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).

Dry suits BCD Jackets Twinsets
Blobs Reels Weighting and Lift
Computers and Tables Depth and Depth limits Ponies
Gases and Gas Switching Rebreathers Masks
Dumpable Weight P-Valves 300 Bar
Training Argon Long hoses and the Hog rig
Failure points Air Horns Marshalling
DIR Go Pro Suit as buoyancy
Technology or skills? Gradient Factors Nitrox
Airway separation Gas Calculations Fill Rates
Oxygen Cells Gills Fatalities
Seasickness Gloves Narcosis
BOV/DSV Conservatism Solo Diving
BSAC Alpinist Stages
- - Strokes


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.

Tendons like steel wires 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.

Ice dive - Yes, it is me 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.

Torn neck seal. 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.

Pesky mice 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.

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
Twin sets 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.

Manifold 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.

Valve diagram 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
DSMB 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.
Reels
Reel 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 he breathed 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.

Weights Let's say you end up with 7.5Kgs of lead and 120bar of gas and you can still go up and 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 Kgms 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? 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 where hold a normal breath means or if you 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 9 or 10Kgs and the gas about another 10Kgs then you want 2 or 3Kgs 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.

20 meters 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.)

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.
Pony 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.

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
Mask with half lenses 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' diaphram 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't get it free because the club compressor is too old but 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.

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 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
Failed 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.

 Horn 1Horn 2
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.

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.

Fill Rates
Needle valve 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
Don't do this at home kiddies It's old but it give 11mV in air 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 oxygen, 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.

cell Not really complicated is it? 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.

Thermistors in circuits always worry me 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.

It looks like an epoxy seal The thermistor is pushed right down into the cell 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.

O2 against depth 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.

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