A prospectus on direct traceability to the kelvin for point-of-use applications

The revised definition of the kelvin has created new interest in alternative paths of traceability to the SI. In particular, the development of methods for the direct realization of the kelvin is ongoing, establishing traceability without the need to go through the international-consensus temperature scale, the ITS-90. These direct realization methods are in contrast to the ITS-90 and the measurement infrastructure of reference materials and artefact thermometer calibration chains. This infrastructure has been built up over a century or more of industrialization and scientific progress, and while very robust, is not well suited to address the requirements of the most demanding of application environments. We examine how direct traceability to the kelvin can be applied to address those applications and assess the relative practical merits of various direct realization approaches. Primary thermometry methods are surveyed from the standpoint of achievable uncertainties over the range 4 K to 1800 K with examples of practical direct-realization technologies. This assessment points to a narrow application space of long-term missions, or those occurring in remote or hazardous environments, as the most compelling cases for the direct realization approach. We present two special application environments where direct realizations are well suited for point-of-use temperature measurement. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.