Formulation consist of set of formulation for different areas, for next values of parameters:
273.15 ≤ T ≤ 1073.15 with p ≤ 100 MPa
1073.15 ≤ T ≤ 2273.15 with p ≤ 10 MPa
The layout of these areas in p-T diagram showed at pic. 1. Areas 1, 2, 5 based on fundamental equation for Gibbs's energy g(ð, Ò), area 3 – fundamental equation for HelmHoltz's energy f(ð, Ò), and the saturation line represented by formula Ps(Ò). This five equations are make up the group of main equations of the Formulation IF-97. Except these equations there are some additional formulas in IF-97, which allow to don't use iterative calculations. Description of them given in [3].
Pic. 1. Areas of usage the Formulation IF-97
Borders of areas are clear from pic. 1, except border between areas 2 and 3. This border represent with next formula:
P = n1 + n2·T + n3·T2 (1)
or
T = n4 + ((P - n5)/n3)(1/2) (2)
Formulations (1) and (2) are real for temperature from 623,15 Ê with pressure 16,5292 MPa to 863,15 Ê with pressure 100 MPa.