Rate of PO2 drop changes with depth true/false

rdmmdr

can we say woops

rick

Genesis

Boy now there's an argument for mandatory training!

NOT!

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cramerdn

The hilighted area is correct but, out of context. It is true that with two breathing loops of the same volume and the same fraction of O2, one at 1 bar and one at 10 bar, the 10 bar one will last 10 times as long.

However, in the context of a rebreather if you have a breathing loop at 10 bar that has a 70% O2 fraction you are breathing a gas with a pO2 of 7 bar!!

So in a real rebreather, two indentical loops that both have pO2s of .7 will have the exact same rate of decresse in pO2 as they are breathed, regardless of the ambiant preasure, the deph, or the loop's volume.

If a loop starts at 70% fraction of O2 at the surface (1bar) and still has .7 pO2 at depth, then no O2 was added to the loop on the way down to maintain the loop's volume, and it must have a fraction of O2 of 7%.

If that 7% FO2 loop is taken to the surface and breathed it will have a pO2 of .07. Somewhat hypoxic.

If the dilutant has O2 in it then that extra O2 will be added to the loop as the volume is maintained and the loop's constiuants will have changed.

UWSojourner

Heh, heh . Disregard my prior posts (maybe on every topic?) as you are all probably thinking I'm out of my marbles (slight pun intended).

However, at the risk of further displaying my ignorance, I'm still not coming to the "same PO2 in loop results in same drop in PO2 per unit time regardless of depth" conclusion. I only get to that conclusion if inert gas is added to the loop as O2 is consumed.

Unfortunately, my calculations seem to indicate PO2 will drop FASTER at depth for a given starting PO2 assuming no change in inert gas in the loop. Maybe someone can tell me where my calcs are off.

Experiments C & D seem to get to the issue fastest. See the attached spreadsheet for the detail of the experiments described below.

Experiment A – 1) Fill loop at the surface, 30% O2. 2) Breathe until O2 exhausted. 3) Fill loop at surface, 30% O2. Descend to 4 ATA WITHOUT ADDING GAS. Breathe until O2 exhausted. Note time needed to drop loop 0.1 PO2

>>>> Time to drop PO2 by 0.1 SUBSTANTIALLY SHORTER at 4 ATA

Experiment B – 1) Fill loop at the surface, 30% O2. 2) Breathe until O2 exhausted. 3) Fill with same mix (30% O2) at 4 ATA. Breathe until O2 exhausted. Note time needed to drop loop 0.1 PO2

>>>> Time to drop PO2 by 0.1 LONGER at 4 ATA, but not by much

Experiment C – 1) Fill loop at the surface, 60% O2. 2) Breathe until O2 exhausted. 3) Fill loop at 4 ATA to get to same PO2. Breathe until O2 exhausted. Note time needed to drop loop 0.1 PO2

>>>> Time to drop PO2 by 0.1 SHORTER at 4 ATA

Experiment D – SAME AS C, but replace used O2 with inert gas to keep loop full.

>>>> Time to drop PO2 by 0.1 is the same. This is the position everyone seems to be taking, but it depends on the loop remaining full by inert gas being added.



Ok. Get your sticks out and take a whack.

Attached Files

PO2 Rate of Decline.zip (13.8 KB, 9 views)

Sutty

Ok, this is getting way past the point I was originally trying to make - that some authorities are suggesting that for a given PO2 your loop would keep you alive for 10 times as long (without adding O2) at 10Atm than at 1 Atm. This is incorrect.


However as I hadn't expected the thread to get into such detail I admit to approximating. The calculations I posted initially ignore the change in loop volume produced by metabolism of O2. It remains true that for the same loop volume and same PO2 the loop contains the same mass of O2 regardless of absolute pressure (which is what I posted). There are some differences produced by the change in volume:

At 1atm:
PO2=0.2, loop volume 10 litres
therefore O2 content is 2 litres (surface equivalent ie a mass unit), inert content 8 litres. FO2 is 0.2 (ie 20%).

We now metabolise 1 litre of O2;
There remains 1 litre of O2 in the loop.
Total loop volume is now 9 litres.
Therefore FO2 is 1/9=0.1111111,
so at 1 Atm PO2 is also 0.1111111 (7 decimals).


At 10atm:

PO2=0.2, loop volume is 10 litres (total surface equivalent volume 100 litres), O2 content is still 2 litres (surface equivalent), FO2 is 2/100 = 0.02 (ie 2%).

We again metabolise 1 litre surface equivalent of O2
There remains 1 litre (surface equivalent) in the loop.
Total loop volume is now 9.9litres (our 1 litre of O2 occupies only 0.1 litre at this 10Atm)
Therefore FO2 is 1/99.9=0.01001001 (again correcting O2 and inert to surface volumes for calculation, 0.1/9.9 if you prefer),
so at 10 Atm PO2 is 0.1001001

Doing similar calculations with more moderate depths & PO2s:
10 litre loop, PO2=0.5, metabolise 1 litre O2 at surface or 20m (3Atm);
at the surface PO2 drops from 0.5 to 0.4444
at 20m PO2 drops from 0.5 to 0.4138

As you see there is a small difference, but not of much note in the real world, and in the direction of the loop becoming hypoxic faster at depth.
The ONLY reason it takes a loop longer to become hypoxic at depth is because you should be starting from a higher PO2.

Neil

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RonMicjan

OK, let me try and sum this up, cause folks are coming at it from different directions.

first, ya cut the best part of that pic from the manual off, that would be the part with me in it

next: If one has a static volume loop, at a loop PO2 of say 1.2 and a loop free volume (at the surface) of 10 liters, then no matter what the depth, the loop has 12 free liters of oxygen in it. Without adding or subtracting anyting, no going up or down, no adding dil or o2, then a diver that consumes 1 liter per minute VO2, will breathe the loop to zero (if that were possible) in 12 minutes. Again, regardless of depth. Suttys original math was right on. You figure the density, the loop %, the free volume and its 12 free liters of o2 at every depth with a constant PO2 of 1.2.

The PO2, will not be a linear change for the 12 minutes, but will go from 1.2-1.1 in 2.22 minutes at a depth of 2ata (33fsw/10meters), but at 10ata depth, that same loop will drop from 1.2-1.1 in just 1.1 minutes. This is just a function of the volume of inert gas skewing the fo2 calculations and Kevins arguments and spread sheet illuminate that fact, however, in either case, its lights out in 12 (or less) minutes.

So, the age old adage of more time to fix problems at depth is a myth, this was true when using an SCR, (fixed fraction of O2 in the loop, higher PO2 at depth) and that is how it got ingrained into my head, cause I was ready to do a slap the monkey on sutty, but did the math myself and found he was absolutly right. Have some green dude.

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Sutty

Thanks Ron!

I wouldn't keep harping on about this but I think anything put out
incorrectly that affects safety and could increase diver complacency at depth has to be sorted.

Neil

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RonMicjan

we all make mistakes, look here is one now...


and please let us know when you are going to demonstrate walking on water...

Now theres an argument for hiring an experienced instructor!! Even one who is wrong occasionally, but is happy to admit it!

quick, stu, close the thread a-fore he can do a three page rebuttal...

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Genesis

The difference is that I admitted mine (and corrected it) about an hour later.

How long has that book been in print? Has it been formally recalled, fixed, reprinted and all those students inaccurately taught contacted at IANTD's expense to make the correction?

Just a bit of a difference there eh? Never mind that I don't have a shingle out claiming that I'm an "expert."

__________________
"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

Tom Rose

quick, stu, close the thread a-fore he can do a three page rebuttal...[/quote]

I have been lurking and shaking my head over this thread.....Seems a number of folks do not understand the difference at depth of Percentage O2 or Partial Pressure of O2.....

Sadly, Darwin will sort it out....

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