Scuba gas switch roulette

By Richard Devanney

I’ve been watching a lot of tech clips on YouTube lately for procrastination research purposes, and it has to be said there are some fantastic presentations, tutorials, and discussions on all manner of subjects ranging from gas physiology to dry glove installation; tips and tricks for everyone. Of course, if you want to keep your faith in humanity I strongly recommend not reading ANY of the comments; trolls will be trolls, and you’re better than that. But as with every yin, there has to be a yang. By yin I mean useful resource and by yang I mean questionable video clip.

A few days ago I happened upon a promo video for tech courses, and, well, let’s just say that the instructor’s trim was equidistant between horizontal and vertical, the sidemount deco cylinder placement was “how did they even get them to do that and stay there”, and propulsion methods used were brave under the circumstances.

But then I saw a gas switch that got me thinking, about gas switching. The clip in question was clearly showing a student gas switching during a course. He rushed it and missed some important steps. In essence, he did all the things you are supposed to do on a course; you mess them up and then incrementally improve on them in a controlled training environment. The only thing that I found strange about the whole thing was that it was in the promo. But it got me thinking about how easy it can be to get lazy about something like gas switching. I’ve seen it too often in some highly trained people. Here’s my personal favourite example. Someone I know had a rebreather problem and bailed out at 60m/200ft. He didn’t end the dive and continued to swim at 60m/200ft for 10 minutes. When he ascended to his gas switch at 21m/70ft, he realised that he had already accidentally bailed out onto his 50% O2 at 60m/200ft, and had been breathing a PPO2 of 3.5 for 10 minutes. I don’t know how he is still alive either, but I think we can all agree he was suffering from a staggering case of normalisation of deviance, and was very lucky. Ignoring the obvious failure to end the dive immediately, a proper gas switch would have prevented unnecessary exposure to CNS oxygen toxicity.

So let’s have a look at gas switching in a little more detail

Here’s what the diver in the video did: he showed the camera the switch signal and stared at his bottom timer whilst turning the cylinder on. He then grabbed the 2nd stage, pulled the hose out, put the 2nd stage in his mouth and began breathing from it. Here’s what the diver didn’t do: check the cylinder contents, trace the 2nd stage back to the 1st stage, check his depth before exchanging regs, or gain (or receive) any confirmation from his team at any stage. To be clear, using the example of the diver in the video is just a good lead in to talking about the process of switching. So let’s look at why each step matters using TDI’s MODS method. These steps begin (in my opinion) when you are neutrally buoyant at the correct depth (Keep It Simple Stupid) rather than during your ascent to the switch depth, and your team positioning is such that you will not lose your ascent line and can communicate easily and clearly:

Mix- Remember that you (should have) analysed and labelled the cylinder on the day of diving. Confirm the tank is yours and it’s the correct gas to switch onto at that point in the dive. Are you still going to switch if you realise that you exited a wreck and accidentally retrieved someone else’s cylinder, or were about to proceed with a switch onto 100% O2 instead of 50% at 21m/70ft?
Open- Are you turning on the correct cylinder? Did you grab the correct 2nd stage? After you’ve confirmed it’s your cylinder, tracing the reg will ensure you’re about to breathe the same gas (the one you confirmed in the previous step). It also makes it easy to turn on with one hand and purge with the other. Purging the reg confirms it is working and clears any crap away. Some people do it as you turn it on to minimise the risk of adiabatic compression, some do it after as they’re more worried about flooding the reg. When diving in colder waters, divers may be trained to not purge at all to minimize risk of freezing the regulator, instead clearing the reg by exhaling and taking a cautious first breath to assure proper function. Do not rely on colour coding of regs- you could have put the right reg on the wrong tank/wrong reg on the right tank.
Depth- Are you at the correct switch depth? Dropping below the MOD increases your risk of a convulsion and alters your on/off-gassing in relation to your plan. Going too shallow increases DCS risk. Show your team the MOD facing them in large writing on the side of the tank for double validation.
Switch- Exchange regs and look to your buddy for confirmation. They should have been watching throughout.  Clip off or stow previous reg, change gas on computer, help buddy validate during their switch, stare at each other awkwardly during stop, maintain buoyancy, positioning, and communication of plan. Clean up/throw previously used cylinder.

Various variables to consider

Obviously if you haven’t done a deco course yet, you need to seek proper training to go through gas switching in much more detail than I have outlined above. There are many other things to factor in, such as how to rig the deco cylinder, how to set up the 1st stage and stow the hose in a streamlined but deployable way, how to turn on the gas, where you will wear the tank(s), awareness, buoyancy and positioning, clear communication, and how the whole thing fits within your dive plan. Knowing these details is one thing. Doing them fluidly together takes time and practice in the water.

There are nuances to the above too. Some will advocate everyone switching at the same time instead of in turn, some will advocate preparing to switch as you ascend to the switch depth, instead of waiting until you hit the switch depth. Some will insist everyone on the team has their deco tanks on the same side, others say it’s up to the diver. We can get on those youtube comments to argue which of those are safe, efficient or simple.

No matter what gas switching procedure you learn, it should include variations of the points outlined above- checking the mix, confirming the correct tank is turned on and you have hold of the correct regulator, checking and re-checking depth and MOD, and having someone to confirm everything as you go along.

Hopefully no one reading this is thinking “well I agree that you need to check depth but the confirmation of mix is not that important, I always know it’s my tank”. Let’s be very clear on this; missing one of the steps may very well kill you, or cause a snowball effect which could then very well kill you. Narcosis, bad visibility, currents, bad awareness, task loading can turn a simple assumption into a costly mistake, and domino a stressful situation into a dangerous one.

Practice makes perfect

Technical diving procedures don’t just appear out of thin air, they are discussed, tested, and implemented by panels of highly experienced and qualified divers that have thousands of dives under their belt in a multitude of different environments. They also use something that is invaluable in helping them create safe procedures- examples of unsafe procedures in the form of diving incident and fatality reports. If you are taught an inadequate gas switching method you are risking your life. If you learn a thorough gas switching method and then get complacent or think you know better, normalisation of deviance will eventually catch up with you and you are risking your life.

Watch this to see the CON in CONVENTID. It’s very sobering to watch, and is mandatory viewing in my advanced nitrox and decompression procedures classes. Don’t be the person that the panel of experts are reading about.

For any prospective technical diving student out there, seek proper training and don’t get lazy or rusty when you are qualified. Make time to practise the techniques you learned whenever you can- don’t take shortcuts. During your courses, don’t just nod your head and copy something shown, ask why you are doing it. You should receive (sometimes more than one) answer that makes sense. As an instructor, be prepared to be able to answer those “why” questions. Sometimes there are “this way” and “that way” answers, and one is not more valid than the other. Sometimes there is very clearly a right or wrong way to do something. Either way, keep the awkward staring underwater, and safe diving.

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