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Use real gas compressibility in planner

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Modify formluas for gas use to take into account the
compressibility correction for real gases. This introduces
also the inverse formula to compute the pressure for a given
amount of gas.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
This commit is contained in:
Robert C. Helling 2017-01-12 21:19:40 +01:00 • committed by Dirk Hohndel
parent fedadc65db
commit 7725842383
3 changed files with 19 additions and 4 deletions
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1
core/dive.h
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@ -136,6 +136,7 @@ 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);
extern double isothermal_pressure(struct gasmix *gas, double p1, int volume1, int volume2);
static inline int get_o2(const struct gasmix *mix)

10
core/gas-model.c
View file

@ -62,3 +62,13 @@ double gas_compressibility_factor(struct gasmix *gas, double bar)
*/
return Z * 0.001 + 1.0;
}
/* Compute the new pressure when compressing (expanding) volome v1 at pressure p1 bar to volume v2
* taking into account the compressebility (to first order) */
double isothermal_pressure(struct gasmix *gas, double p1, int volume1, int volume2)
{
double p_ideal = p1 * volume1 / volume2 / gas_compressibility_factor(gas, p1);
return p_ideal * gas_compressibility_factor(gas, p_ideal);
}

12
core/planner.c
View file

@ -246,7 +246,7 @@ static void update_cylinder_pressure(struct dive *d, int old_depth, int new_dept
if (in_deco)
cyl->deco_gas_used.mliter += gas_used.mliter;
if (cyl->type.size.mliter) {
delta_p.mbar = gas_used.mliter * 1000.0 / cyl->type.size.mliter;
delta_p.mbar = gas_used.mliter * 1000.0 / cyl->type.size.mliter * gas_compressibility_factor(&cyl->gasmix, cyl->end.mbar / 1000.0);
cyl->end.mbar -= delta_p.mbar;
}
}
@ -830,8 +830,11 @@ static void add_plan_to_notes(struct diveplan *diveplan, struct dive *dive, bool
volume = get_volume_units(cyl->gas_used.mliter, NULL, &unit);
deco_volume = get_volume_units(cyl->deco_gas_used.mliter, NULL, &unit);
if (cyl->type.size.mliter) {
deco_pressure = get_pressure_units(1000.0 * cyl->deco_gas_used.mliter / cyl->type.size.mliter, &pressure_unit);
pressure = get_pressure_units(1000.0 * cyl->gas_used.mliter / cyl->type.size.mliter, &pressure_unit);
int remaining_gas = (double)cyl->end.mbar * cyl->type.size.mliter / 1000.0 / gas_compressibility_factor(&cyl->gasmix, cyl->end.mbar / 1000.0);
double deco_pressure_bar = isothermal_pressure(&cyl->gasmix, 1.0, remaining_gas + cyl->deco_gas_used.mliter, cyl->type.size.mliter)
- cyl->end.mbar / 1000.0;
deco_pressure = get_pressure_units(1000.0 * deco_pressure_bar, &pressure_unit);
pressure = get_pressure_units(cyl->start.mbar - cyl->end.mbar, &pressure_unit);
/* Warn if the plan uses more gas than is available in a cylinder
* This only works if we have working pressure for the cylinder
* 10bar is a made up number - but it seemed silly to pretend you could breathe cylinder down to 0 */
@ -840,7 +843,8 @@ static void add_plan_to_notes(struct diveplan *diveplan, struct dive *dive, bool
translate("gettextFromC", "Warning:"),
translate("gettextFromC", "this is more gas than available in the specified cylinder!"));
else
if ((float) cyl->end.mbar * cyl->type.size.mliter / 1000.0 < (float) cyl->deco_gas_used.mliter)
if ((float) cyl->end.mbar * cyl->type.size.mliter / 1000.0 / gas_compressibility_factor(&cyl->gasmix, cyl->end.mbar / 1000.0)
< (float) cyl->deco_gas_used.mliter)
snprintf(warning, sizeof(warning), " &mdash; <span style='color: red;'>%s </span> %s",
translate("gettextFromC", "Warning:"),
translate("gettextFromC", "not enough reserve for gas sharing on ascent!"));