Nitrox and Other Gases

John BantinA reader of Britain’s Diver Magazine recently wrote to me complaining that I had admitted in print to using independent twin tanks, one tank with air (MOD 182 feet at 1.4 bar ppO2) and one tank with nitrox 32 (MOD 108 feet) for a dive to 165 feet. He congratulates me on effectively using a single tank of air at 165 feet and asks what I would have done had my regulator failed? Well, I would have used the one on the other tank!

Bear in mind, regulators and other equipment subject to high gas pressures usually fail when that pressure is at its highest. That is to say at the very beginning of a dive, not half way through it, unless it’s caused by icing. In a history of a great many dives I have yet to experience a true failure in seawater other than within the first moments of turning a tank on.
He goes on to ask what I should tell newly trained divers about going to 165 feet with only one tank. I answer that newly trained divers should stick to the depth limits of their certification and that if a PADI Open Water diver can go to 60 feet on a single tank, I believe a suitably trained and experienced diver with a twinset can go 60 feet below the MOD of the gas in his second tank providing he is breathing a suitable supply of gas in his first.

Even then, the ppO2 limit of 1.4 bar is a limit currently set by training agencies with an eye on litigation. When we started using nitrox around fifteen years ago, we all used at least 1.6 bar as a limit. A limit of 1.6 bar ppO2 gives an MOD for nitrox 32 of 130 feet, so at 165 feet you could say I was only 35 feet deeper with this ‘single’ tank of air.

Breathing a mix beyond its operating depth for long periods is quite unwise but in the event of a sudden interruption of my air supply I would have been happy to breathe the nitrox 32 beyond its MOD for the brief interval that it would have taken me to ascend that first 33 feet. When I originally learnt to dive you had to show you could make a free-ascent from 100 feet in an emergency. That’s without any gas supply whatsoever.

In the event of the total failure of my air supply, I would have continued to ascend with the nitrox 32 at a safe rate all the way to the surface, subject to any stops required. Oxygen toxicity is not as instantaneous as you might think.

Let’s consider diving in the UK 30 years ago. That was in the heyday of the BSAC in the UK. We all used air. We often dived to 165 feet with it. We were routinely made decompression-stops on the way to the surface. Then PADI arrived with its training system devised in the USA for a very different and some would say very litigious society.

PADI simplified diving. They did away with the complications of deco-stops and made no-stop diving the rule. This limited maximum depths because you don’t get a lot of no-stop time at 130feet. The Recreational Dive Planner gives only a few minutes for the whole dive. In fact most PADI divers were limited to 60 feet unless they went in for further training. Even then 130 feet was considered as a very extreme limit, and a 100 feet maximum was normal. PADI wanted to make diving simpler and therefore more popular. The latter cannot be denied.

This gave an opportunity for new technical training agencies to fill the gap in the market left by PADI, to teach people to go deeper. These training agencies commenced by telling people about nitrox but that was not really the answer. Nitrox alone is for longer or safer shallow dives, not for going deeper. So they came up with trimix courses especially for those who wanted to go beyond the recently created recreational diving limit of 100 feet. A new generation of divers emerged who would faint at the idea of using anything other than a helium mix for going as deep as 130 feet and, importantly, were prepared to pay for it. Deep air is now only for ‘drug addicts’, for people hooked on nitrogen narcosis! That’s their message.(I’m not talking about depths greater than, say, 200 feet.)

At least it is in countries where helium is available, but of course there are many more diving destinations where it isn’t, and the old use of air technology still applies. When I mentioned to Fabio Amaral at Bikini Atoll that many divers in the UK would frown on the use of air for diving the 180 feet deep wrecks, he looked me in the eye and said bluntly, “Tell them we don’t want them here!”

I also wish I could say that the use of tri-mix has made diving safer but statistics reveal a different story, because people are inclined to attempt more difficult dives with it. People go way beyond the old air limits.

It’s a bit like motoring. Modern cars are much safer than those of 40 years ago. However, they are also much faster. Incidentally, people also tend to drive them solo.

Solo diving has been one of the greatest frauds perpetrated on us by the training agencies. We are taught from the very beginning that safety is in the hands of our buddy when so often, when the chips are down, the buddy has been found wanting. Now even PADI has introduced a Self-Reliant Diver Course so, although not admitting to the fact that many divers (trainers, photographers, dive-guides) effectively been diving alone, even that cautious training agency can admit that divers should have the option to be self-reliant.

Let’s agree that diving, like a lot of other things we’d prefer not to think about, is dangerous. People drown in the most seemingly benign conditions. You should be well informed about the risks you take. In the mean time I’m keeping my air-bags in service and my seat-belt in place in my car but I don’t have a problem with diving to 165 feet with twin tanks, one with air and the other with nitrox.

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8 thoughts on “Nitrox and Other Gases”

  1. As an instructor it is my job to see that ALL my students recieve trianning of self sufficiency. If they can not achive this within one year,I will refuse certification.If my agence fults me for over informing my divers so be it. As they say ignorance is bliss

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  2. John:
    Whilst the rating agencies, especially PADI, have dumbed it all down, you’re also right they’ve made diving more accessible.
    I too was trained by BSAC in the ’70s when open water shared ascents (from 20m, I think, but it might have been 30m) were a requirement for the 2nd class ticket. As I had a twin-hose, I was made to do 4 of them: 2 as donor, 2 as recipient. Under ideal conditions, starting with both divers in close proximity and a shot-line, it was still tricky. It made up my mind that a free ascent would be default out of air option, unless my buddy was within arm’s reach – then I’d watch for him to inhale, then snatch his reg (Octopuses were still a novelty then). Recently when I’m paired with strangers (shallow Caribbean/Florida dives) I tell them ‘If I run out of air, I don’t expect to share yours. and if you run out, you’re on your own’. About half the supervising DMs nod knowingly, the other half tell me angrily I’m a selfish bastard – to which I happily agree…(I think BSAC stopped practicing OW shared ascents when they realized more divers were lost training than in reality, but that might just be an urban legend..)

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  3. I agree with what you’ve said John. As an instructor I am less than pleased with the ways things seem to keep changing and making a certification more and more easy with less skill and safety involved. I don’t feel like I’m putting out good divers, just glorified discover scuba people with C cards. With that being said, no one being certified by the big agencies, at least OW and Advanced, are qualified to do any of the dives you talk about and I think that a lot of people misunderstand when they get their C-Cards and think they are.

    Btw, I remember the 100 ft free ascent days, they had just stopped doing that when I first got certified in 1974.

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  4. I dive with air and/or mixed gasses, I use what ever mix is safest, for what ever the dive is I’m doing. I personally prefer to be in control of my faculties on a 50mt+ dive and when I need to do manditary stops, I like the Nitrogen and Helium to be out of my system asap. There are risk involved in every thing we do, I try and keep up with the science and the medical side of diving, then I can make up my own mind and make any risks, calulated risks

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  5. “While a lot of the article makes sense in terms of understanding what you do with gas mixes and risk while diving, it does lose a bit of credibility when you refer to free ascents from 100ft.”

    Ask an old Israeli diver!

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  6. The training agencies are the last to understand (or refuse) many gas issues. When I started diving in the 1960’s nitrogen narcosis, according to the training agencies, caused you to throw away your regulator and swim with the fishes. When nitrox hit the scene, they couldn’t tell the difference from nitrous oxide. I was on the board of DEMA at the time and they put up notices at the DEMA show stating
    “DEMA does not endorse or support the use of nitrox”. I was working with Dick Long and the Skin Diving Resource Group (SDRG)trying to to convince the board that nitrox was a good thing. Skin Diver magazine called it the Devils Gas. PADI declared they would never teach it. It appears the training agencies would demonize and, even worse, mislead the diving public on issues it either does not understand or agree with. I wouldn’t hesitate to use a mix of 2.0 in an ascent situation.

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  7. John
    While a lot of the article makes sense in terms of understanding what you do with gas mixes and risk while diving, it does lose a bit of credibility when you refer to free ascents from 100ft.

    Which training agency ever had that as part of even an advanced course except for the Submarine Escape Training Tank in Gosport? That is also in a highly controlled environment, and yet there was still accidents each year.

    Having experienced air at 180ft / 55m in a chamber and the same with tri-mix in open water, I will happily go with tri-mix. Narcosis hits different folk in different ways, and my reaction is always tunnel vision heading to blackout. Hence my preference for deeper dives using tri-mix.

    Just crossed Bikini Atoll off the bucket list too 🙂


    PS thinking I need a new t-shirt with the caption ” I’m not Solo, just self reliant”

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  8. John speaks the objective truth candidly and articulately. When I started using nitrox mixes in 1971 for Navy, commercial and scientific diving, the standard PO2 limit was 2.O ATA. It was later modified to 1.6 ATA simply because with a move away from only two nitrox mixes (then 32% or 36%), it simply was viable to appease the neo-cons and use a lower PO2 since you could work around the depth easily by widening the mix percentages of oxygen. And John’s exactly right: you don’t spontaneously combust and burst into flames if you exceed the PO2 limit briefly. Oxygen toxicity is a “dose” related phenomenon and needs two components to bring on symptoms: time and depth. Together these two factors make the “dose”. At 1.6 ATA PO2 you have 45 minutes of exposure. As you go deeper with the SAME gas, your PO2 increases and your exposure time is decreased. It’s really very simply and logical.

    I also prefer single tanks of appropriate sizes rather than twins. (Incidentally, I set the world depth record in 1990 to 452 fsw on a single tank of air. No helium was available in my remote location and we were exploring some superb systems in the Bay Islands of Honduras. I had no problems. But others certainly would… Don’t do it!)

    John, you have simply fallen victim to the current generation of PADI (and other agencies) divers that have both a “dumbed-down” training curricula and equally “dumbed-down” instructors. They all belong in the shallow end of the pool and should stay there where they have reasonable chance of survival in warm, clear, calm water watched over vigilantly by anxious supervisors.

    As an industry professional for over 40 years (and founder of TD), I’ve already fought all these battles
    and really have just lost all patience with trying to suffer fools gladly. The next time someone tells you what to do or wear for gear, just smile and thank them while getting on with it. We can both then sit back and watch then struggle to assemble their equipment or even to get their fins on correctly. I call it “Diving Darwinism” and it’s pool with a very shallow end.

    Fabio is right: if you can’t handle the demands of a dive in the real world of remote diving where going to 200 foot plus on single tanks of air is routine, “we don’t want you there”. And you have no business even thinking about being in that diving environment.

    Steer those self-righeous cult members straight to places like Grand Cayman where they will be corralled into groups and never allowed to dive deeper than 100 feet, once a day, and never be allowed to have an independent thought. A few more PADI divers will escape death and, at least, they’ll be out of our way when while we’re in the Solomons, Truk, Indonesia, PNG, Cocos, or anywhere else where your ability is not defined by how many stupid PADI merit badges you have sown on your windbreaker.

    Now go try again to learn to clear you mask…

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