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

One of the important tasks of improving the International Temperature Scale (ITS-90) is considered—the replacement of the mercury (Hg) fixed point in connection with the ratification of the Minamata Convention and restrictions on the extraction, use, and transport of mercury. The paper presents the results of experimental calibration of the 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 implement the triple point of carbon dioxide (CO₂) using liquid nitrogen. The proposed method makes it possible to reduce the preparation time of the CO₂ triple point cell from 60 to 30 min, while extending the CO₂ melting plateau from 10 to 40 h. The results of thermometer calibrations using the Hg and CO₂ fixed points are presented. The methods for evaluating the thermometer calibration characteristics based on the calculated sensitivity coefficients are described. The sensitivity coefficients for two thermometer calibration subranges have been analyzed, showing 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 instead of the Hg fixed point. The obtained data are relevant for the manufacturers of standard platinum resistance thermometers, and for organizations conducting verifications and calibrations of such thermometers.