New Bug Reports/Feature Requests
Dirk Hohndel
dirk at hohndel.org
Tue Feb 23 23:28:18 PST 2016
On Tue, Feb 23, 2016 at 11:18:42PM -0800, Linus Torvalds wrote:
> On Feb 23, 2016 15:46, "Linus Torvalds" <torvalds at linux-foundation.org> wrote:
> >
>
> > (b) air is not actually entirely compressible.
> >
> > This is a fairly small factor at 3000psi, but it's a factor.
> > HOWEVER. The rule for cylinder sizing is that the stated cylinder size
> > is basically the "theoretical" size, not the real size.
>
> Actually, doing the math, the compressibility of air is enough to
> bring that 80 cuft down to about 78 cuft. So that may actually be the
> biggest effect.
>
> We currently approximate the gas volume as being linear below 200 bar,
> and eat that up-to-3% error.
>
> Maybe we could do better.
>
> Does somebody have curve fitting software to generate a better
> function for the air compressibility factor? From Wikipedia (staying
> at 300K, which is warm water), we have
>
> bar compressibility
> --- ---------------
> 1 0.9999
> 5 0.9987
> 10 0.9974
> 20 0.9950
> 40 0.9917
> 60 0.9901
> 80 0.9903
> 100 0.9930
> 150 1.0074
> 200 1.0326
> 250 1.0669
> 300 1.1089
> 400 1.2073
> 500 1.3163
>
> and we could probably do better than our current "linear plus
> second-order" approximation.
>
> Somebody with R (or matlab) could probably get a reasonable curve from
> the above data. With a function for the compressibility factor, we
> could improve on our current "gas_volume()" function.
Robert is the usual suspect for stuff like this :-)
> Of course, we could also just do it the stupid way and do the above
> table and just do linear interpolation in between entries. Sometimes
> simple and stupid is good.
Let's see if Robert comes up with something nice, alternatively we'll go
with segment approximation.
In the meantime I have fixed the rounding issue with weights (and oh boy
did we have a serious bug in there for people big enough to dive with 20kg
or more in weight...)
/D
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