Air Bottles

Fancy A Top-Up?

Diving with mixed gases can be extremely costly. In a bid to make trimix diving more economical, we looked at how to get the best out of any expensive leftovers

You’ve had a great day diving on the Saturday. It was a 65m dive on to a wreck and your 18/40 mix gave you a good, clear head. The trouble is, you’ve got 90 bar of the stuff left and you’re not sure what to do with it. One thing is certain – you want to use it, if possible, because in twin 12-litre cylinders this 90 bar represents about £15 worth of helium. What you do with it will depend on a number of factors. The most elementary decision is probably this: are you going to choose a dive appropriate to what you’ve managed to end up with in your cylinders, or are you going to try to achieve a mix suitable for the dive that you would really like to do? Circumstances may make this decision for you in terms of what is available in the way of gas and gas-mixing equipment. 

Assessing content

Let’s have a look at an ideal situation. The exact composition of the original mix is known because, not only was the oxygen analysed but the mixing station was also able to analyse for helium content. Secondly, there is an unlimited supply of helium and oxygen available for topping up and a gas-mixing programme available on a laptop. Given these circumstances, the choice is yours but it’s not a situation in which many divers are likely to find themselves. For a start, helium analysers are expensive and only recently available within the sport diving industry, which means they are rarely seen in dive shops. The usual way of assessing helium content is this – if I’ve done the sums right, if the oxygen is right, and if the fill pressure is right, then the helium is probably right. Not ideal, but it works – at least for the dive which is imminent.

However, I wouldn’t be confident in trying to accurately predict a mix topped up with helium, using the same method if precision were required. Any initial errors might well be unknowingly compounded. Lack of gas may limit your options in using the residual mix as the basis for a new hot mix. Think about it. If you have calculated that 57 bar of helium has to be added to the 90 bar of the residual mix, then you are going to need a helium source in excess of 175 bar. This may be a tall order on a Sunday morning at the coast. In short, a new hot trimix will be out of the question unless you are in the fortunate position of having access to a lot of gas and a helium analyser. 


Air top-ups

Here’s another scenario. What happens if we top up our 90 bar of 18/40 with air? We must consider the possibility of unknown inaccuracies in the initial mix and assess how critical these may be. Probably the best way to look at this is to apply some real figures to it. We’ve had our 65m dive on the Saturday on the 18/40 mix and the original intention was to dive a wreck on air the following day. This wreck is at about 50m, I’ve dived it many times on air, but I have to admit that narcosis has always limited my efficiency as a diver. I know there are things on it that I want to see but have never been able to locate, and just maybe a bit of helium in the mix will help.

I would be reluctant to pay a lot of money for helium to dive something that I have always dived on air but perhaps the 90 bar of 18/40 mix is an opportunity to have a clearer head at no extra cost. If our original mix was indeed 18/40 and we top-off with air to 200 bar then we will have a mix with 18 per cent helium and 19.5 per cent oxygen in it, the balance being nitrogen. If the fill pressure is right and analysis of oxygen proves correct, then the helium content should be about as predicted. So, what is the benefit to the diver? A 38m equivalent narcotic depth (END).

I don’t know about you but I’m very different diver at 38m to the one I am at 50m. Still, how safe is this? We cannot be sure of the helium/nitrogen balance without a helium analyser, so are we playing Russian roulette with our decompression? I don’t think so but try it for yourself. We have predicted that our mix will comprise 19.5 per cent oxygen, 18 per cent helium, and 63.5 per cent nitrogen. Run this on your favourite PC decompression programme for a bottom time of, say, 25 minutes. Even with our somewhat agricultural method of predicting the helium/nitrogen balance, an accuracy of within 10 per cent should be easily achieved.

But to be on the safe side, let’s say we could be 15 per cent adrift; so now run your programme again for 15 per cent helium and 66.5 per cent nitrogen and again for 21 per cent helium and 60.5 per cent nitrogen. You will be surprised at how similar the decompression schedules are and if you programme in a 20 per cent safety factor and plan for the predicted mix you should be okay. I must emphasize that this does work for hot mixes topped up with helium, where accuracy and precision are of paramount importance. It is a way of maximizing your use of expensive helium and maybe using trimix on a dive that you would normally consider an air dive – at no extra cost.



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