The thermodynamic temperature of the point of inflection of the melting transition of Re-C, Pt-C and Co-C eutectics has been determined to be 2747.84 ± 0.35 K, 2011.43 ± 0.18 K and 1597.39 ± 0.13 K, respectively, and the thermodynamic temperature of the freezing transition of Cu has been determined to be 1357.80 ± 0.08 K, where the ± symbol represents 95% coverage. These results are the best consensus estimates obtained from measurements made using various spectroradiometric primary thermometry techniques by nine different national metrology institutes. The good agreement between the institutes suggests that spectroradiometric thermometry techniques are sufficiently mature (at least in those institutes) to allow the direct realization of thermodynamic temperature above 1234 K (rather than the use of a temperature scale) and that metal-carbon eutectics can be used as high-temperature fixed points for thermodynamic temperature dissemination. The results directly support the developing mise en pratique for the definition of the kelvin to include direct measurement of thermodynamic temperature.

The past decade saw the emergence of new temperature sensors that have the potential to disrupt a century-old measurement infrastructure based on resistance thermometry. In this review we present an overview of emerging technologies that are either in the earliest stages of metrological assessment or in the earliest stages of commercial development and thus merit further consideration by the measurement community. The following emerging technologies are reviewed: Johnson noise thermometry, optical refractive-index gas thermometry, Doppler line broadening thermometry, optomechanical thermometry, fiber-coupled phosphor thermometry, fiber-optic thermometry based on Rayleigh, Brillouin and Raman scattering, fiber-Bragg-grating thermometry, Bragg-waveguide-grating thermometry, ring-resonator thermometry, and photonic-crystal-cavity thermometry. For each emerging technology, we explain the working principle, highlight the best known performance, list advantages and drawbacks of the new temperature sensor and present possibilities for future developments.

The International Committee for Weights and Measures (CIPM), at its meeting in October 2017, followed the recommendation of the Consultative Committee for Units (CCU) on the redefinition of the kilogram, ampere, kelvin and mole. For the redefinition of the kelvin, the Boltzmann constant will be fixed with the numerical value 1.380 649 × 10 ⁻²³ J K ⁻¹ . The relative standard uncertainty to be transferred to the thermodynamic temperature value of the triple point of water will be 3.7 × 10 ⁻⁷ , corresponding to an uncertainty in temperature of 0.10 mK, sufficiently low for all practical purposes. With the redefinition of the kelvin, the broad research activities of the temperature community on the determination of the Boltzmann constant have been very successfully completed. In the following, a review of the determinations of the Boltzmann constant k , important for the new definition of the kelvin and performed in the last decade, is given.