A tank of air for the bottom, a tank of nitrox for the deco - it seems a logical arrangement, yet John Bantin's favoured method for a deep air dive still has its critics
One air, one nitrox: what's the beef?
FOR MANY YEARS I HAVE DIVED down to 50m or so using one tank of air plus another of nitrox. For years, too, I have been chastised by various people who denounce this practice as dangerous.
Lately, however, I have noticed that I no longer seem to be alone in my bizarre behaviour. On my past three Red Sea trips, I have seen many divers doing the same thing, and dive centre staff don't bat an eyelid when asked for "one of each".
A while ago I referred in print to this practice of using independent twins, one containing air (Maximum Operating Depth 56m at 1.4 bar ppO2), the other with nitrox 32 (MOD 33m), for a 50m dive.
A reader wrote to congratulate me on effectively using a single tank of air at 50m, and asked what I would have done had my regulator failed? Well, I would have used the one on the other tank!
Bear in mind that regulators, and other equipment subject to high gas pressures, usually fail when that pressure is at its highest. That is to say, at the beginning of a dive, not halfway through it, unless a problem is caused by icing.
In a history of a great many dives, I have yet to experience a true failure in sea water, other than within the first moments of turning a tank on.
The reader went on to ask what I should tell newly trained divers about going to 50m with only one tank.
My answer is that newly trained divers should stick to the depth limits of their certification, and that if a PADI Open Water diver can go to 18m on a single tank, I believe a suitably trained and experienced diver with a twinset can go 18m below the MOD of the gas in his second tank, provided he is breathing a suitable supply of gas in his first.
Even then, bear in mind that the ppO2 limit of 1.4 bar has been set by training agencies with an eye on litigation.
When we started using nitrox some 15 years ago, we all used at least 1.6 bar as a limit. That gives an MOD for nitrox 32 of 40m. So at 50m, you could say I was only 10m deeper with my "single" tank of air.
Breathing a mix beyond its operating depth for long periods is 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 10m.
When I learnt to dive, a novice had to show that he or she could make a free ascent from 30m in an emergency. That is, without any gas supply whatsoever.
In the event of 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, in the heyday of the British Sub-Aqua Club. We all used air, and often dived to 50m with it, routinely making deco 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. It did away with the complication of deco stops and made no-stop diving the rule. This limited maximum depths, as you don't get a lot of no-stop time at 40m. PADI's Recreational Dive Planner gives only a few minutes for the whole dive.
In fact, most PADI divers were limited to 18m unless they went in for further training. Even then, 40m was considered as a very extreme limit, and a 30m maximum normal. PADI wanted to make diving simpler and therefore more popular, and succeeded. 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 agencies started by telling people about nitrox, but that was not 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 30m. A new ge.neration of divers emerged who would faint at the idea of using anything but a helium mix for going deep 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. 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 55m-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 trimix has made diving safer, but statistics reveal a different story, because people are inclined to attempt more difficult dives with it, and go way beyond the old air limits.
It's a bit like motoring. Modern cars are much safer than those of 40 years ago, but also much faster. Incidentally, people also tend to drive them solo.
So 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 apparently benign conditions.
You should be well informed about the risks you take. In the meantime, I'm keeping my air-bags in service and my seatbelt in place in my car, but I don't have a problem with diving to 50m with twin tanks, one with air and the other with nitrox.