Rate of PO2 drop changes with depth true/false

Sutty · 24th December 2006, 21:19

Quote: (Originally Posted by Genesis)

3. Know how fast your PO2 drops when on loop if you have no injection. Don't know? Test it on the couch and remember that for every atmosphere down it drops SLOWER, so this is most dangerous on or near the surface.

This is something I have seen quoted in a number of places including IANTD's training manual for MOD2.
As I see it this is WRONG! The number of molecules of O2 in the loop to acheive a PO2 of eg 1.3 at 10m is exactly the same as the number required to acheive the same PO2 at 110m.
Therefore given the same O2 consumption, the PO2 will decline at the same rate whatever the depth.
I'm concerned that people will be lulled into a false sense of security by thinking that their PO2 will drop slower at depth than they have seen/tested on the surface.

Neil

**fireman** · 24th December 2006, 22:28

Quote:

This is something I have seen quoted in a number of places including IANTD's training manual for MOD2.
As I see it this is WRONG! The number of molecules of O2 in the loop to acheive a PO2 of eg 1.3 at 10m is exactly the same as the number required to acheive the same PO2 at 110m.
Therefore given the same O2 consumption, the PO2 will decline at the same rate whatever the depth.
I'm concerned that people will be lulled into a false sense of security by thinking that their PO2 will drop slower at depth than they have seen/tested on the surface.

No I think you are mistaken: Much more oxygen is required assuming the volume of the loop is the same.

Sutty · 24th December 2006, 23:10

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

Genesis · 25th December 2006, 01:47

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.

***ScubaDadMiami*** · 25th December 2006, 03:28

This is at least part of the reason why the riskier part of the dive is on the surface or while ascending. It's also the reason why I make sure to put a good shot of O2 into the loop as I approach the boat ladder. If there were any kind of delay in getting out of the water, I want to make sure I am well above the hypoxic level as I am busy with other things.

bhackett · 25th December 2006, 03:32

Quote: (Originally Posted by Genesis)

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.

Uh, 10 liters of O2 would be 20%, not .2 PO2.

.2 PO2 / 5ata = .04 FO2. .04 FO2 * 50 liters = 2 liters O2.

Brian

Genesis · 25th December 2006, 03:45

Quote: (Originally Posted by bhackett)

Uh, 10 liters of O2 would be 20%, not .2 PO2.

.2 PO2 / 5ata = .04 FO2. .04 FO2 * 50 liters = 2 liters O2.

Brian

You're right, that's FO2 and not PO2.

Let's think this one through.... and then its going to be time to get out the timer and dump the data from my handset to see why I'm not getting what I expect

Ok, we want a constant PO2.

Let's say its 0.5 for the sake of argument.

At 1 ATA that's 50% of loop volume, or 5 liters in a 10 liter total.

We decend to 5 ATA (132fsw.) The PO2 if we have the same PERCENTAGE would be 2.5 (!)

So to get a PO2 of 0.5, we would need 1 liter of O2 in 9 liters of inert gas (.10 FO2 * 5 ATA = .50 PO2)

This is the same 5 liters at STP, which would mean that it should bleed off at the same rate at 5 ATA that it does at 1 ATA if we shut off the injection, since our metabolic demand is in moles (or liters at STP, which is the same thing using different units.) It also means that in terms of MOLES of O2 in the loop they are the same for a given PO2 irrespective of depth - and both IANTD (and lots of other people, including how I've been looking at it) are wrong.

But - experimentally - it doesn't work that way.

This raises the question "why not?"

Time to go looking for the answer to that one.

(This also means that the injection interval should not be compensated for depth - just error from the desired PO2!)

I'm going to throw some instrumentation at this over the next week or two.

bhackett · 25th December 2006, 03:56

Quote: (Originally Posted by Genesis)

But - experimentally - it doesn't work that way.

...

I'm going to throw some instrumentation at this over the next week or two.

Measuring this would be interesting. I think though there are several confounding variables making it hard to directly compare surface and depth.

At the surface you (well, me) are usually futzing with gear, lines -> task loading and harder breathing. Less monitoring handsets, lower base PO2 (e.g. .5 vs. 1.2) mean less room for error, plus the assumed 'safety' of the surface from OC make the surface a really dangerous place on Rebreather.

But regardless, what you metabolize lowers the ambient PO2 at a constant rate given a constant (loop) volume, regardless of ambient pressure. Math just works that way. What you're measuring experimentally is the effect of the above factors and similar.

Brian

Genesis · 25th December 2006, 04:28

There's more to it than this, or the injection algo wouldn't be working as well as it is.

I'll get to the bottom of it, as this is now one of those puzzles where the math and the empirical evidence don't line up. There IS a reason for it, and ultimately it has to make sense.

BTW, I am diving my unit at 0.9 - 1.1 PO2 at depth, and at 0.5 to 0.7 above 10' (automated setpoint change; 0.5 is the maximum with unvalidated sensors, 0.9 is the best you can get with a surface validation since at the surface all you can produce is a 1.0)

Since I have the source for the K1's code (I wrote it) I can easily insert data logging to see what the deviation between setpoint and actual is, how often it happens, etc. Empirically injection events happen roughly "evenly" independant of depth (expected) and so does (by sound) the time period the solenoid is open, for a given level of work.

I'll stick some logging code into the firmware and see what I come up with after a few dives with it in there.

Sutty · 25th December 2006, 08:46

Could a moderator please split this off to a new thread eg from post 126 - entitled something like "rate of PO2 drop changes with depth true/false".

It all goes back to the definition of PO2 - "the pressure that would be applied by the gas under consideration were it to occupy the volume alone", this means that to produce the same PO2 in the same volume requires the same number of molecules of the gas irrespective of absolute pressure. The "extra" pressure in our case is made up by more diluent molecules.

Quote:

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.

I think where Genesis is going wrong here is that this assumes that the FO2 has not dropped as we descend - yes we do have 5 times the moles of gas, but a higher proportion is diluent if we maintain the same PO2.

Neil

24th December 2006, 21:19	#1 (permalink)
Sutty Classic Kiss diver Current Rebreather/s: Classic Kiss Other Rebreather/s: Join Date: Jun 2005 Location: Glossop, Derbyshire, UK Posts: 775	Re: why do we do it? Quote: (Originally Posted by Genesis) 3. Know how fast your PO2 drops when on loop if you have no injection. Don't know? Test it on the couch and remember that for every atmosphere down it drops SLOWER, so this is most dangerous on or near the surface. This is something I have seen quoted in a number of places including IANTD's training manual for MOD2. As I see it this is WRONG! The number of molecules of O2 in the loop to acheive a PO2 of eg 1.3 at 10m is exactly the same as the number required to acheive the same PO2 at 110m. Therefore given the same O2 consumption, the PO2 will decline at the same rate whatever the depth. I'm concerned that people will be lulled into a false sense of security by thinking that their PO2 will drop slower at depth than they have seen/tested on the surface. Neil __________________ Never forget that life is a finite resource. Last edited by schford : 24th December 2006 at 22:18. Reason: fixed quote
(Offline)

24th December 2006, 22:28	#2 (permalink)
fireman rEvo combat swimmer Current Rebreather/s: rEvo Other CCR Other Rebreather/s: rEvo Other CCR Join Date: Nov 2006 Location: chicago Posts: 442	Re: why do we do it? Quote: This is something I have seen quoted in a number of places including IANTD's training manual for MOD2. As I see it this is WRONG! The number of molecules of O2 in the loop to acheive a PO2 of eg 1.3 at 10m is exactly the same as the number required to acheive the same PO2 at 110m. Therefore given the same O2 consumption, the PO2 will decline at the same rate whatever the depth. I'm concerned that people will be lulled into a false sense of security by thinking that their PO2 will drop slower at depth than they have seen/tested on the surface. No I think you are mistaken: Much more oxygen is required assuming the volume of the loop is the same.
(Offline)

24th December 2006, 23:10	#3 (permalink)
Sutty Classic Kiss diver Current Rebreather/s: Classic Kiss Other Rebreather/s: Join Date: Jun 2005 Location: Glossop, Derbyshire, UK Posts: 775	Re: why do we do it? 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 __________________ Never forget that life is a finite resource.
(Offline)

25th December 2006, 01:47	#4 (permalink)
Genesis Bubbless Box of Death Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Oct 2005 Location: Sunny Florida Posts: 1,394	Re: why do we do it? 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. __________________ "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 Last edited by Genesis : 25th December 2006 at 01:52.
(Offline)

25th December 2006, 03:28	#5 (permalink)
*ScubaDadMiami* Moderator Current Rebreather/s: Optima Other Rebreather/s: Join Date: Sep 2005 Location: Florida, USA Posts: 1,937	Re: why do we do it? This is at least part of the reason why the riskier part of the dive is on the surface or while ascending. It's also the reason why I make sure to put a good shot of O2 into the loop as I approach the boat ladder. If there were any kind of delay in getting out of the water, I want to make sure I am well above the hypoxic level as I am busy with other things. __________________ Howard Packer IANTD CCR Instructor Miami Beach, Florida CCRDiveTraining.com
(Offline)

25th December 2006, 03:32	#6 (permalink)
bhackett New Member Current Rebreather/s: Sport Kiss Other Rebreather/s: Join Date: Apr 2005 Location: San Francisco, CA Posts: 11	Re: why do we do it? Quote: (Originally Posted by Genesis) 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. Uh, 10 liters of O2 would be 20%, not .2 PO2. .2 PO2 / 5ata = .04 FO2. .04 FO2 * 50 liters = 2 liters O2. Brian
(Offline)

25th December 2006, 03:45	#7 (permalink)
Genesis Bubbless Box of Death Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Oct 2005 Location: Sunny Florida Posts: 1,394	Re: why do we do it? Quote: (Originally Posted by bhackett) Uh, 10 liters of O2 would be 20%, not .2 PO2. .2 PO2 / 5ata = .04 FO2. .04 FO2 * 50 liters = 2 liters O2. Brian You're right, that's FO2 and not PO2. Let's think this one through.... and then its going to be time to get out the timer and dump the data from my handset to see why I'm not getting what I expect Ok, we want a constant PO2. Let's say its 0.5 for the sake of argument. At 1 ATA that's 50% of loop volume, or 5 liters in a 10 liter total. We decend to 5 ATA (132fsw.) The PO2 if we have the same PERCENTAGE would be 2.5 (!) So to get a PO2 of 0.5, we would need 1 liter of O2 in 9 liters of inert gas (.10 FO2 * 5 ATA = .50 PO2) This is the same 5 liters at STP, which would mean that it should bleed off at the same rate at 5 ATA that it does at 1 ATA if we shut off the injection, since our metabolic demand is in moles (or liters at STP, which is the same thing using different units.) It also means that in terms of MOLES of O2 in the loop they are the same for a given PO2 irrespective of depth - and both IANTD (and lots of other people, including how I've been looking at it) are wrong. But - experimentally - it doesn't work that way. This raises the question "why not?" Time to go looking for the answer to that one. (This also means that the injection interval should not be compensated for depth - just error from the desired PO2!) I'm going to throw some instrumentation at this over the next week or two. __________________ "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
(Offline)

25th December 2006, 03:56	#8 (permalink)
bhackett New Member Current Rebreather/s: Sport Kiss Other Rebreather/s: Join Date: Apr 2005 Location: San Francisco, CA Posts: 11	Re: why do we do it? Quote: (Originally Posted by Genesis) But - experimentally - it doesn't work that way. ... I'm going to throw some instrumentation at this over the next week or two. Measuring this would be interesting. I think though there are several confounding variables making it hard to directly compare surface and depth. At the surface you (well, me) are usually futzing with gear, lines -> task loading and harder breathing. Less monitoring handsets, lower base PO2 (e.g. .5 vs. 1.2) mean less room for error, plus the assumed 'safety' of the surface from OC make the surface a really dangerous place on Rebreather. But regardless, what you metabolize lowers the ambient PO2 at a constant rate given a constant (loop) volume, regardless of ambient pressure. Math just works that way. What you're measuring experimentally is the effect of the above factors and similar. Brian
(Offline)

25th December 2006, 04:28	#9 (permalink)
Genesis Bubbless Box of Death Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Oct 2005 Location: Sunny Florida Posts: 1,394	Re: why do we do it? There's more to it than this, or the injection algo wouldn't be working as well as it is. I'll get to the bottom of it, as this is now one of those puzzles where the math and the empirical evidence don't line up. There IS a reason for it, and ultimately it has to make sense. BTW, I am diving my unit at 0.9 - 1.1 PO2 at depth, and at 0.5 to 0.7 above 10' (automated setpoint change; 0.5 is the maximum with unvalidated sensors, 0.9 is the best you can get with a surface validation since at the surface all you can produce is a 1.0) Since I have the source for the K1's code (I wrote it) I can easily insert data logging to see what the deviation between setpoint and actual is, how often it happens, etc. Empirically injection events happen roughly "evenly" independant of depth (expected) and so does (by sound) the time period the solenoid is open, for a given level of work. I'll stick some logging code into the firmware and see what I come up with after a few dives with it in there. __________________ "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 Last edited by Genesis : 25th December 2006 at 04:30.
(Offline)

25th December 2006, 08:46	#10 (permalink)
Sutty Classic Kiss diver Current Rebreather/s: Classic Kiss Other Rebreather/s: Join Date: Jun 2005 Location: Glossop, Derbyshire, UK Posts: 775	Re: why do we do it? Could a moderator please split this off to a new thread eg from post 126 - entitled something like "rate of PO2 drop changes with depth true/false". It all goes back to the definition of PO2 - "the pressure that would be applied by the gas under consideration were it to occupy the volume alone", this means that to produce the same PO2 in the same volume requires the same number of molecules of the gas irrespective of absolute pressure. The "extra" pressure in our case is made up by more diluent molecules. Quote: 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. I think where Genesis is going wrong here is that this assumes that the FO2 has not dropped as we descend - yes we do have 5 times the moles of gas, but a higher proportion is diluent if we maintain the same PO2. Neil __________________ Never forget that life is a finite resource.
(Offline)

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode