A 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.