INCONSISTENCY IN THE ITS-90 AND THE TRIPLE POINT OF MERCURY

The triple point of mercury appears to be assigned an incorrect temperature on the ITS-90. This conclusion is based on fixed-point correlations and the disagreement of the a-coefficients for the deviation functions associated with the calibration of platinum resistance thermometers above and below 273,16 K. The disagreement of the a-coefficients is particularly important as it implies a slope discontinuity in the ITS-90 at 273,16 K. The proposed remedy involves modification of the ITS-90 platinum resistance thermometer low-temperature reference function. With the revised reference function, it is also necessary to lower the argon triple point temperature by 0,7 mK to 83,8051 K to preserve acceptable sub-range consistency. The mercury point retains its ITS-90 assigned value of 234,3156 K. With this modification, the a-coefficient obtained from any sub-range calibration above 273,16 K may be used in the low-temperature deviation functions in place of a measurement at the mercury triple point.