Realization of the triple point of carbon dioxide in a transportable cell using long-stem SPRTs

Abstract

The action of the Minamata Convention requests in the near term to develop alternative fixed points to replace the mercury triple point (Hg TP) from the set of the defining points of the International Temperature Scale (ITS) of 1990. Carbon dioxide (CO2) TP is a candidate that exhibits the advantages of high chemical and physical stability and low health risk, and the disadvantage of relatively high pressure at room temperature. We report our study on the realization of the CO2 TP using long-stem standard platinum resistance thermometers (SPRTs). We present a novel design for a CO2 TP cell with thermometer immersion depth extending from 110 –200 mm with no increase in sample gas pressure. The outer dimension of the new cell is suitable for a drop-in compatible replacement of a normal cell for the Hg TP. We realized 14 plateaus of the CO2 TP using two long-stem SPRTs over a span of 12 months. The mean temperature at the CO2 TP is 216.591 30 (36) K at the melt fraction F = 0.5 and 216.591 36 (37) K at F = 1.0. Within the given uncertainties, the uncertainty arising from the transformation to ITS-90 contributes a major part of 0.29 –0.30 mK. The uncertainty formed by the pressure-head effect and the axial heat leak through the thermometer well of the new cell, the impurities of the sample gas and the inaccuracy of the electronic instruments contribute a smaller part of 0.21 –0.22 mK. The plateaus consistently exhibit small slopes of heights of less than 0.21 mK and time spans from 147 –347 h between F = 0.3 and F = 0.75. The small slopes of the plateaus demonstrate the repeatability of 0.056 and 0.059 mK, respectively, for the melt fraction F = 0.5 and 1.0, and the long-term stability of 0.031 mK.