CONVERSION OF TEMPERATURES AND THERMODYNAMIC PROPERTIES TO THE BASIS OF THE INTERNATIONAL TEMPERATURE SCALE OF 1990

Tables of temperature differences between the International Temperature Scale of 1990 (ITS-90) and earlier temperature scales (IPTS-68, EPT-76, IPTS-48, and ITS-27) are presented. These tables also contain values of the derivatives of these differences with respect to temperature. Analytical equations to reproduce the temperature difference (T90 - T68) and its first derivative are also given. This information is needed for the adjustment of thermodynamic results to the basis of the ITS-90. Thus, for the most accurate thermodynamic results, it is preferable to change the temperatures of the original work to the ITS-90 and then recalculate the thermodynamic results on this basis. However, conversion formulae based upon a Taylor expansion of the enthalpy have been derived previously by Douglas (J. Res. Natl. Bur. Stand., Sect. A 73, 451-470 (1969)). These equations are greatly simplified when the differences between the two temperature scales are small. Approximate effects resulting from the conversion from the IPTS-68 to the ITS-90 and from the IPTS-48 to the ITS-90 for existing calorimetric determinations of heat capacity, enthalpy, and entropy have been calculated with the equations of Douglas for ND4ReO4(s), BaSnF4(s), alpha-Al2O3(s), BeO.A12O3(s), BeO.3Al2O3(s), and Mo(s). The results of these calculations are given in tables which can be used to assess conveniently the approximate effects on thermodynamic properties due to the differences in these temperature scales. It is found that only the most accurate thermodynamic results require examination and possible adjustment because of a change in the temperature scale.