Hi Rick, > On 25 Oct 2015, at 02:07, Rick Walsh <[email protected]> wrote: > > Doing VPM-B calculations for dives outside of the planner has not been > possible because real dive data does not record either: > - reference pressure for the Boyle's law compensation (i.e. > first_stop_pressure), or > - deco_time for the vpmb_next_gradient function used to do the CVA > calculations >
This is definitely a step in the right direction. We should be able to use VPM also to say something meaningful for existing dives. > However, we can infer these values to be: > - first_stop_pressure is the deepest ceiling in the dive > - deco_time is dive time remaining It is just that I need some more convincing that this is the right thing to do. I agree that it gives the correct ceiling for planned dives. You know, the current ceiling im VPM is a somewhat ill defined concept: It depends on the allowed tension but that is determined in the CVA (which makes no sense for an existing dive). Forget Boyle compensation for a moment, then the CVA determines the tension by integrating the tension over the decompression time (and adding a surface bit), calling this the volume of free (i.e. in bubbles) gas and comparing this to a critical value. The integration there does not look like one, since the VPM profiles, the tension is a constant during deco (ignoring Boyle), so it’s simply tension times deco time. My concern with taking the deco time of the real dive is that it goes in the wrong direction: Imagine starting with a dive that follows exactly a VPM-B planned profile. Then, as you write, with your code, you get the allowed tension and hence the ceiling as in the planner. But now imagine, the diver cuts the deco short. Then your code sees a shorter deco time and for the product tension * deco time to stay under a critical limit, the tension can actually increase which in turn means the ceiling during the dive goes up (towards the surface). This to me sounds counter intuitive. Also, in the above calculation, only the deco time is supposed to be counted. That is is the time in which the there is off-gassing. During the dive, when there is on-gassing still going on, taking the remaining dive time is certainly wrong. I would think, that traditional ceilings as for Buehlmann just don’t make sense for VPM. There is still something one could call a ceiling: You know that when planning, you can get a red profile when you add a point above the ceiling, i.e. when VPM would compute a first stop deeper than where you currently are. In that case, you would say you violated a ceiling. So for a real dive, you could define a ceiling at time t as: Take the dive up to that point, but then ascent with the ascent velocity to depth d and then let the VPMB planner plan the deco from that point. Find the minimal d for which that planned deco would not turn the profile red. If you know what I mean. One could also try something completely different: After all, VPMB really judges a dive based on the free gas volume (computed as the integral as above, for real dives the integral is more complicated than a product): Is it above or below the critical value. But this is of course a single number for a dive and not one per instant of time (like a ceiling). But maybe, with some further thought, that could also be turned into something plowable (like for example the current bubble volume producing rate). But right now, some younger members of my family want me to prepare breakfast for them. Will write some more later. TL;DR: I don’t think this patch produces something that is meaningful for real dives. I like the code but need more convincing about the philosophy. Best Robert
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