The definition of the kelvin, SI base unit of temperature, is as follows :
The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is define by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10–23 when expressed in the unit J K–1, which is equal to kg m2 s–2 K–1, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
The new definition of the kelvin does not imply any particular experiment for the practical realization of a primary standard, though only four methods are currently officially recognized to be sufficiently accurate for this purpose: acoustic gas thermometry; dielectric constant (or refractive index) gas thermometry; radiometric thermometry; Johnson noise thermometry.
INRiM has so far realized: primary acoustic gas thermometers working over the temperature range between 9 K and 430 K; a primary refractive index gas thermometer for the range between 9 K e 273.16 K; radiometric thermometers for the range above 1235 K. Development of these methods to extend their working temperature range is ongoing.
The primary methods currently available for the realization of the kelvin are complicated and time-consuming making them unpractical for the direct dissemination of the unit of thermodynamic temperature. As an alternative, the National Metrological Institutes, including INRiM, continue to realize, maintain and disseminate the International Temperature Scale according to its most recent definition in 1990 (ITS-90), in close agreement with the best thermodynamic measurements available by that time. The ITS-90 provides a recipe for the realization of highly reproducible and precise temperature standards – based on a set of fixed points corresponding to equilibrium thermodynamic states of a specified set of pure substances – and defines interpolation and extrapolation methods to cover temperature ranges internal or external to these points.
At INRiM, ITS-90 is realized in the:
- Primary Contact Thermometry Lab which realizes 9 fixed points in the range from -190 °C to 962 °C
- Cryogenic thermometry Lab (which realizes 4 fixed points in the range from 4 K to 273.15 K
- Primary Standard Radiation Thermometry which realizes radiation thermometry scales at different wavelengths above 962 °C