Background on O2 toxicity at 80m

[scotdiver]

I was intrigued, to say the least, about the comments that it had been taught it was ok to breath air at 80m with a ppo2 of 1.89 bar (9 x ppo2 0.21).

Which of course completely ignores the effect of narcosis!

So this evening I've gone back and reread the Oxygen toxicity section of Bennett and Elliott, Physiology and Medicine of Diving (http://www.amazon.co.uk/Bennett-Elliotts-Physiology-Medicine-Diving/dp/0702025712/ref=sr_1_1?ie=UTF8&s=books&qid=1246122637&sr=8-1)

On p378 they reproduce a table showing the 1991 US Navy single depth exposure limits for closed circuit oxygen which states an exposure limit of 80 minutes breathing 100% O2 at 9msw = 1.91ATA.

The table was adapted from a large series experiments of 600+ man dives by Butler and Thalmann 1984,1986. The previous exposure limit was 45 minutes, based on some work done in 1954.

So it looks like a military 100% O2 schedule had been "converted" to an equivalent air depth.

A little knowledge is a dangerous thing. :thumbdown:

[Drambeg]

Which of course completely ignores the effect of narcosis!

Which is also more than likely why air used deep doesn't seem to cause the same reactions that exceeding the max depth of oxygen does. A lot of people believe that the anaesthetising effect of nitrogen makes convulsions less likely.

It might seem strange today but in the 90's it wasn't that unusual to be doing 60 or 70m on air on a regular basis, I even ended up at 88m on a training dive (misread my computer, thought it said 70-something). When I did my trimix training you progressed Adv Nitrox (40m) > Deep Air (50m) > Tech Nitrox (60m) which was basically the trimix course done on air > trimix (60m+). Not saying it's right or wrong, only that it was done. Thankfully today we know better and divers don't need to go through air dives at stupid depths. As hypocritical as it sounds, I'd class air at 40m a stupid depth these days, the number of divers you see at the bottom of the Rondo on a single and pony... not for me.

I'd also take military limits with a pinch of salt because conservatism is very often traded off for functionality. 1.9 for 100min might be acceptable when you've a. got a lot of surface support and/or b. have a task to complete.

ATB,

Drambeg