The ITS-90 [4,14] is realized, maintained and disseminated by NIST to provide a standard scale of temperature for use in science and industry in the United States. This scale was adopted by the International Committee of Weights and Measures (CIPM) at its meeting in September 1989, and it became the official international temperature scale on January 1, 1990. The ITS-90 supersedes the IPTS-68(75) [8] and the 1976 Provisional 0.5 K to 30 K Temperature Scale (EPT-76) [15].

        The adoption of the ITS-90 has removed several deficiencies and limitations associated with the IPTS-68. Temperatures on the ITS-90 are in closer agreement with thermodynamic values than were those of the IPTS-68 and the EPT-76. Additionally, improvements have been made in the non-uniqueness and reproducibility of the temperature scale, especially in the temperature range from t₆₈ = 630.74 °C to 1064.43 °C, where the type S thermocouple was the standard interpolating device on the IPTS-68.

        The ITS-90 extends upward from 0.65 K to the highest temperature measurable in terms of the Planck radiation law using monochromatic radiation. The increased number of temperature subranges and alternative definitions, all of equal status, allows for greater flexibility than was available with the IPTS-68. Temperatures on the ITS-90 are defined in terms of equilibrium states of pure substances (defining fixed points), interpolating instruments, and equations that relate the measured property to t₉₀. The defining fixed points and assigned values of temperature, as well as the interpolating equations that are used to define the ITS-90, are given in References 4, 14, and 16.  Table III describes the fixed points of the ITS-90.

        Temperatures on the ITS-90 may be expressed in terms of International Kelvin Temperatures, with the symbol T₉₀, or in terms of International Celsius Temperatures, with the symbol t₉₀. The units of T₉₀ and t₉₀ are the Kelvin, symbol K, and degree Celsius, symbol °C, respectively. The relation between T₉₀ and t₉₀ is:

        A given thermodynamic temperature expressed on the ITS-90 has a value that is different from that expressed on the IPTS-68, except at 0 K and 273.15 K, and at a few other points at which the temperatures on the two scales are fortuitously the same. Monograph 175 and References 4 and 9 give the differences between temperatures on the ITS-90 and the IPTS-68 as a function of t₉₀ in both tabular and mathematical form.