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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 tissue saturation has to 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). This 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. 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. Fixes #549 Signed-off-by: Robert C. Helling <helling@atdotde.de> Signed-off-by: Dirk Hohndel <dirk@hohndel.org> |
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| .. | ||
| animationfunctions.cpp | ||
| animationfunctions.h | ||
| divecartesianaxis.cpp | ||
| divecartesianaxis.h | ||
| diveeventitem.cpp | ||
| diveeventitem.h | ||
| divelineitem.cpp | ||
| divelineitem.h | ||
| divepixmapitem.cpp | ||
| divepixmapitem.h | ||
| diveplotdatamodel.cpp | ||
| diveplotdatamodel.h | ||
| diveprofileitem.cpp | ||
| diveprofileitem.h | ||
| diverectitem.cpp | ||
| diverectitem.h | ||
| divetextitem.cpp | ||
| divetextitem.h | ||
| divetooltipitem.cpp | ||
| divetooltipitem.h | ||
| profilewidget2.cpp | ||
| profilewidget2.h | ||
| ruleritem.cpp | ||
| ruleritem.h | ||