There's been some discussion on gas choice here over the past couple of days, so I thought I would clarify the DIR position on gas selection, both for bottom gas for reactional divers, and decompression gas for technical divers. DIR divers use the same gases. Any ten DIR divers which gas they will use at any given depth between 1 and 100 metres, and they will all give you the same answer. For example, between the surface at 30 metres we breathe EAN32. Between 30 metres and 48 metres we breathe 21/35. A little deeper we breathe 18/45 trimix. Deeper still and it's 15/55 trimix. Very deep indeed and it's 10/70 trimix. Our decompression gases are equally standardised. 100% Oxygen, 50% and 21/35 trimix are the three we use unless diving something genuinely extreme. So why do this. Many people are taught the strategy of "optimum gas". This means picking a "best" mix for a given depth. for example, if you pick 37% nitrox for a dive to 28 metres, this maximises the amount of Oxygen you can have in your gas, and thus limits the amount of Nitrogen. Makes sense to me. Equally, with a decompression gas there is a strategy that suggests something similar, which is why people end up diving gas mixes like 70% and 80%. Nothing wrong with that, it's a tried and tested strategy used by the vast majority of divers But it's not what we do. so we dive standardised gases. 20 metre dive? EAN32. 25 metre dive? EAN32. 12 metre dive? EAN32. why would we do this. Well, let's think about what happens when we standardise a gas. 1. Mixing the gas becomes a doddle For example, 32% is 32 bar of O2 and topped up with air. That was easy wasn't it. It gets better. 21/35 is 70 bar of Helium topped up with, wait for it, 32%. 18/45 is 90 Bar of Helium topped up with 32%, 15/55 is etc etc etc. Mixing like this becomes easy, especially once you find a nice friendly shop that banks 32%. you lean how to mix it and then unless you are doing top ups you dont need tables or calculators etc etc. In my BSAC club we have O2 and Helium and mixing the standard gases for a large amount of cylinders is easy because you're putting the same thing in all the cylinders. 2. We learn patterns in decompression limits Using EAN32, for example, we can do a dive to 30 metres for 30 mins. At 25 metres its 45 mins. At 20 metres its an hour. Ooh, that's nice. Think about that. As long as we now round our dive time up to the next largest 16 minutes, and as long as ascent at the DIR rates, then we know how long we can dive - without tables or computers. You could develop patterns like this with any gas you chose as long as you either ran enough numbers or did enough dives. The benefit comes not from the actual gas, but from the standardisation of that gas. dive the same gas enough, and you'll get a very accurate feeling for the maximum allowable dive time in the recreational range, given any combination of depth and time. 3. Decompression cylinders can be the same size If you use 80% deco gas on one dive and 50% deco gas on another dive, you are either going to have to limit your decompression or buy different cylinders. You are on the 50% a lot earlier and thus obviously need a lot more gas. Standardising the decompression gases means you can have dedicated cylinders, permanently marked up with the appropriate labels, which every technical agency recognises as being the safest way to do things. 4. It's easy to maintain safe limits. All our limits are the same for every dive. In addition to our minimum decompresion limits, our maximum operating depth is the same for every dive. We don't have to think "whats the Max operating depth for this gas" - its the same for every recreational dive. this extends into technical diving as well. Every standard gas has a prescribed range in which it works. 5. Out of Gas! If a decompression gas fails, recalculation is easy because you are not diving a gas that might be new to you. You're diving the same decompression gases you always dive. What's more, everyone else is diving the same thing anyway so you can take someone else's gas and just carry on regardless. 6. It permits ratio deco Ratio deco, something I will write about in the future, allows DIR divers who have been trained to the technical level to execute any dive without the need to rely on computers or tables, and perform the required decompression whatever the depth and time happens to be. to a large extent, this is only possible because so many thousands of dives have been done using the same gases the decompression can be accurately predicted. This would, again, be true for any gas, not just the gases that GUE and UTD select, but the research would need to be redone. 7. It adds a safety factor to gas switches. When do you do your gas switch on an accelerated decompression dive. If you are breathing 70% you might do it at 12 metres. If you are breathing 80% it might be 9 metres. If you follow a strategy of "best mix" this depth could be different on every dive, which we believe could lead to confusion if there was a problem on ascent. We change at either 21 metres. Or 6 metres. On every decompression dive. That brings a certain comfort and familiarity. It also means if someone tries to switch at a different depth, there is no confusion or communication - they are wrong and you just slap the reg out of their mouth. 8. It makes gas planning simpler. what's the depth? that's the complexity of the gas planning. We don't need to run numbers through computers, or work out PPO2 limits or CNS limits or OTUS or Maximum operatiing Depths or anything else for that matter. Stick to the standard gases in the ranges they are prescribed and you avoid all complications like this. Once we know the depth, everyone on the team knows what gases we will use. It just doesn't need discussion. now DIR doesn't have the monopoly on standard gases. Most of the technical divers I know from all agencies use standard gases these days for the above reasons. Most recreational divers use a standard gas - air. The only thing that different about DIR is that we only ever use standard gases, and we accept the gases that are defined by DIR because we recognise that the benefits described above vastly outweigh any cons. What's perhaps more interesting is why DIR chose the gases it did. But that's a different story altogether.