Deep Stops Increases DCS

[Dr Simon Mitchell]

But state the problem you are trying to solve now ??

It's NOT high DCS numbers.

I think you are out of touch with the reality of technical diving in the field. We could argue all day about case numbers and what constitutes "high" or "low", but who cares really? You are not a clinician Ross. I am. And I am involved in the management of enough critically injured divers to know that getting your decompression right is incredibly important to those individuals unfortunate enough to be afflicted in that way. And there is a much larger number (many of whom never get into any sort of databases) who suffer more minor events which can impose inconvenience, anxiety, trip disruption and other consequences. Trying to claim that there is no problem to solve is insensitive and disingenuous, especially coming from someone who is defending a product that, based on best current evidence, is probably not the optimal way of avoiding these problems.

The dcs we get today, all comes from issues we can mostly identify. An excess of model derived tissue pressure (profile) stress is not the issue. Tissue pressure (profile) stress is the ONLY thing a deco model can control. So obviously there is no point in trying to fiddle profile stress any more, because it won't solve any issues !

There is overwhelming evidence, largely the subject of debate on this thread, that this is wrong.

Here is a chart of all known causes of DCS. The deco model / program has control of just 2 items (stops/time) in there. Everything else is from some other influence or control of the diver.

The chart is excellent for the purpose to which it was put by its author, but it shows nothing about proportional importance. The only thing on the chart that you MUST have in order to suffer DCS is a dive profile. All of the other things are risk factors or mitigations, some more important than others.

 

[Dr Simon Mitchell]

DCIEM tables are designed with the full knowledge that VGE exist, that VGE are part of everyday diving, and that VGE are not harmful on their own. Therefore the VGE have been ignored as a cause for concern - officially.

This is what you say about a test program based entirely on VGE grades, with rejection of profiles if a threshold grade was exceeded??!! I give up Ross. It is impossible to debate you. When cornered you just continue to state that black is actually white.

We have all collectively ignored VGE in our deco planning, as a cause of concern.. either in total, or with the knowledge that VGE are not harmful.

Try telling that to the divers who have suffered crippling DCS as a result of VGE crossing right to left shunts.

Simon M

 

[rossh]

This is what you say about a test program based entirely on VGE grades, with rejection of profiles if a threshold grade was exceeded??!! I give up Ross. It is impossible to debate you. When cornered you just continue to state that black is actually white.

Simon M

DCIEM tables, are based on a dissolved gas Kidds Stubbs model. VGE was a secondary measure and check point.


From the Introduction, in the DCIEM diving manual.....

 

[Storker]

DCIEM tables, are based on a dissolved gas Kidds Stubbs model. VGE was a secondary measure and check point.

So the page you're quoting here tells me that the DCIEM tables were based on the DCIEM decompression model (which is based on the Kid-Stubbs model from 1962, i.e. developed before the bubble models and thus a dissolved gas model). And those modeled data were then calibrated/validated using experimental data on Doppler-detected microbubbles after chamber dives. Is that about right?

If I've got it right, it seems like a pretty sound methodology, scientifically speaking. At least if you can't do as they did in the NEDU deep stop study (and, IIRC, like Haldane once did), actually bend a bunch of people.

By the way, back in post #1027 I asked which data that were used to calibrate/validate the VPM-B model. Did you miss that question?

 

[rossh]

So the page you're quoting here tells me that the DCIEM tables were based on the DCIEM decompression model (which is based on the Kid-Stubbs model from 1962, i.e. developed before the bubble models and thus a dissolved gas model). And those modeled data were then calibrated/validated using experimental data on Doppler-detected microbubbles after chamber dives. Is that about right?

If I've got it right, it seems like a pretty sound methodology, scientifically speaking. At least if you can't do as they did in the NEDU deep stop study (and, IIRC, like Haldane once did), actually bend a bunch of people.

By the way, back in post #1027 I asked which data that were used to calibrate/validate the VPM-B model. Did you miss that question?

.

I like the DCIEM tables. Back in my bad old air diving days, I used these extensively.


*************

VPM-B was calibrated and checked to various points in ZHL-C and DCIEM 1994, both N2 and helium. VPM-B was also calibrated to match PADI NDL tables reasonably well.

Of course all four models have different ideas on how to do the same job, so alignment is transient, between all models concerned.

.

 

[Storker]

VPM-B was calibrated to various points in ZHL-C and DCIEM 1994, both N2 and helium. VPM-B was also calibrated to match PADI NDL tables reasonably well.

Of course all four models have different ideas on how to do the same job, so alignment is transient, between all models concerned.

Let me see if I understand: Are you saying that VPM-B was calibrated against other decompression models, but neither calibrated nor validated against experimental data?

And AFAIU the stop distribution is different in those models, so which datapoint was chosen as the calibration value? Total run time?

 

[rossh]

Let me see if I understand: Are you saying that VPM-B was calibrated against other decompression models, but neither calibrated nor validated against experimental data?

And AFAIU the stop distribution is different in those models, so which datapoint was chosen as the calibration value? Total run time?

I think all three base models where matched to experimental data - yes? The basis of decompression stress is supersaturation pressures, and this can be measured in a model independent way, and compared and used as references.

And in many cases, VPM-B is longer and slower and has equal or lower stress, than the ZHL-C equivalent. therefore...

From our web site FAQ: "VPM is experimental. There is no database of proven dives, or scientific results to look over."

 

[Storker]

I can't see that you're answering my questions. Let me try to rephrase:

It seems to me that VPM-B was neither calibrated nor validated against any kind of measured data, but calibrated against other models. Right or wrong?

If VPM-B was calibrated against other models, which feature/datapoint was chosen for calibration? Total run time? Estimated P(DCS)? Something else, and if so, what?

 

[leadduck]

You got it backwards. The CR works in reverse. Bigger CR is more conservative. VPM-B needed lower CR settings, because the missing Boyle law component has larger effect on the overall attributes of the model output.

That's what I wrote ("Baker saw that the VPM-B algorithm became more conservative and suggested to reduce the critical radius and lambda").

Baker wrote in his update explanation for VPM-B:
"The new program tends to be somewhat conservative and the Critical Radii may need to be dialed down a little (try for example 0.55 to 0.7 for N2 and 0.45 to 0.6 for He). Also the Critical Volume parameter Lambda may need to be reduced a little to say 6500."

(In the original VPM code of Baker as in Yount's paper, ICR_N2=0.8 microns and lambda=7500 fsw min.)

My point was: Baker made these numbers up out of thin air, only with total runtime in mind, but having no idea how his schedules and parameter choices relate to DCS risk, just like Yount did 15 years earlier.

 

[rossh]

I can't see that you're answering my questions. Let me try to rephrase:

It seems to me that VPM-B was neither calibrated nor validated against any kind of measured data, but calibrated against other models. Right or wrong?

If VPM-B was calibrated against other models, which feature/datapoint was chosen for calibration? Total run time? Estimated P(DCS)? Something else, and if so, what?

As I said, it used existing data as a basis, and a proper measure of decompression stress, to come to its own conclusions. In many cases, VPM-B has the same or lower stress than other model equivalent of the same dive. But if you want more specifics - sorry - not doing that here.

MultiDeco has all the necessary tools built in to look at this in great detail.

Only military tables have pDCS ratings. Normal models in tech / rec diving do not have this.