A Proposal to Update the International Temperature Scale

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. In 1990, reference functions were introduced both above and below the triple point of water, and the addition of overlapping sub-ranges increased the flexibility of realization. Over the 22 years since its introduction, 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 in a one-step process, through an evolutionary change that maintains the familiar mathematical structure of the ITS-90, by updating the coefficients of the reference functions and the temperatures of the defining fixed points. This route to updating the ITS has relatively modest requirements for implementation. 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.