| Abstract | Since its inception in 1927, the International Temperature Scale (ITS) has changed to meet the needs of the time. The ITS protocol specifies phase transitions with assigned temperatures (the defining fixed points), defining instruments (thermometers), and interpolating (or extrapolating) equations. Since 1927, the selection of fixed points and their assigned temperatures have changed, defining instruments have been added and deleted, and the equations have become more complex.
Since its introduction in 1990, the ITS-90 has served its user community well. However, its departure from thermodynamic temperature is more than is desirable for the most demanding applications. One approach is to continue making measurements on the ITS-90 (T₉₀), and then correct the temperatures for better accord with thermodynamic temperature (T) using the Consultative Committee for Thermometry's best estimates of (T – T₉₀). Alternatively, these shortcomings can be addressed by an evolutionary change that maintains the familiar mathematical structure of the ITS-90, but updates the coefficients of the reference functions and the temperatures of the defining fixed points. The impact on embedded instrumentation is minimal – requiring only an updating of the coefficients of the reference functions and not a complete reworking of the mathematics. |
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