Deco calculation for repetitive dive - strange effect

[Robert Helling]

Hi,

> On 06 Feb 2017, at 19:30, Robert Helling <helling at atdotde.de> 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 http://euve10195.vserver.de/wp/?p=27 <http://euve10195.vserver.de/wp/?p=27> or the German version http://euve10195.vserver.de/wp/?p=46 <http://euve10195.vserver.de/wp/?p=46>)

One of the state variables of this model is the „crushing radius“ (here r_reg) defined by

2(γc−γ)rreg=pmax+2(γc−γ)rcrit
which enters the calculation of the allowed gradient (at least in the first iteration) as

G0=2γγcγc–γrreg.

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.

As far as Subsurface is concerned, I guess we should leave it as is (after all, people want „the real model“, if we would change the behaviour of Subsurface we would break with the original Baker code and people will hate us for this).

Thanks for finding this out. Once I have a bit more time, I will write about this in my blog.

Best
Robert
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