On Tue, Feb 23, 2016 at 11:18:42PM -0800, Linus Torvalds wrote: > On Feb 23, 2016 15:46, "Linus Torvalds" <torva...@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 _______________________________________________ subsurface mailing list subsurface@subsurface-divelog.org http://lists.subsurface-divelog.org/cgi-bin/mailman/listinfo/subsurface