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Contingency Depth August 6, 2009

Posted by Chris Sullivan in Technical Diving.
Tags: , , , , , , , , , , , ,

We learn in Nitrox class that our Maximum Operating Depth is 1.4 atmospheres PPO2 and our contingency depth is 1.4 1.6 atmospheres PPO2. For instance, a 40% mix at 99 fsw (which equals 4 atmospheres absolute) is .4 x 4 or 1.6. I’ve found that divers are somewhat confused about contingency depth and oxygen limits in general so I’m going to venture my personal opinion about it here as I don’t think it is explained particularly well anywhere. If you search contingency depth on the web, you’ll find lots of references to 1.6 PPO2, but very little on what it is used for – I’ve seen “in case you blow your planned depth”, “for deco”, or “margin of error”. That’s not very satisfying.

The main thing to remember about oxygen limits is while the probability of CNS toxicity is quite low between 1.4 and 2.0 atmospheres of Oxygen pressure, the consequences are extremely severe. A convulsion at depth carries a very high risk of drowning, so the limits are designed to carry near-zero risk of CNS toxicity. Contrast this to the no decompression limits, which carry a higher risk of mild DCS, but which is almost always treatable. Consequently, even with the extensive use of Nitrox these days, we don’t hear of many accidents involving a CNS “hit”, and when we do, it is always, in my experience to my knowledge at least, the result of an inappropriate gas switch by a technical diver.

Back in the WWII, much higher limits for PPO2 were allowed, with the result than many such divers had CNS toxicity issues. Well that was war, and people died anyway, but that’s less acceptable in recreational diving so the limits got progressively lower. They also used full face masks, so drowning was less likely after a convulsion (some technical divers do this as well). When I learned to dive in 1982, the limit was 2 ATA, now its reduced. Some reports I’ve read have shown possible symptoms as low as 1.3, although without convulsions . Time is also a factor, and the longer you are exposed the more likely you are to have a problem, so we also learn about the “CNS clock” to manage exposure over time. For what it’s worth my old IANTD Advance Nitrox manual permitted operational  diving up to 1.6 PPO2, while recommending more conservative limits in cases of exertion or stress.

What is apparent is that people have been diving up to 1.4 without problems ever since Nitrox became popular, and I would expect that a good percentage of these have unintentionally exceeded the limit a bit every once in a while. It’s also a fact that pushing this particular limit can easily kill you, so it’s not something to fool around with. Because time is a factor, my own take on the contingency limit, other than for deco, is that if you have good reason for a brief excursion between 1.4 and 1.6, then there is no reason not to do it if you are otherwise OK (not out of breath, shivering, with a headache, overexerting yourself, etc.) For instance, if I were diving Nitrox 36 and dropped a dive light onto a 110′ foot bottom, I’d go down and get it. I wouldn’t hang around there, but a couple of minutes at contingency depth isn’t going to worry me a lot.



1. Kal - April 24, 2011

Great blog. I just stumbled on it today and have been reading the older articles.

This one looks like you have a typo in the first sentence where you have 1.4 when you mention contingency depth is 1.6 multiple times in the first paragraph.


Chris Sullivan - April 25, 2011

Hi Kal,

Good catch. Thanks for noticing it. I’ve corrected it and made a couple of other small changes in which I’ve tried to make things a little clearer.



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