Rate of PO2 drop changes with depth true/false

cramerdn

Yep if you only added black marbles at depth then the rate of white marble depletion will stay the same. So if your dilutant bottle is full of pure nitrogen or pure helium then the PO2 in the loop should decrease linarly regardless of depth, as long as the partial preasure of O2 is high enough to allow it to be metablized.

UWSojourner

Its getting unclear to me who is supporting what position, or whether everyone is saying the same thing but is a bit dull due to consuming a zillion calorie Christmas meal, but what you say above is how I see it (but then again I swallowed the zillion calorie meal).

Anyway, if you added a diluent with 0% O2 to maintain same loop volume (size), your PO2 would remain the same as you descend and you would consume the PO2 you started with at the same rate.

In your experiment, your 30 parts O2 and 70 parts inert gas would be diluted by another 300 of inert gas for a fraction O2 of 7.5% (30/400). Multiplied by your ambient pressure of 4 ATA gets you back to a PO2 of 0.3 which is what you started with at the surface.

If I'm wrong let me know. I'll blame it on the second helping of ham, or potatoes, or butter laden rolls ... it probably wasn't the peppermint ice cream pie with extra whipped cream, nor the candy bar I got in my stocking. Forget the pO2, I'll probably enter a suger-induced coma sometime soon.

Hope everyone had a Merry Christmas. May you all (we all) have a very happy 2007!

nigelh

OK 30% mixture 0.3bar ppO2 at the surface and 1.2bar at depth but the 'jar' has collapsed to a much smaller jar at depth or the absolute pressure would not change. So 120 to 280 so still 30%, ppO2 now about 1.2bar but we have reinflated the 'jar' back to its old size. We seem to have a clash between the constant ppO2 people who argue that in a constant volume loop ppO2 will drop consistantly and this discussion of constant fO2 which puts more oxygen in the loop so it will last longer but, however, the ppO2 will still fall at a constant rate.

The fact that the loop is constant volume to maintain ambient pressure is the unstated but crucial flaw in your argument but that is how the real world works.

(edit)
On the surface we have 30 marbles representing 0.3bar ppO2.
We breathe ten marbles and the ppO2 drops to 0.2bar (20 marbles so 20% at 1 bar)
At 100ft (say 30m)
We have 120 marbles representing 1.2bar
We breathe 10 marbles and the ppO2 drops to 1.1bar (110 marbles)
It should take the same time.

In both of these examples the 'lung' is decreasing by ten marbles worth of space which will be more on the surface.
(/edit)

__________________
nigelh

Sutty

The problem here is that in adding extra "white marbles" you are increasing the PO2, to work the analogy for constant PO2 you only add black marbles. Of course as we descend on a rebreather we do add extra O2 because there is some in the diluent we add, and this is why PO2 rises on descent (as we keep volume constant).

The basic physics still says that for the same PO2 in the same volume the number of O2 molecules remains constant, and as we metabolise them at the same rate irrespective of depth our PO2 will drop at the same rate on the surface or at depth.

I find it worrying that agencies such as IANTD are promoting the myth!

Of course there is a reason that a loop will take longer to become hypoxic at depth - because we generally start from PO2 1.2-1.3 at depth, whereas at the surface it can't be more than 1.0, and is usually about 0.7.

Neil

__________________
Never forget that life is a finite resource.

bhackett

Which CMF mCCRs use a compensated O2 first stage? (i.e. actual IP increases with depth, vs. plugged where IP is constant over all depths)

Brian

wedivebc

Please keep this most interesting thread on topic. CMF does not enter into this discussion. We are talking about whether 1.2 PPO2 at 2atm contains the same amount of O2 as 1.2 at 10 atm.
The fact that diluent adds O2 is a red herring. We should be considering a stable loop at each depth and not a dynamic loop as occurs on descent or when using CMF.
BTW Kevin, I support the same amount of time at either depth end of the argument.

__________________
Cheers,

Dave....

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

dantheman

True. It's SCR that varies (since it's about fO2, not pO2).

The surface is most definitely a more dangerous place, but primarily
for logistic reasons, not mathematical ones: our pO2 is lower than at
depth and our workload is higher.

FWIW, the TDI KISS mod 1 book gets this right in chapter 2.
We also do an in-water drill to determine a baseline for each diver.

I would be interested in where this is found in the IANTD materials.
It doesn't agree with any of the IANTD materials I've seen, and I can't
imagine the 2 outstanding IANTD instructors I've trained with, ever saying this.

--dan

rdmmdr

density of the gas has nothing to do with partial pressure of the of the gas components.

p=p1+p2

and pressure in a fixed vessel is determined by the amount of energy in the vessel, ie the amount of atoms in the vessel and the heat energy carried by those atoms.

since we maintain, hopefully a fixed po2 the and have a fixed loop volume the total energy to the loop contributed by the o2 atoms is fixed therefor the total number of o2 atoms in the loop is fixed.

now in a dynamic situation, ie descent/accent let use a po2 of 1 to make it easier and a loop volume of 10

on descending to ATM we have decreased to volume of the loop to 5 and therefor increase the loop pressure to 2 ATM. but since we can not decrease the loop volume without collapsing a lung we had to add dil to make up the volume. so now we have 60% o2 in the loop and 40% N2 in the loop

p=p1+p2

p=2bara
p1= .8 pn2
p2=1.2 po2

there is your o2 spike on decent.

lets do an accent form 10bara to 5 bara

p=10bara
p1=9 pn2
p2= 1po2

since the volume stays the same, the pressure has to drop. therefore we are venting gas in a rate equal the the proportion of the mix

p=5bara
p1=4.5 pn2
p2=.5po2

and this does not take into account the amount of N2 you are expelling form the blood stream

this is why we spike of descent and drop on accent

rick

Sutty

IANTD "Tek Closed Circuit Rebreather" Page 34

Their colour highlight, not mine!


img001.jpg

__________________
Never forget that life is a finite resource.

nigelh

OK.
You have them cold.

Ten times the number of molecules in the same volume is 10 times the partial pressure so 7bar.

It will last longer than you will.

__________________
nigelh