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

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

Carl, as (Paul?) pointed out the important thing is the partial pressure of oxygen, which is the oxygen fraction multiplied by its pressure.  The air we breathe on the surface has a PPO2 of 0.21 ATA, which is to say 21% times 1.0 ATA.  The only reason 
you don't need as large a fraction of oxygen at 160 fsw, is because of the pressure at that depth.  For the same PPO2, you only need 4 percent oxygen (0.0359 x 5.848 ATA = 0.21).  The demand is the same.  (You can't get something for nothing).

It is important to note that your actual physiological oxygen requirements are not the same as the necessary oxygen level in your breathing medium.  In reality, we only metabolize a small fraction of the oxygen inspired in any given breath.  On a fully 
closed circuit rebreather, the unused oxygen is recycled and used again, such that your consumption rate is the actual physiological consumtion rate.  When breathing open curcuit, as in SCUBA, every breath you take you waste oxygen when you 
exhale.  The deeper you are, the denser the gas has to be to breathe it, so you would consume the contents of a tank much faster at depth (5.848 times faster at 160 fsw).  On the rebreather, the only thing that would affect your gas consumption is your 
workload, stress, etc.  Depth doesn't have any bearing on this, assuming a fully closed circuit system.

Of course, if you breathed oxygen from a tank at 160 fsw, you would tox and die, and hence the tank would last for a very long time...

As far a sub applications, in a 1 ATA dry sub, the life support system is typically functionally identical to the rebreather.  CO2 from your expired breath is removed chemically, dropping the cabin pressure.  Oxygen from high pressure cylinders is added to 
bring the pressure back up to what it was.  The nitrogen is just there, and recycled in and out of your lungs.  PPO2 = 0.21.

-Sean


On Tue, 11 Dec 2001 10:49:03 -0700, Coalbunny wrote:

>Actually, what I was thinking is if you had 10 cu.ft of O2 in a tank on
>the surface, how long would the O2 last in comparison to the 10 cu.ft.
>in the same tank except at 160'?  I figure that the 10 cu.ft. would last
>longer as the demand itself is not as great at 160', correct?  All one
>needs is to find a way to come up with the H or He and I would suppose
>that electrolysis should help with both O2 and H.  
>
>Or am I missing the boat big time?
>Carl