Re: [PSUBS-MAILIST] New means having alot to learn.

["Sean T. Stevenson" <sts@telus.net>]

For the working phase of a dive, the mamimum recommended PPO2 is about 1.4 ATA.  As the dives get longer, you need to reduce this value to manage the oxygen exposure (avoiding the lung damage you refer to).  I typically mix for about 1.2.  You 
can decrease this value, but it doesn't make a lot of sense to do this, since all you are doing in that case is increasing the partial pressure of inert gases, and hence your decompression time.

In the water, oxygen typically becomes toxic at values higher than 1.6 ATA.  This is the maximum recommended value for decompression (not working), and even then should not be applied continuously.  The PPO2 of air hits 1.6 at 218 feet, so that is 
the maximum depth for air from a toxicity perspective, but the narcotic effect effectively precludes its use much deeper than the recreational limits of 100-130 fsw.  At these depths, nitrox or normoxic trimixes are more effective gases to use anyway.  
Personally, I use air for powering tools and inflating tires, and that's it.

-Sean



On Tue, 11 Dec 2001 16:41:08 +0000, Paul B wrote:

>Hi,
>
>>So actually, a tank holding 10 cubic feet of O2 at 1ATM lasts longer (in
>>theory) at 160 feet?
>>Carl
>
>I'm sorry, but what are we comparing it with?
>
>If we're talking ambient pressure subs..
>
>The problem is, you can't breathe pure O2 for long periods of time, 
>especially at high pressure. Same goes for breathing gas with high partial 
>pressure of O2 (That includes air at 200ft+). It will painfully kill you, 
>destroying your lungs, the more the pressure, the faster.
>
>The key is to maintain appropriate partial pressure of O2. Ideally - within 
>0.18-0.40 (if I remember the diving manual correctly). That's why they make 
>different mixtures with different O2 percentage, depending on the depth of 
>your operations. With regular air, 200 ft is pretty much the limit (partial 
>pressure of O2 gets too high and you get nitrogen narcosis), and I wouldn't 
>even recommend going that deep with it. Helium/oxygen and proper equipment 
>may take you as deep as 600 ft. Any deeper ­ you're talking BIG BUCKS.
>
>Sincerely, Paul.