FAQs About IAPWS

IAPWS is not a funding organization. However, IAPWS does fund a few young scientists each year to work in a collaborative way involving two (or more) IAPWS member countries. The amount of money for these projects is rather small (usually around $10,000); the primary purpose is to encourage collaborative research.

In addition, IAPWS issues IAPWS Certified Research Needs (ICRNs) describing areas in which IAPWS judges research is needed. One purpose of these documents is that researchers may include them in applications for funding from other agencies to document the industrial need for the research. Current ICRNs are listed on this page.

The easiest way to get copies of IAPWS documents is to download them from the IAPWS webpage. Separate sections exist for Releases, Guidelines, and Advisory Notes, Technical Guidance Documents for power cycle chemistry, and for IAPWS Certified Research Needs.

The "ASME Steam Tables" many people are familar with is a book first published in 1967, with accompanying software in later editions. The thermodynamic properties in the 1967 ASME Steam Tables book were calculated from a formulation for industrial use known as IFC-67, which was developed and adopted as a standard by the international organization that later became IAPWS. The ASME Steam Tables was just one of many books produced from this international standard; several other countries and organizations have issued books based on IFC-67.

However, the IFC-67 formulation is now officially obsolete, having been replaced in late 1997 by a new formulation known as IAPWS-IF97. IAPWS-IF97 is now the international standard for calculations in the steam power industry. As a result, the ASME has produced a replacement for the 1967 book, titled ASME International Steam Tables for Industrial Use. This book has tables based on the new IAPWS-IF97 formulation. They have also produced software based on IAPWS-IF97. The new book and software, described on this webpage, can now officially be considered the "ASME Steam Tables," and they are based on (and fully consistent with) IAPWS standards. Steam tables books based on IAPWS-IF97, and therefore equally consistent with current IAPWS standards, have also been produced in the Czech Republic, Germany, Japan, and Russia.

Neither the ASME Steam Tables nor the IAPWS-IF97 standard should be confused with the separate standard IAPWS maintains "for general and scientific use." The distinction between this standard and the IAPWS-IF97 industrial standard is explained here.

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.

Yes, any individual can be involved in the activities of IAPWS and attend the annual meetings and Working Group sessions. It is also possible for a country (or group of closely related and/or neighboring countries) to join IAPWS; details of membership can be obtained from the Executive Secretary.

Guidelines are similar to 'Releases', but for properties where the quality or range of the available data is limited. They are therefore not expected to provide as definitive a representation as Releases. Guidelines are expected to require revision when new information is available. 

Find IAPWS Guidelines here

Releases provide carefully evaluated, internationally agreed-upon data and formulations of properties for which measurements of high quality exist over a wide range of states. They are intended to embody the state of the art for representation of the property at the time of the Release.

Find IAPWS Releases here

Cycle chemistry guidelines in the form of Technical Guidance Documents have been developed by IAPWS to provide truly international cycle chemistry guidance for fossil and combined cycle plants. They represent the accumulated experience of members of the IAPWS Power Cycle Chemistry (PCC) Working Group with expertise from 21 countries.

Find IAPWS Technical Guidance Documents here

IAPWS Certified Research Needs (ICRNs) represent the considered opinion of experts in the field that research on a particular subject is needed. They are often developed as a response to the inavailability of adequate data of a particular kind. They consist of a statement of the problem, a statement of the industrial need, a contact person, and an expiration date. No funding is available from IAPWS, but the ICRNs may be copied and attached to proposals to funding agencies. 

Find IAPWS Certified Research Needs (ICRNs) here

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.

IAPWS meets annually and holds International Conferences on the Properties of Water and Steam at roughly 5 year intervals. 

See IAPWS Meetings and Conferences information here.

Questions about the programs and policies of IAPWS should be directed to the Executive Secretary:

Dr. Daniel G. Freind
E-mail: dg.friend@iapws.org

Technical questions should be directed to the Chair of the relevant IAPWS Working Group.

Contact us here to send us your questions or comments.