The Rant
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
to).
Units.
Most of the physical constants I quote come from the CRC Handbook of Chemistry
and Physics 77th Edition (1997) aka The Old Rubber Book.
Throughout
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 slightly temperature and salinity dependant and varies between 1.021
and 1.028 bar. Please don't give me grief about psi, feet or ATA as me laughing
at you may cause offence.
Dry suits
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. I do believe there are companies that make
latex inners with neoprene outers but that might just be a complexity too
far.
I used to use cheap Boots talc on my latex seals. Avoid the baby ones 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.
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 value 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. 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 aprés 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...
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
spin cycle.
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
change.
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 seems to be to use bee's
wax on the zip. 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
down.
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 these.
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 run down 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
surface.
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.
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 unfluffy and chilly.
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
loose heat more rapidly and if you are committed to a lot of deco stops this
may be a problem. Secondly you are loosing 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 and
Weezle overboots and the difference between zipped up and dry and unzipped and
soaked was 7kgms. This means that after a flood I need 7Kgm of extra buoyancy
in the BCD/wing to get back to neutral and ascend. This is the sort of number
you need to factor into your buoyancy plans.
BCDs
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 pool training. However there
are a few things I see people doing that I think are odd though...
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.
Twinsets
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
out.
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
gas.
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
sorted out.
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
DV.
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
loosing 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.
Blobs
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
up.
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
again.)
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
to see.
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
bag.
(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.
Reels
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
boat bitch.
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
in Kilos.
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 kgm 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
do.
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 loose 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 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
breathe again.
To picture my 60 meter Normoxic trimix qualification we need a block of flats.
About 20 stories. I won't bother with a picture as I'm sure you can find
one locally.
(Sorry. That was a lie. I know you will have difficulty
finding a 20 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.
Ponies
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
normally.
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 his fault.
You are not going to go and take a serious risk by looking for him. You are on
your last gas and you are going to get home.
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
worse.
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.
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 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 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 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 the same level of bent on
nitrogen.
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 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 three.
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 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 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 computer and dive that.
I do not subscribe to the tables are best mantra even though for
serious decompression I was taught to dive the plan on my slate. What I want is
redundancy so I carry pre-printed tables and a computer and if one fails I can
dive the other one. How can tables fail? They fail if I hit the bottom of the
shot line and all I see is sand. They fail if part way through the dive I or my
buddy has a serious kit failure and we need shortest time to surface
urgently. They fail if the dive is seriously shallower than planned.
These days I tend to use two computers from different manufacturers running
different algorithms and dive the most conservative at any point. I'll discuss
conservatism setting later.
Rebreathers
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 look at my
Inspiration pages and the
IDA64.
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
fixing it.
Masks
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. If the mask sticks with just the lightest of breath
then it fits. That means it fits your face. Get a 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 and with the same name on them 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 leaks. You just want it tight 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 and by holding the top in you only allow it
to escape at the bottom across your cheeks. If there is water it naturally that
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. So I just tip my head so the water runs to the side then
clear. Easy. ... but I do get some strange looks.
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. After an hour or so in the water an
badly fitting mask becomes evil and you 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
lens sections.
Dumpable Weight
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.
P-Valves
Dehydration is blamed for a lot of DCI hits when the diver was otherwise well
within the tables or computer plan for the dive. However staying well topped up
with liquid when you have some serious diving planned can give rise to
problems. It is said that you can go in a wet suit but I'm far to inhibited but
wearing my lovely fluffy undersuit it would be daft. So we need 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.
OK. The first time you use it it's a bit unnerving. You open the external
valve, hesitate, no sudden cold feeling so no leaks and it didn't run back in.
Then go, hesitate, no 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 suit and a second one way valve to
allow the air in 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 a p-valve to 50meters and never been squeezed that I'd notice 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. 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 - well tedious - otherwise. If
you 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
with it is diving on a cold Stoney morning I stepped into the gents to pop the
consumables on and it was so cold I needed a size smaller than I was carrying.
Oh and always assemble the O3 clip before fitting. Somehow it becomes ten times
harder if you forget.
Update! Custom Divers now produce a thing called a 'Hewee'. I'm not describing
it here 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 six hours which
covers a weekends gas diving in one hit. As a further plus it's washable so you
don't need a big box of the consumables.
300bar
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 here for the
Windows version.
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 to heavy for
me." Well give up diving wimps. Leave it to the girls and old men who can
handle it.
While I'm on the subject...
...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?
A bar 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.
Small?
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.
Medium?
A typical old steam railway locomotive ran at 15-20 bar. Those things always
impress me. Watch one pulling out of a station pulling how many hundreds of
tons of stock. And it's doing it 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
energy packed
into a filled scuba tank is comparable to that in a hand grenade.
Training
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.
Now in conventional beginner level scuba you know where the way out is. It is
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. Up 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 direction marker.
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.
The long hose, seven feet of it, is the get out of a cave trick. The first idea
is that the OOA diver goes first. If it's 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 divers length ahead.
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 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. Breathe. Live.
Argon
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
difference.
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
cold.
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
high.
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.
Failure points
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
boost.
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.
Air Horns
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
checked regularly.
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.
|
 
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Marshalling
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
enjoy.
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
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
library.
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
face.
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
admiration.
Suit as buoyancy
"I was told to use the BCD on the surface and the suit for buoyancy in the
water".
<sigh>
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
enough.
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.
Go Pro
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
diving:
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 for their test pretty assured 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 low. It's not hard, it doesn't pay well, there are lots of
them and not many people 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 weekends even if they had never
discovered diving. If that's not you get real now. That 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
more common?
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.
probably more...
Gradient Factors
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
explanation.
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 probably bent. 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 20 here. Stay well away from the bent/not bent 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 20 and the surface at 80 factor the conservatism by depth so half way it is
(80+20)/2 = 50.
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, 20/80? 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
needlessly.
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 20/80 or 20/70 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.
Nitrox
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
Air.
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
cleared.
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
question.
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
of trouble.
OK so this sets a Maximum Operating Depth (MOD) for any gas containing
oxygen.
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 22m.
Now my favourite Air tables give me 17 minutes no stop at 30m and 36 minutes no
stop at 22m. We have probably gone from being limited by the no stop time to
being limited by the gas consumption.
Not a bad 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
deco.
Airway Separation
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.
Easy.
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.
Gas Calculations
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
idea.
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
divide.
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 others.
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
on?
Right 18/40 trimix is 18% Oxygen, 40% helium and hence the remainder is 42%
nitrogen.
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
there.
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.
Fill Rates
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 big Air compressor. No, not the electric one the
monster with the big petrol engine. 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 booster.
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
second.
You're not sure what 12000cms/second is? 12000cm/sec is 120 meters/second so
about 270mph.
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
same speed.
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
you).
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 milli-volts 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 higher.
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.
Calibrating Cells
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:
SPECIFICATIONS:
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
50% R.H.
10) SHELF LIFE: 24 MONTHS.
11) WEIGHT - 1.2 OZ (32 GRAMS).
12) LOAD - 10K REQUIRED.
ALSO:
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).
©Telidyne 2000
Got that? No? OK let's do it slowly.
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
humidity
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
it.
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
own.
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 it off.
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 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
problem?
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 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 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
whole process.
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 a tiny part of the rebreather and saving it 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 DPV '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. Top
predator ready to strike and kill. Where does he get all his energy
from?
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 goes to the muscles with none left for
direct heat production.
When mammals re-entered the sea they could become the biggest creatures and the
most efficient predators ever known just because they retained their air
breathing systems. A 4 meter, 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 24 meters, 130,000Kgs and pushing four tons of
krill a day into its tummy.
We mammals can't make do on gills. We have gone way beyond that sort of trick.
Centrifuging air or oxygen out of seawater just won't work for us.
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. When we have no details it is
right not to speculate but 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, think
about it, discuss it and 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.
If the conclusion has to be drawn that I did something silly and won a Darwin
Award for it say so.
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
daft error that people no-longer make so be it.
Seasickness
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 lost it.
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
you.
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 enough diesel and exhaust and 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
me.
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.
Gloves
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
on...
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.
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.
Narcosis
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
from' solution.
BOV/DSV
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 class of beginners, do this all the
time?
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. 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
on.
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.
Solo Diving
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,
solo diving.
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
that common.
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.
BSAC
This is based on something I posted to the "Change one thing" thread in the
BSAC forum.
It tries to describe a problem in the way the British Sub-Aqua Club works in
practice.
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
own".
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
done'.
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's lots 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
organisers.
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
crossovers.
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?
Alpinist
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 it 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
adequate.
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 while others say all rebreather divers are a couple of
sandwiches short of the picnic. 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
irrecoverable.
That is when you become an alpinist.
Stages
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, the regulators, rigging kit and valve gear are
'typical'. All that changes is the volume, the cylinder weight and the
material.
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
water1.
Now the first deduction is that 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 because if I
breathe it all I will become buoyant and do an inadvertent ascent. Since they
each weigh 13.1Kgs each out of the water this seems a bit mean but that's
physics.
The second trick is that I can suggest a simple approximation for the 'weight'
of an empty rigged stage in fresh water.
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 and most Alis and several
steels are. If they are buoyant then you need to consider adding lead to your
belt so that if you do need to breathe the gas you don't have to release the
now buoyant cylinders before they lift you to the surface prematurely.
For
use in the sea the differences are 2.7% of total weight so 350 grams for mine
and, frankly, you'll never notice. 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 loose 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
help you.
Incidentally: While I had the scales out 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 expected.
1 For limited meanings of 'fresh water'. You really don't
want to know what lives in my rain water butt.
Semi Closed Rebreathers
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 but often attract the question "Should I
get an SCR as a 'starter' rebreather?"
Can I answer that question as clearly as possible. No.
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
jet, sourcing Sofnolime, cleaning the system far more carefully and generally
fafing about.
So I obviously don't like SCRs. Their one advantage is they are cheap 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 for no 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.
Internet Divers
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 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 an
hour.
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 number has
not relaxed back into obscurity yet, but the newbie diver, who emulates our
views without knowing the road we have walked to get there, is still
about.
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 vis range and no line) but nobody is doing that
on my watch as a club Dive Manager.
Helium
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
quote).
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
very good.
I have some figures that say oxygen is about double 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 and, until we have a Narco-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
here.
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 pressure?
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 =
4.2bar.
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 fills alone.
However, cost aside, the real problem with helium is that it is a 'fast'
gas.
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
four and your leg is a compartment 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 goo 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
nitrox dive.
Until you miss stops. Then those ten minutes you missed on your 40 meter dive
is effectively half an hour off 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
nitrogen.
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
three.
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 two
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
it.
Spanners
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
Diameter/Cross Section)
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.
| What I call it | What it 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 BSP | -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" |
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 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
each wrist.
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.