The International Temperature Scale of 1990 (ITS-90) down to its defined minimum temperature of 0.65 K has been realized at the National Metrology Institute of Japan (NMIJ), using a 3He vapor pressure thermometer (VPT) from 0.65 K to 3.2 K, a 4He VPT from 2.2 K to 4.2 K, an interpolating constant-volume gas thermometer (ICVGT) from 3 K to 24.5561 K and standard platinum resistance thermometers (SPRTs) above 13.8803 K. The scales realized by these different thermometers in the overlapped temperature ranges differ among each other. These differences, also known as the non-uniqueness of the ITS-90, are estimated and reported in this paper. The difference between the scales of the SPRTs and ICVGT has a maximum of about 0.6 mK at temperatures close to 15 K, though it is about half of the propagation of the calibration uncertainties of the SPRT at the fixed points. The difference between the vapor pressure scales of 4He and 3He shows temperature dependence, which is consistent with that reported previously and has a maximum of about 0.2 mK at temperatures close to 3.1 K. The differences between the ICVGT scales with different lowest calibration temperatures are also estimated. The difference between the ICVGT scales with or without corrections for the aerostatic pressure head, the dead-space volume effect and the thermomolecular pressure difference has a maximum of about 0.17 mK.

This content is only available via PDF.
You do not currently have access to this content.