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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