Study of the instability of the fixed-point cell temperature of working standards: determining the optimal time intervals between comparisons

Metal reference freeze-point cells represent an important part of the certified (working) temperature standards of the zero category. These standards are intended for reproducing the unit of temperature (kelvin) in accordance with International Temperature Scale (ITS-90), which is currently the main method for reproducing the unit of temperature in the range from −189 to 1084 °C. A transfer of the temperature unit occurs by comparing the working standard cells with the state secondary temperature standard cells. In order to develop recommendations for increasing the interval between comparisons and to modify the requirements of the state verification schedule for the temperature measurement instruments, which are imposed on the characteristics of cells that are part of the working standards of the zero category, the instability of the cell temperature in the intervals between the comparisons was studied. Such instability is one of the most important characteristics of the working standard cells. The paper summarizes the results of studying 26 fixed-point cells for tin, zinc, and aluminum, with the service life of 2 to 10 years (from 2013 to 2023), and the time interval between comparisons ranging from 2 to 5 years. Possible causes of instability of the reproducible temperature were analyzed, and uncertainties arising during the comparisons between the working standard cells and the state secondary temperature standard cells were calculated. It was established that over a long period of time (2013–2023), the temperature instability of the phase transitions, reproducible by using the cells, did not exceed 1.02 mK for tin, 1.75 mK for zinc, and 6.47 mK for aluminum. It was presumed that the most probable cause of temperature deviations between the working standard cells and the state secondary temperature standard cells was the difference in purity of the metals used. It is noted that the destruction of the quartz shell of the cell without any mechanical action is unlikely, and therefore, there is no explicit effect of pressure on the phase transition temperature. A comparison was conducted between the observed instability and uncertainty of the temperature measurements during the comparisons of the working standard cells and the state secondary temperature standard cells, as well as with the values required by the state verification schedule for temperature measuring instruments. Based on the results of the comparative analysis, it was concluded that the interval between comparisons for tin and zinc cells can be increased to five years. Such an increase in the interval has a positive economic effect due to a reduction in costs associated with operating the state secondary temperature standard and transporting cells to the comparison site. For aluminum cells, an individual approach to setting the interval between comparisons was recommended based on analyzing the results of previous comparisons. The obtained results will be useful to various metrology center specialists utilizing the working temperature standards, as well as to all scientists studying the processes occurring at the metal freezing fixed-points.