The triple point of mercury appears to be assigned an incorrect temper
ature on the ITS-90. This conclusion is based on fixed-point correlati
ons and the disagreement of the a-coefficients for the deviation funct
ions associated with the calibration of platinum resistance thermomete
rs 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 th
e argon triple point temperature by 0,7 mK to 83,8051 K to preserve ac
ceptable sub-range consistency. The mercury point retains its ITS-90 a
ssigned 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.