Re: Decompression calculation - Bühlmann settings
Robert Helling
helling at atdotde.de
Wed Feb 15 02:19:03 PST 2017
Stefan,
> On 14.02.2017, at 23:53, Stefan <sjti at gmx.net> wrote:
>
> I was wondering about differences between the deco stops calculated by
> my OSTC 2N and by subsurface for a while. After recently studying the
> Bühlmann model and looking at different implementations I found a few
> lines in subsurface which are irritating to me:
>
> In the current "core/deco.c" lines 39-41 read:
>> struct buehlmann_config {
>> double satmult; // safety at inert gas accumulation as percentage of effect (more than 100).
>> double desatmult; //! safety at inert gas depletion as percentage of effect (less than 100).
>
> Then, lines 49-51 state:
>> struct buehlmann_config buehlmann_config = {
>> .satmult = 1.0,
>> .desatmult = 1.01,
>
> Obviously a value of 1.01 is not < 100 %
Thanks for finding this. Will fix this. But the the effect in fact is minimal, we are talking about two percents of total deco time, roughly: The effect of the satmult is to multiply the saturation time (mostly bottom time), while desalt time multiplies the decompression time. So, for 20min bottom time, we are talking about 20min 12s instead (not something I would worry about in a dive) as well as for one hour deco, we are talking about about 30 extra seconds.
Decompression models are only very crude approximations of what is going on in the human’s body and there are much larger systematic uncertainties than a few percent uncertainties/errors in some constants.
>
> On most OSTCs these values are adjustable, so in the long run it
> would be great to allow the same in subsurface. Yet, on the OSTC 2
> (old as well as the newer models running on hwOS) the defaults are
>> char_I_saturation_multiplier = 110;
>> char_I_desaturation_multiplier = 90;
> so it would be nice to incorporate these as new defaults.
These are fudge factors in any case (that have no good justification) and I don’t think changing the defaults would be a good idea (actually, I think it would be more honest to make those 1.0). And yes, one could make all kinds of parameters user configurable but, honestly, our user interface already has too many knobs so adding more will have the effect of confusing users and possibly computing with ill advised values without noticing. I will need a very convincing argument to add more configurable parameters.
>
> By the way - the OSTC code also uses the 1a Variant of ZH-L16C, i.e.
> the one with the first compartment having a 4min halftime instead of
> the 5min (1b) one as on the DR5.
Again, this makes very little difference (if at all then for dives with very short deco), since usually this tissue desaturates already during the ascent without having to wait for it.
If you are really worried about the difference between what we do and H&W, you should notice that we handle gradient factors differently (and I would argue that we do the “correct” thing. You can see this in their ceiling jumping when the leading tissue changes, this was at least true last time I looked at their code but that was a while ago). But those differences go away for setting gradient factors to 100/100.
Best
Robert
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Robert C. Helling Elite Master Course Theoretical and Mathematical Physics
Scientific Coordinator
Ludwig Maximilians Universitaet Muenchen, Dept. Physik
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