Dissemination of thermodynamic temperature by gas thermometry below 300 K

The redefinition of the kelvin motivates the realization of thermodynamic temperature standards and their improvement up to the stage where they become sufficiently accurate, reliable and practically useful for the purpose of disseminating the unit by direct calibration of temperature sensors. We review progress milestones in the development of gas-based primary thermometry methods, assess their current state of the art and discuss the remaining challenges and the perspectives of using thermodynamic methods as an alternative to traditional dissemination based on the realization of the International Temperature Scale. We give an account of research initiatives which are underway to test the maturity of this perspective, including an international blind comparison of thermodynamic calibrations of capsule-type resistance thermometers using different methods of primary gas thermometry in the range 4 to 300 K. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.