The determination of gas quantity, or volume is carried out under operating conditions (metering conditions). The result is an operational flow VB (TB, PB) as a function of temperature and pressure. This operational flow needs to be converted to standard conditions (TN = 0 ° C, pN = 1.01325 bar) in order to compare volumes and so that it can be used as an input for gas billing. Since the model for an ideal gas is only approximately valid for real gases at low pressures, a compressibility factor Z (T, p, xi) is introduced into the equation of state for ideal gases. The compressibility factor is mathematically approximated by a series expansion of the molar density (virial approach). The calculation of standard volume is thus given by equation 8. 3:

V(TN > Pn ) = TnPb Zn (3)

V(TB, Pb) TbPn Zb

The ratio of the compressibility factors is called the gas law deviation factor.

Two methods for calculating compressibility factors are given in G 486 including the supplementary sheets: The standard GERG-88 virial equation and the AGA8-DC92 equation of state. The former requires input parameters of p, T, HS, N, p, xCO2 and XH2, the latter the mole fractions. The AGA8 equation of state requires a full analysis by means of a process gas chromatograph.