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Deep Air IS Stupidity
Please note: This article was originally posted on The Divesite, the predecessor of BlueBeyond. It has been reposted here for the benefit of our readers, and has been somewhat updated.
Prior to the advent of technical mixed gas diving, many sport divers used air to dive to depths beyond 40m. Many rationalised this by saying that as long as they kept the partial pressure of oxygen to under 2.0ata, air could be used safely to 90m (assuming air has 20% O2).
There are MANY flaws with this approach, and many divers have DIED following it. For an interesting account of one famous diver who apparently died pushing deep air limits that he himself publicly promoted, read this article about Rob Palmer in the UK's Diver magazine.
Firstly, air has 20.9% oxygen, which changes the point where a PPO2 of 2.0ata is reached to 85.6m.
Secondly, as we learnt more about oxygen toxicity, we also learnt that a more real maximum PPO2 is 1.6ata or less. In fact most agencies agree that 1.6ata should only be used for contingency purposes, the real maximum should be 1.4ata or even less. Using a PPO2 of 1.4ata, the maximum depth limit would be 56.6m. Progressing beyond 56m on air is really playing Russian Roulette with oxygen toxicity limits, and should be avoided.
Thirdly, the above logic ignores half of the equation. To determine the maximum depth, we should consider the partial pressure of all component gases in the breathing gas. In air, we have approximately 79.1% nitrogen (N2) and 20.9% oxygen (O2). We all know that nitrogen becomes narcotic with increasing depth, and many experts now suggest a maximum PPN2 of 4.0ata for most divers, and 4.5ata for properly experienced, training and equipped divers.
For most divers, then, a PPN2 of 4.0ata would indicate a maximum depth limitation of 40.5m on air. For people trained and experienced (probably at least 200 dives, with technical training) the real depth limit on air would be 46.8m. By the way, a limit of 40m for most divers looks somehow familiar, doesn't it.
What happens as you push these PPN2 limits? Well, nitrogen is probably more forgiving than oxygen, which explains why so many divers have survived dives beyond these limits. Unlike oxygen toxicity, nitrogen narcosis is unlikely to cause an effect that will immediately lead to your death. It probably doesn't even cause you to do stupid things. But it WILL limit your ability to initiate the correct response when things do go wrong.
Most diving accidents, when analysed later on, start as a series of small problems that snowball into a more serious problem. If you've ever had a near miss or been involved in an actual accident, you will probably relate to this. Nitrogen narcosis is consistently shown to be a major contributing factor in diving fatalities.
Just say no to deep air.
What do you do if you want to go deeper than 40m? Firslty if you're appropriately trained and experienced, use lean EANx mixes to a maximum PPO2 of 1.4ata, and a total pressure in the range of 5.5-6.5ata, depending on your training and experience levels. At 5.5ata, this equates to a limit of 45m (the limit for the TDI Deco Procedures and Advanced Nitrox Diver courses).
As you want to move beyond 45m, you have a couple of choices. To move up to the 50-55m range, consider taking the DSAT TecDeep Diver course , which has a limit of 50m (6.0ata), or the TDI Extended Range Diver course, which has a limit of 55m. Beyond that range, you're absolutely in a range where you need a lower-narcotic breathing mix, such as Trimix, which is based on Helium. However, even in the 45m and beyond range there is a strong story for Trimix, and some groups advocate its use as shallow as 30m.
For my money, 50m is the extent of my air/nitrox based diving, and as I want to go beyond that, Trimix is the best choice.
Deep air is stupidity. Don't do it.
In this day and age, it amazes me that some circles are still promoting extended range air diving to 66m - a PPO2 of 1.59ata, and a PPN2 of 6.01ata!!!!! This is just plain stupidity. In the days when we didn't have a better choice, this was maybe forgiveable. Maybe. But not when we have the right gases and the accident statistics to prove there is a problem. This is not acceptable, especially when some of the leading figures in the circles that promote deep air have died on deep air dives.
This discussion should highlight that for most divers, 40m is the right maximum depth on air, for experienced divers it is perhaps a little deeper. But not much. Make sure you keep all gases in your breathing gas of choice under check. Oxygen should be kept to a partial pressure of 1.4ata at the most, and nitrogen should be managed to your exposure to narcosis manageable, within the limits of your training, experience and equipment constraints.