The Immersion Profile in a Triple Point of Water Fixed Point Cell for Different SPRTs of the Same Type

摘要

The defining fixed-points of the International Temperature Scale of 1990 (ITS-90) are reference points for temperature calibration. The measured temperature inside the fixed-point cell depends on thermometer immersion, since measurements are made below the surface of the fixed-point material and additional effect of the hydrostatic pressure has to be taken into account. Also, the heat flux along the thermometer stem can affect the measured temperature. Since the measurement of the immersion profile is rather lengthy and time consuming (several hours for measurements at single temperature), the laboratories measure it for every type of SPRT calibrated, even only for particular SPRT design, but not for every thermometer under calibration. Even the small differences in manufacturing procedure can contribute to the different response of the particular SPRT of the same type to the immersion profile. Also, usage of the thermometer, measurement set up, can change its characteristics and response to the immersion profile. Thus the uncertainty budget of the thermometer due to perturbing heat exchanges is at the end different to every particular thermometer and measurement set-up, and should be determined.