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gas pressures: use an actual compressibility table for air

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We could in theory make this dependent on the gasmix, but for now let's
just assume (incorrectly) that everything we breathe acts like air.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
This commit is contained in:
Linus Torvalds 2016-02-24 12:14:53 -08:00 • committed by Dirk Hohndel
parent adaeb506b7
commit 45ed0ec757
2 changed files with 49 additions and 11 deletions
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59
subsurface-core/dive.c
View file

@ -850,24 +850,61 @@ static void update_min_max_temperatures(struct dive *dive, temperature_t tempera
* At high pressures air becomes less compressible, and
* does not follow the ideal gas law any more.
*
* This tries to correct for that, becoming the same
* as to_ATM() at lower pressures.
*
* THIS IS A ROUGH APPROXIMATION! The real numbers will
* depend on the exact gas mix and temperature.
* NOTE! The compressibility doesn't just depend on the
* gas, but on temperature too. However, this currently
* just follows the 300K curve for air, and ignores the
* gasmix. And the temperature we don't really even have
* a way to try to take into account.
*/
static double surface_volume_multiplier(pressure_t pressure)
#define ARRAY_SIZE(array) (sizeof(array)/sizeof(array[0]))
double gas_compressibility_factor(struct gasmix *gas, double bar)
{
double bar = pressure.mbar / 1000.0;
static const struct z_factor {
double bar, z_factor;
} air_table[] = {
{ 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 }
};
const struct z_factor *n;
int i;
if (bar > 200)
bar = 0.00038 * bar * bar + 0.51629 * bar + 81.542;
return bar_to_atm(bar);
for (i = 0; i < ARRAY_SIZE(air_table); i++) {
const struct z_factor *n = air_table+i;
double frac;
if (n->bar < bar)
continue;
if (!i)
return n->z_factor;
/* How far from this one? */
frac = (n->bar - bar) / (n->bar - n[-1].bar);
/* Silly linear interpolation */
return frac*n[-1].z_factor + (1.0-frac)*n->z_factor;
}
/* Over 500 bar? We make shit up */
return air_table[ARRAY_SIZE(air_table)-1].z_factor;
}
int gas_volume(cylinder_t *cyl, pressure_t p)
{
return cyl->type.size.mliter * surface_volume_multiplier(p);
double bar = p.mbar / 1000.0;
double z_factor = gas_compressibility_factor(&cyl->gasmix, bar);
return cyl->type.size.mliter * bar_to_atm(bar) / z_factor;
}
/*

1
subsurface-core/dive.h
View file

@ -129,6 +129,7 @@ extern int units_to_sac(double volume);
/* Volume in mliter of a cylinder at pressure 'p' */
extern int gas_volume(cylinder_t *cyl, pressure_t p);
extern double gas_compressibility_factor(struct gasmix *gas, double bar);
static inline int get_o2(const struct gasmix *mix)