Pursuit gradient factors

**UWSojourner** · 1st April 2008, 05:15

From my perspective trying to "match" VPM with a GF setting, especially on deeper dives, may not yield the best of results.

Two profiles are shown in the graphs below. The profile was CCR sp1.2 for 20 min @280fsw (Dil 10/50; O2@20ft). GFs were chosen so that the 1st stop depth and total runtime were the same as a VPM profile with nominal conservatism.

The 1st chart shows depth and time. In general you see the two profiles are within about 10fsw for most of the profile. This may lead to the conclusion that the profiles are about "matched".

However, consider the 2nd chart which shows the actual gradient(fsw) vs. time. The gradient is pressure in the tissue compartments above ambient pressure. The VPM and GF methods use gradients as a fundamental component of risk management.

The point, I think, is that the gradient chart better illustrates risk differences in the profiles. The maximum allowed gradient under the "Equivalent VPM" GF method is 25% higher than the actual VPM profile and remains higher for a large part of the decompression.

Whether this "equivalent" GF schedule will suffice is anyones guess. But from a fundamental perspective the profile violates both methods. First, it violates Buhlmann's M-values (upon which GFs were built) because the profile surfaces the diver in excess of 110% of Buhlmann's M-values. Second, it violates the nominal VPM profile because the limited bubble growth gradients calculated by VPM are exceeded by some 25%, and remain larger than the VPM gradients for a large part of the decompression.

I'm not trying to be alarmist; of course, there is grey area in decompression. But give it some thought before you "match VPM" (or another bubble model) with GFs. Additional risk may be incurred even though you match runtimes.

jhaaja · 1st April 2008, 09:11

Did not see this linked here. Quite good description of GFs:

http://www.rebreather.ca/Library/Deep%20Stops.pdf

JH

Meng_Tze · 1st April 2008, 14:53

Quote: (Originally Posted by UWSojourner)

From my perspective trying to "match" VPM with a GF setting, especially on deeper dives, may not yield the best of results.

Two profiles are shown in the graphs below. The profile was CCR sp1.2 for 20 min @280fsw (Dil 10/50; O2@20ft). GFs were chosen so that the 1st stop depth and total runtime were the same as a VPM profile with nominal conservatism.

The 1st chart shows depth and time. In general you see the two profiles are within about 10fsw for most of the profile. This may lead to the conclusion that the profiles are about "matched".

However, consider the 2nd chart which shows the actual gradient(fsw) vs. time. The gradient is pressure in the tissue compartments above ambient pressure. The VPM and GF methods use gradients as a fundamental component of risk management.

The point, I think, is that the gradient chart better illustrates risk differences in the profiles. The maximum allowed gradient under the "Equivalent VPM" GF method is 25% higher than the actual VPM profile and remains higher for a large part of the decompression.

Whether this "equivalent" GF schedule will suffice is anyones guess. But from a fundamental perspective the profile violates both methods. First, it violates Buhlmann's M-values (upon which GFs were built) because the profile surfaces the diver in excess of 110% of Buhlmann's M-values. Second, it violates the nominal VPM profile because the limited bubble growth gradients calculated by VPM are exceeded by some 25%, and remain larger than the VPM gradients for a large part of the decompression.

I'm not trying to be alarmist; of course, there is grey area in decompression. But give it some thought before you "match VPM" (or another bubble model) with GFs. Additional risk may be incurred even though you match runtimes.

Very impressive work on the comparative charts. I do however have some comments/questions related to the posed conclusions.

Given that a diver would do this dive and follow VPM out (let's assume this diver is very fit and can handle the con.=0). This same diver would be in a similar state (all things being equal) when they would do the GF profile. Depth, time, ascend profile, gas and diver are the same.... the only thing that changed is the way we 'assume' bubble growth vs. dissolved gas loading/ off loading of tissues (put simply). The true gradient didn't even change, the diver is following the exact same profile and the ambient pressure difference on the divers tissues is the same (give or take that 10ft).

Quote:

Whether this "equivalent" GF schedule will suffice is anyones guess. But from a fundamental perspective the profile violates both methods. First, it violates Buhlmann's M-values (upon which GFs were built) because the profile surfaces the diver in excess of 110% of Buhlmann's M-values. Second, it violates the nominal VPM profile because the limited bubble growth gradients calculated by VPM are exceeded by some 25%, and remain larger than the VPM gradients for a large part of the decompression.

I do not feel that this is a conclusion that can be drawn in its full sense.

The 'equivalent' GF schedule does bring the diver out with higher GF (110%) and this may be more than the basic model intended. If coming out with more than 100% m-value is a violation, then yes this first part is a violation.

The 'equivalent GF' schedule can never violate the bubble growth gradient (VPM) since the way the bubble growth is calculated based on the gradient is different than the dissolved model. The gradient is the same in both models, what is done with that gradient and the subsequent outcome is different (bubble growth volume control vs maximum allowed pressure on M-value).

I would bet that if a straight forward Buhlman gradient graph would be put next to this same VPM gradient graph, one would also 'violate' the other. It is a matter of perspective and comparing two different models.

I do agree with the statement that there is more to it than just matching RT. But again..... if empirically one successfully does a series of VPM profiles and matches up Buhlman GF profiles (or visa versa)....... one would still come out okay. I am by no means an expert on this matter and hope to be corrected if I am mis stating something.

**UWSojourner** · 1st April 2008, 17:54

Quote: (Originally Posted by Meng_Tze)

Given that a diver would do this dive and follow VPM out (let's assume this diver is very fit and can handle the con.=0). This same diver would be in a similar state (all things being equal) when they would do the GF profile.

The true gradient didn't even change ...

The second chart shows the gradients at the start of each stop (connected to form a line). As you can see the “equivalent VPM” GF method drives gradients well in excess of the VPM profile. So, I’m unclear what you mean by “similar state”. In both deco methods (GF and VPM), higher gradients imply more risk.

Quote: (Originally Posted by Meng_Tze)

The 'equivalent GF' schedule can never violate the bubble growth gradient (VPM) since the way the bubble growth is calculated based on the gradient is different than the dissolved model.

The profile violates VPM in that the “equivalent schedule” allows the diver to push gradients well in excess of VPM’s allowable gradients (2nd chart).

VPM can also “violate Buhlmann” in that the method may recommend surfacing gradients well in excess of Buhlmann’s M-values. The theory is that the VPM profile with lower early gradients (longer deeper stops ) limits bubble growth so the shallow time is not required to be as long.

Quote: (Originally Posted by Meng_Tze)

The gradient is the same in both models ...

No, see the 2nd chart.

Quote: (Originally Posted by Meng_Tze)

I would bet that if a straight forward Buhlman gradient graph would be put next to this same VPM gradient graph, one would also 'violate' the other. It is a matter of perspective and comparing two different models.

Yes, Buhlmann/GF profiles often “violate” VPM because they push higher gradients early. But then they “make up for the violation” by completing a longer shallow time (or so the theory goes). [These are general statements ... many VPM profiles don't surface the diver with gradients exceeding Buhmann's m-values.]

My comments are more about risk. IMO the risk of the profiles shown in the charts above are not the same even though RT is matched. That’s my point … they are not “equivalent”.

So (IMO) the strategy of choosing GFs by 1) picking the low GF to generate a 1st stop equivalent to VPM and 2) picking the high GF to match total RT is not a good general strategy. The strategy becomes more problematic (IMO) the more extreme the dive.

Mark Chase · 1st April 2008, 18:12

Running deco planner you can switch between GF and VPM and compare the profiles. I don't think it necessary to attempt to match the overall run times in order to emulate the VPM precisely.

The fact is if you add a load of deep stops to a conventional GF profile you will extend the shallow stops. Most agree that this is unnecessary when using rich helium mixes. This does NOT apply to Nitrox mixes. If I were running air dill up to say 21/25 Id run more like straight Buhlman profiles in order to get out of the water fast. Its the method Buhlman uses to deal with Helium that we try and combat using extended deep stops and short shallow stops.

Id be interested to see a graph for VPMB 0/1/2/3 over laid with 10/100/110/120/130

ATB

Mark

gerstl_ossi · 1st April 2008, 18:22

15/85 for me
Cheers Ossi

**UWSojourner** · 1st April 2008, 18:40

Quote: (Originally Posted by Mark Chase)

Id be interested to see a graph for VPMB 0/1/2/3 over laid with 10/100/110/120/130

I can only do 6 at a time automatically. So GF 10/100, 10/115, 10/130 and VPM+0, VPM+2 and VPM+3 are shown below. The chart shows gradients (tissue tensions in excess of ambient pressure).

Meng_Tze · 2nd April 2008, 13:57

Quote: (Originally Posted by UWSojourner)

The second chart shows the gradients at the start of each stop (connected to form a line). As you can see the “equivalent VPM” GF method drives gradients well in excess of the VPM profile. So, I’m unclear what you mean by “similar state”. In both deco methods (GF and VPM), higher gradients imply more risk.

The profile violates VPM in that the “equivalent schedule” allows the diver to push gradients well in excess of VPM’s allowable gradients (2nd chart).

VPM can also “violate Buhlmann” in that the method may recommend surfacing gradients well in excess of Buhlmann’s M-values. The theory is that the VPM profile with lower early gradients (longer deeper stops ) limits bubble growth so the shallow time is not required to be as long.

No, see the 2nd chart.

Yes, Buhlmann/GF profiles often “violate” VPM because they push higher gradients early. But then they “make up for the violation” by completing a longer shallow time (or so the theory goes). [These are general statements ... many VPM profiles don't surface the diver with gradients exceeding Buhmann's m-values.]

My comments are more about risk. IMO the risk of the profiles shown in the charts above are not the same even though RT is matched. That’s my point … they are not “equivalent”.

So (IMO) the strategy of choosing GFs by 1) picking the low GF to generate a 1st stop equivalent to VPM and 2) picking the high GF to match total RT is not a good general strategy. The strategy becomes more problematic (IMO) the more extreme the dive.

I think we are talking cross purposes here.

I wanted to highlight this:

One diver does the same dive twice (your original chart1 with the ascend profile). Assuming this diver does the VPM profile first and comes out fine. Then the diver does the 'equivalent GF' profile (all things being equal). The diver should come out the same (given the same profile, gradient push etc, etc). This is what I meant by similar state and same gradient (same profile, same pressure difference between ambient and tissues)

My point was that even though according to BM GF he was 'pushing' the gradient (25% above the other), the same profile was taken up. Your physiology does not change according which deco algorithm you choose.

I'll give you a real live example. I did a 4 hr dive recently and even though I have a SW GF, I used the X1 (VPM) as the main computer to get me out (I am not going to discuss why I use a computer in caves in this thread). I had set the X1 at cons 3 because I felt that more aggressive settings were just, well too aggressive. I had set the SW GF to 15/85 for comparison. Both computers computed their deco but I 'bent' the SW GF like a mofo.

Now the question I have is: Both computers calculated the same dive and profile. One was bent like a mofo and the other was fairly quick in getting me out. I personally felt like a million dollars and have had no adverse signs of DCS of any kind. Had I set the SW GF to 10/100 (or there about) I would have been a lot closer with that profile but probably still bent it.

Had I set the SW GF to 10/130 (as an example) I would have pushed the BM GF gradients above and beyond the VPM and standard BM GF intentions, 'violated' them. Would I be at more risk? Not according to VPM and my physiology but yes according to BM GF.

I hope this clarifies my point.

darthmuhl · 2nd April 2008, 21:45

Any recommendations for PC software that will allow you to compare the different deco algorithms?
I have V planner, but would like something to emulate the Shearwater Pursuit Buhlmann GF algorithm.

Carl

Meng_Tze · 3rd April 2008, 00:01

Quote: (Originally Posted by darthmuhl)

Any recommendations for PC software that will allow you to compare the different deco algorithms?
I have V planner, but would like something to emulate the Shearwater Pursuit Buhlmann GF algorithm.

Carl

DecoPlanner, Nautilus will allow you to do that.

1st April 2008, 09:11	#32 (permalink)
jhaaja Custom Title Allowed! Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Mar 2005 Location: Finland Posts: 883	Re: Pursuit gradient factors Did not see this linked here. Quite good description of GFs: http://www.rebreather.ca/Library/Deep%20Stops.pdf JH
(Offline)

1st April 2008, 14:53	#33 (permalink)
Meng_Tze 246 Bubbleless Cove Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Jan 2007 Location: FL, USA Posts: 106	Re: Pursuit gradient factors Quote: (Originally Posted by UWSojourner) From my perspective trying to "match" VPM with a GF setting, especially on deeper dives, may not yield the best of results. Two profiles are shown in the graphs below. The profile was CCR sp1.2 for 20 min @280fsw (Dil 10/50; O2@20ft). GFs were chosen so that the 1st stop depth and total runtime were the same as a VPM profile with nominal conservatism. The 1st chart shows depth and time. In general you see the two profiles are within about 10fsw for most of the profile. This may lead to the conclusion that the profiles are about "matched". However, consider the 2nd chart which shows the actual gradient(fsw) vs. time. The gradient is pressure in the tissue compartments above ambient pressure. The VPM and GF methods use gradients as a fundamental component of risk management. The point, I think, is that the gradient chart better illustrates risk differences in the profiles. The maximum allowed gradient under the "Equivalent VPM" GF method is 25% higher than the actual VPM profile and remains higher for a large part of the decompression. Whether this "equivalent" GF schedule will suffice is anyones guess. But from a fundamental perspective the profile violates both methods. First, it violates Buhlmann's M-values (upon which GFs were built) because the profile surfaces the diver in excess of 110% of Buhlmann's M-values. Second, it violates the nominal VPM profile because the limited bubble growth gradients calculated by VPM are exceeded by some 25%, and remain larger than the VPM gradients for a large part of the decompression. I'm not trying to be alarmist; of course, there is grey area in decompression. But give it some thought before you "match VPM" (or another bubble model) with GFs. Additional risk may be incurred even though you match runtimes. Very impressive work on the comparative charts. I do however have some comments/questions related to the posed conclusions. Given that a diver would do this dive and follow VPM out (let's assume this diver is very fit and can handle the con.=0). This same diver would be in a similar state (all things being equal) when they would do the GF profile. Depth, time, ascend profile, gas and diver are the same.... the only thing that changed is the way we 'assume' bubble growth vs. dissolved gas loading/ off loading of tissues (put simply). The true gradient didn't even change, the diver is following the exact same profile and the ambient pressure difference on the divers tissues is the same (give or take that 10ft). Quote: Whether this "equivalent" GF schedule will suffice is anyones guess. But from a fundamental perspective the profile violates both methods. First, it violates Buhlmann's M-values (upon which GFs were built) because the profile surfaces the diver in excess of 110% of Buhlmann's M-values. Second, it violates the nominal VPM profile because the limited bubble growth gradients calculated by VPM are exceeded by some 25%, and remain larger than the VPM gradients for a large part of the decompression. I do not feel that this is a conclusion that can be drawn in its full sense. The 'equivalent' GF schedule does bring the diver out with higher GF (110%) and this may be more than the basic model intended. If coming out with more than 100% m-value is a violation, then yes this first part is a violation. The 'equivalent GF' schedule can never violate the bubble growth gradient (VPM) since the way the bubble growth is calculated based on the gradient is different than the dissolved model. The gradient is the same in both models, what is done with that gradient and the subsequent outcome is different (bubble growth volume control vs maximum allowed pressure on M-value). I would bet that if a straight forward Buhlman gradient graph would be put next to this same VPM gradient graph, one would also 'violate' the other. It is a matter of perspective and comparing two different models. I do agree with the statement that there is more to it than just matching RT. But again..... if empirically one successfully does a series of VPM profiles and matches up Buhlman GF profiles (or visa versa)....... one would still come out okay. I am by no means an expert on this matter and hope to be corrected if I am mis stating something. __________________ Homo Bonae Voluntatis Last edited by Meng_Tze : 1st April 2008 at 14:56.
(Offline)

1st April 2008, 17:54	#34 (permalink)
UWSojourner Pacific Northwest Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Feb 2005 Location: Portland Oregon Posts: 556	Re: Pursuit gradient factors Quote: (Originally Posted by Meng_Tze) Given that a diver would do this dive and follow VPM out (let's assume this diver is very fit and can handle the con.=0). This same diver would be in a similar state (all things being equal) when they would do the GF profile. The true gradient didn't even change ... The second chart shows the gradients at the start of each stop (connected to form a line). As you can see the “equivalent VPM” GF method drives gradients well in excess of the VPM profile. So, I’m unclear what you mean by “similar state”. In both deco methods (GF and VPM), higher gradients imply more risk. Quote: (Originally Posted by Meng_Tze) The 'equivalent GF' schedule can never violate the bubble growth gradient (VPM) since the way the bubble growth is calculated based on the gradient is different than the dissolved model. The profile violates VPM in that the “equivalent schedule” allows the diver to push gradients well in excess of VPM’s allowable gradients (2nd chart). VPM can also “violate Buhlmann” in that the method may recommend surfacing gradients well in excess of Buhlmann’s M-values. The theory is that the VPM profile with lower early gradients (longer deeper stops ) limits bubble growth so the shallow time is not required to be as long. Quote: (Originally Posted by Meng_Tze) The gradient is the same in both models ... No, see the 2nd chart. Quote: (Originally Posted by Meng_Tze) I would bet that if a straight forward Buhlman gradient graph would be put next to this same VPM gradient graph, one would also 'violate' the other. It is a matter of perspective and comparing two different models. Yes, Buhlmann/GF profiles often “violate” VPM because they push higher gradients early. But then they “make up for the violation” by completing a longer shallow time (or so the theory goes). [These are general statements ... many VPM profiles don't surface the diver with gradients exceeding Buhmann's m-values.] My comments are more about risk. IMO the risk of the profiles shown in the charts above are not the same even though RT is matched. That’s my point … they are not “equivalent”. So (IMO) the strategy of choosing GFs by 1) picking the low GF to generate a 1st stop equivalent to VPM and 2) picking the high GF to match total RT is not a good general strategy. The strategy becomes more problematic (IMO) the more extreme the dive.
(Offline)

1st April 2008, 18:12	#35 (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: 2,522	Re: Pursuit gradient factors Running deco planner you can switch between GF and VPM and compare the profiles. I don't think it necessary to attempt to match the overall run times in order to emulate the VPM precisely. The fact is if you add a load of deep stops to a conventional GF profile you will extend the shallow stops. Most agree that this is unnecessary when using rich helium mixes. This does NOT apply to Nitrox mixes. If I were running air dill up to say 21/25 Id run more like straight Buhlman profiles in order to get out of the water fast. Its the method Buhlman uses to deal with Helium that we try and combat using extended deep stops and short shallow stops. Id be interested to see a graph for VPMB 0/1/2/3 over laid with 10/100/110/120/130 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)

1st April 2008, 18:22	#36 (permalink)
gerstl_ossi Custom Title Allowed! Current Rebreather/s: RB80 / Clone Home Build Other Rebreather/s: Not Bought Yet RB80 / Clone Home Build Join Date: Dec 2006 Location: Italy Posts: 319	Re: Pursuit gradient factors 15/85 for me Cheers Ossi
(Online)

2nd April 2008, 13:57	#38 (permalink)
Meng_Tze 246 Bubbleless Cove Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Jan 2007 Location: FL, USA Posts: 106	Re: Pursuit gradient factors Quote: (Originally Posted by UWSojourner) The second chart shows the gradients at the start of each stop (connected to form a line). As you can see the “equivalent VPM” GF method drives gradients well in excess of the VPM profile. So, I’m unclear what you mean by “similar state”. In both deco methods (GF and VPM), higher gradients imply more risk. The profile violates VPM in that the “equivalent schedule” allows the diver to push gradients well in excess of VPM’s allowable gradients (2nd chart). VPM can also “violate Buhlmann” in that the method may recommend surfacing gradients well in excess of Buhlmann’s M-values. The theory is that the VPM profile with lower early gradients (longer deeper stops ) limits bubble growth so the shallow time is not required to be as long. No, see the 2nd chart. Yes, Buhlmann/GF profiles often “violate” VPM because they push higher gradients early. But then they “make up for the violation” by completing a longer shallow time (or so the theory goes). [These are general statements ... many VPM profiles don't surface the diver with gradients exceeding Buhmann's m-values.] My comments are more about risk. IMO the risk of the profiles shown in the charts above are not the same even though RT is matched. That’s my point … they are not “equivalent”. So (IMO) the strategy of choosing GFs by 1) picking the low GF to generate a 1st stop equivalent to VPM and 2) picking the high GF to match total RT is not a good general strategy. The strategy becomes more problematic (IMO) the more extreme the dive. I think we are talking cross purposes here. I wanted to highlight this: One diver does the same dive twice (your original chart1 with the ascend profile). Assuming this diver does the VPM profile first and comes out fine. Then the diver does the 'equivalent GF' profile (all things being equal). The diver should come out the same (given the same profile, gradient push etc, etc). This is what I meant by similar state and same gradient (same profile, same pressure difference between ambient and tissues) My point was that even though according to BM GF he was 'pushing' the gradient (25% above the other), the same profile was taken up. Your physiology does not change according which deco algorithm you choose. I'll give you a real live example. I did a 4 hr dive recently and even though I have a SW GF, I used the X1 (VPM) as the main computer to get me out (I am not going to discuss why I use a computer in caves in this thread). I had set the X1 at cons 3 because I felt that more aggressive settings were just, well too aggressive. I had set the SW GF to 15/85 for comparison. Both computers computed their deco but I 'bent' the SW GF like a mofo. Now the question I have is: Both computers calculated the same dive and profile. One was bent like a mofo and the other was fairly quick in getting me out. I personally felt like a million dollars and have had no adverse signs of DCS of any kind. Had I set the SW GF to 10/100 (or there about) I would have been a lot closer with that profile but probably still bent it. Had I set the SW GF to 10/130 (as an example) I would have pushed the BM GF gradients above and beyond the VPM and standard BM GF intentions, 'violated' them. Would I be at more risk? Not according to VPM and my physiology but yes according to BM GF. I hope this clarifies my point. __________________ Homo Bonae Voluntatis Last edited by Meng_Tze : 2nd April 2008 at 14:13.
(Offline)

2nd April 2008, 21:45	#39 (permalink)
darthmuhl New Member Current Rebreather/s: Classic Kiss Azimuth Other Rebreather/s: Azimuth Join Date: Apr 2005 Location: NJ, USA Posts: 7	Re: Pursuit gradient factors Any recommendations for PC software that will allow you to compare the different deco algorithms? I have V planner, but would like something to emulate the Shearwater Pursuit Buhlmann GF algorithm. Carl
(Online)

3rd April 2008, 00:01	#40 (permalink)
Meng_Tze 246 Bubbleless Cove Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Jan 2007 Location: FL, USA Posts: 106	Re: Pursuit gradient factors Quote: (Originally Posted by darthmuhl) Any recommendations for PC software that will allow you to compare the different deco algorithms? I have V planner, but would like something to emulate the Shearwater Pursuit Buhlmann GF algorithm. Carl DecoPlanner, Nautilus will allow you to do that. __________________ Homo Bonae Voluntatis
(Offline)

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