Implementing the revised kelvin

Temperature is one of the most frequently measured physical quantities in science and industry, and many industrial processes rely on high accuracy thermometry.

It is proposed to redefine the kelvin (K), the SI unit of temperature later this year and replace one based on the triple point of water i.e. the temperature at which solid, liquid and vapour all co-exist, with one based on a fixed value of the Boltzmann constant. The Boltzmann constant relates (among other things) the average kinetic energy of a gas molecule to its temperature. Its value at the time of the redefinition will be based on prior extensive international research, including previous Euramet research projects funded by the EU to enable the kelvin redefinition.

To support the introduction of the redefined kelvin and to meet on-going user traceability requirements into the 2020s, the EMRP has funded two projects; Implementing the new kelvin and Novel techniques for traceable temperature dissemination. New direct thermodynamic temperature measurement methods have the potential to replace the ITS-90 and PLTS-2000 but require both development and evaluation. Improvements at high temperatures, for example, will be facilitated by new high temperature fixed points which have been developed for temperature realisation and dissemination above 1100 °C. Further research to support on-going user traceability continues in a follow on EMPIR project Implementing the new kelvin 2.

Thermodynamic temperature

Think about the air in the room around you. The individual air particles are all moving at a variety of speeds, they are passing each other and colliding with each other (and you and the room walls) all of the time. The thermodynamic temperature of the air represents the average kinetic energy (the energy associated with the velocity and the mass of the individual particles) of the gas particles in the room. If sun shining through a window heats the air, then the heat energy transferred by the sunlight to the gas particles causes their average speed to increase, so the kinetic energy increases, and hence their temperature increases. Thermodynamic temperature represents the average thermal energy associated with the motion of particles and is a fundamental quantity.

ITS-90 is a practical and close approximation to thermodynamic temperature based on defined temperature fixed points such as the freezing points of silver or tin. ITS-90 is used throughout the world to ensure temperature measurement is performed reliably and uniformly which is vital for a wide variety of users in industry, medicine, weather and research.