International Temperature Scale of 1990
The International Temperature Scale of 1990 (International Temperature Scale of 1990, ITS-90) defined temperatures in units of Kelvin and Celsius and supports the comparability and compatibility of temperature measurements. Although the Kelvin is defined by means of the absolute zero (0 K) and the triple point of water ( 273.16 K), it is cumbersome to use this definition at temperatures that are far away from the triple point of water, because appropriate procedures for determining the absolute temperature are connected extremely complex and highly uncertain.
The ITS-90 establishes specific temperatures, so-called fixed points (mostly temperatures of certain phase transitions of some substances ), starting at 0.65 K (upward ) that determines in the past with an absolute method (including noise thermometer, gas thermometer, Acoustic Thermometers, Radiometer ) were. Between these temperature values are interpolated using defined thermometer ( very rarely extrapolated ), which were previously calibrated at the fixed points. To cover the gamut, you need: helium vapor pressure thermometer, helium gas thermometer, platinum resistance thermometer and Monochromatic Radiation thermometer. In between there are several temperature ranges that overlap partially, and thus lead to an inherent ambiguity of scale ( because different methods and different fixed points probably not be exactly the same temperatures yield ).
The default values ( the fixed points ) on which the ITS-90 is based, speak only as of 1990 again, and can be refined in a future scale ITS -XX ( to be expected after 2010). Similarly, the interpolation formulas and even the thermometry method can be adapted, as well as the ITS-90 is just an evolution of its predecessor, the IPTS -68 and the EPT -76 ( low temperature range ). Just as today historical precision measurements can be converted into each other, the ITS-90 ensures traceability of temperature data in our time for the future, which (still) can not afford the absolute method.
The following table shows some of the values of the scale.
In the range between 3 K and 5 K, respectively, -270.15 ° C to -268.15 ° C no fixed points, but the vapor pressure of helium are used.