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Use Schreiner water vapour pressure for VPM-B

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Inspired gas loading equations depend on the partial pressure of inert gas in
the alveolar.
P_alv = (P_amb - P_H2O + (1 - Rq) / Rq * P_CO2) * f
where:
P_alv	alveolar partial pressure of inert gas
P_amb	ambient pressure
P_H2O	water vapour partial pressure = ~0.0627 bar
P_CO2	carbon dioxide partial pressure = ~0.0534 bar
Rq	respiratory quotient (O2 consumption / CO2 production)
f	fraction of inert gas

In our calculations, we simplify this to use an effective water vapour pressure
WV = P_H20 - (1 - Rq) / Rq * P_CO2

Buhlmann ignored the contribution of CO2 (i.e. Rq = 1.0), whereas Schreiner
adopted Rq = 0.8.
WV_Buhlmann = PP_H2O = 0.0627 bar
WV_Schreiner = 0.0627 - (1 - 0.8) / Rq * 0.0534 = 0.0493 bar

Buhlmann calculations use the Buhlmann value, VPM-B calculations use the
Schreiner value.

Concept explained here:
http://www.divetable.de/workshop/V1_e.htm

Signed-off-by: Rick Walsh <rickmwalsh@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
This commit is contained in:
Rick Walsh 2015-08-29 21:43:32 +10:00 committed by Dirk Hohndel
parent becc1e6dc4
commit 7d09f418f8
1 changed files with 26 additions and 4 deletions
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30
deco.c
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@ -115,7 +115,28 @@ const double buehlmann_He_factor_expositon_one_second[] = {
const double conservatism_lvls[] = { 1.0, 1.05, 1.12, 1.22, 1.35 }; const double conservatism_lvls[] = { 1.0, 1.05, 1.12, 1.22, 1.35 };
#define WV_PRESSURE 0.0627 // water vapor pressure in bar /* Inspired gas loading equations depend on the partial pressure of inert gas in the alveolar.
* P_alv = (P_amb - P_H2O + (1 - Rq) / Rq * P_CO2) * f
* where:
* P_alv alveolar partial pressure of inert gas
* P_amb ambient pressure
* P_H2O water vapour partial pressure = ~0.0627 bar
* P_CO2 carbon dioxide partial pressure = ~0.0534 bar
* Rq respiratory quotient (O2 consumption / CO2 production)
* f fraction of inert gas
*
* In our calculations, we simplify this to use an effective water vapour pressure
* WV = P_H20 - (1 - Rq) / Rq * P_CO2
*
* Buhlmann ignored the contribution of CO2 (i.e. Rq = 1.0), whereas Schreiner adopted Rq = 0.8.
* WV_Buhlmann = PP_H2O = 0.0627 bar
* WV_Schreiner = 0.0627 - (1 - 0.8) / Rq * 0.0534 = 0.0493 bar
* Buhlmann calculations use the Buhlmann value, VPM-B calculations use the Schreiner value.
*/
#define WV_PRESSURE 0.0627 // water vapor pressure in bar, based on respiratory quotient Rq = 1.0 (Buhlmann value)
#define WV_PRESSURE_SCHREINER 0.0493 // water vapor pressure in bar, based on respiratory quotient Rq = 0.8 (Schreiner value)
#define DECO_STOPS_MULTIPLIER_MM 3000.0 #define DECO_STOPS_MULTIPLIER_MM 3000.0
#define NITROGEN_FRACTION 0.79 #define NITROGEN_FRACTION 0.79
@ -270,7 +291,7 @@ double he_factor(int period_in_seconds, int ci)
double calc_surface_phase(double surface_pressure, double he_pressure, double n2_pressure, double he_time_constant, double n2_time_constant) double calc_surface_phase(double surface_pressure, double he_pressure, double n2_pressure, double he_time_constant, double n2_time_constant)
{ {
double inspired_n2 = (surface_pressure - WV_PRESSURE) * NITROGEN_FRACTION; double inspired_n2 = (surface_pressure - ((in_planner() && (prefs.deco_mode == VPMB)) ? WV_PRESSURE_SCHREINER : WV_PRESSURE)) * NITROGEN_FRACTION;
if (n2_pressure > inspired_n2) if (n2_pressure > inspired_n2)
return (he_pressure / he_time_constant + (n2_pressure - inspired_n2) / n2_time_constant) / (he_pressure + n2_pressure - inspired_n2); return (he_pressure / he_time_constant + (n2_pressure - inspired_n2) / n2_time_constant) / (he_pressure + n2_pressure - inspired_n2);
@ -468,7 +489,8 @@ double add_segment(double pressure, const struct gasmix *gasmix, int period_in_s
int ci; int ci;
struct gas_pressures pressures; struct gas_pressures pressures;
fill_pressures(&pressures, pressure - WV_PRESSURE, gasmix, (double) ccpo2 / 1000.0, dive->dc.divemode); fill_pressures(&pressures, pressure - ((in_planner() && (prefs.deco_mode == VPMB)) ? WV_PRESSURE_SCHREINER : WV_PRESSURE),
gasmix, (double) ccpo2 / 1000.0, dive->dc.divemode);
if (buehlmann_config.gf_low_at_maxdepth && pressure > gf_low_pressure_this_dive) if (buehlmann_config.gf_low_at_maxdepth && pressure > gf_low_pressure_this_dive)
gf_low_pressure_this_dive = pressure; gf_low_pressure_this_dive = pressure;
@ -504,7 +526,7 @@ void clear_deco(double surface_pressure)
{ {
int ci; int ci;
for (ci = 0; ci < 16; ci++) { for (ci = 0; ci < 16; ci++) {
tissue_n2_sat[ci] = (surface_pressure - WV_PRESSURE) * N2_IN_AIR / 1000; tissue_n2_sat[ci] = (surface_pressure - ((in_planner() && (prefs.deco_mode == VPMB)) ? WV_PRESSURE_SCHREINER : WV_PRESSURE)) * N2_IN_AIR / 1000;
tissue_he_sat[ci] = 0.0; tissue_he_sat[ci] = 0.0;
max_n2_crushing_pressure[ci] = 0.0; max_n2_crushing_pressure[ci] = 0.0;
max_he_crushing_pressure[ci] = 0.0; max_he_crushing_pressure[ci] = 0.0;