Quote: (Originally Posted by
Sutty)
At 10m, pressure 2atm:
PO2=1.3
FiO2 = 1.3/2 = 0.65 ie 65%O2
For example total loop volume is 10 litres, therefore oxygen content of loop is 10x0.65x2 (x2 to convert to surface-equivalent litres ie mass).
=13 litres O2
At 110m, pressure 12atm:
PO2=1.3
FiO2 = 1.3/12 = 0.10833333
With same 10 litre loop volume, O2 content, adjusted to surface is:
10x0.10833333x12
=13 litres O2
Therefore the mass of O2 is the same in both examples, so given the same oxygen consumption your PO2 will decline at the same rate at whatever depth.
If I am missing something please tell me, if not then a potentially dangerous myth is being promoted.
Neil
Here's what you're missing..... its not liters, its
moles.
(BTW, for the chemistry buffs 1 mole of O2 is ~22.4 liters at STP.)
Let's say your total loop volume is 10 liters (just for easy math's sake.) At 1 ATA (surface) with a .20 PO2 (simple math again remember!) we have 2 liters of O2 and 8 liters of inert gas (nitrogen or some nitrogen/helium mix).
Let's say your body consumes 1 liter at STP per minute for metabolism with light work (swimming but not hard.)
In 1 minute at the surface with no injection your PO2 will be .10 - you're dead.
Ok, now we decend to 5 ATA (33 * 4 = 132fsw.) During the decent we must make up loop volume as the gas compresses to 1/5th of its original volume.
We arrive at 5 ATA and wish to operate with the same .20 PO2 (I know, that's stupid, but the math is easier if we leave it alone.)
So we now have 5 times the
moles of gas in the loop, although the volume is the same. That is, we have 10 liters of O2 (at STP) in the loop and 40 liters (at STP) of diluent. Since the gas is at 5 ATA it occupies 10 liters, not the 50 it would occupy on the surface.
Our body still consumes 1 liter of O2 at STP, but at 5 ATA the
volume of that 1 liter at STP is only 1/5th of a liter!
So we now breathe for
five minutes before the PO2 drops to .10 and we pass out and expire.
The error in your thought process is that metabolically we do not consume oxygen by volume, we consume it molecularly (that is, we require a certain number of
moles of O2, not a certain
volume of O2.) The numberr of moles of O2 that fit into a given volume is directly proportional to the pressure (at reasonable pressures, yes, I know that doesn't account for the 'z' factor, but if you're THAT deep then me thinks you have other problems to worry about!)
BTW this is
exactly what I experimentally verify if I turn off my PO2 injection (well, ok, not completely off, but set it to a PO2 of .20) and then run it up manually to .8 and breathe it down. At 100fsw (about 4 ATA) it requires four times as long to drop to .20 as it does on the surface. On the couch a couple of minutes at MLV puts me in the danger zone while at depth I'm still good at the 5 minute mark.
It takes a LOT more inattention for a loop to go hypoxic at depth - at the surface its quite easy to have it happen if the injection system isn't working right.