ITS-90

from Wikipedia, the free encyclopedia

The International Temperature Scale of 1990 ( International Temperature Scale of 1990 : ITS-90 ) was established in 1989 by the International Committee for Weights and Measures ( CIPM adopted). It defines temperatures in the units Kelvin and degrees Celsius and supports the comparability and compatibility of temperature measurements. It is needed because measuring the thermodynamic temperature using a primary thermometer is very complicated and time-consuming. In contrast, the temperature determination with secondary thermometers is easier. However, these must be calibrated, which the ITS-90 enables.

construction

The ITS-90 defines from 0.65 K up to the highest measurable temperatures, the temperatures in Kelvin and in degrees Celsius, which only represent a (as good as possible) approximation of the real thermodynamic temperature. For this purpose, the ITS-90 defines special temperatures, so-called fixed points (mostly temperatures of certain phase transitions of some substances), which were determined in the past with a primary thermometer. Defined thermometers are used to interpolate (very rarely also extrapolated ) between these temperature values , which were previously calibrated at the fixed points . The scale is divided into individual areas in which a certain interpolation thermometer is prescribed. The individual temperature ranges overlap and thus lead to an inherent ambiguity in the scale, since different processes and different fixed points usually do not result in exactly the same temperatures.

To cover the whole range, you need:

The specified values ​​(the fixed points) on which the ITS-90 is based only reflect the status from 1990 and can be refined in a future ITS-XX scale. The interpolation formulas and even the thermometric procedures can also be adapted, just as the ITS-90 is only a further development of its predecessors, the IPTS-68 and the EPT-76 (low temperature range). Just as historical precision measurements can be converted into one another today, the ITS-90 ensures the traceability of temperature data in our time for the future, which absolute methods cannot (yet) achieve.

For lower temperatures, the ITS-90 is supplemented by the provisional low temperature scale from 0.9 mK to 1 K ( Provisonal Low Temperature Scale from 0.9 mK to 1 K , PLTS-2000).

Fixed points

The following table shows the fixed points on the scale.

fixed point Temperature ( K ) Temperature ( ° C )
Triple point of hydrogen 0013.8033 −259.3467
Triple point of neon 0024.5561 −248.5939
Triple point of oxygen 0054.3584 −218.7916
Triple point of argon 0083,8058 −189.3442
Triple point of mercury 0234.3156 0−38.8344
Triple point of water 0273.16 0000.01
Melting point of gallium 0302.9146 0029.7646
Melting point of indium 0429.7485 0156.5985
Melting point of tin 0505,078 0231,928
Melting point of zinc 0692.677 0419,527
Melting point of aluminum 0933,473 0660,323
Melting point of silver 1234.93 0961.78
Melting point of gold 1337.33 1064.18
Melting point of copper 1357.77 1084.62

In the range between 3 K and 5 K (−270.15 ° C to −268.15 ° C) no fixed points are used, but the vapor pressure of helium.

Web links

literature

Individual evidence

  1. a b c d Joachim Seidel, Jost Engert, Bernd Fellmuth, Joachim Fischer, Jürgen Hartmann, Jörg Hollandt, Erich Tegeler: The International Temperature Scales ITS-90 and PLTS-2000 . In: PTB-Mitteilungen . tape 117 , no. 3 , 2017, p. 16-22 .
  2. ^ A b c d e Walter Blanke: The new "International Temperature Scale of 1990" . In: Physics in Our Time . tape 22 , no. 1 , 1991, p. 13-19 .