[PATCH] Fix up SAC calculations for ATM/bar confusion

[Dirk Hohndel]

On Feb 24, 2013, at 2:38 PM, Chris Lewis wrote:

> . ]
> >>
> > I have noticed something which surely distords SAC-calculation when using 300 bar tanks. Subsurface is not taking into account the air unlinear compressibility above 270 bar.
> >
> > Function bar_comp = -0.0007*bar*bar + 1.109*bar + 0.8
> > would work from 250 to 300 bar very well (= close to wikipedia's table at temp 27 C)

Umm - where do you dive with tanks at more than 250bar? I happen to be certified as a cylinder inspector (don't ask) and gas blender and most stations aren't even able to fill 4350psi (many can't even fill 230bar/3440psi tanks completely). Same goes for hydrostatic testing. A quick Google search shows few places able to test scuba sized tanks at 500bar / 7350psi (which you need for the 5/3 testing).

Long story short - yes, this is theoretically interesting but to me provides little or no practical value here.

> I suppose if accuracy in this calculation is important we should use a full Van dear Waals method.  However this might be more hassle than its worth, I'm afraid I'm no programmer so am completely unable to even try to help :-(

Yes we could. But why. And oh-by-the-way. None of these formulas are valid unless you know the temperatures involved. Which could be vastly different from the temperature measured by your dive computer.

/D
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