Realization of the triple point of carbon dioxide evaluated by the ITS–90

Abstract

We performed a precise temperature measurement of the triple point of carbon dioxide (CO2), which is one of the secondary reference points of the International Temperature Scale of 1990 (ITS–90), using an adiabatic calorimeter with five capsule-type standard platinum resistance thermometers (SPRTs). Impurity analyses showed that the purity of the CO2 sample was 99.999 37%. A thermal treatment process before the realization of the triple point reduced the width of the melting curve of the triple point of CO2 and increased the linearity of the melting curves with respect to the inverse of the melting fraction F. We determined the triple-point temperature of CO2 at the liquidus point by extrapolating the six melting curves obtained after the thermal treatment process. The triple-point temperature of CO2 measured in this work is 216.590 90 K 0.36 mK (k = 1). This result indicates that the triple point of CO2 can potentially be used as a fixed point based on the ITS–90 to replace the triple point of mercury.

Towards Realising the Redefined Kelvin

“…The three main candidates for replacing the mercury point (-38.8344 • C) are the triple points of Xe (~-111.744 • C) [49], CO 2 (~-56.558 • C) [19] and SF 6 (~-49.595 • C) [50]. Investigations so far have mainly been for capsule SPRTs, which is a significant limitation as nearly all commercial calibrations are for long-stem SPRTs.…”

“…Scale non-uniqueness will be investigated, with the objective of reducing its uncertainty by 30 % [16]. A possible replacement fixed-point for the mercury triple point will be identified, constructed and tested and the issue of integration within the ITS-90 will be addressed [17][18][19]. 4) To reduce the uncertainty in different gas-based primary thermometry methods, approved for use in the MeP-K-19, and so extend their applicability for temperature realisation and dissemination into the temperature region above 25 K. This will be facilitated through reducing the uncertainties of the measured and calculated thermophysical properties of gases (e.g.…”

ITS-90 non-uniqueness and evaluation of characteristics of new 1000 Ω type platinum resistance thermometers for low-temperature measurement

“…In recent years, precise temperature determination of secondary reference points [16][17][18][19][20][21][22], e.g. the triple points of SF 6 , CO 2 , and Xe, based on the ITS-90 becomes important to find a candidate for alternatives to the triple point of mercury to retain low-temperature scales of the ITS-90 because the use of the triple point of mercury has been restricted due to strict regulations for mercury based on the Minamata convention on mercury [23].…”

“…the triple points of SF 6 , CO 2 , and Xe, based on the ITS-90 becomes important to find a candidate for alternatives to the triple point of mercury to retain low-temperature scales of the ITS-90 because the use of the triple point of mercury has been restricted due to strict regulations for mercury based on the Minamata convention on mercury [23]. Recently, NMIJ/AIST has specifically emphasized exploration of a better alternative candidate for the triple point of mercury to maintain a temperature scale based on the ITS-90 [21,22]. Precise determination of temperatures of secondary reference points based on the ITS-90 will require elucidation of type III non-uniqueness of the CSPRTs used for investigation.…”

“…This study compared seven CSPRTs with a reference CSPRT using the comparison system to assess type III nonuniqueness at low temperatures between the temperatures of TPeH 2 and TPW. Five of them were used to estimate temperatures of the triple points of CO 2 and SF 6 [21,22].…”