Deco artefacts with low GFlow, ticket #549

[roberto forini]

This maybe could help to understand your great explanation..
Il 17/giu/2014 06:11 <joejoshw at gmail.com> ha scritto:

>
>
>
> Sent from my BlackBerry 10 smartphone.
>   Original Message
> From: Robert C. Helling
> Sent: Tuesday, 17 June 2014 12:25 AM
> To: Subsurface Mailing List
> Subject: Deco artefacts with low GFlow, ticket #549
>
> Hi,
>
> I have investigated this for a seizable number of hours but am still not
> in the position to send a commit that fixes this problem in all
> circumstances.
>
> Here is a progress report on what happens: In a dive, when you choose a
> very low GFlow (like 5 or 9) and a trimix with quite some He (12/48 in the
> example) and descend fast, the ceiling seems to do strange things in the
> first minutes of the dive (very very deep for example or jumping around).
>
> To understand what is going on we have to recall what gradient factors do
> in detail: Plain Buehlmann gives you for each tissue a maximal inert gas
> pressure that is a straight line when plotted against the ambient pressure.
> So for each depth (=ambient pressure) there is a maximally allowed
> over-pressure.
>
> The idea of gradient factors is that one does not use all the possible
> over-pressure that Buehlmann gives us but only a depth dependent fraction.
> GFhigh is the fraction of the possible over-pressure at the surface while
> GFlow is the fraction at the first deco stop. In between, the fraction is
> linearly interpolated. As the Buehlmann over-pressure is increasing with
> depth and typically also the allowed overpressure after applications of
> gradient factors increases with depth or said differently: the tissu
> saturation has be lower if the diver wants to ascent.
>
> The main problem is: What is the first stop (where to apply GFlow)? In a
> planned dive, we could take the first deco stop, but in a real dive from a
> dive computer download it is impossible to say what constitutes a stop and
> what is only a slow ascent?
>
> What I have used so far is not exactly the first stop but rather the first
> theoretical stop: During all of the dive, I have calculated the ceiling
> under the assumption that GFlow applies everywhere (and not just at a
> single depth). The deepest of these ceilings I have used as the “first stop
> depth”, the depth at which GFlow applies.
>
> Even more, I only wanted to use the information that a diver has during
> the dive, so I actually only considered the ceilings in the past (and not
> in the future of a given sample).
>
> But this brings with it the problem that early in the dive, in particular
> during the descent the lowest ceiling so far is very shallow (as not much
> gas has built up in the body so far).
>
> This problem now interferes with a second one: If at the start of the dive
> when the all compartments have 790mbar N2 the diver starts breathing a
> He-heavy mix (like 12/48) and descents fast the He builds up in the tissues
> before the N2 can diffuse out. So right at the start, we already encounter
> high tissue loadings.
>
> If now we have a large difference between GFhigh and GFlow but they apply
> at very similar depth (the surface and a very shallow depth of the deepest
> ceiling (which for a non-decompression dive would be theoretically at
> negative depth) so far) it can happen that the linear interpolation as
> opposite slope then in the typical case above: The allowed over-pressure is
> degreasing with depth, shallower depth do not require lower gas loading in
> the tissue (i.e. can be reached after further off-gasing) but but tolerate
> higher loadings. In that situation the ceiling disappears (or is rather a
> floor).
>
> So far, I got rid of that problem, by stating that the minimum depth for
> GFlow was 20m (after all, GFlow is about deep stops, so it should better
> not be too shallow). Now the dive reported in ticket #549 takes values to
> an extreme in such away that 20m (which is determined by
> buehlmann_config.gf_low_position_min in deco.c) was not enough to prevent
> this inversion problem (or in a milder form that the interpolation of
> gradient factors is in fact an extrapolation with quite extreme values).
>
> I have now a patch that gets rid of the problem for the dive described
> above but still it is possible to find (more extreme) parameter choices
> that lead to non-realistic ceilings. I am still working on this.
>
> Let me close by pointing out that all this is only about the descent, as
> it is about too shallow depth for GFlow. So no real deco (i.e. later part
> of the dive) is inflicted. This is only about a theoretical ceiling
> displayed possibly in the first minutes of a dive. So this is more an
> aesthetically than a practical problem.
>
> Best
> Robert
>
> PS: I also copy this text to trac.
>
>
>
> --
>
> .oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oO
> Robert C. Helling Elite Master Course Theoretical and Mathematical Physics
> Scientific Coordinator
> Ludwig Maximilians Universitaet Muenchen, Dept. Physik
> print "Just another Phone: +49 89 2180-4523 Theresienstr. 39, rm. B339
> stupid .sig\n"; http://www.atdotde.de
>
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