IAPWS has two thermodynamic property formulations. Which one should I use?

What is the difference between the IAPWS-95 formulation "for general and scientific use" and the IAPWS-IF97 "industrial" formulation?

The most accurate state-of-the art formulation recommended by IAPWS for the thermodynamic properties of water is that in the "IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use." We will refer to this formulation as IAPWS-95. IAPWS-95 is, as the title suggests, recommended for most uses, especially in scientific applications.

However, the steam power industry, which has traditionally been a major focus of IAPWS, has some special needs. This industry needs a formulation that is designed for fast computation, because steam properties are often called millions of times within design calculations. They also need a formulation that will not be changed for many years (even if it falls a bit behind the state-of-the-art). The results of these calculations are integral to so many aspects of the industry that changing the formulation requires many other changes, so one only wants to make such a change every 30 years or so.

In order to meet these special needs, IAPWS has established a separate formulation for industrial use. The current "industrial" formulation is the "IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam" (known as IAPWS-IF97). This was adopted in 1997, replacing the previous formulation for industrial use (known as IFC-67) adopted in 1967.

The industrial formulation IAPWS-IF97 was actually fitted to match the IAPWS-95 formulation; the differences between the two are small for most purposes. The only places where IAPWS-IF97 is notably inferior are in the very near vicinity of the critical point and in the metastable supercooled liquid. IAPWS-95 also covers a somewhat larger range of temperatures and pressures. Whereas IAPWS-95 is a single equation, IAPWS-IF97 is divided into 5 separate regions, producing small discontinuities at the region boundaries.

For all of the reasons given above, the properties from IAPWS-95 are generally preferred, except in the specific cases where use of IAPWS-IF97 is called for. The most important such case is the steam power industry, where IAPWS-IF97 is the standard for contracting, testing, etc. Because of this, it makes sense to use IAPWS-IF97 in other calculations related to the steam power industry. The second case where IAPWS-IF97 might be preferable would be calculations where speed of computation was important, such as finite-element calculations of water or steam flows. The user would have to judge whether the savings in computer time justified the (usually small) sacrifice in accuracy.

Both of these formulations, along with books and software based on the formulations, are descibed on this page.

Updated July 13, 2000