[PATCH]Re: Deco calculation for repetitive dive - strange effect

[Rick Walsh]

On 8 February 2017 at 07:24, Robert Helling <helling at atdotde.de> wrote:

> Hi,
>
> On 07 Feb 2017, at 19:53, Stefan Fuchs <sfuchs at gmx.de> wrote:
>
> 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?
>
>
> I guess (as so often) Linus is right, we should reset the state between
> dives (except of course tissue saturations) to get rid of this anomaly, and
> if other code does that even better.
>
> I attach a patch that does this.
>
> This is entirely sensible and rational and makes sense to me. But...

In the last paragraph of an article by Eric Baker, he states, "The
"crushing" (i.e. compression) and slow regrowth of gas nuclei providee an
explanation of the common qualitative observation that greater pressure
reductions can be tolerated on repetitive dives than on first dives."  He
then discusses that no decompression limits may be greater (!) for
repetitive dives, and a hypothetical commercial diver who tolerates
multi-day repetitive dives but gets bent on the first dive after returning
from vacation.
http://www.archeonet.org/sub/biblio/Baker.%20Collection%20of%20Narrative%20Explanations%20About%20the%20Varying%20Permeability%20Model%20%28VPM%29.pdf


>
> There is probably no point in arguing about the original model since that
> IIIRC does not consider repetitive dives but I have to check the docs again.
>

Eric Baker's Fortran VPM-B program, which I assume you mean by the
"original model", does indeed consider repetitive dives and surface
intervals.  Input is in the form of a text file containing details of the
dive/s to be planned.  I compiled it on my laptop at home, but I'm at work
now and won't have a chance for a few days to test the effect.  Given
what's written in the article I linked above, I suspect the crushing radius
is not reset in between dives.

I tested similar profiles to Stefan's test case with GUE Deco Planner 3.1.4
(funnily enough, I couldn't work out how to set the final ascent rate to
1m/s, even though this is GUE standard practice).  First dive (18/45 to
60m) had leaving 6m (last) stop at 83min (same as Subsurface).  Second dive
(air to 40m) had leaving 3m stop at 52min for either 47:30 surface
interval, or 1hr surface interval.  Doing the 40m air dive without a
previous dive, the 3m stop ends at 51min (same as Subsurface), so a slight
penalty is paid for doing it as a repetitive dive.  Note that Deco Planner
treats surface intervals unconventionally: it assumes the surface interval
is performed at 1m depth, so tissues never fully de-saturate.  I am not
aware of any other planning software doing this.


In any case, I would say this still shows how strange the model is: We have
> to artificially reset parameters between two dives. But of course „between
> dives“ can be somewhat arbitrary: If you only surface for a few minutes and
> then descend again, are these „two dives“ (which require a reset in between
> and how does your body know that it has to reset itself, after all this is
> what we are supposed to simulate) or is this a single dive (without reset)?
> What I take away is the impression that this does not make a lot of sense.
>
> But yes, please take this patch for master.
>
> I would be interested to see a comparison with V-Planner/Multideco and the
Fortran program, but no harm in making the change now - there's plenty of
time until next release.

Cheers,

Rick
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