NACH OBEN

The IAPWS-95 Formulation for Water

In 1995 the International Association for the Properties of Water and Steam (IAPWS) released a new scientific-standard equation of state called "The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use" or IAPWS-95 for short. The comprehensive article on the IAPWS-95 formulation (Wagner and Pruß [1]) contains detailed information about the following topics:

  • Necessity of a new scientific-standard equation of state for water
  • Phase equilibrium of water
  • Data used in the development of IAPWS-95
  • Details on the development of IAPWS-95
  • Numerical description of IAPWS-95 and further details
  • Uncertainty of IAPWS-95
  • Comparison of IAPWS-95 with experimental data
  • Recommendation on an improvement of the data base
  • Annex: Steam table based on IAPWS-95

The following diagrams show the estimated uncertainties of IAPWS-95 for the properties density, speed of sound, isobaric heat capacity in the homogeneous region and for the properties vapour pressure, saturated liquid- and vapour density and the liquid-vapour phase boundary.

zww_iapws95_en_p1

Uncertainty of IAPWS-95 in density, in the critical region in pressure (left diagram), and in speed of sound (right diagram).

zww_iapws95_en_p2

Uncertainty of IAPWS-95 in isobaric heat capacity (left diagram) and on the vapour-liquid phase boundary in the three properties vapour pressure ps, saturated-liquid density r' and saturated-vapour density r" (right diagram).

The three diagrams for the homogeneous phases also show the range of validity of IAPWS-95. Apart from this formal range of validity, IAPWS-95 can be reasonably extrapolated for the properties pressure, density and enthalpy up to extremely high pressures, temperatures and densities (about 80000 MPa, some thousand kelvins and about 2500 kg m3).

Software

There is an extensive software package available for the use of IAPWS-95, namely as the program FLUIDCAL in the version for the substance water, which is described in the section software of this website.

Due to the scientific relevance of IAPWS-95, the software for the calculation of thermodynamic properties of water and steam based on IAPWS-95 is available for universities free of charge.

Details on the software FLUIDCAL can be found here.

Reference

[1] Wagner, W. Pruß, A. The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use. J. Phys. Chem. Ref. Data 31 (2002), 387-535. https://doi.org/10.1063/1.1461829