/* 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.
*
*/
static 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
static double redlich_kwong_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))));
}
/*
* Generic quintic polynomial
*/
static double quintic(double bar, const double coefficient[])
{
double x0 = 1.0,
x1 = bar,
x2 = x1*x1,
x3 = x2*x1,
x4 = x2*x2,
x5 = x2*x3;
return x0 * coefficient[0] +
x1 * coefficient[1] +
x2 * coefficient[2] +
x3 * coefficient[3] +
x4 * coefficient[4] +
x5 * coefficient[5];
}
/*
* These are the quintic coefficients by Lubomir I. Ivanov that have
* been optimized for the least-square error to the air
* compressibility factor table (at 300K) taken from Wikipedia:
*
* bar z_factor
* --- ------
* 1: 0.9999
* 5: 0.9987
* 10: 0.9974
* 20: 0.9950
* 40: 0.9917
* 60: 0.9901
* 80: 0.9903
* 100: 0.9930
* 150: 1.0074
* 200: 1.0326
* 250: 1.0669
* 300: 1.1089
* 400: 1.2073
* 500: 1.3163
*/
static const double air_coefficients[6] = {
+1.0002556612420115,
-0.0003115084635183305,
+0.00000227808965401253,
+1.91596422989e-9,
-8.78421542e-12,
+6.77746e-15
};
/*
* Quintic least-square coefficients for O2/N2/He based on tables at
*
* http://ww.baue.org/library/zfactor_table.php
*
* converted to bar and also done by Lubomir.
*/
static const double o2_coefficients[6] = {
+1.0002231211532653,
-0.0007471497056767194,
+0.00000200444854807816,
+2.91501995188e-9,
-4.48294663e-12,
-6.11529e-15
};
static const double n2_coefficients[6] = {
+1.0001898816185364,
-0.00030793319362077315,
+0.00000327557417347714,
-1.93872574476e-9,
-2.7732353e-12,
-2.8921e-16
};
static const double he_coefficients[6] = {
+0.9998700261301693,
+0.0005452130351730479,
-2.7853712233619e-7,
+5.5935404211e-10,
-1.35114572e-12,
+2.00476e-15
};
static double air_compressibility_factor(double bar) { return quintic(bar, air_coefficients); }
static double o2_compressibility_factor(double bar) { return quintic(bar, o2_coefficients); }
static double n2_compressibility_factor(double bar) { return quintic(bar, n2_coefficients); }
static double he_compressibility_factor(double bar) { return quintic(bar, he_coefficients); }
/*
* We end up using specialized functions for known gases, because
* we have special tables for them.
*
* For air we use our known-good table. For other mixes we use a
* linear interpolation of the Z factors of the individual gases.
*/
double gas_compressibility_factor(struct gasmix *gas, double bar)
{
double o2, n2, he; // Z factors
double of, nf, hf; // gas fractions ("partial pressures")
if (gasmix_is_air(gas))
return air_compressibility_factor(bar);
o2 = o2_compressibility_factor(bar);
n2 = n2_compressibility_factor(bar);
he = he_compressibility_factor(bar);
of = gas->o2.permille / 1000.0;
hf = gas->he.permille / 1000.0;
nf = 1.0 - of - nf;
return o2*of + n2*nf + he*hf;
}
