Future traceability of practical primary thermometry and self-validating thermometry

Many new techniques for ensuring traceable temperature measurements at the point of use are being developed and some are approaching maturity. The aim of this study is to examine the formalism associated with traceability to the SI kelvin for these practical techniques, as well as to identify areas of research which should be a priority. First, the status quo of thermodynamic temperature realization and dissemination is summarized. Then the state of the art of two main types of thermometry which can potentially provide in situ traceability is discussed. These are self-validating thermometers which make use of the phase change of materials, and practical primary thermometers, examples of which are given in order of decreasing commercial readiness: relative primary radiometry, acoustic gas thermometry (AGT), Johnson noise thermometry (JNT) and Doppler broadening thermometry (DBT). It is shown that relative primary thermometry is, in general, much more likely to become a day-to-day practical reality than absolute primary thermometry, and that this has a significant bearing on what the formalism might look like regarding metrological traceability and demonstrations of equivalence. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.