The Formulation consists of equation sets for various regions within such parameters as:
273.15 ≤ T ≤ 1073.15 at p ≤ 100 MPa
1073.15 ≤ T ≤ 2273.15 at p ≤ 10 MPa
Figure 1 shows location of these regions in p-T diagram. Regions 1, 2, 5 are based on fundamental Gibbs energy equation g(p, T), region 3 on fundamental HelmHoltz energy equation f(ρ, T), and the saturation line is described by equation Ps(Ts) and Ts(Ps). These five equations are the main in the Formulation IF-97. Besides, some additional equations are used that allow to avoid iterative procedures. Description of that is given in [3].
Pic. 1. Regions of application the Formulation IF-97
The borders of regions are clear from fig. 1, except for that between regions 2 and 3 which is close to the isentropic curve and is represented by equations:
P = n1 + n2·T + n3·T2 (1)
or
T = n4 + ((P - n5)/n3)(1/2) (2)
Equations (1) and (2) are valid from temperature 623,15 K and pressure 16,5292 MPa to temperature 863,15 K and pressure 100 MPa.