Buhlmann questions

[wedivebc]

You may want to re-read Erik Baker's paper a few times and think again.

I am familiar with his work. The point I was trying to make was that bubble models were originally thought to reduce overall decompression time and VPM was actually tweaked after finding it was too aggressive. I am not sure if RGBM was ever modified the same way. Many users apply GF numbers to approximate curves generated by v-planner (for example) . If you are approximating a profile that is not conservative you are not improving safety, just like diving nitrox to the limits of the EAD does not increase decompression safety.

 

[BlueTrin]

Remember, at ambient pressure you're not off-gassing at all, so there's zero risk of any DCS, and at M-value line you're at the point where risk of DCS becomes "too great".

It follows that anywhere in-between the risk is acceptable.



Some actually can explain GF in 70..85 range and GF Low at 83% of GF High (or was it 86%?) -- of course their ideas may later turn out like deep stops: "sounded good at the time".

Yes I understand that somewhere in the middle seems intuitive: if you need to be more conservative than ZHL , it has to be somewhere between the ambiant line and the ZHL line, because at the ambiant line you are not off-gasing.

I was more wondering whether it was obvious whether the shape of the curve should be a straight line and why we are using straight lines in the first place.

I would like to know the explanation for the GF factors though, that you mentioned in the second part.

 

[dmaziuk]

I am familiar with his work. The point I was trying to make was that bubble models were originally thought to reduce overall decompression time and VPM was actually tweaked after finding it was too aggressive. I am not sure if RGBM was ever modified the same way. Many users apply GF numbers to approximate curves generated by v-planner (for example).

And the point I was trying to make is that the paper is very clear on the fact that GFs will not reduce overall decompression time, and that when used to generate deeper stops GFs will add conservatism at the top end.

This:

GFs were a way to bend the Buhlmann curve to correspond to the curve generated by deep stop algorithms such as RGBM and VPM. I don't think conservatism was a factor since in it's initial iteration these models were suppose to get you out of the water faster. Sadly that turned out to be a bad idea.

is misleading to the point of being untrue and everybody keeps repeating it instead of just trying to read and comprehend the actual paper. That is very clear on both points:

1. the curve was not meant to "correspond" to "deep stop" profiles, it was meant to add a deep stop at the expense of longer ascent time, and
   
2. conservatism was the factor. The entire write-up leading to the sentence I quoted earlier explains exactly that.

 

[dmaziuk]

I would like to know the explanation for the GF factors though, that you mentioned in the second part.

Doolette's rationale for 83%: Gradient Factors in a Post-Deep Stops World

Fraedrich's Venn diagram showing 3 models agree in ~GF70-85 area: Fraedrich follow-up – The Theoretical Diver

 

[KenGordon]

Yes I understand that somewhere in the middle seems intuitive: if you need to be more conservative than ZHL , it has to be somewhere between the ambiant line and the ZHL line, because at the ambiant line you are not off-gasing.

I was more wondering whether it was obvious whether the shape of the curve should be a straight line and why we are using straight lines in the first place.

I would like to know the explanation for the GF factors though, that you mentioned in the second part.

A simplistic mental model of ZHL can be deceptive. If there was only one compartment and the risk was instantaneous then it works how people think. However there are many compartments and the risk is cumulative over time. This means that the apparently limiting tissue might not really be the one that matters if slower tissue are being ramped up but not becoming the limiting one. This matters when thinking about what conservatism actually means. What seemed more conservative due to lower fast tissue limits (because of low GF low) actually turns out to be less conservative as the accumulation in slow tissues adds more risk. This is why it is a bit bogus thinking about GF numbers. The same GF numbers with different profiles will result in quite different risks, only the leading tissue is controlled by the algorithm but the rest still count for risk. This means that a GF hi of 85 is a different thing on a 4 dives a day week long Red Sea trip, two dives a day uk day trip or a week in Scapa.

There are more sophisticated models that try to minimise the overall risk, but it may be the case that the extra sophistication doesn’t really help in the face of all the other personal and conditions variations.

 

[Mod63]

Thanks but that doesn't change what I said: I still don't know anything about it.

Ahh, language barrier, I read it like you had never heared of ZH-L8.
The 5th edition of Tauchmedizin has a chapter where there is a description of ZH-L8 but not to much detail. We learn that compartment 1 is 5 min and 8 is 640 min but nothing much else is given in regards to real numbers. I think it is mostly due to the fact of Bühlmanns death and partialy due to ZH-L8 being somewhat intelectual property of Uwatec.
Accroding to a paper on decopression written by A. Salm, the compartments that are affected by the ADT algorythm are 40, 80 and 160 min; their halftime is prolonged by recalculation based on changed perfussion.

 

[dmaziuk]

... This means that the apparently limiting tissue might not really be the one that matters if slower tissue are being ramped up but not becoming the limiting one. This matters when thinking about what conservatism actually means. What seemed more conservative due to lower fast tissue limits (because of low GF low) actually turns out to be less conservative as the accumulation in slow tissues adds more risk.

Well this is where Buhlmann and Baker part ways with the others: with GFs you keep loading your slower compartments at deep stop, if and when you load them enough they become leading, they'll define longer shallow stops.

Bubble models and USN's Thalmann effectively modify the M-value line instead. It's no longer a Workman's straight line there.

 

[dmaziuk]

There are more sophisticated models that try to minimise the overall risk, but it may be the case that the extra sophistication doesn’t really help in the face of all the other personal and conditions variations.

SAUL lets you choose your risk and back-calculates from there (personal conditions etc. notwithstanding), but it's a bit meaningless without a clear way to relate his "risk" numbers to something concrete. Like, this number is "as risky as DSAT" and this one is "as risky as Zoop".

 

[scubadada]

SAUL lets you choose your risk and back-calculates from there (personal conditions etc. notwithstanding), but it's a bit meaningless without a clear way to relate his "risk" numbers to something concrete. Like, this number is "as risky as DSAT" and this one is "as risky as Zoop".

I already had these. First, clean dive NDLs and probability of DCS (in %) from SAUL Recreational Dive Planner, includes a 3 min SS SAUL Recreational Dive Planner | Modern Decompression Shearwater NDLs vary a little depending on atmospheric pressure

100/% = Rate

 

[dmaziuk]

View attachment 587496

DSAT report claimed 1/5000 for rec diving back in 1993 when they were using Navy tables ("3. Decompression testing and statistics"). On face value this would correspond to SAUL's pDCS of .02 and GF in the 75 range -- that I don't think quite matches up to USN tables.