[PATCH 0/3] Some cylinder pressure handling cleanups

[Linus Torvalds]

This is a lot more questionable than my previous set of patches (the 
pressure interpolation round), but let's see if people see any issues with 
it.

The first patch in the series doesn't actually change anything, it just 
makes sure a function that will be changed is private, and removes another 
one that isn't actually used and is questionable to begin with. So that 
patch is a no-brainer, I think.

The second patch came about from the noticing that our 
"match_standard_cylinder()" logic actually tried to take the 
compressibility of gases into account. That happened to not matter for 
2400 and 3000 psi cylinders, because our old gas compressibility 
heuristics made the differences there unnoticeable, but if we change our 
compressibility thing to give the full 3% change at 3000 psi, then that is 
a difference of a couple of cuft.

There is actually a theoretical argument for why the nominal imperial 
cylinder sizes should *not* take compressibility into account: it depends 
on the gas in question. The compressiblity of oxygen is noticeably 
different from that of air, for example.

So from an imperial cylinder naming perspective, it actually kind of makes 
sense to say "AL80 means 80 cuft of _ideal_ gas at STP".

It is also worth noting that the *other* places where we create cylinder 
sizes (notably get_volume_string()) are not using the compressibility 
factor, so the change in patch #2 really is mostly about being internally 
consistent (but also about getting rid of uses of that odd "surface 
factor", which is how I noticed this in the first place).

The third patch is the one that gets rid of our current slightly hacky 
surface_volume_multiplier(), which is now no longer used anywhere else but 
by the "gas_volume()" helper. 

Instead of that surface_volume_multiplier(), it introduces the notion of 
compressibility Z factor and makes gas_volume() use that instead for its 
calculations, and a function to calculate it (called, not surprisingly, 
"gas_compressibility_factor()").

NOTE! That function takes a gasmix, but then ignores the hell out of it, 
and just uses the Wikipedia table for air at 300K.

That third one should be looked at critically. I got the linear 
interpolation wrong (mixed up "frac" and "1-frac") in the first version, 
and wouldn't have noticed at all if I hadn't done some testing with hacky 
debug code. The compressibility factor isn't really noticeable in any 
normal way.

Linus Torvalds (3):
  gas pressures: do some initial cleanup
  gas pressures: do not use gas compressibility for cylinder naming
  gas pressures: use an actual compressibility table for air

 subsurface-core/dive.c | 81 +++++++++++++++++++++++++++++++++++++-------------
 subsurface-core/dive.h |  2 +-
 2 files changed, 61 insertions(+), 22 deletions(-)

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2.7.2.334.g7c0da37