Rate of PO2 drop changes with depth true/false

AD_ward9

Seconded. All this thread is doing at the moment is showing how training is wasted on rebreather users. After training, a surprising number believe that if they squirt some O2 into a bag on the surface, then take it to 132ft, low and behold, some more O2 appears by magic!
Supports my case that in the interests of safety, the equipment must not let the diver control things

Alex

sadave

Easily fixeded...

Keep the discussion going, interesting stuff guys!

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nigelh

PV=nRT
Partial pressure depends on the total volume it has to play about in V, the number of mols of gas n, the Gas constant R and the absolute temperature T.
Absolute pressure is not in the equation.
As I consume oxygen at a constant molar rate the pp will decrease linearly.
Also independently of absolute pressure.

R is 0.0831451 if you want to do sums in bar and litres.

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nigelh

Genesis

How about disbanding the agencies?

After all, they're the ones teaching this stuff......

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"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket

grahamsp

Absolutely correct.

For a given constant PO2, the mixture in the loop is the variable. Thus, to give a PO2 of 0.2 at the surface, the loop gas will be 20/80. To give a PO2 of 0.2 at 90 metres, the loop gas must be 2/98. It follows then that at all depths, the mass of oxygen in the loop is also constant. It follows in turn that the rate at which the oxygen in the loop is depleted must be depth independent.

Graham

paulraymaekers

you're right for the full 100%

people keep having wrong idees about PPO2: with the same PPO2, the same amount of O2 molecules in that specific volume, independent of depth.

this is what PPO2 means: the pressure that a gas would have IF IT WERE ALONE IN THAT VOLUME: so if that gas is alone in a volume, and you have the same pressure (same PPO2): means the same quantity of that gas in that volume

regards
paul

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UWSojourner

Question 1: "Does my O2 consumption change with depth?"

Answer 1: No. Our bodies consume (metabolize) the same amount of O2 regardless of depth.

Question 2: "Will the following experiments take the same amount of time?" Experiment 1: Fill my loop with some mix of O2 at the surface. Breathe until it drops 0.1 ppO2. Experiment 2: Fill my loop with same mix of O2 at the surface. Descend to 100ft WITHOUT ADDING ANY GAS. Breathe until it drops 0.1.

Answer 2: Yes. The experiments will take the same amount of time. Of course you would have to start with a pretty big loop at the surface so you could actually breathe at 100ft. The crux of the issue is that in both experiments you had an identical amount of O2 in the loop so, and since you consume it at a constant rate, the drop is the same per unit of time.

Correction: No. The experiments will take different times. It will take a SHORTER time to breath the loop down 0.1 PO2 at 4 ATAs. But, you are also starting at a higher PO2. You will breath all the O2 out of the loop (if it were possible) is the same time.

Question 3: "Will the following experiments take the same amount of time?" Experiment 1: Fill my loop with some mix of O2 at the surface. Breathe until it drops 0.1 ppO2. Experiment 2: Fill my loop with same mix of O2 at the surface. Descend to 100ft but add gas with the same mix per usual rebreather practice to keep the loop breathable. Breathe until it drops 0.1.

Answer 3: No. The experiments will yield different times. More O2 is in the loop on experiment 2 (otherwise what were you adding?) and, since you still consume it at a constant rate, the drop per unit time is slower.

Addition: You start from a higher PO2 AND it will take longer to drop 0.1 PO2 at 4 ATAs.

Maybe an analogy is in order. Suppose you have 30 white marbles and 70 black marbles in a jar. You subtract 1 white marble per minute at the surface. Now take those 30 white marbles and 70 black marbles to 100ft and subtract 1 per minute. You will exhaust the white marbles in an identical amount of time.

However, if you have 30 white marbles and 70 black marbles at the surface, descend to 100ft, and then add 90 white marbles and 210 black marbles, but still only subtract 1 white marble per minute, your 120 white marbles will last longer (even though you still subtract an identical amount per minute). In addition, the % drop per unit time at 100ft will be slower since 29/99ths is less than 119/399ths. Of course, in this analogy the white marbles are oxygen and the black marbles are whatever inert gas you're breathing.


At least, that's how I see it.

marekm

I suspect the issue here is confusion between changes in PO2 related to changes in depth and changes associated with O2 metabolism/loop O2 content.

In regards to depth changes, suddenly ascending 10m from a depth of 10m to surface will cause a drastic 50% change in PO2, while asceding the the same 10m from a depth of 90m to 80m will only cause a 10% shift in the PO2. Thus when quickly changing depths with a relatively low PO2 in the loop, doing so at depth is safer than doing so closer to the surface, hence the perceived notion that the Rebreather diver is 'safer' when breathing off the loop at depth.

Somehow, people forget that rapid changes in depth play a part in this, but remember that you can pack more gas into a fixed volume when at depth due to the increased pressure, and probably make the (incorrect) assumption that the increased safety of opearting the loop at depth is because there is more O2 packed into the loop, which is not the case if the PO2 has remained constant.

Marek

wedivebc

Seems to me there is the same number of O2 molecules in the loop at PPO2 1.0 at the surface as there are at PPO2 1.0 at 100m. There is just a whole bunch of other molecules of inert gas but the O2 didn't change.

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Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

AD_ward9

One other reason some may have been misled to believe in the magic O2 fairy, is that if they are using Constant Mass Flow valves (or even solenoids where there is a constant opening time), then more O2 is injected at depth. I can illustrate this with the picture below, showing the flow through a 100um orifice at different depths (some quite deep), with 4bar, 9 bar and 14bar intermediate pressure from a compensated first stage regulator.

Alex

Attached Images

Example orifice flow2.png (5.3 KB, 139 views)