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

[Chris Lewis]

On Feb 24, 2013 10:51 PM, "Dirk Hohndel" <dirk at hohndel.org> wrote:
>
>
> 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

In the UK at least 300 bar tanks are getting more and more common.

I have a lovely pair of manifolded 7l 300 bar cylinders, getting a 300 bar
fill is not that difficult hear (although I will admit its not the norm by
a long way)

Also if you know a good dice shop most will pump your 232 tanks to about
240-245 so they cool nicely to ~230

Chris
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.hohndel.org/pipermail/subsurface/attachments/20130224/ba15a09a/attachment.html>