Comparison of Five Isotope-Corrected Water-Triple-Point Cells with the NMIA-2002 WTP Ensemble

A comparison of NMIA’s new water-triple-point (WTP) ensemble with a previously established ensemble is reported. Until 2007, the kelvin in Australia was defined as the average of an ensemble of WTP cells that were selected for stability and purity and collected over a period of several years from a variety of sources. As a result of the recent CCT-K7 comparison, a clarification of the SI definition for the kelvin was adopted, explicitly specifying the isotopic composition of the water in WTP cells. Although NMIA’s results were within the estimated uncertainties, NMIA initiated a project to acquire cells with isotope information from several manufacturers and batches to establish a new ensemble. We find that the standard deviation of the isotope-shift-corrected temperatures of five cells from three manufacturers to be 6 μK, which is significantly lower than that of the cells in the previous ensemble, which was 24 μK. The average temperature of the new ensemble is found to be approximately 107 μK higher than that of the previous ensemble. This difference is consistent with the findings of CCT-K7, which identified a group of laboratories controlling isotope effects, and is displaced 73 μK from the mean of the other laboratories.