Thermodynamic Temperature Measurements from the Melting Point of Gallium Down to the Triple Point of Mercury

An acoustic gas thermometry system, which introduces a one-liter quasi-spherical resonator (QSR) made of oxygen-free copper, built at the National Metrology Institute of Japan (NMIJ/AIST), has been employed to measure thermodynamic temperatures from the triple point of water down to the triple point of mercury. The measurement adopted a thermostatic bath operated down to 263.15 K and a new one that can operate down to lower temperatures. The present acoustic gas thermometry system measured the speed of sound in argon on the isothermal curves of the triple point of water, 268.15 K, 263.15 K, 253.15 K, 243.15 K and at the triple point of mercury under the pressure range from 500 kPa down to around 60 kPa. Based on the measured speed of sound, the thermodynamic temperatures at the mentioned isotherms were determined relatively from the speed of sound at the triple point of water. Using the measured thermodynamic temperature T, the difference between T and the temperature T90, based on the International Temperature Scale of 1990 (ITS-90), (T − T90), along with the associated uncertainties, u(T − T90), were determined to be − 0.4 ± 1.1 mK for 268.15 K, − 1.0 ± 0.9 mK for 263.15 K, − 1.9 ± 0.9 mK for 253.15 K, − 2.5 ± 0.9 mK for 243.15 K and − 2.7 ± 0.9 mK for 234.3156 K. The present (T − T90) values were found to be consistent in all cases within the estimated uncertainty with the currently reported values existing in overlapping temperature range.