International Practical Temperature Scale of 1968 IPTS

Approximately every twenty years, the international temperature scale is updated to incorporate the most recent measurements of the equilibrium thermodynamic temperature of the fixed points, to revise the interpolation equations, or to change the specifications of the interpolating measuring devices. The latest of these scales is the international temperature scale of 1990 (ITS-90). It supersedes the earlier international practical temperature scale of 1968 (IPTS-68), along with an interim scale (EPT-76). These temperature scales replaced earlier versions (ITS-48 and ITS-27). 

The International Practical Temperature Scale of 1968 (IPTS-68) is currently the internationally accepted method of measuring temperature reproducibly. A standard platinum resistance thermometer is the transfer medium that is used over most of the range of practical thermometry. 

The establishing, or fixing, of points for temperature scales is done so that anyone, anywhere can replicate a specific temperature to create or verify a thermometer. The specific temperature points become (in effect) the International Prototypes for heat. The General Conference of Weights and Measures accepted the new International Practical Temperature Scale of 1968 (IPTS 1968) with 13 fixed points (see Table 2.29). The new (IPTS 1968) scale was a revision from the IPTS 1948 (which had been amended in 1960). 

The International Practical Temperature Scale of 1968 (IPTS-68) has been replaced by the International Temperature Scale of 1990 (ITS-90). The ITS-90 scale is basically arbitraiy in its definition but is intended to approximate closely the thermodynamic temperature scale. It is based on assigned values of the temperatures of a number of defining fixed points and on interpolation formulas for standard instruments (practical thermometers) that have been cahbrated at those fixed points. The fixed points of ITS-90 are given in Table 1. 

There have been three international temperature scales The International Temperature Scale of 1927, The International Temperature Scale of 1948 which was renamed the International Practical Temperature Scale of 1948 (IPTS-48) in 1960, ° and The International Practical Temperature Scale of 1968 (IPTS-68).22... 

Temperature values to be used for the currently valid thermodynamic formulas (EOS-80) of seawater (Fofonoff and Millard, 1983 Mamayev et al., 1991) need to be expressed in the International Practical Temperature Scale of 1968 (IPTS-68). Practically measured temperatures are assumed to be in IPTS-68 before January 1, 1990, and in the International Temperature Scale of 1990 (ITS-90) afterward (Mamayev et al., 1991). Temperatures required for the thermodynamic potential functions for water, ice, and seawater need to be expressed in ITS-90, see Section 20.2.1. 

A new temperature scale, the International Temperature Scale of 1990 (ITS-90), was officially adopted by the Comite International des Poids et Mesures (CIPM), meeting 26—28 September 1989 at the Bureau International des Poids et Mesures (BIPM). The ITS-90 was recommended to the CIPM for its adoption following the completion of the final details of the new scale by the Comite Consultatif de Thermometrie (CCT), meeting 12—14 September 1989 at the BIPM in its 17th Session. The ITS-90 became the official international temperature scale on 1 January 1990. The ITS-90 supersedes the present scales, the International Practical Temperature Scale of 1968 (IPTS-68) and the 1976 Provisional 0.5 to 30 K Temperature Scale (EPT-76). 

Table 10.7 The Fixed Points of IPTS-68, the International Practical Temperature Scale of 1968 ...

ASTM technical publication 565, "Evolution of the International Practical Temperature Scale of 1968." Philadelphia, PA, 1974 "The International Temperature Scale, Amended Edition of 1975." Metrologia 7, 17 (1976). The new scale, ITS 90, initiated on Jan. 1, 1990 is described by H. Preston-Thomas, Metrologia, 27,3 (1990). See there also for the conversion of the IPTS 68 and earlier scales to the ITS 90. 

In 1954 the General Conference wanted to redefine the temperature scale using various primary points in addition to the two points of freezing and boiling water. The triple point of water (at 273.16 K) proved easy to obtain and very accurate (one part in a million). In 1960 the triple point of water and five other fixed points were accepted for an International Practical Temperature Scale. This scale was superseded in 1968 by the International Practical Temperature Scale (IPTS 1968), which added eight more fixed points. The current scale is shown in Table 2.29. 

An mtemationaUy agreed upon secondary, empirical temperature scale is described in Fig. A.8.1 It is called the International Temperature Scale of 1990, in short, the ITS 1990 (Tjo in kelvin, K tgo in degrees Celsius, °C). It makes use of the higher precision possible with secondary thermometers. The given fixed points have been agreed upon internationally to achieve as close a match to the kelvin scale as possible. The values for the fixed points are to be corrected when better absolute measurements become available. Earher international temperature scales were accepted in 1927 (ITS 27), 1948 (ITS and IPTS 48), and 1968 (IPTS) (IPTS is the abbreviation for International Practical Temperature Scale , the second version of the 1948 scale and the 1968 scale). 

The conversion of p, V, T data published earlier on to the 1968 International Practical Temperature Scale will require a painstaking review of the whole literature a method of proceeding has been discussed by Angus. In this article we mainly deal with / , K, T data that relate to differences between real- and ideal-gas properties and as these differences are generally not established to an accuracy such that changes in definition of temperatures are relevant, we avoid the problem of revision here. The larger problem of converting measured thermodynamic properties that depend on temperature from IPTS 48 to IPTS 68 has been discussed by Rossini. ... 

In 1968, an international agreement was reached about the definition of an official (practical) scale of temperature for T> 14 K. This temperature scale IPTS-68, corrected in 1975 [11], was defined by reference fixed points given by transitions of pure substances. To extend the low-temperature range of IPTS-68, the EPT 76 [12-13] gave nine reference temperatures defined by phase transition of pure substances in particular the superconductive transition (between 0.5 and 9K) of five pure metals was introduced. Moreover,... 

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