[PATCH 2/2] Difference between Buhlmann and Schreiner water vapour pressure

[Rick Walsh]

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 at gmail.com>
---
 deco.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/deco.c b/deco.c
index c6738dc..fcaed3e 100644
--- a/deco.c
+++ b/deco.c
@@ -115,8 +115,27 @@ const double buehlmann_He_factor_expositon_one_second[] = {
 
 const double conservatism_lvls[] = { 1.0, 1.05, 1.12, 1.22, 1.35 };
 
-#define WV_PRESSURE 0.0627 // water vapor pressure in bar
-#define WV_PRESSURE_SCHREINER 0.0493 // 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 NITROGEN_FRACTION 0.79
 
-- 
2.4.3