Study of the instability of the fixed points cells temperature of working standards: establish the optimal time intervals between the comparisons

The cells of fixed freezing points of metals are an important component of the working standards of temperature of the 0th category. These standards are intended to reproduce the unit of temperature kelvin in accordance with ITS-90, which is currently the main method in the temperature range from –189 to 1084 °C. The transfer of the temperature unit is carried out by comparing the cells of the working standards of temperature with the cells of the state secondary standard of temperature. In order to develop recommendations for increasing the interval between comparisons and changing the requirements of the State Verification Scheme, one of the most important characteristics of the cells of the working standards of temperature was studied – the instability of the cell temperature in the interval between comparisons. The paper summarizes the results of the study of 26 cells of fixed points of tin, zinc and aluminum, the service life of which was from 2 to 10 years, and the interval between comparisons was from 2 to 5 years. Possible causes of instability of the reproducible temperature are described and the uncertainty of comparisons of working standard cells with secondary standard cells is calculated. The analysis of the results showed that the instability values over a long period of time do not exceed 1.02 mK for tin cells, 1.75 mK for zinc cells and 6.47 mK for aluminum cells. The most probable cause of deviation of the temperature of working standard cells from secondary standard cells is the different purity of the metals used. It is noted that the destruction of the quartz shell of the cell without mechanical action is unlikely, and the effect of pressure on the temperature of the reference point is absent. Based on the results of the comparative analysis, it was concluded that it is possible to increase the interval between comparisons for ampoules of tin and zinc to five years. Such an increase in the interval has a positive economic effect by reducing the cost of operating the state secondary temperature standard and transporting ampoules to the comparison site. For aluminum cells, an individual approach to setting the interval between comparisons is recommended based on the analysis of the results of previous comparisons. The materials published in this paper may be useful to specialists of metrological centers working with working temperature standards, as well as to all scientists studying the processes occurring in cells for measuring the solidification temperature of metals.