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/* 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))));
}
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