 12+12 twinset is 6.6Kgs, it really isn't.
The suit as buoyancy only really works on a single and you are pushing your
luck even then. However when you learn to dive a dry suit learn to control
buoyancy with the suit. This becomes your get-out-of-jail-free card to manage
a BCD fault so it is a skill to acquire but, in my opinion, not a skill to use
every dive. You may chose to do your ascent, after most of the gas has been
consumed, on the suit but that is a personal decision.
Some people will say that having two sources of buoyancy to adjust makes the
job to hard for poor beginners but that really isn't true. Set the autodump
wide open, put some gas in when the suit starts to compress on you and
otherwise ignore it. Auto dumps really are auto used like that. Anyway what's
the problem with two sources of buoyancy to control? Many times in a dive I may
decide the suit is a bit tight and I put a bit extra in and bubble some off
from the wing. It's hardly a skill because I'm controlling my position in the
water by how I breath (well not on the rebreather as how I breath does not
effect buoyancy so I have to get the suit and the wing spot on).
I'd say use the BCD, that's what it's there for, but be able to do both.
This was actually written for somebody I considered was being a bit simplistic
in their ambition to become a diving instructor when they has barely started
Instructing and diving are totally different. I'm PADI trained and I know the
way the system sweeps you along but once you reach RD level you have options
ahead and 'Go Pro' is only one. It doesn't work out well for many people.
I used to be an Institute of Advanced Motorists Observer on motorcycles (called
Observer not Instructor for, I suspect, insurance reasons). Training somebody
up to a standard where they pass the pursuit test with a Police motorcycle
instructor was a personal thrill but it bore no resemblance to riding a bike
fast and well myself. I would never want to be a professional bike instructor
as I like to set my own pace and work the victim, oops, student hard and send
them off for their test pretty assured that they will ace it. No way would a
customer put up with what I, as an unpaid volunteer, could hand out but nobody
ever complained and I just got recommendations.
For diving however I don't see it. If I want to instruct I'd go the club route
and keep my lucrative full time job but only if it is extra time in the water.
I'm not losing the diving that I, like you, love to mess about shallow doing
the drill again, not because they got it wrong, but because I think they need
a couple more goes to be relaxed with it.
Ask the question "What do I want to spend my time in the water doing?". There
are the people who get their kicks out of 'Colourful things' who are best
suited to warm water holidays. Camera geeks who have masses of fun but are best
left alone as they are oblivious to the world. Wreckies, both the salvagers and
the lookers, archaeologists (specialist wreckie) and finally instructors. Pick
one or several of those options and have a great time the rest of your life. OK
I confess I'm a wreckie with aspirations to being an Archaeologist so my
subsequent training is to enable me to get to the wrecks I want to go to and to
gain the archaeological techniques needed to understand them.
There is the side issue that setting your life's ambition to be a dive
instructor is aiming rather low. It's not hard, it doesn't pay well, there are
lots of them and not many people can make a life's work out of it. The
drop-out-rate is huge as it just isn't something you can do week by week for
years. Frankly the people I see having the most fun instructing are the totally
'people' oriented people who would be teaching and helping people on their
weekends even if they had never discovered diving. If that's not you get real
now. This is something that is built in to your basic character and no training
will make it happen.
If you want to do it then do it. But don't burn any boats as they say.
Technology or skills?
The question if often posed: Should divers substitute technology fixes for good
in water skills?
Now this is a loaded question normally used to decry some latest device that
bypasses a skill that was once trained. Yes. We all approve of divers having
the skill set to manage a dive on much simpler equipment than many of us carry
but does that mean we should avoid the gadgets and devices that are getting
I say we should embrace good technology:
The weakest link in my dive kit is me. If I can just use
me to monitor the technology that is good in my book.
Hence the tables and depth/timer back up to the computer.
The self seal blob backs up the self inflate.
The CCR is all electronic with manual and SCR modes as a fallback.
The twinset was manifolded with an isolator not independents.
Autodump not cuff dump.
Octopus not buddy breathing.
Originally written on Yorkshire Divers
There are lots of mathematical explanations of Erik Baker's Gradient Factors
and as a mathematician I like them but let me try and do a words
There is Professor A A Bühlmann's deco model. Tried tested. You can
'overpressure' yourself, ie let the gas tensions in your tissues go right up to
the Bühlmann a/b limits and that draws the line between 'probably not
bent' and 'at risk'. That is '100'. Fast out - probably not bent.
Then there is a another way of using Bühlmann's compartment model and that
is never to let the tissue inert gas tensions exceed the absolute pressure.
This is '0' definitely not bent but so slow. You are decompressing solely on
the oxygen content of your breathing mix.
A first stab at a conservatism system is to pick a number between 0 and 100 and
plan on that. However that doesn't really address the problems of the
compartment model. Better is more conservative deep, so you introduce short
deep stops to suppress bubble formation. This is where the gas flow graphs are
all steep so small changes in physiology make big differences so there is more
likely hood of an error (the maths is OK, it's your body that is at fault) so
use 50 here. Stay well away from the bent/at risk line.
Shallow, where the graphs are flatter, you can be less conservative, say use
80, because off gassing is nice and predictable here. Between your deepest stop
at 50 and the surface at 80 factor the conservatism by depth so half way it is
(50+80)/2 = 65.
Right so how does this work out in practice?
Start from the bottom at highest conservatism so deco stops are called to keep
you well away from bubbles. This means that blood jostling about doesn't flash
up much in the way of microbubbles and any that do form stay small and get
flushed out in the lungs. However shallow the conservatism drops off so you
don't do an extra hour of stops where microbubbles and turbulent flow have
ceased to be a problem.
So what do you choose in a GF set? 100/100 is raw Bühlmann. Fast out and
probably safe for any one dive but if you do 100 dives a year you can probably
expect to get bent a couple of times and you will usually feel pretty rough
after a dive. 0/0? Take a lot of gas. You won't get bent but don't expect to
be invited to dive with those guys again. 30/70, 40/80? The smaller, deep
number will switch in deep stops and the larger, shallow number is still more
conservative than raw Bühlmann but not drag out the time
Being more conservative deep does not extend your dive time much, these are
typically two minute stops and having run times for leaving depths is a good
tool to control your ascent rate (slow is good). Hanging on a string at 6
meters is very predictable so you don't need so much extra here.
GF is a good system. If I'm cutting tables I use 50/80 or 50/70 but I confess I
do most of my diving on a computer set to zero conservatism and I add deep
stops/extra stop time in my head. The computer is on, effectively, 100/100 so
if something all goes horribly wrong I can abandon the slate and do shortest
time to surface.
Well it looks like BSAC are going to make Nitrox part of entry level diving so
it seems reasonable to discuss it here.
I like Nitrox. It has its down side but the
advantages really stack up. Nitrox is not the deep water diving gas, that is
Trimix, but it offers better no-stop time and more safety for diving down to
about 35 meters.
The problem is Nitrogen. Nitrogen is a nasty gas to dive. It does nothing good
for you. It dissolves in your blood and if you overstay your time or ascend too
fast it can spontaneously undissolve and produce bubbles that block things up.
We have lots of vital organs and a bubble blocking something reasonably trivial
like the blood supply to a piece of skin may be bad but a bubble in the brain
has the potential to be worse than death. Nastier still the Nitrogen, when
dissolved, acts as an aesthetic and takes your brain away. I have dives below
40m on air in my log but none in my memory. The only thing in Nitrogen's favour
is that it is dirt cheap as you get 790ccs free with every Litre of
OK so the simplest way to reduce the Nitrogen in your breathing gas is to
replace it with Oxygen. Now Oxygen isn't the nicest stuff but we'll come back
to that. Your body can cope with Oxygen. Dissolved Oxygen is fuel for your
cells so even if a bubble does form and the blood supply is reduced then the
tissues rapidly become oxygen starved and the oxygen in the bubble gets used up
supplying the short-fall and that reduces the bubble in the process. An
excess of dissolved oxygen is rapidly depleted as the cells use it and don't
bother getting it from the blood. Decompression calculations virtually ignore
oxygen because in live animal tests (goats) forced oxygen bends spontaneously
Right so why don't we dive 100% oxygen all the time? Well sadly oxygen has its
dark side too. There has been a lot of work done with oxygen only systems as
the military like the idea that you can recycle it giving no nasty
tell-tale bubbles. However pure oxygen systems, once you got to 40 feet (13
meters) and below, often put the diver into convulsions. That was 2.3 bar of
Oxygen and convulsions, if you have to keep a mouth-piece in your mouth
to breathe, can be annoyingly fatal. If you test your divers you can get ones
who can reliably do 45 feet (15m) and below but it's not a good idea.
Quick Maths Recap.
The pressure at the surface of the air we breathe is 1 bar.
As we descend we gain an extra 1 bar every 10 meters so at 35 meters we have
4.5 bar of pressure. Pressure = depth/10 + 1
Partial Pressure is the fraction of the total pressure made up of the gas in
So at 30m (4 bar pressure) with 30% Oxygen 70% Nitrogen you have an Oxygen pp
of 4 * 0.30 = 1.2 bar and a Nitrogen pp of 4 * 0.7 = 2.8 bar.
If you want to know the partial pressure pp = total_pressure * fraction
If you want to know the fraction f = partial_pressure/total_pressure
If you want to know the total pressure p = partial_presure/fraction.
The current thinking is that 1.6bar of oxygen is a good, safe limit. 1.4bar is
better. There are tables that you can work out your exposure on if you are
planning a dive but on conventional scuba mixes those limits will keep you out
OK so this sets a Maximum Operating Depth (MOD) for any gas containing
If we are talking about Air with 21% Oxygen the total pressure at that
depth will be max_ppO2/fractionO2 = 1.6/0.21 = 7.6bar so 66 meters.
If we prefer 1.4bar max Oxygen then 1.4/0.21 = 6.7bar so 57 meters.
For N% nitrox convert N to a fraction (35% => 0.35) and divide it into 1.6 (or
1.4 or whatever limit you want). That gives the depth in bar. Subtract 1 for
the atmosphere at the surface and multiply by 10 to get depth in meters.
eg: 35% -> 0.35 -> 1.6/0.35 = 4.57bar -> 35.7meters
We tend not to push Oxygen limits. It is so easy to stay safe. You may be a
hard man who can take 2bar for extended periods but the only way to find out is
to do the dive and not drown. So what? You keep diving deeper until you don't
come back and they engrave your personal limit on your tomb stone? Lacks
something that does.
So what advantages do we get from Nitrox?
Well the whole trick is that we can ignore oxygen in the deco calculations. So
let's consider a 35% mix. That's 35% oxygen and 65% Nitrogen.
Always work out the MOD. MOD = 1.6/0.35 = 4.57bar = 35meters.
Let's say we're diving 30 meters.
At 30m our ppN2 will be 4.0bar * 0.65 = 2.6bar and that is the equivalent of
diving on Air to 23meters (2.6/0.79 = 3.29bar) so we can plan the no stop time
with Air tables as if we were at 23m.
Now my favourite Air tables give me 17 minutes no stop at 30m and 36 minutes no
stop at 23m. We have probably gone from being limited by the no stop time to
being limited by the gas consumption.
Not so bad a deal.
How do we use it in practice?
We are diving the City of Waterford out of Brighton.
The guide book lists it as 28meters.
We are diving on a high slack of 5meters so 33meters to the sand.
MOD? Let's go for 36m so that's 4.6bar so 1.6/4.6=0.347 so 35%.
Get 35%, measure it on an oxygen meter you trust and sticker 35% 36M on
the tank. This trick is important as 35% might just kill you at 45meters.
The figures are going to be quite near my example above and when we dial up 35%
on our Nitrox computer it will work out our real dive times and, as the City of
Waterford is about 8m high in places, we should get a good 40 minute dive with
the opportunity to cut-and-run if there is a problem as we have never gone into
We had finished the pool session for the evening and adjourned to the bar. They
were doing OK overall but I had seen them having problems tonight. I asked what
the difficulty was. It was water up the nose causing sneezing, spluttering,
must stand up and stop this drill syndrome. Yup. That's what I saw. It happened
every time you tried.
I mentioned 'air way separation'. I knew they didn't have a clue so I laid it
on a bit thick, telling them they had a valve in their head that could switch
their breathing between mouth, nose, both, neither, to get to the "I can't do
that" stage. They couldn't. They were convinced.
Yes you can. You do it all the time you just don't know you do. Let's practice.
Hold your breath and blow. Right now do it with your mouth open so your cheeks
don't blow up. See? I can see your chest tighten. That's the off position. The
important thing about 'off' is not to do it ascending. Best not to do it ever
as it can catch you out. Well unless you are snorkelling.
Now breathe in and out through your nose with your mouth open. See? Pinch your
nose a bit with your fingers so you can feel it is only going that way.
Now the one you want in the pool. Just breathe through your mouth. Close it
down to a kiss so you can feel it is only going through there not your nose.
Like when you blow up a balloon. Now the problem is to just hold that
configuration because you tend to blow out like that and then relax it as you
breathe in. Blow-suck-blow-suck on just the kiss. If you do that on a just a
reg no water enters your nose. If you need to clear your nose either pop a hand
up and pinch it so you can blow out or switch to nose breathing and back
mid-exhale. Don't stop though or you always get some back.
I've done lots of mask off drills and the 'blow through a kiss' mentality sets
things up just fine. Head down is too easy and head level works too. I suspect
that if I looked up I might regret it.
It's not quite that. It's one of those things you explain and watch the eyes so
you know when to stay on a point and when to move on. I expect you could
formalise it but I've never tried. It's a "realise you can" sort of thing not
something you learn.
A lot of people complain that when they get to doing nitrox and trimix diving
they get bogged down with the calculations. There are so many formula and they
all look so much the same that it's hard to remember which one to use and to be
sure that you've actually got all the details right. This shouldn't be like
that. Let me introduce you to depth in bar. This is the magic trick
that makes all the formula simple.
We all know that the pressure rises as we descend through the water and in
metric units it is very easy to work out. You start with 1 bar of air pressure
at sea level and gain an extra 1 bar for every 10 meters you descend.
So the depth in bar is the absolute pressure at that depth. So divide by ten
and add one.
0 meters = 1 bar
7 meters = 1.7 bar
18 meters = 2.8 bar
72 meters = 8.2 bar
That was easy wasn't it?
Now do it the other way.
3.6 bar = 26 meters ... OK I won't bother with another list as you've got the
So how does this make life easy?
What is the oxygen partial pressure of 27% Nitrox at 26 meters.
26 meters is 3.6 bar deep. 27% of 3.6 bar is 0.27*3.6 = 0.97 bar
Yes that simple.
What about EADs? Equivalent air depths.
What do we want here? We want to know how deep we would be diving on air to be
breathing the same amount of nasty nitrogen as we have in our Nitrox so we know
what depth to use in our tables.
Lets say we are diving 35% O₂ at 27 meters.
27 meters = 3.7 bar.
How much nitrogen? 65%. So 65% of 3.7 bar is 2.4 bar.
If we were breathing air with 79% nitrogen how deep would that be?
We multiplied to go from depth to fraction so to go from fraction to depth we
2.4/0.79 = 3.0 bar deep. So 20 meters. So although we are at 27 meters we can
plan our dive times as an air dive to 20 meters. That should help increase the
no stop time.
OK what about the MOD for our Nitrox? The Maximum Operating Depth before oxygen
toxicity becomes a problem and nitrox stops being friendly. If we have 40%
O₂ what do we write on the cylinder?
Let's say you use the rule of never break 1.4 bar O₂. I know there are
1.4/0.4 = 3.5 bar so 25 meters.
Do you see how everything so far has reduced to a single steps of multiply and
divide? All the business of multiplying and dividing by ten and adding ten, or
was it one, or was it subtract, has gone. You couldn't remember them
anyway and who wants failure points in their mathematics?
Let's do a big one. We are diving a rebreather with an Oxygen setpoint of 1.3
bar and a diluent of 18/40 trimix and we are at 60 meters. What is going
Right 18/40 trimix is 18% Oxygen, 40% helium and hence the remainder is 42%
We are 7 bar deep so that is what our gases must add up to.
Well the oxygen is easy. If the rebreather is working it is 1.3 bar.
Right 5.7 bar to go.
Now the rest of the gas came out of the diluent bottle so it is in the same
ratio as only Oxygen has been added. So He:N2 is 40:42 so add them
together is 82 so the Nitrogen is 5.7/82*42 = 2.9 bar and the helium
is 5.7/82*40 = 2.8 bar.
We are breathing 1.3 bar of O₂, 2.9 bar of N2 and hence 2.8 bar of helium
and a quick add up checks that is 7 bar.
END? Equivalent Nitrogen Depth. How deep would we dive on air to be that
narked? 2.9/0.79 = 3.2 bar. 22 meters. Even I still have my head in gear
You want to do that one again because you think Oxygen is narcotic?
Well you're probably right so:
O2+N2 = 1.3+2.9 = 4.2. Think of it as 4.2bar of air although it's really 31%
nitrox. 4.2bar = 32 meters.
I was asked about filling a cylinder. Particularly filling with oxygen. If it
is clean why do I fill so slowly?
The short answer is that I'm a coward.
The long answer is time for some maths:
Let's say we're using my old Air compressor. No, not the nice electric one the
monster with the big petrol engine that I got rid of. It does a 12L cylinder in
about 10 minutes while the electric one takes 25. Let's work on 20bar/minute
fill rate and see what's going on. As I can just open the valves between my
O₂ bank and a cylinder I could fill faster than this if I don't need the
20 bar per minute into a 12L is 12*20 litres/minute so 4L per second.
Now if you've had a valve out of a tank you'll know what I mean by a 'debris
tube'. Basically it is a bit of pipe that feeds the valve that means that if
the cylinder is upside down any solid matter does not go into the valve and
into your regs and possibly into you.
A debris tube is about 6mm inside diameter so 0.3cm internal radius so, using
the pie-r-squared we get about 0.3square cms of pipe. Now 4000 cu cms per
second going through 0.3 sq cm must go at about 4000/0.3 about 12000 cms per
You're not sure what 12000cms/second is? 12000cm/sec is 120 meters/second so
Now when oxygen is 1.3 gram/litre at air pressure it gets to about 40grams/L at
30 bar. A pretty standard 15 grain airgun pellet is 1 gram and flies about the
Right so the last part of the equation is oxygen. For something you need to
breathe moment by moment every hour of your life oxygen is not your friend.
Do you remember the formula of gun powder? Charcoal, saltpetre and sulphur
wasn't it? You've got it. You are adding a good oxygen source (saltpetre) and a
catalyst (sulphur) to liberate the oxygen fast. This converts those brick
things you can never get to light on your barbie into the stuff Guy Fawkes
planned to blow up parliament with (Come back Guy Fawkes - your country needs
The final part of the review is using a gas-axe. I love that tool. An
Oxy-acetylene flame to heat a metal up, eg: steel, and then squeeze the lever
and dump huge amounts of extra oxygen into the flame. Steel burns beautifully
in pure oxygen.
Are you getting the idea? We have all the components of a very nasty incident.
Oxygen fires do happen on scuba gear, a boat was burnt out to the water line
not many years ago and its picture was well aired on the scuba forums at the
time. It is not my intention to end up in the obituary notices of the forums I
contribute too as burnt to death or blown up. That just lacks finesse.
Current limiting and Oxygen Cells
There is an awful lot of rubbish spouted on the internet about the galvanic
oxygen measuring cells we use in rebreathers and it makes you want to weep at
times. Instrumentation is my business. Making instruments that stay in
calibration and knowing when they won't is bread and butter.
OK. All galvanic oxygen cells current limit. Yes all of them. That is
how they work. A cell is a battery and a battery's chemistry has a predicted
voltage that is derived from the materials used. In theory they should give
that voltage from the day they are made except that one chemical that is part
of the planned reaction has been left out of the assembly. Oxygen.
Oxygen is one of the fuels of the cell so the more oxygen there is in the
equation the more electricity is generated. The chemistry sets the voltage and
the fuel, the quantity of oxygen available, sets how much current it can give.
If you put a circuit on the cell that draws current you can draw up to this
current but ask for more and, like putting too higher power bulb in your torch,
you don't get more and the voltage from the cell fades.
Look at the graphs. This is for a naked cell without the little circuit board
attached. These are V/I plots where volts are in millivolts vertically and
current is in micro-Amps horizontally. I am applying a greater and greater load
and watching the voltage and the current change. This has to be done slowly as
the cell takes time to settle to its new conditions, about six seconds.
The blue line is a cell in air. Notice how it starts at about
660mV when I am applying no significant load. As I try and take current from it
at first it drops to its 'working voltage' of about 500mV and then slopes in a
way that implies an internal resistance of about 700 ohms. Then it starts to
run into its current limit. It is clearly not going to give more than 120uA no
matter what I do.
Now the same cell but installed in the system shown on the right with the
Oxygen valve open and plotted in magenta. We have the same starting voltage and
the same working voltage but now the internal resistance looks like about
300 ohms and the current is limiting point is going to be about 550uA.
To make this work in a practical measuring cell they are wired into a quite low
value resistor which will always try to draw more current than the cell can
deliver and will drag the output voltage down. This limited current is sampled
by a second resistor so a voltage can be read and this has a temperature
compensating varistor/thermistor across it to make up for the fact that the
uncompensated cell is also quite a good thermometer. I have represented this by
the yellow line on the graph which is a V/I plot for a 100ohm resistor. The
output voltage readings for a given Oxygen ppO₂ will be based on the
point where the V/I plot for the cell intersects that line.
So in a specific pressure of oxygen the cell gets a certain density of fuel
so it can give a certain amount of current which we read as the output. The
problem comes when, due to age, the other components of the chemistry start to
become exhausted. The battery is going flat. Now a second factor is limiting
the current and, regardless of the oxygen, it will not provide more than a new
limit on current. There was always a limit like this but before it was the
equivalent of several bar of oxygen. To a rebreather user this becomes
significant when this new current limit starts to become comparable to the
oxygen readings we are expecting. The cell will now read correctly up to a
certain ppO₂ and then will give no more current regardless.
So what do we do? We want to be sure the cells are correctly controlling the
setpoint. So we need to know that they will detect that we are on or above the
maximum ppO₂ and stop the inject. We calibrate them at 1 bar of O₂
or there about and so if we inject oxygen at depth so the displays read 1.4 bar
or above we know that they are still working well enough. A cell becomes
dangerous when it gives a fictitiously low reading which may cause the system
to inject extra oxygen, boosting the loop oxygen fed to the diver higher and
Now this is why cells get 'used up' faster if we leave them in pure oxygen. The
other materials in the cell are being eaten up at five times the rate. It is
also the reason why I cap off the cells in my oxygen analyser when it is not in
use. The cells last longer once they have consumed the oxygen inside the cap
and the reaction stops.
On the rebreather I just flush it through with DIL (if I remember) as taking
things to bits is too much of a chore. Incidentally the electrical load from
the rebreather or a nitrox analyser is trivial compared with the internal
current so unplugging the cell will not significantly extend its life.
You cannot test for 'current limiting' at any value of oxygen less than the
normal loop values. The millivolt readings in air or at 1 bar O₂
quantitatively tell you very little. There is a slight clue that a cell that is
running out of stuff will respond more slowly to changes in the oxygen but this
is a rather ephemeral effect. I have long since decided to change them every
12 months and check I can get them well above the setpoint either, incidentally
as they spike on a descent, or by pushing the O₂ inject on a 6 meter
stop. If an oxygen flush will go up to 1.5 bar, I can never get 1.6, I know
they are safe for the next few dives.
What's all this about? Well the way a galvometric cell is produced means that
the output is not predictable so the makers do not sell them with a
certification sheet that says something like 50mV = 1 bar ±2% or
something as it just wouldn't happen. What they do say is:
Got that? No? OK let's do it slowly.
1) OUTPUT - (8-13MV) IN AIR AT 25°C, SEA LEVEL.
2) RANGE - 0-1 ATM PO2, 0-2 ATM PO2
3) ACCURACY - WITHIN +/-1% OF FULL SCALE AT CONSTANT TEMPERATURE
AND PRESSURE (0-1 ATM), ±2% FULL SCALE AT CONSTANT
TEMPERATURE AND PRESSURE (0-2 ATM PO2) WHEN
CALIBRATED WITH 100% OXYGEN.
4) RESPONSE TIME - LESS THAN 6 SECONDS FOR 90% OF FINAL VALUE.
5) OFFSET - LESS THAN 0.5% OF OXYGEN EQUIVALENT AT 25°C (77°F) IN
ZERO GAS AFTER 36 SECONDS.
6) HUMIDITY - 0 TO 99% R.H. (NON-CONDENSING).
7) OPERATING TEMPERATURE RANGE - 0 TO 40°C (32 to 104°F).
8) STORAGE TERMPERATURE - 0-50°C (32 TO 122°F).
9) AVG. EXPECTED CELL LIFE - 36 MONTHS IN AIR AT 25°C AND
10) SHELF LIFE: 24 MONTHS.
11) WEIGHT - 1.2 OZ (32 GRAMS).
12) LOAD - 10K REQUIRED.
3. TEMPERATURE COMPENSATION ERROR IS ±5% OF FULL SCALE OVER
THE OPERATING TEMPERATURE RANGE. WORST CASE TRACKING ERROR
(WITHIN THE FIRST HOUR AFTER A MAXIMUM TEMPERATURE STEP) IS
±7.5% OF FULL SCALE. (GAS SAMPLES MUST BE BROUGHT TO AMBIENT
TEMPERATURE) PERCENT READOUT IS ONLY WITHIN ±1% AT CONSTANT
PRESSURE (E.G. A 10% INCREASE IN PRESSURE WILL RESULT IN A 10%
INCREASE IN READING).
Line 1): the output is anywhere between 8 and 13mV (not mega volts) in
air. Any value in this range is right. The variation is primarily down to the
rate oxygen penetrates the cell and to make it more consistent would make the
cells a lot more expensive. Also it changes through the life of the cell. A
cell might go up in voltage over its life but more likely will go down.
Calibrating is the business of putting a known level of oxygen on a cell and
measuring the voltage. Then you use this remembered number to convert later
voltages to the appropriate partial pressures of oxygen.
Right so we expose the cell to air like we do our Nitrox analysers?
So you know how much O₂ there is?
Yes. It's 20.9% I learnt that on my nitrox course.
<sigh>If it's cool and dry yes. If the
is 100% and the temperature is 45°C it's actually 18.9%
Now 18.9% to 20.9% isn't a 2% error it is a 10% error.
So if we want it right we need something better than air and in the rebreather
business that's pure oxygen. Now the ideal would be to flood the cells with
enough oxygen to be certain they are getting absolutely 100% but that's
actually quite hard to manage. On the Inspiration the factory's best guess is
that you'll get 98% but it is only a guess. However this, and the current
barometric pressure gives us a fixed point. The ppO₂ is barometric
pressure multiplied by oxygen fraction so if it is 1037mbar and 98% that's
1.016 bar of O₂ so if a cell reads 58mV we then know we are getting
57.08mV per bar O₂ and can calculate the oxygen readings from that cell
by dividing by 57.08 so 88.5mV is 1.55 bar and time to do something about
Now the cells from the same batch may well be similar but they will not be
identical so each one has to have it's own calibration number. You can't use
the cells without these numbers as without them all it tells you is more or
less minute by minute. Everything depends on these numbers and the biggest bug
I see is the section 6) in the specification that say the humidity is
NON CONDENSING. It isn't. When I take my scrubber head off it's dripping
wet. Actually diving it is borderline non-condensing and good rebreather design
puts the cells well inside, where they are kept warm, but moisture on the cell
faces reduces the oxygen transfer into the cell and makes them read low.
Now we have hydrophobic (water hating) membranes on the cells to try and
reduce this but it usually gets the blame for once cell drifting off on its
A final note on accuracy.
We've already wondered what our calibration gas is and accepted the best guess
of 98% but what is 3) telling us? Errors are quotes against Full
Scale and full scale is 2 ATM (2.0265 bar) so 2% is 0.04 bar. So anywhere
on the scale there can be 0.04 error, either way.
An aside to this is checking your nitrox analyser. Calibrating in air as 20.9%
is OK but it doesn't test things. Every now and again calibrate on pure
O₂ then read air. If the air reads high the cell was limiting at 1 bar of
O₂. It was trying to say 80% or such because it couldn't go higher but
you turned the knob to make it read 100% so back on air it reads high. If it
reads right it's good for quite a while yet testing 36% especially if you cap
BTW: Do you know how much Combro would charge for me doing
even a short form analysis like that?
Gills, oxygen and seawater
A little while ago on the UK diving web forums we had some nutter trying to
sell his idea for centrifuging the air out of sea water to escape from having
to carry cylinders when we dive. He was trying to obtain sponsors to fund this
scheme and we all had a good laugh at his expense. I assume he now has sponsors
and is living it up on their money.
To debunk this we needed numbers and, at the time, they were a bit
hard to obtain so I thought I'd put the figures I found here to save me having
to go through the angst of searching them out again and explaining in words of
one syllable why this is a stupid idea suitable only for comic books.
We are used to the idea that stuff dissolves in water and we see fish swimming
about powered by a metabolism based on dissolved gases so what's the
Section 14 of the Central Rubber Council's Handbook of Chemistry and Physics
has a nice section on seawater. The salinity in the ocean varies from about 32
to 37 ppt (parts per thousand by weight) except in the polar regions and near
shore where it may be less than 30 ppt. The average salinity of the world's
oceans is 35 ppt, which is the same as 35 grams of salts, mostly sodium
chloride, in each kilogram of water. Incidentally 30ppt seawater freezes at
-1.67°C, 35ppt at -1.92°C and 40ppt at -2.21°C.
Dissolved gases, however, are a harder problem as they vary with depth and
temperature. If it was an equilibrium situation where the partial pressure of
each atmospheric gas equalled tension of that dissolved gas we would expect
12.5mg/Kg of Nitrogen, 7mg/Kg of oxygen and 0.4mg/Kg of Argon. Colder sea water
can hold more gas. We would expect even less CO₂ but this reacts with
water and we end up with 90mg/Kg.
The best I can find for oxygen is some research stuff that gave the numbers for
the graph opposite. Depth in meters vertically and oxygen concentration in mL/L
horizontally measured in a sample brought to the surface so that is mL at 1
bar. They were more interested in the oxygen poor zone extending from 200m to
1000m but I'm more interested in the peak at 40m to about 4.5mL/L. We are not
going to get more than 4.5mL/L (incidentally 4.5ml is 5.8mg)
Right what can we do with 4.5mL/L? Well at 20 meters deep on my standard scuba
set I use about 20 Litres per minute to breathe, of which 21% is oxygen,
multiplied by the 3 bar pressure. Now assuming I can get nitrogen out of
seawater in the same ratio as we find it in air that means I need about
3L/minute of oxygen so dividing 3L by 4.5mL and then multiplying by 3 bar gives
me about 2000L of water per minute that I must process through the magic box if
it runs at 100% efficiency.
This raises three problems in my mind.
Firstly is size. If the magic box is a reasonable size, say a volume of working
water is the size of my scuba tank it will have 12L of water to work on at any
one moment. So 2000L/minute means that the water must be in, processed and out
in 12*60/2000 seconds. That's just over one third of a second to complete the
Secondly there is the fact that 2000L/minute worries me when I consider that my
old 20HP outboard had an 20cm diameter prop on it. To move 2000L/minute it
would have to knock the water through at 2000/31.4=63m/s, otherwise 142mph in
old numbers. So my second problem is that this is a huge amount of thrust for a
diver who does not wish to zoom about like a torpedo and the third problem is
that that sort of power isn't going to come from ordinary batteries.
So ordinary Open Circuit scuba is a non-starter but on my rebreather I
estimate that I use 1L of oxygen per minute but that's conservative and it's
really more like 0.5L/minute unless I'm zooming about or ascending or
descending. This would allow us to divide the consumption by eighteen, although
it does leave us the problem of separating the recovered gases which might take
some creative physics. So then what does this save us? Well it saves us a
cylinder 50cms long 10cms diameter and weighing 4Kgs which holds 5 to 10 hours
worth of oxygen. I could use one a third the size and get a good few hours
diving out of it. This is such a tiny part of the rebreather and is a trivial
saving. To do that I still have to process 100L/minute of water so expect a
flow rate comparable with 3 meters/sec and that is a lot more than my nice
Diver Propulsion 'scooter' does at full power.
OK, the idea is a joke but that leaves us one remaining problem. Fish.
We are all familiar with the picture of the shark endlessly patrolling. Apex
predator ready to strike and kill. Where does he get all his energy
Well the simple answer is that he doesn't. We have a fantastic adaptation that
we are warm blooded and burn a lot of our fuel to maintain that heat. That's
where most of the oxygen we need goes. The supposedly cold-blooded land
reptiles are amazingly warm blooded when a biologist glues a thermometer to
them. They have to bask in the sun to get their bodies up to working
temperature and then regulate their bodies with sun, shade and water to stay on
the right temperature. The shark can generate a little heat through muscle
power but must otherwise stay in a bit of the sea that supplies the
temperatures he can work with. The other thing he does is swim about with his
mouth open and water streaming through his gills all day every day. Only little
fish can sleep. Seawater supplies so little oxygen that to maintain a big body
you have to swim until you die and the oxygen you get goes to the muscles with
none left over for direct heat production.
When mammals re-entered the sea they could become the biggest creatures ever
(eat your heart out 70 ton dinosaurs) and the most efficient predators ever
known just because they retained their air breathing, warm blood design. A 4
meter long, 3000Kg Great White may look impressive and a bit frightening but it
eats one big kill every couple of months and is not in the same league as a
Blue Whale at 30 meters long at 180,000Kgs (that's nearly 200 tons) who is
pushing four tons of krill a day into its tummy.
We mammals can't make do on gills. We have evolved a long way beyond that sort
of trick. Centrifuging air or oxygen out of seawater just won't work for us.
A note added years after the original text.
We have seen a couple of mouth mounted tube assemblies aimed not at the scuba
replacement but the snorkelling arena and working in the 'highly oxygenated'
upper couple of meters.
So I looked for any published papers on oxygenation in the surface of the sea
and found it is higher. There was a graph showing how it varied by season. The
graph maxed out at below 14mL/L. That's three times as much as my numbers
above. So for the OC system you only need to process 700L/minute. Hence if your
gas separation window is say 25cm by 6cm and again you work at 100% efficiency
you need to swim at 46 m/sec to pass 700L through the system. That's 103mph.
Of course this is still all wrong
What we really need to do is to stop all this messing about with disolved
gasses, which are hugely variable, and split the exhaled CO₂ back into
carbon and oxygen. Then we have the ultimate rebreather and a green human
Of course we still need energy for this operation. The raw chemical energy is
actually quite high. Based on 33KJ/gm release for combustion of carbon we would
need about 88KJ/gm for the oxygen released. 1L of oxygen weighs in at about
1.46gms so that's 128KJ per minute or 2.1Kw. OK this means the batteries are
going to be a bit big but draw them small on the prospectus and assume the
technology will catch up before we have to deliver anything.
Now I've done the basic sums I'll leave coming up with a process for splitting
CO₂ to the class as a homework exercise. As a start think about a coal
fired power station and run it backwards. Remember we need 100% efficiency or
the battery has to get bigger and we don't want that.
Reporting fatal accidents
Diving kills people. OK over the years we have gone from heroics to hobby and
the death rate has fallen but every year divers will die.
Now these incidents are often reported in diving forums and the response is
always mixed. There are, naturally, expressions of regret from those that knew
the person, or knew of them, and those that didn't are saddened and post
messages expressing that. However there is often a tendency to treat reporting
the details of the incident as insensitive and I have seen people who
have posted a factual account criticised.
I think this is wrong. Badly wrong. Yes. Respect is important. A dead diver is
a dead diver and there, but for the grace of God, go you or I. However a
fatality is a wake up call to us all. We need to think these things through and
put the skills in place so that if a similar incident starts to unfold for us
or our buddy we see it early and resolve it. It is not insensitive to report
the facts, as well as possible, without attributing blame.
If the topic is ever Nigel Hewitt has died diving write it up
publically, think about it, discuss it and then don't make the same mistake.
The only relatives I have that might read a diving forum are my children and I
anticipate that they would correct any factual errors for you even if they did
feel bad about it. Feel free to speculate if the details are not clear, they
rarely are, and consider the 'what if' scenarios of incidents like mine. If the
conclusion has to be drawn that I did something silly and won a Darwin Award
for it then say so but if somebody realises they can make a change to the way
they dive and be safer because of it that can only good. Respect is remembering
my views and considering them, the ultimate disrespect is ignoring my clearly
expressed opinions just because you feel uncomfortable about it.
Currently I fear that doing a Hewitt involves launching your breakfast
overboard and missing the day's diving but if it becomes the mnemonic for some
other daft error that people no-longer make so be it.
Conversely, if all you can deduce is that I just kept diving and diving as I
got older and older until one day, finally, I was just too old for it and fell
over dead carrying the kit back to the car then you can all remember me, say
"It's the way he would have wanted to go," and come to the Wake.
This has to be my specialist subject. Yes. I have a very silly hobby for
somebody who gets seasick big time.
So what is seasickness?
Personal observation indicates that it happens when the input to your balance
system from your eyes does not match that from the this way is up
sensors in your ears.
For example: Ride in a car and drive or watch the road as if you were driving
and you are never carsick. Try and read a book and you lose it very quickly.
So what's happening? While things agree there is no conflict. All is as normal.
When there is a different input, your ears say you are moving and swaying but
your eyes see a nice stable book on your lap, somehow something spills over
into other parts of your brain and motion sickness rolls over
Now I am told that motion sickness is a poisoning response. Your body assesses
the growing balance problem as a bit of bad foraging, you really shouldn't have
eaten that dead squirrel you found under a log, and in a desperate attempt to
keep you alive it is throwing the rest of the meal overboard. It sounds
plausible but who knows?
There is the added point that you can train yourself both not to be motion sick
(get your sea-legs) and to be sick. I see a lot of people training themselves
to be sick by not taking precautions and choosing the worst, most fatalistic
way to face it.
So what are the tricks?
Tablets. Find a brand that suits you. These slightly dehydrate your ears so
they don't report the detail so much. This makes you less sensitive. Then buy
the cheap generic chemist version of the product. Avoid patches. They come off
when you perspire and sitting, kitted up for a deep dive in the full exposure
suit for over an hour's deco, I cook.
Some people recommend Ginger and there
were some proper studies done. Ginger scored better than a placebo but not as
well as tablets. Scopolamine did very well but the side effects are a bit
unpredictable and include dizziness often enough to exclude driving from things
you should do and I wouldn't dive if I wasn't fit to drive.
Stay away from the exhaust and the fuel. These things predispose you towards
feeling queasy and deepen the pit you feel you are going down. They are
different inputs and I am not sure they effect seasickness directly but they
are pushing the same way. I suspect that with enough diesel and exhaust you'd
be sick on a boat sitting on blocks in the boat-yard.
Now visual tricks. Stand up. This forces your feet to work at keeping you from
falling over and you can't do this without both inputs agreeing. Also look
round so you are seeing the horizon, stationary, not the boat which moves. The
alternative is lie down and close your eyes switching off one input. I can
sleep through a storm at sea.
Now the point where it always got me was kitting up because this was the point
when I had to take my attention from the distant world and point it to the rig
I was about to dive. I counter this by setting the system up before we ropes
off. On the Rebreather it costs me a bit more in batteries but I get more
dives so it is value for money.
Practice. I cox the club RIB and I'm never seasick when I drive. I start the
season taking people diving and not getting to dive myself. This teaches my
system that going to sea doesn't mean seasick and also puts some brownie points
in the bag so that when, later in the season, I get it wrong and just collapse
in a miserable heap and don't help pull the kit back in people forgive
Finally just learn to be seasick. It isn't a big deal. Throw the breakfast,
grab your water bottle and first flush your mouth out and then drink a lot to
rehydrate yourself. You feel masses better in 60 seconds and you just bought
four or five minutes that can get you kitted up and in the water. When you come
down to it sea-sickness is only as debilitating as you let it be. The worse
part is the anticipation. Being sick isn't hard just a bit uncomfortable and,
provided you watch out for putting the fluid back, it doesn't really affect
your diving. I can't make it enjoyable but it's not really that bad.
I see too many people struggling to get their nice, tight fit, wet gloves on,
especially for the second dive of the day.
Now I like wet gloves. Dry gloves are too much messing about and if they leak
they cost you too much warmth as the water gets back up the path that you
arranged to equalise the pressure. Also I've never been cold, even ice diving,
in 5mm wet gloves. However mine are tight.
So how do you get tight gloves on? This is always the snag. You see divers
making a real meal of it. Great gloves when they are on but getting them
The answer is Garage gloves. Well mine don't come from a garage but the sort of
ultra cheap check your oil and throw away style gloves they give away
there in the hope of selling you some oil. They are made from flat sheets of
plastic sheet and a hand shape is cut out with a hot blade to seal them. Please
notice these are not the nice hospital type which are hand shaped
because they fit too well and have some grip. You want the absolute pits of a
glove, the cheapest, the nastiest things imaginable.
OK humour me. Pick some up next time your filling up and then take your
tightest oldest pair of wet gloves and put one under the tap. Really make it
yucky and totally impossible to get on. Then blow a garage glove up like a
balloon. Not too hard as they are cheap and nasty and will split on you given
half a chance but you want to get the fingers open or it's a pig getting them
on. Put them on first and make sure your fingers are right in then slide your
hands into both the soggy wet glove and the dry one. Slide.
Believe brother. Once you've done it you never want to go back to the squeezing
and the pulling and the struggling. You can use vet lube to slide them on but
I've tried it and it seems not quite so good. I suspect the plastic crunching
up makes for a better insulation budget than a nice even film of relatively
Something else gloves need to be, once they start to get thick, is hand shaped.
If they are all floppy they don't have enough cell rigidity to actually provide
proper insulation but conversely if your relaxed hand isn't half closed you
have to work against the spring in a thick glove to grip anything. It is
amazing how many gloves are either too floppy to have any long term warmth or
try to spread your hand out flat because it works for thin gloves and it's
cheaper to make them that way.
A word on dry gloves. I confess I'm not a fan. I've never been cold diving
under ice or in Finland with my nice thick wet gloves/under gloves. So why do
dry gloves get used. Well my opinion is that it is inadequate undersuits. If
your undersuit is not up to the job you will get cold peripheries. That's cold
feet and cold fingers. If your gloves look OK but your fingers are cold beef up
the undersuit with an extra layer. Mine is an old M&S Cashmere and Silk
pullover bought by a friend as a present. This is thin enough to not hit my
weight requirements but still adds some upper body warmth and my hands are
warmer. If you don't start with this sort of fix nothing is going to give you
warm fingers and your gloves options will get more and more extreme as you try
to fix the problem in the wrong place.
I am personally quite receptive to nitrogen rapture. I like it and fear it
like doom. It destroys the instinct of life.
Jacques-Yves Cousteau. The Silent World. p43
Narcosis is a worry to me. Sitting under the relative protection of a helium
mix I have watched a nitrox diver I both trust and respect take five minutes to
inflate and launch a DSMB sliding several meters down the side of the hull of
the wreck as he did so. Back on the boat I asked what the problem was.
"What problem?" he replied. He was not aware that a routine I would expect him
to perform in thirty seconds had taken that long.
Another instance when we were at 50 meters another buddy, who incidentally
teaches knot tying to Scouts and Cubs, failed to tie a bowline despite several
attempts on a 28m END while I, on a 20m EAD, could.
Perhaps I am fortunate that narcosis makes me nervous and I become a stickler
for rules. I watch the slate, I watch the computers and if I don't write 'take
photographs' on the slate it doesn't happen. However even then, at 60m with a
36m END when faced with a problem that took me out of the normal I fixated on
that and responded to but did not act on the most basic 'UP' sign. I was
grabbed and lifted a few meters and normal service was resumed but I can still
worry about that one.
So what do I do about Narcosis? Like Cousteau I fear it like doom. I try never
to dive with an END (Equivalent Nitrogen Depth ie: the depth you would go to on
air to be breathing that level of nitrogen) of 30m or more. I suspect that
almost always being sludged up with seasickness remedies doesn't help but even
shore-diving taking my brain near 30m slows it down and makes me silly.
I dive helium mixtures shallow, the rebreather makes this inexpensive, but this
means I see how much divers are slowed down even at relatively small depths.
Never believe you aren't narked at 30 meters. If you think you aren't you are
as deluded as the guy that thinks a few pints make him a better driver. Being
on the inside of a brain full of nitrogen you feel good, confident, on the ball
but these is just the symptoms of the problem. Another good friend tells how
the first time he dived to 40m on trimix he was scared. It was the first time
he had an accurate perception of just how deep that really was.
I quite like the way that trimix is promoted for shallower diving now. I think
the days where 'deep air' was considered acceptable should be considered over.
There are courses that fill in the details for using trimix and these are a
good thing especially for rebreather users where the solution to a problem may
require more thinking about than the OC 'find a better regulator to breath
The Bail Out Valve or Demand/Surface
Valve is a piece of rebreather equipment that gets a very varied
response amongst even experienced rebreather divers. Some think it is vital, a
piece of equipment that gives you your life back when you need it and others
just can't see the point.
So what does it do?
On a rebreather the gas you exhale is kept, cleaned up, topped up and recycled
to become your next breath. This has many advantages but also the snag that
the main gas path starting at your mouth and going all the way round through
the works and back to your mouth again needs to stay dry. There are all sorts
of tricks to try and keep it dry but the obvious first line of defence is a
control on the mouthpiece that seals things off if you are going to take it out
of your mouth in the water.
Clearly if you are still under the water at this point you need something else
to breath from and this is normally a separate scuba regulator. A common trick
is to build this regulator into the mouth piece so switching the valve to keep
the loop dry also connects you to the regulator and normal breathing is resumed
at once. This is the BOV.
Some rebreathers have a BOV as standard, some have it as an option and some do
not have one. All rebreather training emphasises that if you suspect a problem
switch to ordinary scuba for a couple of breaths and see if this clears things.
You cannot taste low or high oxygen nor high carbon dioxide which are the
dangers of a rebreather that seems to be breathing correctly.
So why the controversy? Surely divers can take one mouthpiece out of their
mouth and put in another? Techies, like rebreather divers, who always seem to
be festooned in more tanks than a whole class of beginners, do this all the
Here hangs the problem. If your rebreather loop has an excess of carbon dioxide
your body cannot get rid of the CO₂ it is producing. The body reacts by
increasing the breathing rate as this is what normally works. Now this is a
pretty low level design feature designed to stop you dying suddenly and when it
takes hold it isn't negotiable. Try holding your breath until you die to
experience it. It just doesn't happen.
So our rebreather diver begins to breath deeper and faster. At first they won't
notice as this is just automatic processes adjusting as they normally do. It
grabs your attention when you can't breath enough though. This is the point
when the penny drops, you say CO₂ hit and reach for the bail out reg.
Problem. You have to hold your breath while you swap to the bail out gas supply
because you are under the water. Stopping breathing has become something you
are not allowed to do. You might as well try to stop your heart beating. Divers
have died in this situation.
OK I've been there. Your mind is horribly clear but 'breathe' has moved to a
'knee jerk' reflex position. So what did I do?
Well I have an ADV in my loop, a device to automatically top up the gas in the
loop on descent etc. I exhaled through my nose rather than my mouth, which was
hard enough but at least possible and when I inhaled the ADV refilled the loop
and I got fresh gas not the CO₂ laden stuff that was circulating. Don't
think of this as a gentle bubble like a mask clear - this was a mask flooding
rush. However the desperate 'must breath' lessened and I could pull out the
mouthpiece and stuff the bail out into my mouth and then seal the loop. I was
still panting but I paused, reseated my mask and cleared it. My buddy's face
was in mine. He had hold of my harness. Another couple of seconds and I would
have been CBL'ed.
I bought a BOV. I hope I never feel that again but if I do it's a one quarter
turn on the big knob and I'm breathing OC gas.
Conservatism on deco and dive computers
I have always been taught on dive courses that conservatism is good.
Now in part I agree. Getting things right to the limit on decompression
planning is just looking for trouble. We all know people who have had
undeserved deco hits and none of us want to join the list. So for a lot
of people this involves setting their dive plans and dive computers to use one
or another system to add extra time to the decompression profile so they stay
further away from the bent/not bent line.
The problem is that when I'm setting things up I don't know if the dive will go
well and I'll be doing a happy ascent reminiscing about what a good time I've
had or if it will be a total disaster and I'll want fastest time to surface
with enough deco to stop my bad day from getting seriously worse.
So my conservatism comes out of my head and is not pre-planned. Deep stops are
easy, you just do some. Giving it another 10 or 20 minutes at 6m, especially
if I have got a buddy to annoy is easy too. What I don't want to do is hit the
surface with something serious broken and not know if the N minutes of stops
on the computer that I just blew away were real or just a precautionary add
OK so I set the computer to its lowest conservatism so it is showing the
fastest time to surface. Both computers actually. However if I dive this to the
letter regularly at my age I'm probably pushing my luck. So my first head
conservatism factor is a slow ascent. I will save hurry for the day
something breaks. Then add deep stops. I did run with the Richard Pyle trick of
taking half the distance to your first mandatory stop and waiting there until
the minutes on your computer click twice but now the VR3 wants more than that
and more works for me.
A VR3 deep stop is sometimes washed out by my slow ascent but if I've seen a
depth listed I like to do a Pyle two ticks on the minute counter so it's
between one and two minutes. This takes me slowly up to 6 meters where I settle
down for a snooze.
Now I'm diving a rebreather so at 6 meters the 1.3 bar ppO₂ already gives
me an 81% Oxygen mix but by pushing some injector buttons I can push it higher.
If I know I am nowhere near high CNS levels I will try and flush it to pure
Oxygen but that never quite happens as I have 0.0567bar of water in the loop.
This is another layer of conservatism because although the Vision electronics
in the rebreather knows what I'm up to the VR3 doesn't. Add some extra time and
I'm doing what I consider a conservative profile.
Now this might not suit everybody as a non-rebreather diver must watch their
gas consumption in a way I don't have to. It's no good doing a slow ascent and
then finding that the time left on the computer is getting tight on the SPG
hitting zero. I'm fortunate that if I start full and nothing breaks I'm using
at tops 60bar from both of my built in 3L cylinders and deco diving I have two
7L cylinders of the same again, with inject whips, and they are not in any plan
other than emergency bail out.
This is just the way I do it. I'm not recommending it as it's not an agency
view but until I get bent I'll have to say it works for me. Conservatism
out of my head gives me the margin I like to stay well away from the
edge of the tables while still meaning I have numbers available to me to know
how serious a problem is that is likely to force me to the surface.
Of course deco on a computer is a pretty much a heretical idea to some. I
should be doing tables and a slate. <shrug> So shot me.
One other thing. It is often said that "We don't really understand deco".
This is rubbish. We have been deco diving on tables for over a hundred years
and we can formulate nice predictable results. The problem is not the tables
the problem is the divers.
Tables are formulated for a young, fit, healthy, male diver who is eating and
drinking properly. The divers who have problems may claim to be most of those
things but I just don't see it. We are now aware of the subtle effects of a
slight heart defect, the PFO. We are aware of the problems with being
dehydrated. We are aware that fit and young are important.
OK, so the next time some obese old buffer says he's 'Dive Fit' to me I will
roll on the floor laughing. Darn it I'm over 60 and I take that as a warning to
do slow conservative ascents and think twice about doing that second dive,
especially if it's not really very interesting. Certainly I don't go out on the
lash the night before a deep dive so my body needs to use a lot of its water
content to wash all that poisonous ethyl alcohol out of my system, in fact I
have equipped myself with a p-valve so I can pour lots of nice water down my
neck before I hit the water without any fear of later discomfort. Even the PFO
is often signalled by migraines and normally only shows a problem with a
Valsalva nose clear so perhaps it's the second dive that caused the
So yes. I would agree there is probably lots still to be learnt about the
detail of decompression but let's not rubbish what we know. Ninety nine times
out of a hundred that's just an excuse.
OK so I'm a hypocrite.
I don't approve of solo diving...
...but I rather enjoy it.
Yes, that is deep, sea water, wreck penetration, decompression, rebreather,
Diving with my club or my friends there is no problem. It is when I travel to
somewhere because I've been offered a dive on a specific wreck that I'm
interested in that a buddy often doesn't work out. I know I'm a mug for pier
pressure so I prefer not to get into that situation if I've taken a day off
and driven 200 miles. My deco schedule is based on my mix and, more
significantly, my bailout tanks. That alone often makes me and my
laissez-fair attitude to dive planning incompatible with other people.
If I turn up on a boat and I don't know anybody I don't push it. I am a quiet,
shy, middle-aged geek with a rebreather. I'm easy to ignore. I can solo and
nobody notices. When (if?) I become inordinately rich I will always take a
buddy along. They will be on the payroll and it will be their job to swim along
with me with all the spares I might need. I'm sure getting paid to dive all
over the world will ease the burden of having to put up with my dive plan (and
carrying my kit) but, until then, forcing my dive on somebody else seems a bit
mean. Somebody with the same aspirations and interests is great... but not all
If you dive with somebody they deserve an input into what you do together but
that can be at a huge cost. What if they are not equipped or prepared to enter
a silty wreck? What if they view fish and weed as important and think that my
attacking things with a wire brush to find out what it really underneath is
wildlife vandalism? Are they really ready to do the dive in terms of gas and
bailout? Have they a clue what they are diving? "It's a wreck? Huh?". Often
the restrictions I put on myself solo diving are worth it to get to see what I
came to see.
I don't solo all the time, in fact I don't solo by choice as a buddy, a good
buddy, is lots of fun and I like sharing a dive with a like minded friend but
it does have its place in my system of doing things. I'm not sure I should
even recommend it although most of my problems diving have been when I have
had a buddy along. Alone I am listening to the rebreather, I am feeling the
dive. Alone I am exquisitely sensitive to my kit and my body in a way I never
can be with somebody else around. I have stopped a solo dive early just on the
suspicion of a fault where with a buddy I would have demanded proof before I
cut their dive short. Solo is unsafe in very many ways but in some strange way
I think that for me it is often the safer choice.
I quite like not having to take onboard somebody else's model for what the dive
should be so the plan becomes more fluid and what I do depends far more on
what I find while it's hard to communicate subtle choices with just your
hands. The final ascent profile can be very conservative without worrying
about somebody else's gas consumption/state of bladder/tea withdrawal
symptoms. I know I can be a real pain when I have a camera in my hands and
I've gone into wreckie mode and I really love not having to chase somebody who
wants to do the whole wreck in the first ten minutes or whose fascination with
fish/crustaceans bypasses my fixation on boilers and mechanicals.
For soloing I have developed a somewhat more complex kit setup than normal. The
whole dive is planned on the rebreather but I carry enough open circuit
bailout to complete the dive on a total failure of my main systems. Most of
the essential clutter is duplicated (masks, DSMBs, reels, torches, buoyancy
systems, stuff to breath) and if something breaks the dive is over and I am on
the way up. Nothing is allowed to stress me as I only have one of me.
This is based on something I posted to the "Change one thing" thread in the
It tries to describe a problem in the way the British Sub-Aqua Club works in
Now I admit I'm a BSAC fan and I think we do a lot of things well but this is
an area where what we used to do needs leaving behind.
I am discriminated against in the BSAC system because I am not a loud person.
That is I will not push myself to the front. I certainly will not push myself
in front of somebody else.
To paraphrase a quote in a forum thread: you "Hand hold OD trainees, lead SDs,
expect DLs to run a bit of their own training but for AD they're on their
This selects for a certain personality type in the higher grades and it is a
type that thinks it obvious that if you aren't like them you aren't Dive Leader
material. To them it is obvious that to be AD or such you need that sort of
'stand up for yourself' aggression and they can't see why it should be
otherwise. Think assertive. Think military. Think 'getting things
I have to question this.
This type of personality selection closes the doors to a good percentage of our
members and these are the type of person who are choosing to do the new look
'hobby' diving rather than the old 'hero' diving. People aren't coming into
scuba now 'to boldly go' but because it looks a lot of fun. These should be the
people that will rise up and run our trips and run our branches but the current
system tends to drop them at SD so that's where they stick. Divers but not dive
Many will have good jobs and good organisational skills because diving tends to
appeal to that type. They will slide into the non-diving committee positions
and we run the risk of having well organised, well financed branches but no
ADs, no AIs, no FCDs etc. Our diving will become increasingly segregated into
the mundane club dives while the teckies, like myself, are going out and doing
specialist stuff outside of BSAC and the training is reduced to just accepting
We need to beware of characterising our divers with an out of date model. If
you want something done in my office it goes, not to rufty tufty Don with his
military 'make things happen' ethos but to sweet little Pauline who finds out,
works out, arranges and then makes sure it is happening. How is a diving club
and dive planning different from that? The distinction is subtle however
because Don will pester me for something he wants but Pauline won't. Similarly
I will not force my branch to use my ADI nor to finish my DL. I'm an old enough
diver with other qualifications that it doesn't really matter either to them or
to me as it doesn't restrict my diving. However what about the new people
coming through? We don't have a big pool of ADs to draw from so our DO had
better look after himself and not get promoted or offered a big rise to go and
work somewhere with less silt in the sea. (I wrote this before we elected a DL
to the post in a one candidate election).
I've had problems with DL, the stories I've heard of AD imply it is even harder
to get the training and the approvals, the rumours of FCD imply it is almost
deliberately made impossible to accomplish even before you pick up a manual or
zip up your suit. If that is the policy can it be stated and monitored so
everybody gets the approved amount of grief? If not can it be changed?
An Alpinist, in rebreather diving terms, is just somebody who has decided that
the problems they get from carrying bail out cylinders are more than the
problems they would solve. It is a necessary value judgement. Rebreathers have
some failure modes that involve finding something else to breath from but they
are not common and carrying 'enough' bail out can also be problematic. For me
personally, for a dive with serious decompression time, Alpinist is a step too
far. However for no-stop and short deco dives I will happily dive with just the
onboard gas supplies as my bail out and when I've used it it has been
Now a lot of people have taken onboard the idea that rebreathers are dangerous
so perhaps we ought to discuss this first.
If you are diving open circuit equipment a fault is hard to miss. You are
diving along happily when suddenly you are surrounded by your own personal
Jacuzzi of bubbles or suddenly you go to breathe in and you either get nothing
or, more distressingly, you get sea water. On a closed circuit system a fault
with the oxygen control systems can mean that although you can keep breathing
in and out comfortably the oxygen levels go up or down and, if you don't
notice, the world just goes painlessly away. The old OC idea that if you could
breathe everything was OK has gone and you need to know what you are
breathing minute by minute.
This generally means that you need to stay in touch with your gas mix with
instrumentation so that, when the evil day comes and something important
breaks, you notice and do something about it in a timely fashion. In theory
this isn't hard but hour after hour of boringly correct readings can take their
toll and we get complacent. If you combine complacent with lucky the system
just keeps right on going and you are unworried however, having never won the
lottery or such, my luck isn't something I'd trust my life too so a little
paranoia is a good thing.
OK so where does this leave us with the Alpinist question? Some people say
Alpinists are mad and although, others may say that all rebreather divers are a
couple of sandwiches short of the picnic, that is debatable. What you have to
do is decide how safe is safe.
The 'dead set' faults on a rebreather are loop floods and scrubber
breakthrough. You can manage anything else, stay on the loop and keep using
only a small amount of gas. Now both these problems are normally down to bad
rebreather care or bad planning. If you are confident that the rebreather is
not going to throw either at you the bailout is extra weight, in the way and a
total pain moving about out of the water. There must come a point where the
complications with managing bailout gas exceeds the dangers of not carrying it
for the vanishingly few instances where you will have an undeserved loop flood
or and unexpected scrubber break through during a part of the dive where it is
That is when you become an alpinist.
Specifically getting your weight right after adding stages to your kit.
Again, I'm afraid, you get a lot of twaddle passed about by divers so let's do
some sums and find out what the real story is.
The first thing we need to know is the volume and 'stamp weight' of our
cylinder. Both of these numbers should be punched into the metal. In the
illustration you should be able to see 7.0L WT8.8KG. You need to pick it
out from all the test marks and the serial numbers and it should be under a
nice protective layer of paint.
So this cylinder consists of 8.8Kgs of steel and has a 7 litre hole in the
middle. Let us assume that it, my regulators, rigging kit and valve gear are
'typical'. All that changes is the volume, the cylinder weight and the
material the cylinder is made of.
So I put on the regs and stuff and get out the digital scales and dangle the
fully rigged stage in the rain water butt. It weighed 3.54Kgs but that was with
210bar of 100% O₂ in it which contributes 2.2Kgs leaving
me with 1.34Kgs for the cylinder and all its fittings on empty in fresh water.
Now the first deduction is that if I promise myself the cylinders are never
going to be 'staged' or handed off then all I can remove from my weight belt
for my two stages is 2 or maybe 2.5Kgs. I can't allow anything for the gas they
contain because if I do and then I breathe it all I will become buoyant and do
an inadvertent ascent to my well deserved Darwin Award. Since they weigh
13.1Kgs each out of the water this seems a bit mean but that's physics.
Obviously if you are planning to leave them somewhere or lend them out you can
The second trick is that I can now suggest a simple approximation for the
'weight' of an empty rigged stage in fresh water using the Kg and Litre
Steel: weight = 0.7 + stamp_weight*0.87 - vol
Ali: weight = 0.7 + stamp_weight*0.63 - vol
If they come out negative the cylinder is buoyant when empty and most Alis and
several modern steels are. If they are buoyant then you need to consider adding
lead to your belt so that if you do need to breathe all the gas you don't have
to release the now buoyant cylinders before they lift you to the surface
For use in the sea subtract 2.7% of total dry weight so 350 grams for mine and,
frankly, you'll never notice. OK, if you use chain for your rigging kit it's
going to be more and more complex regs will need further changes. However this
will only change that 0.7 number at the front.
The other useful formula we can derive from this little experiment concerns the
problem of changing your known cylinder for an unknown.
Again use the
stamp_weight*constant_for_metal - volume and calculate a value for each
cylinder. If it is bigger, ie: 'more positive', then you can lose belt weight
and if it is less you need to add weight. If you end up with two negative
numbers and you are not quite sure what 'more positive' means get a grown up to
Incidentally: While I had the scales out I did a couple of other tests.
First I weighed a pony (Faber 3L/232/steel) and in the butt it was 1.7Kg empty.
Then I tied my 5mm Hydrotech gloves and my 5mm Otter hood up in a bundle with a
belt weight. Out of the water they weighed 2.11Kgs and in the water 0.61Kgs so
1.5Kgs of buoyancy which is, I confess, quite a bit more than I
‡ For limited meanings of 'fresh water'. You really
don't want to know what lives in my rain water butt. Darn it I'm not sure I
want to know what lives in my rain water butt.
Let's talk about the types of rebreather out there and, just for now, put the
generic, all rebreathers will kill you, scare stories to one side.
There are basically three types of rebreather in common use. The most common in
UK scuba circles is the the fully closed type (CCR) in which the breathing gas
loop is measured and either automatically or manually topped up with pure
oxygen to maintain the desired breathable levels and everything else, well
except for the CO₂ which is chemically removed, goes on round and round.
The second type is a simplification on that which, by using pure oxygen in the
loop, avoids the need for measuring but adds a depth restriction as pure oxygen
is not nice stuff to breathe deep. The final type is the semi-closed type (SCR)
where the loop is being constantly topped up with a flow of nitrox, oxygen rich
air, and that causes it to bubble off an equal volume every breath.
SCRs are somewhat cheaper than CCRs and so often attract the question "Should I
get an SCR as a 'starter' rebreather?"
Can I answer that question as clearly as possible. No.
Only buy an SCR if you are planning the sort of diving to which an SCR is
suited. What sort of diving is that? Well it's the sort of diving you could do
on a single 15L tank with the added fuss of getting the right nitrox, setting
the right jet, sourcing the Sofnolime, cleaning the system far more carefully
and generally fafing about. Oh and they still bubble.
So I obviously don't like SCRs. Their one advantage is they are cheap to buy
initially so if you are wanting a rebreather for ideological reasons they are
attractive but I can't see why else...
Let me be emphatic on this one. An SCR is not a rebreather with training wheels
that will help you learn some arcane skills that introduce you to the black
arts of rebreather diving it is just all the fuss and extra costs for zero
payback. They use a clever idea to get round having to measure the Oxygen in
the loop but nobody in their right mind dives one without something to do just
that because too much or too little oxygen kills you dead.
OK having got that one off my chest let's move on to Oxygen rebreathers. Hans
Hass did fantastic work on Oxygen rebreathers but that was back in the days
when we weren't all such pussies about oxygen. They are probably perfectly safe
if you use them properly but, frankly, we don't have to now as we have the more
modern CCRs so why mess about with being able to detect the oxygen twinges and
do a rapid ascent before you go into a completely oxygen convulsion. I have an
old ex-Russian military Oxygen rebreather. It is light, simple and really good
to dive but I don't take it deep because I have better ones for that.
OK so that brings us back to the Blendermatic, the oxygen injecting CCR. These
come in three flavours: well two and a half way version.
eCCR The Electronic CCR is basically computer controlled. You normally
have three galvanic oxygen cells measuring the partial pressure of oxygen in
the breathing loop and a bit of computer stuff working out how long to hold the
oxygen valve open to keep that up to the required numbers. The galvanic cells
are considered the weak link in the chain, which is why you have three, but you
are still dependant on the electronics to do the sums to keep you alive so you
are trained to keep checking the displays to see that you like the numbers. I
have had the displays freeze up on me and I promptly put the OC backup system
in and surfaced. It may have been once in fifteen years but you do need to do
I have often said that rebreathers favour the paranoid. With OC gear it is easy
to spot the fault when it happens. You go to breathe in and get seawater.
You will notice that. You only worry about the mix you are breathing when you
measure your Nitrox/Trimix before you dive and when you do a gas switch. With a
CCR you must have some brain cells permanently on the job of knowing what you
are breathing minute by minute as a key life expectancy issue. If you aren't up
to this don't dive CCR. It's not common that things fail but they only need to
kill you once and all the rest of us safe paranoids get tarred with the
'dangerous divers' brush.
mCCR The Manual CCR takes a different approach to keeping the oxygen in
the loop constant. By using a nice trick called a sonic orifice it can arrange
a constant gas flow of oxygen (in litres/minute) into the loop regardless of
depth and this is set to be at just about life support level. You are then
expected to check the cell readings and top it up to the desired numbers by
pushing a valve. Now the proponents of manual control say that this is
the way to learn 'proper' monitoring and control but to me it always
sounds a bit like 'real men dive mCCR'. Frankly you are providing a
system that will drift off the number to force you to check it while an eCCR
just needs looking at to see that it is still on the job. I rather feel that
mCCR is just a crutch for people without the proper paranoia to dive a real
hECCR The Hybrid CCR is a bit of both. This is the sitting on the fence
rebreather. Let's have the orifice from the mCCR and the electronics from the
eCCR but we'll probably set that computer oxygen setpoint low and then manually
keep it topped up the the point we want. Basically by buying a Hybrid what you
are saying is 'I want a real man rebreather but can mummy come with me in case
it plays rough?'. Is a hybrid is an mCCR with the 'real man' button? No. It's
an eCCR with a 'poser' button.
We are internet divers. We may not be good divers but we are enthusiasts to be
on a web forum at all and running a website is giving in to it. We have
exposure to ideas that most divers don't care about and probably incorporate
concepts into our diving that are just not part of most diver's worlds. This
has a good side and a bad side.
The whole 'technical diving' thing is our biggest kick. Most divers would think
that 40 plus meters down is way too deep and they're probably right. Most
divers can't hold a stop and I admit I couldn't until I got used to doing deco
with a buddy where we took turns, dive by dive, one to run the blob and the
other just to hold station with them.
Doing deco like that forces you to sort out your buoyancy control and forces
you to sort out your trim. Real deco, not just a few minutes of stops, forces
you to think about 'what do I do if something breaks?' because you are looking
up at the route home but it's not safe to go there yet and won't be for another
However just reading about this stuff and becoming familiar with the ideas is
the bad side of the internet diver. These are the ones that suffer from the
arrogance of knowledge. We seem to have lost the stroke shouting DIR enthusiast
these days (aka rec.scuba news group), although his opposite number, the DIR
hater, has not relaxed back into obscurity yet. However the newbie diver, who
emulates our views without knowing the road we have walked to get there, is
So if you've dived a lot of deco profiles and you see a pattern and you want to
just stick to that then do it, but that isn't the same as running a lot of deco
programs on a computer. I may do solo deco dives on a rebreather with a green
water ascent (nothing within my visual range and no line) but nobody is doing
that on my watch as a club Dive Manager.
When you really come down to it there are two sorts of Technical diver:
Those with an abiding interest in something underwater who need the
technology to give them the depth and duration to investigate it.
Those who are barking mad.
(NB: There appears to be a considerable overlap between these two groups.)
I talked a while back about narcosis and the way it takes your brain away. So
what is our preferred solution?
Well the first thing is to understand the problem so go to the medical text
books and look up the Meyer-Overton hypothesis. This correlates lipid
solubility of an anaesthetic agent with potency and suggests that onset of
anaesthesia occurs when sufficient molecules of the anaesthetic agent have
dissolved in the cell's lipid membranes. (my hack on a Wikipedia
This would lead us to believe that nitrogen is indeed an anaesthetic when under
pressure but, perplexingly, oxygen is even more so. However it gives helium as
a poor dissolver so very good from the 'avoiding narcosis' point of view.
I have some figures that say oxygen is about double the solubility of nitrogen
but, since it is consumed in your tissues it will be lessened at the actual
point of action while helium should be about 16% of that of nitrogen.
What we tend to do is treat oxygen and nitrogen as equally narcotic while
helium is considered not narcotic at all but, until we have a Narc-o-meter to
plug into somebody's head and read out that they are 28% narked, that is good
enough as the day to day and person to person differences are bigger variables
So. We introduce the idea of END, Equivalent Narcotic Depth, which is
the depth you would have to dive on air/nitrox to be that narked.
The sum is very simple. What is the partial pressure of the O₂/N₂
part of your gas? How deep would you have to go to find that as an absolute
For example diving 18/40 Trimix (18% O₂, 40% He and hence
42% N₂) at 60 meters.
You have 42+18=60% narcotic gases.
The absolute pressure at 60m is 7bar so the partial pressure is 7*0.6 =
Your Air/Nitrox dive to 4.2bar absolute pressure is 32 meters deep.
So diving 18/40 to 60m is roughly, and I emphasise roughly, the same as diving
32 meters on air from a narcosis point of view. (Please note this is not
anything like EAD and decompression. Cutting your gas with O₂ can reduce
your deco time, helium will probably increase it.)
That seems good but Helium costs me well over a penny (UK) a litre buying it in
bulk so to fill my 10L/300bar twinset with 40% is over £30. Far more if
you go to a dive shop and they cost in all the work that I do for myself and
the equipment I have. This is on top of any oxygen needed to make up the mix
and the usual price for an air fill. A weekend's helium diving can easily cost
a hundred quid in store fills alone.
However, cost aside, the real problem with helium is that it is a 'fast'
So what do I mean by fast?
On the Bühlmann decompression model there are 16 'compartments' which
represent overlapping tissue types. It isn't that your spleen is a compartment
type four and your leg is a compartment type six just that all 16 overlap
enough for everybody to get represented. Also when one compartment feeds
another it is just represented by a slightly longer compartment. The system
dates from Haldane and it's good physics.
For nitrogen they have one set of 'half times' for each compartment
representing how long gases take to dissolve and release but for helium they
are about a third as long.
This implies that helium dissolves in your tissues at nearly three times the
rate of nitrogen so, in very approximate terms, a dive on helium is the
equivalent of three times as long. However, as your deco is three times as long
too so it evens out and a helium dive isn't all that much different from a
Until you miss stops. Then those ten minutes you missed on your 40 meter dive
is effectively half an hour of missed stops on a dive with three times the
bottom time and your problem has moved up an order of magnitude.
Conversely when they haul you messed up carcass into a chamber all those helium
bubbles redissolve at three times the rate and you then are back to doing the
re-deco at 'normal' rates. Your untreated bend is much worse but, if they get
to it in time, the prognosis is much better than for the same level of bent on
One of the consequences of this is that your ascent needs to be spot on. Deep
stops can have some compartments off gassing while others are still on gassing
so hitting those numbers is much more important. The fact that a only two
minute stop is called for doesn't make it trivial (think six minutes). I much
prefer diving a computer on helium precisely because in the deep ascent stage
of the dive it is recalculating 'on the fly'. If I were diving tables I would
have to select a far more conservative algorithm because I know I'm not going
to hit my numbers to the second and all my errors are multiplied by
To me helium is an essential part of deep diving. Since I use a rebreather some
of my diving decision making is more complex than for an open circuit diver so
a clear head is a life expectancy issue but happily my main gas tank is three
litres not twenty so my helium costs are moderated. I tend to run the
rebreather either on 18/40 where Oxygen toxicity is not a problem or 12/65
where it is. I have bailout cylinders with the same mix so I can swap about if
I have a problem and my training included switching both my decompression
computers back and forth between the rebreather and the bailout.
I even use a tag line on web fora of Helium, because I'm worth
So why the picture of Sussex Heights in Brighton? It's 102 meters from the roof
to the road. A good trimix dive.
No, not really spanners. I just wanted to list the thread names we use on Scuba
gear as I can never remember them.
O-rings are identified by their 'dash number' and ID/CS dimensions. (Internal
Notice that the DIN and A-clamp O-rings are listed as different but a DIN ring
seems to work in every A-clamp reg I see. This is not true the other way round.
They seem to seal OK but fall out at every opportunity. I think everybody uses
the DIN size and thinks they are the same.
So while we are talking about O-rings perhaps we need to explain them a bit:
|What I call it||What it really is ||O-ring|
|Standard low pressure port||3/8" UNF||-011 .301" .070"|
|The Apeks bigger version||1/2" UNF||-013 .426" .070"|
|Standard high pressure port ||7/16" UNF||-012 .364" .070"|
|Standard 'DIN' thread||5/8" 14TPI BSPP||-112 .487" .103"|
|New funny EU nitrox thing||M26x2||Apparantly as DIN|
|A clamp||-014 .489" .070"|
|HP swivels||-003 .056" .060"
|2nd stage connection||9/16" UNF||-010 .239" .070"|
An O-ring is not just a plug, like the caulking on a boat, that fills up the
holes to stop stuff getting in. They are far more sophisticated than that and
knowing how they work should control our thinking on how to maintain them.
The first O-ring patent dates from 1896 so it is quite late into the age of
machinery. In its simplest form an O-ring is placed in a rigid mounting that
applies a uniform load deforming it. The seal is produced because the fluid
cannot pass as long as the contact pressure of the O-ring exceeds the applied
pressure, ie: the fluid cannot push hard enough to lift the O-ring from the
wall it is sealing against. The magic trick is that the pressure of the fluid
pushing against the exposed face of the O-ring further deforms it away from the
fluid and into the wall increasing the seal pressure so the applied pressure
can never 'catch up' with the seal. Basically an O-ring will hold until either
the housing fails or the O-ring is physically extruded through the gap it is
So why do O-rings get greased? Two reasons. Firstly a lot of O-ring materials
have plasticisers and other semi-volatile components and greasing stops these
escaping but secondly as the O-ring deforms under pressure it must not adhere
to the housing wall, even under extreme load but must deform uniformly or the
surface tears away from the body and the ring fails as the pressure now rather
than increasing the seal penetrates the ring.
The down side of greasing a ring is that it becomes a dirt magnet and an O-ring
cannot perform its deforming to seal better and better trick against an uneven
surface. The general rule is clean the ring fastidiously and if it is the sort
of ring that likes/needs grease apply it very sparingly.
Bailout on a rebreather
There is always a lot of discussion when 'bailout' is mentioned. So we're clear
what I'm talking about here I'm using 'bailout' as a noun that describes your
provision of gas for a broken rebreather scenario.
Obviously you need something as your CCR 101 class taught you the quick check
of a 'sanity breath' to check your body by getting it off the loop for a
moment. This is good but how long you can stay off the loop depends on how much
gas you have available.
Now on my MOD 1 (basic CCR) I was taught to use the onboard 3L tank of diluent
on my Inspiration Classic for my sanity breaths and my quick trip to the
surface if all is not well. MOD 1 was a very limited (eg: 5 minutes) deco
ticket so a 3L would get you home undrowned.
MOD 2 is Normoxic trimix and you do deco, quite serious deco so we were taught
'team bailout'. We were all carrying 7L stages of trimix that could be breathed
at all points of the dive and the drill was that if somebody had an equipment
failure they switched to the stage, signalled their team <UP> with
urgency and started to ascend. When that cylinder got down to half contents
they swapped it with another team member so they had a full one again and the
donor had a half full one so they had something to reach for if they had a
subsequent problem. The idea is that the team carried enough gas to get one and
a half divers safely back from the worst point on the dive and the procedures
meant that everybody always has some gas hanging from them.
MOD 3 is hypoxic trimix and moved the emphasis to self sufficiency. You carried
enough gas to get yourself out on a total rebreather failure and you only
looked at a buddy if you had subsequent failures.
However this is 'worst case' but not all cases are worst. What is
Well let's start by thinking through what is a 'dead' rebreather. In the basic
class you learn to manage the oxygen inject system manually and to set it up
properly and that prevents or manages the vast majority of reported
People die on rebreathers but when the results come out they are often the
simplest things. The old mantra 'Always know your ppO₂'
would have saved most of the lives lost. Diving with the system simply switched
off has killed far too many people, I can't save you from that Darwin Award,
but what faults are next on the line?
A loop flood. Something unexpectedly broke and suddenly the plumbing is
irretrievably full of water. This is not 'it has some water in it' because you
should be able to manage that and clear the loop and resume the dive or at
least finish the dive on the loop. I am thinking about a breathing hose
flapping about in the tide when it should be screwed in behind your head sort
CO₂ break through. Most of these are operator error, mine
was, but recriminations are no good under water so you have to get off the loop
(which might be a problem see BOV).
If the loop isn't dead I hold that you should stay on it and use your training
to manage the gas in the loop. We didn't have bailout in MOD 1 and it worked
but once you commit to the bailout you now have only one path home. There is no
bailout from the bailout. The only snag comes if you don't know what the gas in
the loop is either because your displays are trashed or because they are all
wildly different and you don't know who to trust even after doing the
procedures to work out who is telling porkies.
While I'm on the subject of Rebreathers let's explore the much maligned 'Temp
It seems very trendy in trendy circles to be rude about the temp stick but,
although it has its limitations, I think it's a good addition to a
So what is it? Well it is a rod that passes down the centre, usually the
centre, of the rebreather's Carbon Dioxide scrubber with a number of
temperature sensors in it. Now the scrubbing reaction is pretty vigorous,
exothermic in chemist speak, so as the 'lime reacts with CO₂ it gets hot
so we have a tell-tale signature of heat generation.
This means that the heat being generated at the reaction front will
progressively warm the bed upstream so we should see a kick where the scrubber
is working. The graph should be simple. Right? Explain the graph on the right
then. That isn't simple.
This is an hour long December dive in Lake Geneva on a part used scrubber and
you can't see a five minute pre-breathe. The sensors in order from the top of
the stack are numbered 1 through 8 and the thin line is the depth. The depth
scale in meters is correct but I don't believe the temperature scale, in fact
I'm not sure they are even zeroed so only look at the up down trends not the
So what has happened? Well it has been dived before so we can divide the
scrubber into three general areas. At the top (this is a Sentinel and the
exhaled gas comes in at the top) is scrubber that is mostly exhausted. It might
have some life but it is not going to be making a serious contribution to
scrubbing or heat generation. In the middle let's call it 'part used' while at
the bottom there is largely unused scrubber material.
The working zone of a scrubber on the surface is surprisingly narrow. When
CO₂ hits good lime it gets zapped. Most of this stuff is sold into the
very competitive medical market where cost is an issue and not going wrong and
getting sued comes a close second so it is very efficient and well tested. If I
was just to sit on the shore and breathe the working band would creep down over
time leaving dead stuff behind but with untouched stuff below. Getting my
fingers on the can and feeling the heat I'm going to guess it's less than an
The heat is another matter. The gas coming in is fresh from me so it is at body
temperature so my scrubber that has been cooling from the nice warm car as we
have changed and been briefed gets warmed up again on the prebreathe. Sensor
1 just reads this body temperature for the whole dive as it is in
amongst some 'previously enjoyed' scrubber material so no reaction and hence no
extra heat. Remember that a sensor tells you about the reaction in the section
of bed before it so when I speak about something happening at sensor 3 I
am talking about the section of the scrubber between sensors 2 and
3. I think the prebreathe reaction really happened between sensors
2 and 3 and 2 was heated by conduction sitting waiting to
dive. Certainly 2 blips a bit as we start the dive but I suspect is
reading pretty much the same as 1. 3 is certainly on the job at
the start of the dive and from them on the heat generated here is carried on
down the can in the nice warm gas warming the layers below that have yet to be
called on to work.
The next thing that happens is a progressive descent to 35 meters. Now this
increases the absolute pressure from 1 to 4.5 bar and hence the mass flow goes
up by 4.5 times. Now remember that the gas flow in litres per second hasn't
changed nor has the velocity of the gas but we are just packing more molecules
into the same space. Hence the chance of a CO₂ molecule getting picked up
goes down as there is just 4.5 times as much other stuff getting in the way so
the working zone becomes wider. The scrubber material is still scrubbing but
less efficiently. If this was a well used scrubber and the working zone
actually reached all the way to the bottom some CO₂ would make it right
through. (We'll discuss this later). What you see is that sensor 4 that
wasn't showing much interest in being prebreathed is now getting in on the
action and showing heat, then 5 and 6. 4 actually sags
through the deepest segment as it is being cooled by the incoming gas more than
it is being heated by its reacting with CO₂. I would say that 4
and 5 are doing the work here and progressively heating the gas, and
hence themselves. 5 is doing the most work as it is generating a big
temperature step while some is getting through to 6 which isn't
generating much heat. 7 and 8 are just getting warmed by the nice
warm gas coming through.
Then we ascend back to 20 meters and for the rest of the dive you can see
4 dropping out of the game as used up while 5 and later 6
coming back on line. 7 and 8 never seem to get involved and are
just heated by the gas flow. That put the scrubber at about three hours so it
went in the bin.
This whole business of the working band widening at depth is the reasoning
behind the APD mantra:
Be shallower than 50m at all times after 100 minutes of scrubber use
Be shallower than 20m at all times after 140 minutes of scrubber
Your worry is that the CO₂ will start to break through as this now puts
more into the cycle next time round and even a small fraction of more is even
more so it begins to wind up. There is no 'almost OK' about a scrubber but the
fix is to get shallow and reduce the depth of the reaction front. The second
best fix is a flush and, if you have the gas, a bit of semi-closed on the
ascent or the getting out of the wreck phase. For dive planning the image I
carry in my head is of the scrubber like an iPhone battery picture. Full is 180
minutes. The working section on the scrubber is about 13 minutes worth of lime
on the surface - unlucky 13. Put the pressure up six times and it is now 13*6
so over 72 minutes of scrubber are in use, that has to be subtracted from the
battery remaining. When the working section hits the end you are in the kill
zone. This works for me but the APD one is easier to remember.
I liked the Sentinel temp stick display because it gave me a little block
graph of the sensors (top left in the picture). OK it also gave me a number
that is developed on some frightfully clever computer algorithm but it is
usually so wrong you want to laugh. I'm looking at the steps. The big step is
where it's working. I know how big my working zone is and I use that to warn me
to give up early. I don't use it to extend my time as I've had one run in with
CO₂ and it really spoils your day.
Attitudes to Oxygen Clean
A story I was once told was about somebody picking up a cylinder from a shop
and wiping the face of the (A-clamp) valve off with his finger. The shop
assistant berated him that this had compromised the 'Oxygen Clean status' of
the tank and it should now be recleaned before it was used for nitrox. I rolled
about the place clasping my tummy before I realised that it wasn't being told
as a joke.
OK let's put some numbers to 'oxygen clean'. Oxygen clean is different from
Oxygen compatible. You must use the right materials for high pressure oxygen
environments. This is just the way it is. However we have this routine of
cleaning and much of it is just plain wrong.
Oxygen clean is not surgically clean. Surgically clean worries that the one
microbe on the one scalpel you failed to autoclave multiplies and multiplies
and wipes out half of East Africa. Oxygen clean is just chemistry.
Back in the days of the uk.rec.scuba news group we worked out the energy in
scuba tanks based on their pressure and in simple terms:
12L at 230 bar is aprox 1242kJ This is the working pressure of a tank.
12L at 346 bar is aprox 1868kJ This is the test pressure of a tank.
So let's get the energy from a fire:
We get 4163kJ/mol burning hexane (eg: petrol)
so (1868-1242)/4163 so 0.15 mols of hexane
at 86gms/mol is 13gms at 0.65gm/cc is 20ccs
This is not half an inch in the bottom but neither is it just *dirty* or *you
touched it - it's infected*.
Interestingly a toxicology site gave 20ccs of petrol as a lethal dose if
inhaled or injected. So a tank with a deadly level of contamination is not
quite an oxygen fire risk?
OK it's funny farm time again as people talk more nonsense about the pros and
cons of Bungeed Wings than most subjects in diving. Firstly what are we talking
A 'Bungeed Wing' is one that has an elastic thread wrapped round it to collapse
it when it is not inflated. The rational is that this offers more streamlining
and hence less drag. Now it can be well done and it can be badly done. Lots of
tight bungee with the inflated wing bulging out between looking for all the
world like some macabre creation of the balloon animal man at a children's
party in not just bad but very obviously silly. A bit of discreet bungee
arranged to tuck the wing away when it is not in use is more what I'm aiming to
Let's talk about streamlining because as usual 'no clue' comes from not
understanding what is going on. Now my fluid mechanics was a second year
physics degree course and for water it is 'incompressible flows'. It wasn't
very interesting at the time but it comes back to haunt me as a working
industrial physicist far more than the exciting quantum mechanics which has
only ever been a hobby interest. If by streamlining you mean drag reduction the
rules are quite simple.
Streamlining rule one. Fill in all the holes. Absolutely. If you are
moving a pipe through water put a flat end cap on it rather than let the water
go down the hole in the middle. You would be horrified to work out how short
the pipe has to be for this not to apply any more. It is getting to be a ring
not a pipe by then.
Streamlining rule two. Flatten the surfaces. What you are after is not
letting energy be dissipated so creating turbulence is your enemy. Any
turbulence you leave behind you in the water is energy you had to put into it.
A lot of flat surface is much better that a little bit of bumpy. Anything that
makes the smooth boundary layer break away from the surface costs you. You want
to see the ultimate in streamlining? Look at a Zeppelin. How can a thing that
is that big and that light need brakes? But they had real problems with
stopping even into wind.
Streamlining rule three. Reduce the surface area. A sphere is very
streamlined and a dart isn't.
So let's look at a diver from a streamlining point of view. No! Stop rolling on
the floor holding your tummy. I know it's bad but we might at least be able to
improve some bits of it. The first entertaining thing we notice is that
clipping equipment in close rather than letting it dangle makes absolutely no
difference. As the boundary layer is measured in millimetres streamlining is
putting it in a pocket or velcroing it hard too you - see rule one. If you want
to carry a lot of clutter efficiently buy a bag. That is a bag with a drain at
the bottom not a string bag as, from a hydrodynamic point of view, a string bag
is just a big pile of clutter dragging through the water next to you with added
Now the cylinder and wing. Didn't you have tank wrap on? Oh, you decided it
might not be quite so good. Well if it was canvas it wouldn't be a problem but
it did have rather a lot of holes to apply rule one too. What about the wing?
Well it has X amount of area and that isn't going to change as it is not a
balloon, it just unfolds as you inflate it. How you stow it makes virtually no
difference, well, unless you ruck it up into lumps and put bungee in the
boundary layer to break up that laminar flow.
So that's it. If the wing just collapses bungeeing it can't make it any better
and the bungee side effects can only make things worse. My smooth old DiveRite
Classic twinset wing might flap a bit when fully deflated but I can't change
the exposed surface area unless I roll it up (rule three). Squeezing it down
can only start getting me into rule two problems and make matters worse. So can
you now see why I'm not a fan of bungeed wings?
So one last question. Does bungee make the wing less likely to snag on things?
I'm not sure I can give a definitive answer here as it depends on what sort of
snag you are snagging on. However if I am venturing into a place with hookie
things that might hook on my bungees I definitely know the answer. If they
aren't hookie aren't they just going to slide over my nice smooth wing? When
you come down to it I'm not brave enough to dive a bungeed wing.
We were discussing 'being blinded by helmet torches' and I realised that I
needed to do a test so I knew what I was talking about. This is also the
argument for black kit so you don't reflect my torch back at me and blow my
The test needs to represent me trying to see with my helmet torch or torches
after getting a full frontal exposure from somebody else's lamp and see what
the recovery time is. Let's make it a big hit not just a flash too. Basically:
is this something I need to worry about having it happen to me or doing it to
somebody else? We have been quoted a 30 minute recovery time which sounds bad
but I just don't quite believe it.
So I am sat in my 'office', which is really just the more respectable end of
the home workshop, and I find my Petzl caving helmet which has a switchable
LED/incandescent light on it. I compare the LED side to the SL6 torch on my
diving helmet and the SL6 is way brighter. Good I will test on the Petzl
because it is so much more comfortable to wear for not being stripped of all
its internal padding to fit over a neoprene hood.
I black the place out, turn off the computer screens and hide the laser mice. I
do have to leave some stuff on so I have points of light about me but I have a
company web server and the mail host running here and they shouldn't go off. I
look round on the Petzl light and start waiting to adjust. Ten seconds later I
feel very adjusted so I turn the helmet light out to see if that helps. I wait
five boring minutes and turn the helmet light back on and I can still see all
my tests but nothing more. I ought to be adjusting to the head light but it's
more interesting to have it off. Using the helmet light the hardest thing to
read is some light blue text on a grey box at about four meters. If I don't
have the light pointed just right I can't see it.
The hardest thing to see without the light on turns out to be the castored legs
of the other office chair just lit by the LEDs on the stereo system (I'm an old
buffer who thinks they haven't built proper hi-fi in thirty years so I share my
office with a Quad 606 amp and a pair of '57 Electrostatic loud speakers.)
Interestingly it takes nearly 25 seconds for the castors to fade back into view
after turning the helmet light off. I try this a few times and wait a lot
longer but I don't get any more detail. This must be my maximum setting.
Remember your eyes have an iris that opens and closes to give a small but fast
adjustment that caters for you glancing from light to shade and chemical
adjustments for wide range adjustment that takes time but that can adjust from
direct sunlight to starlight. Think of it like film speed. It is this chemical
adjustment, not the iris, that we are trying to measure here. My iris, see the
pictures, contracts to 44% by diameter from dark to light so it factors the
light by a mere 5 to 1.
I finish my 30 minutes pre-adjustment period (in case 30 minutes is right) and
reach for the Fa-Mi can lamp. 75 watts spread across six LEDs. The helmet light
is out but I can run this kit by feel. I start listening to the clock tick in
seconds, push the button on the stop watch and turn the lamp on. It's in my lap
pointing at my face. Sheesh that is bright. Don't cheat, look at the bulbs.
-8-9-10 turn it off. OK I can't see anything, well other than this purple blob
with the tracks of the six LED head in it. The LEDs on the stereo come back
but, interestingly, they are orange not green. I try the helmet lamp. Hey I can
see. The centre of my vision field is a bit fuzzy but it works. 35 seconds on
the stop watch - yes I can read the stop watch. I can read the text on the box
That was the test, remember the number 25 seconds from when the big lamp went
out until I read the watch. It might have been less but I'm not doing it again
to find out. However there is one more thing to try. Turn the helmet light out
again and try for the ultimate readjustment to max sensitivity. My orange
lights are greener. Actually it's the slightly luminous blob that is making
them orange. They go green. The castors of the chair finally fade back in.
Check the watch. 3 minutes and a few seconds so less than three minutes from
when the light went out.
Conclusion? Dazzle isn't a disaster and certainly it isn't 'blinded'. This gets
the 'Nigel Rating' of 'inconvenient' (the next level up is 'annoying'). I
really don't expect to have your torch in my face for ten seconds without me
closing my eyes and turning my head away but even if I did twenty five seconds
of glowy blobs isn't a great hardship to get my 'torch vision' back.
One of the constant subjects that comes up in diving forums is that of Reverse
So what is a reverse profile? It is simply when you do two or more dives in a
day and you don't do the deepest dive first.
This seems to come up in a lot of diving instruction that this is wrong and is
repeated as doctrine but is it justified?
Firstly from a deco point of view deepest dive first is a good idea. If you are
going into a deep dive already with a considerable dissolved gas loading it
will increase your decompression time considerably. So if the second dive is
the deepest dive then you will end up spending more time that day dangling on a
string than if you were able to do them the other way round.
Conversely if the extra time isn't a problem that's all it is: extra deco time.
If, on the other hand, the first dive is to check your equipment and procedures
are all in place that might make the second deeper dive much safer and the
extra time is more than worth it. Also the nonsense about setting a limit on
the second holiday dive because it 'mustn't be deeper' is best laughed at and
somebody who seriously suggested doing a quick touch-and-go to 40 meters on the
first dive so as 'not to restrict' the second dive was to my mind detracting
from safety rather than adding to it.
However in 2000 the American Academy of Underwater Sciences ran a Workshop on
reverse profiles where a lot of learned people came and presented papers and
the procedings are
available and the Abstract says:
I was horrified.
1. Historically neither the U.S. Navy nor the commercial sector have prohibited
reverse dive profiles.
2. Reverse dive profiles are being performed in recreational, scientific,
commercial, and military diving.
3. The prohibition of reverse dive profiles by recreational training
organizations cannot be traced to any definite diving experience that indicates
and increased risk of DCS.
4. No convincing evidence was presented that reverse dive profiles within the
no-decompression limits lead to a measurable increase in the risk of DCS.
We find no reason for the diving communities to prohibit reverse
dive profiles for no-decompression dives less than 40 msw (130 fsw) and depth
differentials less than 12 msw (40 fsw).
Given findings 1, 2, 3 and 4 the conclusion just boggles the brain.
The conclusion should have a full stop after the word 'profiles', end of
paragraph, end of document, go home pleased with a job well done.
They should be ashamed of themselves.
Specifically how much drive gas does my booster use?
I'm not going to do an exact model because the important bits get lost in all
the adjustments but I'll admit to the approximations as we go. Consider the
diagram as they say:
Two chambers both with a piston in them. The pistons are connected by a solid
connecting rod. We blow low pressure drive gas into the left hand side pushing
on the large piston so we get a force on the rod that is basically the drive
pressure Pa multiplied by the piston area. This
acts on the smaller piston so if the reverse force Pb multiplied by the smaller pistons area is less than this
force the piston will move to the right compressing the gas.
The ratio of piston areas, which is naturally the same number as the ratio of
the swept volumes, is called the drive ratio of the booster N. It sets
the theoretical maximum pressure output as being N times the available
Let us consider one cycle of the booster:
We connect it to a supply tank that is at pressure Pin through a one way valve so that means that any time
Pb falls to Pin the
valve will open and allow gas in so Pb never
falls below Pin.
We also connect it to the target tank which is at pressure Pout through another one way valve so that means that any
time Pb rises to Pout the valve will open and allow gas out so Pb will never go above Pout.
Here is my first approximation. One way valves don't open magically as soon as
the pressure is 'equal'. They need a certain pressure difference to compress
their springs. This is usually at least one bar and if you want small and such
it can be more. I'm going to ignore that loss for now or everywhere I want to
write Pin I'll need to write Pin-Pvalve loss etc.
We are going to feed drive gas into the left hand side and I'm not really
bothered about the pressure available there provided it is enough, we'll
discuss 'enough' later. We are going to let the gas in gently so our expensive
pistons don't bang about and hence the pressure rises smoothly.
So we start with the left hand side pressure, Pa,
at ambient so no force on the piston from that side but the right hand side
has pressure Pb = Pin so its piston is pushed hard to the left.
We start to let in some drive gas and the pressure Pa starts to rise. The first key event is when the drive
gas exerts a force on its piston that equals the force from the other side.
From then on the gas we let is is going to start to move the piston to the
This will happen when the force on one piston is equal to the force on the
other piston so Pa*N is equal to Pin.
Now things are moving. As drive gas comes in the piston moves progressively
towards the right and the pressure in the output cylinder rises closing the
inlet one way valve. Since the piston is only moving comparatively slowly we
can estimate that it is in balance so Pa*N
will be equal to Pb so both pressures climb in
Our next key event is when the pressure in the output cylinder reaches
Pout and the output valve opens. Now the piston's
movement just pushes gas out so the pressure doesn't rise any more. This is the
point when Pa*N equals Pout and from now on the pressure will not change on either
side significantly as the booster is now moving gas. Finally the pistons reach
their maximum rightwards travel and they trip the reset.
Let's take stock.
We started with a quantity of gas in the output cylinder Vb at pressure
Pin. Using the 'surface litres' trick that is
just Voutput = Vb*Pin litres of gas.
The drive side has swept the same distance as the output but at N times
the area so Va = Vb*N.
The final drive pressure was still governed by Pa*N equals Pout so we
can rearrange it as Pa = Pout/N.
The volume of drive gas we have used, in 'surface litres' is Va*Pa which is, using the two
Vdrive = Vb*N * Pout/N
The N cancels out and we get
Vdrive = Vb *
Before we go any further let's admit to more approximations. The first is that
no pump is perfect. The pistons can't get quite flat up against the one-way
valves so not all the gas gets pumped. Now this is an important matter to pump
makers so they do their best to make this volume small. Looking at the output
side we have some 'dead' volume so the gas we put in was really (Vb + Vdead) * Pin and the gas we got out was that less Vdead*Pout left inside. To
give you a feel for the problem if the dead volume was 5% of the swept volume
and we were multiplying the pressure by 3 we would get 9% less gas per stroke
than we expected.
The second approximation is more subtle. We are using Boyle's law and we don't
really have the 'fixed temperature' it assumes. Actually the instantaneous
heating helps us at the right hand end to get more of that 'dead space' gas out
but the overall heating of the whole unit reduces the throughput. Conversely
the drive gas expanding and hence cooling means we need a bit more of it.
However it is in the single figures of a percent and, like the rest of the laws
of physics, it's unavoidable.
Let's pick the two results up and think them through.
Voutput = Vb * Pin
Vdrive = Vb *
So the first one tells us that regardless of how much gas we are pumping the
drive gas used per stroke depends entirely on the output pressure.
The second one tells us that the amount of gas transferred per stroke depends
entirely on the input pressure.
To me the most delightful point is that the ratio of the pump just cancels out.
This becomes more funny when people start to compare boosters. I remember
reading a comparative review and to me the most obvious point was that the
reviewer was maxing out his little 'hobby paint sprayer' drive pump. So, as you
will now understand, several very different boosters all came in at virtually a
dead heat in the cylinder filling time test. Yes the ratio does matter because
it sets the maximum pressure you can boost to for a given drive gas and a
higher ratio pump will do more cycles and hence more wear to move the same
volume of gas. Mine is a 75 and, frankly knowing that I can boost to 745 bar
and destroy virtually anything I own does me no good but I have it and it works
and, frankly, pumping 3L rebreather cylinders I really don't care.
Let's just mention 'enough' pressure. Well obviously you have to have 'enough'
drive pressure to push against Pout. If your
booster is limited to 10 bar of drive, as most are, that means a 35 ratio unit
will pump to well over 300 bar but if your ratio is only 20 you'll be
struggling to get close to to 200bar.
And the last point. How much drive gas was that to fill a cylinder?
Well fortunately everything is roughly linear so we can play averages rather
than integrals. Let's say we are filling a 10L of O2 from
a 50L J with 120bar in it and we want 220bar final pressure.
Well the first bit is free. We connect up the empty and open the valves and the
gas just runs straight through our one way valves until it all evens up. That
is it goes from 50L * 120bar into 60L * 100bar.
However we want a total of 10L * 220bar so 2200L which is 44bar out of our J.
So we are going to pump our input pressure from 100 down to 120-44=76bar.
Average input is 88bar. Average output pressure goes 100 to 220 so 160bar
Now something tells me that the number of strokes of the output side will equal
the number of strokes of the input side so I can just assume that just cancels
out and I can eliminate it and Vb by dividing my
two equations giving me:
Vdrive = Pout/Pin * Voutput
Bang in the numbers and it's Vdrive = 160/88*1200 = 2182
Not bad for the big chunky air tools type compressor pushing out 400L/min
(14CFM) that I have but not quite so happy on an airbrush driver rated at
25L/min. Still, provided the compressor doesn't die horribly after being on
solidly for over an hour, it will be a nice cool fill.
OK I'd like to whinge about how some people want to make a big issue out of
something simple but take that as read and we'll do the sums.
Well there are two sorts of gas plan. There is 'I'm going on a normal dive, how
long have I got?' and there is 'I'm going deco diving, I want to live'. The
sums are the same but the imperatives are less on the first as if you keep
watch on the SPG discovering you have done your sums wrong is merely
OK, so we all know we draw more from the tank at depth and let's just check we
agree on why. Your body is dumping Carbon Dioxide into your blood as a waste
product and this makes your blood more acidic. Acidic blood triggers the little
'I must breathe more' automatic system so you push more nice CO₂ free air
through your lungs and wash it out. However getting the nasty stuff out of all
those little alveoli takes a predictable volume of gas and if you are deeper
there are more grams of gas in a lungful but you still need the requisite
number of lungfuls a minute going through to stay nice and acid free. You can't
cheat the system because its design assumes that breathing more is safe so
breathe long and slow so the flushing works efficiently to use the least gas to
keep it all happy. The fact that you push considerable amounts of oxygen
through does you no extra good as if you have enough that's enough.
Right so the first thing we need to do any meaningful sums is that required
number of lungfuls or, more usefully, litres a minute. I'm sorry but you have
to do this test in the water. All those mammalian things about water kick in
and the numbers, if measured just sitting in front of your computer of an
evening, will be so wildly out it's funny. Now some people like to talk about
average depth and stuff but averages are a pig to work out unless you do square
dives so take a square section of a dive. One of those boring flat bottom ones
will do nicely. You know the type, second dive of the day and it's on some
silty 'reef ' (pile of rocks) and it's covered in 'life' (weed) but you paid
for a day's diving so you're darn well doing it.
Get to the bottom and pull out your wet notes and note the depth, time and SPG
reading. Mooch about until the boredom begins to hurt and if you've done enough
time to be respectable log those same numbers again, blow the DSMB and get back
on the boat for some tea.
So what do we discover? We were at 18 meters for 25 minutes and we used 125bar.
Roughly. The other thing we need is the tank size and mine's a 12 litre.
OK 125bar at 12L is 1500L of gas breathed in 25 minutes to 60L/minute.
However we were at 18 meters so that's 2.8 bar deep (see depth in
bar if this sounds strange) and we want to factor out the density change so
divide by 2.8 and get 21.4L/minute.
This is your SAC ie your Surface Air Consumption. It's the
number we can multiply by our depth in bar to tell us what's coming out of our
tank minute by minute. You don't need it accurately. I'd put this down as
Unfortunately your SAC isn't something fixed for you like your birthday it's a
best guess, if you're lucky, on a good day so we need some margin on this but
we'll talk about that later. Also remember that we picked a reasonably benign
dive so you weren't going up and down and hitting the drysuit and BCD buttons.
What we are going to do is to work out how much gas you use on a simple dive
and then you can decide how much gas over the 'what you should use' you are
going to take.
Simple dive: We descend to 30 meters in three minutes, we swim about for twenty
minutes and then we come up (gropes for tables) no stop. OK we'll bung in a
three minute safety at 6 meters.
Descent: Average depth 15m for 3 minutes, the ascent will be the same just with
a break at six in the middle. So 15m is 2.5bar deep so 22SAC * 2.5bar * 3
minutes is 165L out of the tank.
Bottom time: 30m is 4 bar so 22 * 4 * 20 is 1760L
Ascent: another 165L plus a stop at 6 so 22 * 1.6 * 3 = 106
Add it all up and get 2196L. Divide by 12L and get 183bar out of the
Well I don't know about you but I think that sucks. We all know that our nice
warm fill was 230 bar in the shop, 215 by the time we had got home and let it
cool down and is going to be more like 200 when we check the gauge at the top
of the shot rope as the tank cools some more to sea temperature (if a fullish
tank loses 10°C it goes down by about 7bar). A plan that means we get out
with 17 bar has not got any built in life expectancy. Quick! Recut that bottom
time or borrow a 15L.
Back to our 2196L and on a 15 that's 146bar. That's more like it. It might not
be a third but it's a plan to get out of the water with more than 50 bar. Watch
the SPG and the magic number to store in your head is the ascent usage of
165+106 = 271L is 18bar so add in a 50bar margin for error and you need to be
on your way up by the time you hit 68. That's on your way not just reaching for
A deco plan is pretty much the same tricks to work out the volumes but now you
really need to think about the 'what if I break something?' question. 'Can I do
the whole dive on back-gas?' is a good thing to ask. Frankly accelerated deco
gets you out sooner and feeling better but you don't need much gas hanging on a
string at 6m so a fall back plan renders breaking your deco reg merely
annoying. Also, if your buddy has a gas problem, it means you can stay with
them which isn't a bad move.
Let's plan a deco dive. 45 meters for 25 minutes and let's plan it for air.
My planner gives me 2mins at 9m and 22mins at 6m and I'm going to ignore the
descent time so it doesn't matter how fast we get down there.
45m is 5.5bar deep so 22*5.5*25 = 3025L for the bottom part.
Ascend to 9m (avg=27m=3.7bar) in about 4 mins = 22*3.7*4 = 326L
9m stop 22*1.9*2 = 84L
Ascend to 6 22*1.65*0.5=18L
6m stop 22*1.6*22=775L
Nice slow final ascent 22*1.3*2=57L
Add it all up and we have 4285L so on my twin 10s that's a notional 214bar.
Good thing they are 10L/300bar twins.
I'm not hanging about at 6m for 22 minutes! That means the total ascent is
longer than the bottom time. I've got a 3L of 80% and that knocks it down to 10
minutes. Well it does if I can find some more 80% for my buddy. What will I
22*1.6*10=352L so 117 bar.
I could now do something horribly complicated using trimix with multiple deco
gases and several bailout scenarios but the sums are just the same. SAC *
depth_in_bar * time, add the bits up then divide by the cylinder volume to get
bar. Then look at the numbers and say 'but is it safe?'. Now I'm not going to
tell you you should have a third left at the end of the dive although I think
that a good plan but I do think you should start the ascent with at least
double what you require and then some so if you suddenly have to stick your
secondary/octo in your buddy's mouth it all still gets you both out. If you're
the type that gets a bit puffy under stress allow for that as there is nothing
quite like a mouth full of seawater and having to do an abrupt swim to your
buddy on empty lungs when you really want to cough the stuff out to get your
buddy's octo to blow your cool. As an experiment I have tried and I managed to
hit 100L/min SAC as a sustained breathing rate and I don't think I could
physically hoover more gas down my neck but that would make a pretty big hole
in a gas plan pretty quickly. You have to make a personal call on how much
extra you need but comparing your pre-dive plan with the post dive facts is a
good start. If it's coming out wrong you might need to adjust that SAC rate up.
All dive gear needs routine cleaning or you are going to regret it. A lot of it
is just a quick hose down with tap water but some things need more attention.
Compare the two DIN threads on the right. As you can guess one does up easily
and the other grinds and grates.
There are two main deposits you get on dive gear: salt and chalk. Salt goes on
fast as seawater deposits it and salt crystals are a nasty abrasive but a
good rinse après dive sorts it out but I live in a 'hard water' area so
I also get lime scale in my home plumbing and on my nicely rinsed dive
So what gets off lime scale? An acid. It doesn't even need to be strong so you
could use vinegar or even lemon juice but I already have some stuff on the
bathroom cleaning shelf to do the inevitable streakies on my big glass shower
door. This has the advantage that it is cut with a line of detergent so it
rinses off well plus, I am assured, it doesn't smell like a chip shop on a
Friday night (OK. I have little sense of smell so I worry about things like
that). An old toothbrush works as an applicator but some little brushes sold to
apply plumbers flux seem to be my tool of choice although next time I have a
plumbing job to do about the house and they are all in the diving tool box I
shall be annoyed.
My preferred rinse after cleaning on dive gear is 'deionised water'. Look for
the label for 'Car Batteries and Steam Irons'. This does not deposit a film of
salts or chalk as it dries so at least things leave the cleaning bench pristine
and beautiful. If I am putting them away for a bit or taking them on holiday
this seems a good idea. I know it only lasts until the next time I hose things
down but it's the thought that counts.
One point to note when doing kit like this is to orient things so the cleaning
compounds do not go inside the thing being cleaned while you work. The DIN
valve was cleaned pointing down and three, no effort applications and a rinse
later it was a lot better. The picture on the left is the same fitting.
Things that go in my mouth used to get done in washing up liquid (if it works
for a plate I eat my dinner from...) but these days I have stuff to clean
rebreathers so I use that. I use it neat. It gets squirted on, toothbrushed or
massaged about then rinsed off. I avoid the stuff that is sold to clean baby
bottles and stuff like that because it may do a good job on glass and rubber
but it doesn't have a good reputation on metal. If I'm worried that there is
really nasty stuff there, eg: phlegm or vomit, I reach for the bleach. Bleach
makes a serious mess of anything biological but then washes off nicely after it
has killed everything in sight. Also, if you fail to rinse it out well you can
taste that there is a problem at once.
I was talking on a forum, as I do, and disagreeing with people, which I do far
too often, and the subject of crack bottles and water ingress came up. Now
somehow the idea that shaking a thick walled aluminium vessel would let you
hear all the water that had got into it sloshing about seemed pretty dubious to
me but the only way to be sure was to try it and see.
This one would be easy. Take my crack bottle, pop off the end, put in various
amounts of water and give it a shake, listen to it and record the results.
Pop the end off. OK when the heavy industrial strap wrench gave up on it as no
contest maybe not so easy. The final solution was clamping the cylinder in a
big machine vice using balsa wood padding to stop it leaving teeth marks in the
paint and using a 300mm long adjustable wrench on flats of the valve. I really
don't take no for an answer from things.
It had powder in it. Stuffing my endoscope into it revealed more deposits
inside but it looked totally dry. Time to detour into working out what is going
on here. Is it salt? Am I wrong about water not getting in? Or is it Aluminium
oxide (Al₂O₃)? Is my 2005/04 stamped crack bottle eroded and
It is stamped 0.21Kgms but the scales tell me it is 0.2152 so nothing much to
deduce there. So I tasted the powder and it isn't salt. I'm going with the
Aluminium Oxide theory for now so it's clean time then do the water
Water? Well I have good old deionised so I won't leave deposits behind when I
dry it out for a reassemble. Right so 20ccs of water goes in. Screw in the
valve. Shake it and you can hear it. I was wrong. Back on the forum to
One last observation. If you have water in an older crack bottle like mine
there is no debris tube and the threaded part of the valve does not protrude
into the body of the cylinder. Fill it, point it straight down and open the
I am informed that later ones, no I don't know the date, do have a debris tube
so that trick won't work. However, after being stripped, cleaned and lubed as I
reassembled it it works so much more smoothly.
Oh well. It needed to come to bits so I could work over the push button as it
was getting stiff. I guess I shall continue to think about sending it up with
the valve open but I'm going to keep doing it.
Inscribed on tablets of stone is the Mantra
Thou hast 180 minutes
Thou shalt be shallower 50m after 100 minutes of use
Thou shalt be shallower than 20m after 140 minutes of use.
Well that's how it is for my Inspo. Perhaps we need to discuss it because it
seems to worry some people.
Point one: I am going to assume you really do have CO₂ production
rate that really does max out a scrubber in exactly 180 minutes. I know I can
do longer but I would never plan on it. From the point of view of this
discussion at 181 minutes you start to feel your breathing rate rise and you
then proceed to die horribly and painfully. Nobody is in the slightest bit
sympathetic as they all know that doing 181 minutes on a scrubber is a well
deserved Darwin Award.
Point two: I am going to assume you metabolic rate is constant so your
CO₂ production rate is constant. Yes, we all know that the CO₂
exhaled is dependant on your level of exertion and how cold you are but we
can't factor in everything to make a general rule.
It is an approximation to say that a working scrubber can be divided into three
parts, the used zone, the working zone and the unused zone. I spent quite a
long time playing with the statistical mechanics and it's not quite true but it
actually works well enough. I am also going to assume that I can ignore the
fact that top of the working zone cannot really move back as I get shallower. I
can justify this assumption mathematically provided it never reaches the top
but not here.
It is a further approximation to say that the length of the used zone depends
on your CO₂ production so, to all intents and purposes, it depends on
your time in the water. Take this in the sense of the Point one and Point two
cop-outs that I started with. In my scrubber the used area grows by one unit
The width of the working zone however depends on you depth. Why the size of the
working zone depends on your depth was so wonderfully explained by Gordon
Henderson in a message
from the Planet Zorg that I'm not going to try and improve on that. I am
going to give you the rule of thumb that the width of the working zone is the
equivalent of 13 minutes of use multiplied by the ambient pressure in bar. This
one is not an approximation, this one is wrong. However it is right enough to
help me explain things.
Time to draw some scrubbers
So, here is a scrubber after 20 minutes at 20 meters (3bar). I've drawn it 220
units wide to represent 220 minutes of time. We'll come back to why later.
Exhaled gas comes in at the left and after 20 minutes we have used the red bit
of the scrubber. Now I've drawn it flat but it will be slightly conical
although scrubber designers work hard to get it as flat as possible.
The red bit is used scrubber. The pink bit is working scrubber. There is
CO₂ in the gas as it enters the pink and it is progressively scrubbed and
by the time you get to the white, that's the unused, pristine material, there
is effectively no CO₂ left.
What is going to happen is that as your dive progresses the red bit will
steadily lengthen and the width of the pink bit will expand as you go deeper
and contract again as you ascend.
So what's the plan? Well obviously we want to keep the pink from reaching the
top because when that happens some of the CO₂ is getting through
un-scrubbed. This is called 'break through' and is universally considered a bad
move. If just a small fraction of the CO₂ comes round again there is more
next time round the loop to not get scrubbed and so on.
Let's consider some significant 'break through' moments
So here is a scrubber after 180 minutes total use and currently at 20 meters
The working zone is 3*13 so 40 minutes wide and is touching the top. You do not
want to be here. If I am climbing the ladder and I am going to sit on my bench
puffing a last hit of nearly pure O₂ I can stretch it out another few
minutes but realistically you want to be out of the water. Remember that even
breathing it sitting on deck the width of the pink does not go to zero.
This sets the limit for the 0 to 20 meter zone of 180 minutes.
Here is a scrubber after 140 minutes currently at 50 meters (6bar ambient).
It's the same sort of thing. The working zone is 6*13 so 80 minutes and again
it is touching the top. You need to already be shallower.
This sets the limit of 140 minutes for the 20 to 50 meter zone.
Right. Last one. 80 meters (9bar ambient) and 100 minutes. Again it's on the
This sets the limit of 100 minutes for the sub 50 meter zone.
Now we could draw up a big table with lots of values and even derive a formula
but when you start thinking about a deco schedule then the three rules at the
top are enough. Yes, if you are for example shallower than 20 meters there is a
bit more life but this is not an accurate enough model to make it worth writing
down the extra rules. If I said that you need to be shallower than 110 meters
after 60 minutes of scrubber time it would never limit your diving because the
others would limit you by insisting you get your deco into 180 minutes.
Right. Since I have written this HTML scrubber gadget let's do a 100 meter 25
minute dive. That's 183 minutes run time on 25/90 GF.
And after all that the limiting feature is the 180 minute scrubber.
|Leaving the bottom after 25mins|
|Arriving at the 6m last stop|
These rules don't actually have a huge impact on a new scrubber on a single
dive. Where they do kick in is where you want to do two dives on the same fill.
Don't pull a 60 meter dive with 110 minutes run time, including setting up and
prebreathing the thing, in the morning. Then have lunch and expect to break 50
meters in the afternoon on the same fill just because you reckon you can do it
in an 60 minute run time and "that's less than three hours innit?" By the time
you've set it up and prebreathed it again you're at 120 minutes and you
shouldn't even be thinking about 20 minutes below 20 meters. The pink will get
That is trim as in being horizontal in the water.
OK nobody is going to argue that holding your body dead horizontal looks cool
and we can all see that it suits diving as we want to look forward and down and
go forward so it is practical too. However it doesn't come naturally at first.
Most people I see, the moment they stop finning, flop into head up feet down.
Definitely not cool and actually making diving harder work and less
The first snag with looking cool is that it feels very funny. You've spent your
whole active life feet down head up and suddenly you are not only lying flat on
your face but with your knees bent your feet are actually above your head. You
feel like your feet are floating and you imagine yourself being dragged to the
surface fins first.
Key Point: trim should not be hard work. No bending, twisting or working at it.
This is the most comfortable way to dive so go with comfort.
My preference is to set things up in the pool:
The magic trick is a couple of quids worth of cheap supermarket polyprop
washing line and your shears (a dive knife sucks at cutting rope). You want
something chunky enough to tie and untie knots in with wet fingers. Get in the
shallow end with some weights on the side and the full kit on, drysuit and
undersuit, I tend to put the hood on then push it down round my neck so I'm not
deaf and since my gloves have Velcro straps I Velcro them to my wrists. Since
my gloves and my hood contribute 1.5Kgs of buoyancy this is a significant part
of the deal. It's a good idea to have a couple of weights that you've measured
on the kitchen scales so you know what they are. Have a big one and a little
one if you have choice. Wetnotes round the deal off or, by the time you get
home, you forget pertinent details.
So take a bit of washing line and tie your measured weights to your belt. First
trim your weight so you are neutral with the suit inflated then allowed to dump
and with nothing in the BCD/Wing. Just spend some time getting this just
Now try and hover horizontally just off the bottom with your knees bent so your
fins are horizontal. Freeze, think cool and just hold the right amount to
breath to stay there. It probably won't work. Your knees or your nose will hit
the tiles. I'll put money on it being the knees. If it's your nose you need
ankle weights. If you don't have any move one of your measured weight to an
ankle and tie it on, and try again. If it's you knees take one or both of your
measured weights in your hands and hold them to your tummy, your chest and your
shoulders. There should be a place where you stay horizontal.
Now there won't be a place that makes you go horizontal because this is
balance. What you are looking for is the weight distribution that means you
just stay where you put yourself. Once you have this clasp the one or two
weights to yourself and remember the position. Now stand up and work out where
on your rig you can attach them. Get out. Rebuild things. Fudge it with bits of
washing line for now and then get back in the pool. Still works? Great.
Now trundle off down the deep end. I want you to try a fin-pivot and fail. When
you breathe in you discover you just rise evenly from the bottom. No pivot. Now
you can play. Head stands, back floats, lie on your side, practice the old
drysuit summersault where you tuck, fin and when you've gone all the way round
to heads up spread out in a big star. It's all easy. Feel smug. You deserve
Right then. It's real wild water time. You splash into the sea, swim to the
shot line, agree <down> <OK> with your buddy and suddenly
you feel horribly foot light. No. This is balance. You can swim nose first down
the shot and when you level out at the bottom you just stay
OK what can go wrong? Well the biggest problem is diving overweighed. You have
to bang lots of air somewhere and you put it in your suit it rushes to your
boots and makes you foot light. Cure one is don't dive over weight. Cure two is
put the extra buoyancy in your BCD/Wing. Now some times you have to dive a bit
overweight because your twinset has 6Kgs of gas in it at the start of the dive.
So don't try and do it all on the suit - do some and some with the BCD.
If you do get it wrong always remember that the short way to 'suit dump value
at the top' is not forcing your head up, that won't happen, it is tuck and fin
- the old dry suit somersault you learnt back in drysuit class.
PS. Stuff a couple of two foot bits of washing line in the bottom of your
drysuit pockets. You will be amazed how useful a bit of 'throw away' rope can
Pressure and height
I was discussing pressure and height with a friend and he was having a problem
because our standard scuba model of gases didn't seem to explain pressure
falling with altitude. We postulated a 5km tall tank. Would the pressure be the
same at the top as the bottom? I quite liked the reasoning I came up with so I
thought I'd save it here so I can use it again.
OK, stop thinking about a gas as a thing in itself and start
thinking the particles involved.
They zoom about at a thousand miles an hour or so, doing high speed billiard
ball impressions. The 'pressure' is the force exerted on the walls of the
chamber as they bounce off.
Now for a small chamber the average velocities near all the walls are as good
as equal but the best way to think of a big chamber is to think about it having
just one molecule in it, all on its own. It bounces off the bottom and just
happens to go straight up (it could rattle off the sides but the effect is the
same) it is going up with an energy of ½mv², that's the
usual kinetic energy formula half mass times velocity squared. However on the
way up gravity is dragging it down so by the time it gets to the top it has
lost mgh, that is mass times gravity times height, worth of energy. This
has to come off the v² term so it dings into the top with less
v so less pressure.
Put more molecules in and they just bang off one another but the effect is just
like the one molecule (look at the Wiki page for Newton's cradle to get an idea
how motion travels 'through' bulk free particles). So a tall chamber has less
pressure at the top than the bottom and, what is more, slower molecules equates
to less temperature, so it's colder at the top too.
This happens whether the chamber is sealed or not. Remember molecules are dumb.
They don't know you closed the lid.
If you really want the sums the average kinetic energy of a gas molecule is
3/2*kT where k is Boltzmann's constant and T is the
absolute temperature but the drop in pressure comes out pretty much like the
Oh and useless facts, well I like them, to get down to half a bar pressure you
need to ascend to 5500 meters, that's 18000ft in aeronautical speak. The
highest town in the world is La Rinconada in the Peruvian Andes at 5100m so you
can apparently live there and mine gold. The pressure at the top of Mount
Everest at 8848 meters is 0.337 bar total of which 0.07bar is oxygen.
We tend to assume that the minimum safe ppO2 when diving is 0.16 bar but you
get down to that at a mere 2500 meters. The capital cities of Ecuador, Bhutan
and Colombia are higher than that and La Paz, the capital of Bolivia, is at
3640m so you're breathing less than 0.14 bar.
Let's talk about breathing because it's a problem to divers. We'd all like to
breathe as little as possible from our tanks to make them last longer but most
tricks we try have horrible side effects that can quite ruin our day. We need
to understand what is going on and why.
Breathing does two good things for us. It puts Oxygen into our blood so it is
available to all the organs as they all need it and it removes Carbon Dioxide,
the waste product of being alive. While we only have to eat on a daily basis
these two gases need management on a minute by minute schedule.
Let's start with oxygen. As I have said elsewhere oxygen is a nasty reactive
gas and, despite the fact that we need it moment by moment to live, it is quite
dangerous stuff. It is a good source of oxygen that converts those charcoal
briquettes that you can never make burn on your barbie into gunpowder, the
stuff Guy Fawkes tried to blow up parliament with. Our bodies need quite a bit
of oxygen and have developed some quite sophisticated tricks to manage
Our lungs are a mass of tubes dividing and subdividing until they end up at
little 'balloons' called alveoli. This is a trick to get more area because the
gas transfer mechanism needs a lot of space to work. To put some numbers to
this an alveoli is about 0.2mm diameter relaxed and gets bigger when it
inflates (as you breathe in) and you have about 700 million of the things. This
adds up to 100 to 200 square meters of gas transfer area for two lungs, about
thirteen parking spaces.
Now the rules for gas transfer are simple: gases like to pass through thin
membranes so there are gas molecules going both ways. However there are more
molecules on the side with more gas so the average trend will be from the
richer side of the membrane to the poorer side. So when we inhale a lung full
of nice fresh air and on the other side of the alveoli wall is blood rather
short of oxygen it starts to move across dissolving in the blood plasma.
However we have a bonus trick in our Red Blood Cells in that they can hold
oxygen bound to a complex molecule called haemoglobin. So as the oxygen goes
into solution in the plasma the red cells promptly soak it up and the blood
stays oxygen poor until they are satisfied keeping up the inwards flow. This
means that blood can carry lots more oxygen than if it were just
Blood full of oxygen, now bright red, is pumped off to service its customers
and as they pull the oxygen held in solution out the plasma the red cells start
dumping their load and the blood keeps all the organs working just fine. As
most cells consume oxygen to metabolise sugars and such so they convert their
oxygen and sugars into water and carbon-dioxide and release the energy they
need. Most of this unwanted carbon-dioxide is actually reacted with water to
become bicarbonate ions ⁻HCO₃ by an enzyme in the red cells, a
little more is carried in solution and yet more is carried in the red cells
binding to the haemoglobin. This tends to make the blood darker, often said to
be blue but if it's blue you're dying, and you can buy little toys, I have one,
that use LEDs to read the colour of the blood going through a fingertip and
tell you how well oxygenated it is. I use mine more as a pulse checker as it
only ever says 98%. Perhaps I just have OK lungs.
Now this oxygen poor carbon-dioxide rich blood is pumped back to the lungs
where the CO₂ transfers out and more oxygen transfers in.
Now bicarbonate ions look like carbonic acid and the pH (the acidity measure)
of your blood when it is full of carbon dioxide swings into the acid zone. We
don't actually have a set of reflexes wired to blood oxygen so if you try and
breathe, let us say, pure nitrogen the world just drifts quietly away with the
minimum of discomfort but we do have sensors for blood acidity.
As I understand it we have a breathing reflex that is designed to keep us on
track by monitoring blood pH and running our breathing rate to suit. This does
rather make the assumption that if we are breathing enough to remove the
CO₂ we are taking in enough oxygen but that has worked since mammals
evolved so if we mess up it's our Darwin award. However since conscious control
of breathing is useful, so you can blow on things, this is left to tick over in
back ground and we can kid ourselves we are in control. The system is actually
pretty good. Without us having to think about it exhaled 'air' normally
contains a remarkably constant 5 to 6% CO₂ with our respiration rate
going up and down to keep it there. Of course if the 'air' we inhale is richer
in CO₂ than the expected 0.035% then the transfer isn't going to work so
well and the increasing blood acidity will call for more flow to solve the
problem because that is all it knows to do.
In the extreme if the blood pH starts to drift too far from acceptable limits
the conscious control is clearly not working and it gets turned off. Now that
breathing has become a matter of life or death it has to become non-optional.
This is the CO₂ hit of the rebreather dive with a scrubber problem. The
scrubber isn't taking out the CO₂ and the breathing rate as quietly
increased to compensate but now the loop gets more and more CO₂ rich the
CO₂ isn't leaving the blood and so the override kicks in. Breathing has
become mandatory, which is a pig if you gave a reg of nice clean air in your
hand but you can't stop the laboured pant-pant-pant to pull the loop out of
your mouth and get in the bailout without risking a lung full of seawater. One
run in with the much feared CO₂ and I decided I needed a bailout valve
because I never want to do that again. It is still a problem with an OC diver.
If you don't breathe enough to clear the CO₂ the reflex will strike. This
is where you hear about silly breathing rates. Now the diver is locked into a
fast pant that is not clearing the lungs, is going through their gas like mad
and getting no benefit from it.
So what does this all tell us? Well being out of control on a dive is about as
bad as it can get and being out of control of your breathing is the worst of the
worst. I suppose it sums up as "don't mess with CO₂" so don't try tricks
to reduce your breathing rate artificially. Do the relax, chill, slow down by
all means so you produce less but don't skip breathe. Yes, we know that at
depth you have vast amounts of oxygen in every breath so you don't need to
breathe as fast for that but as you descend the air becomes more viscous so you
aren't clearing your lungs so well so you probably need to breathe a bit more
to keep the dreaded CO₂ troll under its bridge. If you want to make the
most efficient use of your gas go for a good flush so breathe deep, long and
slow. Get every last little bit of that evil CO₂ out. Most of all be
aware of how you are breathing so you stay in control.
Incidentally one of the most perceptive comments on the subject I have read was
to "let your breathing rate control how much effort you exert not the other way
round." Basically stay in control.
Fins are another of those things where divers and diver's attitudes make me
want to bang my head against the wall. There is no one right and true fin. No
fin and no fin kick is ordained and sanctified from on high.
Theory check. The object of a fin is to push water in one direction so it
pushes you in the opposite direction. However the idea fin is impossible so
like a fishes tail, a ships screw or an oar we need something that at least
propels some water one way while staying with you.
Let's look at some fin types:
Solid fins. Rigid, probably negatively buoyant,
black. Having little or no flex these rely on the ankle acting as a trailing
hinge when up down kicking so not very efficient. These come into their own if
you frog kick and being rigid are the fin of choice for tricks like helicopter
turns and fining backwards. Jetfins are the classic model of this type. This is
the fin of choice for the fining guru and I admit some of them are very
Flexy fins. Not rigid so in an up down kick they
bend producing a lagging blade so they push a good deal of water backwards.
This more emulates fish and whales. A good fin for covering distance in but apt
to kick up the silt. The long free diving fins are the ultimate form of this fin
offering enormous fining power provided you can provide the leg power to drive
Split fins. Normally a variant of a flexy but
with a slit up the centre of the blade. Again the intention is to make the
shape of the fin in an up down stroke more apt to push the water backwards.
Efficient but very hard to use the more complex tricks of reverse fining and
tight turns. Frowned on by the purist for dumbing down fining which is probably
true but not everybody needs smart tricks for a few simple dives.
Force fins and the like Hugely flexible and very
small. The fin bends a lot and is designed that when fluttered up and down at
the ankle they produce a very efficient propulsion. They are an acquired taste.
As I said at the beginning fins divide divers. Little will cause another diver
to despise you before you open your mouth than the fins you use. Split fins
label you as a tourist and merely by bringing Force fins onto the boat you have
diss'ed their Jetfins. I moved from the Jetfin camp through flexies to Force
fins and I now have some of each. They suit the different types of diving I do.
(So now everybody can hate me).
How you kick really depends on your fins.
If you have big solids then frog kicking is really good and you can feel the
punch forward as they come together. That doesn't work with flexies, especially
split flexies, as they spill the big push. Flexies are designed for an up down
kick and split fins only work that way.
I either use my big Gara free dive fins that want a huge scissor kick so they
flex a lot and provide enormous power but they trash your legs quite rapidly or
my little stubby Force Fins that want a funny little flutter kick but then
motor you along with great efficiency and very little effort. If it's a camera
dive I'll probably take the solids as they are the only ones I can go backwards
in to frame the shot with two hands still on the camera.
As with much of diving there is no one 'right all the time' way.
What is technical diving?
Many years ago the infamous John Bantin, I think it was, published a nice
little piece in Diver magazine entitled something like 'We are all Techies
now'. I've searched the web but the copy has been removed or maybe I remembered
the name poorly.
What this did was to ask the question 'where does recreational diving change
into technical diving?' Since I find the distinction pretty arbitrary I was
amused by the article but as my archive of old copies of Dive has gone the way
of all recyclables I can no longer call on his headings so here are mine
roughly based on my recollection but coloured by my personal prejudices. The
list isn't quite ordered as that would imply that each case included all the
cases above it but it's not far off.
Nitrox divers: Those that deviate from simple air
in their cylinders. They now have to make a calculated decision as to what mix
to use for a specific dive, what advantages they will gain and what risks they
Twinset divers: Those that have more than one
cylinder. I'm not really thinking of a simple pony bottle here as that is a
problem management tool.
Decompression divers: Mandatory stops. When the
'glass ceiling' closes over your head and you can no longer just quit the dive
and go for the surface then we have moved up another step.
Accelerated Decompression divers: Add other gases
to your set so you change the mix on the fly to one more suitable for
decompression. This mix may be very dangarous if you breathe it at depth.
Overhead environment divers: I am lumping cave
and wreck penetration together here although specific cave courses are usually
more detailed. 'Up' is no longer the way out so a 'silt-out' goes from annoying
to deadly if you don't have a plan for it.
Rebreather divers: Now move to the state where
you are blending the gas you breathe as you dive and you must monitor and
control the process. This has big advantages but it brings new problems.
Trimix divers: Add Helium to reduce narcosis to
allow extra depth. Factor this into your decompressuion plans.
Hypoxic Trimix divers: Increase the depth to the
point that a safe mix at the bottom is not a safe mix on the last stages of
your ascent. Now you have a gas that can kill you at all points of the dive.
Hypoxic Trimix Rebreather divers: Do hypoxic on
your blendermatic. Never forget the a rebreather diver has to have all the open
circuit techniques at his finger tips as this is his 'get out of jail free'
card for when he has to bail out of a broken unit.
So where do you draw the line? Who is a techie? Who is not? All of us or none
of us? I don't see a point where the line can be drawn so the term becomes
Deep air diving has been one of those macho things in diving. It sorted, so
said the self professed 'men', the men from the boys. Now I never considered
that things with that sort of reputation in any field were a good idea as they
all eventually caught you out and the men hospitalised themselves or worse. So
why don't I like deep air?
Well the first problem is narcosis. We are getting into the very narked region,
see my other item on trimix for more details. Forty meters was my limit on OC
when the nitrox stayed home and I wanted helium in the mix. Certainly even
diving really deep I would not want the effective nitrogen depth much into the
thirties or I'd forget to take photographs or something. This is stupid
However there is more. Professor Simon Mitchell, one of the good guys in
diving, did a talk at the 2015 BSAC Diving Conference where he put up some soon
to be published work by Gavin Anthony on dive outcomes plotted against
breathing gas density. They were doing a study on gas viscosity and Carbon
Dioxide and it makes interesting reading. Without going into details the data
plotted a graph of dive objective failure that starts low sloping up gently as
gas density increases, much as we would expect, and then suddenly turned up at
about 6gm/litre into the 'objective mostly goes fails' region. This was plotted
on experienced professionals not us amateurs.
What is 6gm/L on air? About 40 meters.
Strokes and Strokery
This is getting far too serious so I feel I need to finish with description of
the fine art of diving strokery. Now having been called a stroke for diving my
inverted twins, for using helmet torches and for diving the 'should be sold
with a shovel' Inspiration Rebreather I feel I am an authority on this.
Stroke buoyancy and trim consists of doing an hour of stops flat on your back
(trim) holding a reel and blob (buoyancy), reading a book by alternate pages as
they tend to disintegrate as you turn them and looking at your computers every
ten minutes or so to see how the deco's going (dive planning).
For the working stage of the dive buoyancy and trim consists of hanging onto
the wreck like grim death because we're on springs, slack lasted about four
minutes but we've a new shot of 'lime in the can so it must be good for four
hours. Hence we're trying to memorise a rivet pattern by feel because it might
just help id this pile of scrap despite the zero vis.
Probably solo because we lost them before we were half way down the
shot in all this yoghurt.
If you need something take two - two different brands of dive computer, one on
Shutdown drill - turning the DIL off when the AutoAir freeflows.
Force Fins - your flexible friends.
Steel Stages - to stay below your and not bob about like some demented puppy.
Variable kit configuration - stops people messing with my gear in the water.
Boots on cylinders - better the no rust you can't see than the obvious rust
from dragging them along the floor.
Boots on gauges - keep the grit out.
Standard mixes - 18/40, nuff said.
Helmet torches - don't attract my attention, I'm not interested.
Oh well. That should have stomped on enough sensitive areas to get me lynched
by virtually anybody who reads this.
Who am I?
Please excuse the web tagged intro but I'm playing with Google 'Authorship' so
it can look me up as genuine:
Well my name is Nigel Hewitt
, but I tend to skulk
on the net under the pseudonym of nigelH
My personal web stuff is mostly in http://www.nigelhewitt.co.uk
but you'll also find me
rambling in some of the better diving web forums.
I live at
Bighton in Sussex, UK
and I earn a crust working as a Designer
at Combro Ltd.
. We make instrumentation
systems with a nice line in ballistics.
I am to be found diving around much of the UK, mostly deep and dark but often
out with my local BSAC club from the Marina here.
I've been at this diving lark for quite a few years now and, as a working physicist, I
confess that a lot of the nonsense I hear on the web gets right on my nerves.
This web page is the result.
by Nigel Hewitt