gas model: split up gas compressibility into a file of its own

The gas compressibility is such a specialized thing that I really prefer
having it separate.

This keeps Robert's Redlich-Kwong equation as-is, but let's experiment
with other models soon...

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

1 files changed, 48 insertions, 0 deletions

diff --git a/subsurface-core/gas-model.c b/subsurface-core/gas-model.c
new file mode 100644
index 000000000..9cffd5e47
--- /dev/null
+++ b/subsurface-core/gas-model.c
@@ -0,0 +1,48 @@
+/* gas-model.c */
+/* gas compressibility model */
+#include <stdio.h>
+#include <stdlib.h>
+#include "dive.h"
+
+/*
+ * This gives an interative solution of hte Redlich-Kwong equation for the compressibility factor
+ * according to https://en.wikipedia.org/wiki/Redlich–Kwong_equation_of_state
+ * in terms of the reduced temperature T/T_crit and pressure p/p_crit.
+ *
+ * Iterate this three times for good results in our pressur range.
+ *
+ */
+
+double redlich_kwong_equation(double t_red, double p_red, double z_init)
+{
+	return (1.0/(1.0 - 0.08664*p_red/(t_red * z_init)) -
+		 0.42748/(sqrt(t_red * t_red * t_red) * ((t_red*z_init/p_red + 0.08664))));
+}
+
+/*
+ * At high pressures air becomes less compressible, and
+ * does not follow the ideal gas law any more.
+ */
+#define STANDARD_TEMPERATURE 293.0
+
+double gas_compressibility_factor(struct gasmix *gas, double bar)
+{
+	/* Critical points according to https://en.wikipedia.org/wiki/Critical_point_(thermodynamics) */
+
+	double tcn2 = 126.2;
+	double tco2 = 154.6;
+	double tche = 5.19;
+
+	double pcn2 = 33.9;
+	double pco2 = 50.5;
+	double pche = 2.27;
+
+	double tc, pc;
+
+	tc = (tco2 * get_o2(gas) + tche * get_he(gas) + tcn2 * (1000 - get_o2(gas) - get_he(gas))) / 1000.0;
+	pc = (pco2 * get_o2(gas) + pche * get_he(gas) + pcn2 * (1000 - get_o2(gas) - get_he(gas))) / 1000.0;
+
+	return (redlich_kwong_equation(STANDARD_TEMPERATURE/tc, bar/pc,
+				       redlich_kwong_equation(STANDARD_TEMPERATURE/tc, bar/pc,
+							      redlich_kwong_equation(STANDARD_TEMPERATURE/tc, bar/pc,1.0))));
+}