Triple point of carbon dioxide: using a new calibration point of the International Temperature Scale ITS-90

Abstract

One of the important tasks of improving the International Temperature Scale ITS-90 is considered – the substitution of the fixed point of mercury (Hg) in connection with the ratification of the Minamata Convention and restrictions on the extraction, use and transportation of mercury. Reported results of experimental calibration of long-stem standard platinum resistance thermometers using a new calibration point from the list of secondary fixed points of the International Temperature Scale ITS-90. A method has been developed to realize the triple point of carbon dioxide (CO2) using liquid nitrogen. The method reduced the preparation time of the cell from 60 to 30 minutes and increased the duration of melting plateau from 10 to 40 hours. Presented results of two calibrations cases: using the fixed-point of Hg; using the triple point of CO2 instead of fixed-point Hg. Described methods to evaluate propagation of calibration uncertainty based on the fixed-point sensitivity coefficients. The fixed-points sensitivity coefficients for two subranges of standard platinum resistance thermometers calibration are analyzed and it is shown that the calibration uncertainty using the triple point of CO2 is reduced by half in the range of –189.3442...+ 0.01 °C and by 1.5 times in the range of –60...+ 60 °C. The results confirm the possibility and prospects of using the CO2 triple point as a new calibration point to substitute the fixed-point Hg.