Off gassing at 4 vs 6m (from the complacency thread)

jorgey · 25th March 2008, 10:26

Quote: (Originally Posted by UWSojourner)

It’s difficult to determine if this discussion is trying to get at what is happening physiologically or practically (i.e. how your dive watch actually performs the calculations).

The latter is quite clear. Buhlmann algorithms and VPM both use the tissue half time concept. Tissues are loading according to the following equation:

TC1 = TC0 + (InspiredPressure – TC0) * HTFactor.

The important thing to determine is what changes each symbol.

TC1 and TC0 are the dissolved gas pressures of the tissue compartment at time 0 and time 1. The value TC0 only depends on what happened in the past, not your current stop depth. So whether you are stopped at 6m or 3m, TC0 does not change (it is assumed your tissues have TC0 pressure at the start of the process and you’re starting at 6m or 3m based on the original question).

HTFactor is dependent on the half-time of the compartment and the elapsed time from time 0 to time 1. Again, it is not dependent on depth.

InspiredPressure is the pressure of the inspired inert gas. It is directly dependent on depth, but only if an inert gas is in the breathing mix. Once you are breathing O2, InspiredPressure is 0 at all depths. In this case (breathing O2), TC1 is not dependent on depth because TC0, HTFactor, and InspiredPresssure are not depth dependent, and your Buhlmann/GF and VPM dive computers will not reflect any differences in speed of offgassing based on depth (i.e. TC1 will only be dependent on elapsed time from time 0 to time 1, the half-time of the compartment, and TC0).

If the question is physiological, then all the complex issues surrounding how a body responds to decompression come into play and I'll simply wait for Dr. Mitchell to stop by at some point and provide a case study illustrating why Dr. is not in front of my name.

Hi,
I'm following this discusion with great interest. I wanted only "to hear" because the people here have an experience I don't have.

But in my very VHO the formula you mention give the inside pressure of an inert gas in tissue in a given exposition time, but the Bühllmann algorithm takes this pressure and then says:

P amb. tol. = (TC1 - a) * b

P amb. tol. : tolerable ambient pressure for a given tissue
a and b : tissue dependent factors

I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not?

Interesting enough, Bühllmann says, that breathing 100% O2 between 0 and 15m produces no change in tolerable limits of He and N2 (if I understand right not the offgassing speed, only the gradient limits).

I would say, that you are offgassing more of a given inert gas if you breath less of this inert gas, but how deep you do this it's very relevant for the tissue gradients, and has its limits no matters what you are breathing because it depends of what is your previous tissue load and how much gradient the leading tissue can take.

But I don't have a "Dr" in front of my name either...

jorge

**UWSojourner** · 25th March 2008, 15:35

Quote: (Originally Posted by Mike)

It should be noted that David Teubner also has 'Dr' in front of his name, and does some of that unbending of diver stuff too.

Mike, et al, apologies if it sounded likemy post was a critique of anything anyone else posted. Evidently add self-deprecating humor to the list of things that don't translate well on a forum

. The comment was all about me. I know next to nothing about physiology (except what I've learned from 25 years of marriage and my first prostate exam).

**UWSojourner** · 25th March 2008, 15:47

Quote: (Originally Posted by jorgey)

I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not?

No need to have Dr. after your name to post

. See above.

The supersaturation limits recommended by a decompression method depend on depth (since supersaturation is a function of depth). For Buhlmann & VPM, a tissue compartment model is used to model dissolved gases in the diver, then another method to determine the dissolved gas supersaturation limits.

jorgey · 25th March 2008, 17:25

Quote: (Originally Posted by UWSojourner)

No need to have Dr. after your name to post

. See above.

no problem with that and I do think, we could have now someone of the medical department here

Quote:

The supersaturation limits recommended by a decompression method depend on depth (since supersaturation is a function of depth). For Buhlmann & VPM, a tissue compartment model is used to model dissolved gases in the diver, then another method to determine the dissolved gas supersaturation limits.

was that a yes? ...

j o r g e

Mark Chase · 25th March 2008, 18:28

So basicly if i clear my VR3 and do an extra 5 i should be fine?

ATB

Mark

**UWSojourner** · 25th March 2008, 18:37

Quote: (Originally Posted by jorgey)

was that a yes? ...

What was the question?

"I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not?

"

Yes, the computer might not allow ascent to 3m if its SS limit is holding you to 6m. I suppose the question (on this thread) is, if the computer allows ascent to 3m, AND YOU ARE DECOMPRESSING ON O2, will ascending to 3m clear you sooner to the surface than staying at 6m? The answer is no because the rate at which you are offgassing does not change (i.e. in the mathematical model).

To confirm this, go into your favorite modeling software and make your last stop 20fsw (6msw) on O2. Then run the same profile and make the last stop 10fsw(3msw) and recalculate. The time at which you're cleared to the surface should be the same (Buhlmann and VPM).

Janos · 25th March 2008, 21:18

Quote: (Originally Posted by dteubner)

Your can of coke analogy should be looked at like this. The fastest way to get the CO2 out of the coke is to explosively decompress it (by opening it). If you don't want bubbles to form then you put it in the chamber and open it at pressure.

But bubbles are formed on every dive. Do a Doppler if you don't believe me. However as long as these bubbles are small, there's no problem. And they contribute towards decompression.

Janos

Janos · 25th March 2008, 21:21

Quote: (Originally Posted by UWSojourner)

What was the question?

"I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not?

"

Yes, the computer might not allow ascent to 3m if its SS limit is holding you to 6m. I suppose the question (on this thread) is, if the computer allows ascent to 3m, AND YOU ARE DECOMPRESSING ON O2, will ascending to 3m clear you sooner to the surface than staying at 6m? The answer is no because the rate at which you are offgassing does not change (i.e. in the mathematical model).

To confirm this, go into your favorite modeling software and make your last stop 20fsw (6msw) on O2. Then run the same profile and make the last stop 10fsw(3msw) and recalculate. The time at which you're cleared to the surface should be the same (Buhlmann and VPM).

Breathing pure O2 throughout:

1) Some people believe that you are better off being at 6m rather than 3m (the oxygen window)
2) Some people believe that it makes no difference
3) Some people believe that the process of moving from 6m to 3m offgasses.

Me? I have some symptathy for (2) and (3), but not enough to bet my health on it.

Janos

Mdemon · 25th March 2008, 21:30

Quote: (Originally Posted by Mark Chase)

So basicly if i clear my VR3 and do an extra 5 i should be fine?

ATB

Mark

Yeah, she'll be right.

**UWSojourner** · 25th March 2008, 21:50

Quote: (Originally Posted by Janos)

Breathing pure O2 throughout:

1) Some people believe that you are better off being at 6m rather than 3m (the oxygen window)
2) Some people believe that it makes no difference
3) Some people believe that the process of moving from 6m to 3m offgasses.

Me? I have some symptathy for (2) and (3), but not enough to bet my health on it.

Janos

Gotta love certainty.

I only believe it won't matter to my GF and VPM dive computer.

25th March 2008, 10:26	#41 (permalink)
jorgey New Member Current Rebreather/s: Azimuth Home Build Other Rebreather/s: Azimuth Home Build Join Date: May 2007 Location: Austria / Argentina Posts: 57	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by UWSojourner) It’s difficult to determine if this discussion is trying to get at what is happening physiologically or practically (i.e. how your dive watch actually performs the calculations). The latter is quite clear. Buhlmann algorithms and VPM both use the tissue half time concept. Tissues are loading according to the following equation: TC1 = TC0 + (InspiredPressure – TC0) * HTFactor. The important thing to determine is what changes each symbol. TC1 and TC0 are the dissolved gas pressures of the tissue compartment at time 0 and time 1. The value TC0 only depends on what happened in the past, not your current stop depth. So whether you are stopped at 6m or 3m, TC0 does not change (it is assumed your tissues have TC0 pressure at the start of the process and you’re starting at 6m or 3m based on the original question). HTFactor is dependent on the half-time of the compartment and the elapsed time from time 0 to time 1. Again, it is not dependent on depth. InspiredPressure is the pressure of the inspired inert gas. It is directly dependent on depth, but only if an inert gas is in the breathing mix. Once you are breathing O2, InspiredPressure is 0 at all depths. In this case (breathing O2), TC1 is not dependent on depth because TC0, HTFactor, and InspiredPresssure are not depth dependent, and your Buhlmann/GF and VPM dive computers will not reflect any differences in speed of offgassing based on depth (i.e. TC1 will only be dependent on elapsed time from time 0 to time 1, the half-time of the compartment, and TC0). If the question is physiological, then all the complex issues surrounding how a body responds to decompression come into play and I'll simply wait for Dr. Mitchell to stop by at some point and provide a case study illustrating why Dr. is not in front of my name. Hi, I'm following this discusion with great interest. I wanted only "to hear" because the people here have an experience I don't have. But in my very VHO the formula you mention give the inside pressure of an inert gas in tissue in a given exposition time, but the Bühllmann algorithm takes this pressure and then says: P amb. tol. = (TC1 - a) * b P amb. tol. : tolerable ambient pressure for a given tissue a and b : tissue dependent factors I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not? Interesting enough, Bühllmann says, that breathing 100% O2 between 0 and 15m produces no change in tolerable limits of He and N2 (if I understand right not the offgassing speed, only the gradient limits). I would say, that you are offgassing more of a given inert gas if you breath less of this inert gas, but how deep you do this it's very relevant for the tissue gradients, and has its limits no matters what you are breathing because it depends of what is your previous tissue load and how much gradient the leading tissue can take. But I don't have a "Dr" in front of my name either... jorge Last edited by jorgey : 25th March 2008 at 15:16. Reason: some english corrections, for the sake of understanding :-)
(Offline)

25th March 2008, 15:35	#42 (permalink)
UWSojourner Pacific Northwest Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Feb 2005 Location: Portland Oregon Posts: 556	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by Mike) It should be noted that David Teubner also has 'Dr' in front of his name, and does some of that unbending of diver stuff too. Mike, et al, apologies if it sounded likemy post was a critique of anything anyone else posted. Evidently add self-deprecating humor to the list of things that don't translate well on a forum . The comment was all about me. I know next to nothing about physiology (except what I've learned from 25 years of marriage and my first prostate exam). Last edited by UWSojourner : 25th March 2008 at 15:50.
(Online)

25th March 2008, 15:47	#43 (permalink)
UWSojourner Pacific Northwest Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Feb 2005 Location: Portland Oregon Posts: 556	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by jorgey) I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not? No need to have Dr. after your name to post . See above. The supersaturation limits recommended by a decompression method depend on depth (since supersaturation is a function of depth). For Buhlmann & VPM, a tissue compartment model is used to model dissolved gases in the diver, then another method to determine the dissolved gas supersaturation limits.
(Online)

25th March 2008, 17:25	#44 (permalink)
jorgey New Member Current Rebreather/s: Azimuth Home Build Other Rebreather/s: Azimuth Home Build Join Date: May 2007 Location: Austria / Argentina Posts: 57	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by UWSojourner) No need to have Dr. after your name to post . See above. no problem with that and I do think, we could have now someone of the medical department here Quote: The supersaturation limits recommended by a decompression method depend on depth (since supersaturation is a function of depth). For Buhlmann & VPM, a tissue compartment model is used to model dissolved gases in the diver, then another method to determine the dissolved gas supersaturation limits. was that a yes? ... j o r g e
(Offline)

25th March 2008, 18:28	#45 (permalink)
Mark Chase I go down for ages Current Rebreather/s: Classic Kiss Other Rebreather/s: Inspiration Classic Join Date: Jan 2005 Location: Kent Posts: 2,700	Re: Off gassing at 4 vs 6m (from the complacency thread) So basicly if i clear my VR3 and do an extra 5 i should be fine? ATB Mark __________________ Is it supposed to make that noise ? I took my unit to the dive shop and demanded they bolt on every thing that would fit. Join my elite diving teem and get a Tshirt "Doing It Chasey"Hammerhead Eccr Advanced Diving System
(Offline)

25th March 2008, 18:37	#46 (permalink)
UWSojourner Pacific Northwest Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Feb 2005 Location: Portland Oregon Posts: 556	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by jorgey) was that a yes? ... What was the question? "I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not? " Yes, the computer might not allow ascent to 3m if its SS limit is holding you to 6m. I suppose the question (on this thread) is, if the computer allows ascent to 3m, AND YOU ARE DECOMPRESSING ON O2, will ascending to 3m clear you sooner to the surface than staying at 6m? The answer is no because the rate at which you are offgassing does not change (i.e. in the mathematical model). To confirm this, go into your favorite modeling software and make your last stop 20fsw (6msw) on O2. Then run the same profile and make the last stop 10fsw(3msw) and recalculate. The time at which you're cleared to the surface should be the same (Buhlmann and VPM).
(Online)

25th March 2008, 21:18	#47 (permalink)
Janos "Two Sheds" Current Rebreather/s: Classic Kiss Other Rebreather/s: Classic Kiss Join Date: Feb 2005 Location: East Surrey Posts: 601	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by dteubner) Your can of coke analogy should be looked at like this. The fastest way to get the CO2 out of the coke is to explosively decompress it (by opening it). If you don't want bubbles to form then you put it in the chamber and open it at pressure. But bubbles are formed on every dive. Do a Doppler if you don't believe me. However as long as these bubbles are small, there's no problem. And they contribute towards decompression. Janos __________________ You can lead a horse to water but you can't climb a ladder with a large bell in both hands - Vic Reeves www.hellfins.com/shed
(Offline)

25th March 2008, 21:21	#48 (permalink)
Janos "Two Sheds" Current Rebreather/s: Classic Kiss Other Rebreather/s: Classic Kiss Join Date: Feb 2005 Location: East Surrey Posts: 601	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by UWSojourner) What was the question? "I mean, the depth would be determining the tolerable gradient between tissue and ambient pressure and a Bühllmann based computer would be reflecting it, if it comes over the tolerable limit. Or not? " Yes, the computer might not allow ascent to 3m if its SS limit is holding you to 6m. I suppose the question (on this thread) is, if the computer allows ascent to 3m, AND YOU ARE DECOMPRESSING ON O2, will ascending to 3m clear you sooner to the surface than staying at 6m? The answer is no because the rate at which you are offgassing does not change (i.e. in the mathematical model). To confirm this, go into your favorite modeling software and make your last stop 20fsw (6msw) on O2. Then run the same profile and make the last stop 10fsw(3msw) and recalculate. The time at which you're cleared to the surface should be the same (Buhlmann and VPM). Breathing pure O2 throughout: 1) Some people believe that you are better off being at 6m rather than 3m (the oxygen window) 2) Some people believe that it makes no difference 3) Some people believe that the process of moving from 6m to 3m offgasses. Me? I have some symptathy for (2) and (3), but not enough to bet my health on it. Janos __________________ You can lead a horse to water but you can't climb a ladder with a large bell in both hands - Vic Reeves www.hellfins.com/shed
(Offline)

25th March 2008, 21:30	#49 (permalink)
Mdemon SiegeEngine II Current Rebreather/s: Inspiration Classic Home Build Other Rebreather/s: Inspiration Classic Home Build Join Date: Feb 2007 Location: SWUK Posts: 1,927	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by Mark Chase) So basicly if i clear my VR3 and do an extra 5 i should be fine? ATB Mark Yeah, she'll be right. __________________ www.southwestmafia.com "small minds talk about people, Average Minds Talk About Events, GREAT MINDS TALK ABOUT IDEAS!" The WRONG Attitude will get you killed. "Once the agenda-monkeys and perfect-worlders have moved on, perhaps we can do some diving?"
(Offline)

25th March 2008, 21:50	#50 (permalink)
UWSojourner Pacific Northwest Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Feb 2005 Location: Portland Oregon Posts: 556	Re: Off gassing at 4 vs 6m (from the complacency thread) Quote: (Originally Posted by Janos) Breathing pure O2 throughout: 1) Some people believe that you are better off being at 6m rather than 3m (the oxygen window) 2) Some people believe that it makes no difference 3) Some people believe that the process of moving from 6m to 3m offgasses. Me? I have some symptathy for (2) and (3), but not enough to bet my health on it. Janos Gotta love certainty. I only believe it won't matter to my GF and VPM dive computer.
(Online)

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