New Bug Reports/Feature Requests

Linus Torvalds torvalds at linux-foundation.org
Tue Feb 23 23:18:42 PST 2016


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.

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.

                 Linus


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