Implementing a New Group of TPW Cells as National Standard: Impact on Calibration Services

The definition of the kelvin is based on the triple-point temperature of highly pure water having the isotopic composition of ocean water (more specifically, the isotopic composition is equivalent to that of VSMOW). Belgian national metrology realizes the triple point of water (TPW) as the mean of temperatures measured in three sealed cells. In order to take into account the isotopic composition effect on TPW temperature, the ensemble of cells was replaced in 2006. Three new cells, with isotopic analysis of the contained water, were bought from different manufacturers. The new group of cells was compared to the old TPW national realization in order to quantify the effect of moving towards a new reference. Two different standard thermometers were used in all the cells to take 10 daily measurements on two different ice mantles. The measured resistances were corrected for hydrostatic head, self-heating, and isotopic composition (when available) before calculating the difference. A difference of about 87 μK was found between the old and the new national references. This difference is transferred to customers’ thermometers and cells through calibrations, and the change has to be documented in each new calibration certificate. An additional consequence of the new ensemble cell implementation is the significant reduction in the spread of deviations of individual cells from the mean temperature. The maximum difference between two cells of the ensemble is 96 μK for the old reference cells and 46 μK for the new reference cells corrected for isotopic composition effects.