Why Molecular Weight and Compressibility are Missing

A large section of the standard deals with molecular weight (Mw), and compressibility (Z). The reason we do not need them is they are used to convert the upstream pressure (P1), and the upstream temperature (T1) to density.

The formula for doing so is:

With the density in kg/m3, the pressure in kPa (absolute) and the temperature in K.

The molecular weight is in kg/kmole.

R is the universal gas constant R = 8.314 kJ/kmole K.

Z can be estimated using the Redlich-Kwong Equation of state (only Pressure, Temperature and the critical Pressure and critical temperature of the gas are required for estimating Z). If unknown, assume Z = 1.0.

Another program, zrk, performs the calculation of Z and the conversion of pressure and temperature to density for any gas when the gas critical properties are known.