ITS-90 in practice — and what the 2019 kelvin redefinition did not change

Ask a calibration laboratory how it measures temperature and the honest answer is: against a scale, not a definition. The International Temperature Scale of 1990 (ITS-90) assigns exact temperature values to a ladder of highly reproducible fixed points — the triple point of water at 273.16 K among them — and specifies the interpolation instruments, chiefly the standard platinum resistance thermometer, that connect points on that ladder. Realize the fixed points, calibrate the SPRT at them, and every industrial thermometer downstream inherits its traceability.

In 2019 the kelvin itself was redefined by fixing the Boltzmann constant, cutting the unit loose from any particular artifact or substance. Yet ITS-90 did not go anywhere — and that is by design. The redefinition legitimized primary thermometry (measuring temperature directly through the Boltzmann constant, as acoustic gas thermometers and Johnson noise thermometers do) via the mise en pratique, but for the overwhelming majority of practical work the 1990 scale remains the working realization, because nothing beats a triple point cell for reproducibility per dollar.

The practical takeaways for a lab: your SPRT certificates and fixed-point cells are as valid as ever; the phrase 'traceable to ITS-90' remains correct and expected; and the known small differences between ITS-90 and thermodynamic temperature only matter at uncertainty levels most industrial work never approaches.