Realization of the triple point of carbon dioxide at NPL

In the temperature range from 13.8033 K to 273.16 K the International Temperature Scale of 1990 (ITS-90) is defined by means of platinum resistance thermometers calibrated at defining fixed points and using specified interpolation procedures. Long-stem Standard Platinum Resistance Thermometers (L-SPRTs) used in the subrange from 83.8058 K (triple point of argon) to 273.16 K (triple point of water) must be calibrated at the triple points of argon, mercury (234.3156 K) and water. Increasing restrictions on the use of mercury may prevent its use for scientific purposes in the future. This raises the necessity of looking for an alternative fixed point to replace the hazardous mercury. The potential candidates are xenon, sulfur hexafluoride (SF6) and carbon dioxide (CO2). At NPL, a high purity carbon dioxide cell for use with SPRTs was designed and manufactured circa 1995. Recently, the cell was brought out of long-term storage and placed into a well-stirred oil bath for realization of the freezing and melting plateaus. Despite the long storage period the CO2 was still present, and during the investigation process more than ten plateaus have been realized which showed reproducibility of 0.5 mK using techniques which are readily adapted to routine calibration services. The length of the plateaus was typically about 20 hours, with a temperature range of 0.2 mK or smaller. The triple point temperature realized by the cell was found to be 216.5917 K ± 0.00070 K (coverage factor k = 2) as measured by a calibrated SPRT, which is consistent with the value measured 25 years ago. In this paper new realization techniques to improve reproducibility and reduce uncertainty are described, the determination of the triple point temperature is outlined, and the realization uncertainty is evaluated.