Calibration of Radiation Thermometry Fixed Points Using Au/Pt Thermocouples.

NMIA, radiation thermometers are calibrated by comparison with a number of reference radiation thermometers which are themselves calibrated using fixed-point cells on the ITS-90 temperature scale (In, Sn, Zn, Al, Ag, and Au). The suitability of NMIA fixed-point cells used for standard platinum resistance thermometers (SPRTs) is evaluated by the comparison of ensembles of cells at each fixed point, and by participation in the international BIPM Key-Comparisons K3 and K4. However, the NMIA fixed-point cells used for radiation thermometry are typically much smaller (only 110 mm in length) and the thermowell length immersed in the metal much shorter (85 mm) than those used for SPRTs. Further, the insulation at the front of the crucible needs to accommodate the F/10 viewing cone of the radiation thermometers, so significant temperature gradients exist near the top of the crucible. As a consequence, the conduction errors obtained using SPRTs are too large to be of practical use. A convenient methodology based on the use of a Au/Pt thermocouple, together with a protective tube assembly to reduce conduction errors, has been developed. This allows the convenient measurement of the phase transition temperature traceable, at the 30 mK level, to the fixed-point cells used at NMIA to realize and maintain the ITS-90 scale. As the measurements are made in situ, the temperature environment, and hence the geometry and formation of the liquid-solid interface during melting and freezing, are similar to that occurring when used with radiation thermometers. Results are presented for ITS-90 fixed points up to Ag, establishing formal traceability of radiation thermometry fixed-point cells to NMIA's primary ITS-90 cells. [ABSTRACT FROM AUTHOR]