Deco calculation for repetitive dive - strange effect

Stefan Fuchs sfuchs at
Tue Feb 7 10:53:30 PST 2017

Hi All,

Am 07.02.2017 um 09:09 schrieb Rick Walsh:
>>     On 06 Feb 2017, at 19:30, Robert Helling <helling at
>>     <mailto:helling at>> wrote:
>>     a no progress report: I guessed it’s the max_ambient_pressure but
>>     now I know, it is not. Tonight, I will not have further time to
>>     look into this (we have visitors), so if someone else wants to
>>     give it a try: There are a few other state variables of VPM-B,
>>     all defined as global variables in deco.c. I would next try
>>     max_crushing pressure (by resetting this between dives and see if
>>     this makes the deco almost independent of the day. In Buehlmann,
>>     one can see, that the difference in saturation is only minimal).
>     I think I understand this now. This is a „feature“ of the VPM-B
>     model. (For some background on this model see
>     <> or the German
>     version
>     <>)
>     [...]
>     If you are doing repetitive diving, this is tracked between dives
>     and since p_max (the maximal over pressure during the dives)
>     enters the maximal depth encountered is relevant for the deco.
>     Your first dive has a depth of 60m so it sets this variable also
>     for the second dive which is only 40m. Thus, according to these
>     formulas, it allows for a bigger gradient which means it is more
>     liberal with respect to over pressure in the tissues which results
>     in shorter deco.
>     And no, I don’t think this makes sense. But it is the model. It is
>     not the first oddity of this model we come across. 
> That quirk is indeed odd.  I can see how it could have this unexpected
> effect, but are you sure that's the entire cause?  I found no change
> at all when the surface interval was in the range 10min to 47hr. 
> Surely there ought to be some variation with surface interval, even if
> the smaller "crushing radius" remains.
I'm still here listening (reading) carefully ;-)

I understand that we don't want to tweak the VPM-B implementation to
make the results look nicer w/o knowing if this is then still in line
with the ideas of "the real model" and I fully agree to this. But maybe
based on Linus finding there is until now really a misunderstanding
regarding the implementation for this specific detail?

There is not much I can contribute to the discussion beside one idea:
Can s.o. once check exactly the same dive scenario in another VPM-B
based tool? I heard there is another tool on the market... ;-)
I unfortunately don't have a license. But I also could ask a friend of
mine. But I'm 100% sure one of you has a license.

Best regards


Stefan Fuchs
E-Mail: sfuchs at <mailto:sfuchs at>

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