Primary state standard of the temperature in the range from 0 to 3200 °С, GET 34-2020: practical implementation of the new definition of kelvin

The article considers the necessity of ways of modernization the Primary standard of the temperature GET 34-2007. Special attention is paid to the transition to a new definition of kelvin. Taking into account that the new definition of kelvin does not directly affect the status of the current international temperature scales ITS-90 and PLTS-2000, but there are significant advantages for measuring thermodynamic temperatures below 20 K and above ~1300 K, the main focus of the modernization of the GET 34-2007 in the range from 273.15 K to 1235 K was focused on improving the methods and means of implementing the International Temperature Scale ITS-90. As part of the modernization of the Primary standard in the range above 1235 K, a set of equipment has been created that allows the reproduction of kelvin in accordance with its new definition by two methods recommended by the Consultative Committee: the method of absolute primary radiometric thermometry and the method of relative primary radiometric thermometry. The basic principles of the implementation of these methods, composition and metrological characteristics of the Primary standard are described. The results of key comparisons of the developed standard in the range from 273.16 K to 692.477 K and the results of temperature measurements of a number of high-temperature fixed points and a comparison of the results with the published results of leading national metrological institutes are presented.

Improving fixed-point cell realization by modifying furnace heater shape

Recent changes to the SI make it possible to set up a primary temperature scale using established values for certain high-temperature fixed points. As the furnace used with the fixed points can itself have a significant impact on measurements, improving furnace temperature uniformity can help to reduce uncertainties. A thermal model was used to redesign heaters to reduce temperature gradients where the fixed-point cell is positioned in the furnace. A heater optimised for 1325 °C was compared to the standard one with a cobalt carbon high-temperature fixed-point cells, where the cell was installed in the middle, and also moved 10 mm to each end. The modified heater showed reduced melting range, improved plateau run-off and less sensitivity to fixed-point cell position. The improvements will reduce the uncertainties associated with this type of furnace.

Dissemination of thermodynamic temperature above the freezing point of silver

The mise-en-pratique for the definition of the kelvin at high temperatures will formally allow dissemination of thermodynamic temperature either directly or mediated through high-temperature fixed points (HTFPs). In this paper, these two distinct dissemination methods are evaluated, namely source-based and detector-based. This was achieved by performing two distinct dissemination trials: one based on HTFPs, the other based on absolutely calibrated radiation thermometers or filter radiometers. These trials involved six national metrology institutes in Europe in the frame of the European Metrology Research Programme joint project ‘Implementing the new kelvin’ (InK). The results have shown that both dissemination routes are possible, with similar standard uncertainties of 1–2 K, over the range 1273–2773 K, showing that, depending on the facilities available in the laboratory , it will soon be possible to disseminate thermodynamic temperatures above 1273 K to users by either of the two methods with uncertainties comparable to the current temperature scale.

Thermodynamic temperature assignment to the point of inflection of the melting curve of high-temperature fixed points

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.