git bisect vs libdivecomputer interface

Robert Helling helling at lmu.de
Tue Jan 10 05:23:54 PST 2017


Hi,

> On 10.01.2017, at 14:01, Anton Lundin <glance at acc.umu.se> wrote:
> 
> I don't have a solution to your libdivecomputer issues, but other
> goodies.
> 
> I saw the issue you created and got a idea, and found a probable
> explanation for the issue.
> 
> It looks to be the pressure / volume calculations work done by Linus who
> "broke" the sac-tests. The sac-tests are planned for a ideal gas, but
> now when we do better sac calculations, we're off by a bunch.
> 
> The proper fix would probably to fix pressure-track-code to use real
> volumes and not ideal gases, and also fix the sac-test.xml pressures to
> match with reality.


you were spot on. When I make gas compressibility factor always return 1.0, I get back the 20l/min SAC.

Which worries me a bit: I am surprised that the effect of using real gas compressibility vs ideal gas law makes such a difference. 18.5 vs 20 is an 8% difference in SAC rate!
If I print out the compressibility I get

z = 1,037447 at 202,600000
z = 0,995009 at 101,300000

which does not really look like it should make an 8% difference. Maybe we should double check the math…

And even if it’s correct, we should point out to the users (maybe in the FAQ) that we do these real gas calculations since otherwise I am sure many will be convinced that we compute the SAC wrongly.

Best
Robert

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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics
                      Scientific Coordinator
                      Ludwig Maximilians Universitaet Muenchen, Dept. Physik
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