ACOUSTIC APPARATUS FOR THERMODYNAMIC TEMPERATURE MEASUREMENTS FROM 234 TO 400 K

摘要

A collaborative project between IEN and IMGC is being developed with the aim to compare the thermodynamic temperature as determined by means of an accurate acoustic thermometer with the temperature of the International Temperature Scale of 1990 (ITS-90) in the range from the mercury triple point to 400 K. The experimental method consists in measuring the acoustic resonances of a spherical cavity filled with argon to obtain the zero pressure limit of speed-of-sound ratios between the temperatures T_(x) of interest and the water triple point temperature. This procedure allows to determine the ratio (T_(x) / 273.16 K). The acoustically determined thermodynamic temperature is compared with the ITS-90 temperature T_(90) as measured by two standard platinum resistance thermometers embedded in the resonator poles. Before and after the comparison, the two thermometers are calibrated at the mercury and water triple points, at the gallium melting point and at the indium freezing point. The thermal expansion of the cavity is inferred from microwave resonance measurements under vacuum. The main features of the experimental apparatus which allow to obtain a high accuracy in the acoustic, microwave and ITS-90 temperature measurements are described.