Focus on Temperature and Thermal Measurement (TEMPMEKO 2019)

Radiation thermometry based on the measurement of two fixed points, the so-called n  =  2 scheme, is studied in this work. Verification of the scheme was undertaken for various measurement conditions using seven radiation thermometers with nominal centre wavelengths of 650 nm, 890 nm, 900 nm, and 1600 nm. Comparison between the signals calculated using the n  =  2 scheme and the signals measured directly with fixed-point cells was studied. For temperatures between the two reference fixed points, the equivalent temperature difference was less than 0.14 K in the temperature range from the In to Cu fixed points, and less than 0.3 K in the temperature range from the Cu to Re–C fixed points. The maximum normalised error was 0.35, confirming the validity of the n  =  2 scheme; the consistency was observed not only in the interpolation temperature range, but also in the extrapolation range. The results also indicate that temperature realisation in the range from 150 °C to 2500 °C can be achieved by maintaining only the Zn, Cu, and Re–C fixed-point facilities. The n  =  2 scheme would be of great benefit to developing national metrology institutes in terms of time consumed for scale realisation and cost for maintenance of the measurement system.

This study determined the triple point temperature of sulfur hexafluoride (SF₆), as well as the associated uncertainty, to evaluate the potential use of this value as an alternative to the triple point of mercury as a fixed point defined in the International Temperature Scale of 1990 (ITS-90). The triple point temperature was obtained using an adiabatic calorimeter incorporating a sealed cell made from oxygen-free copper. The cell was filled with 99.9992% SF₆ and included three standard platinum resistance thermometers that had been calibrated at eight fixed points defined by the ITS-90 protocol. Applying a thermal treatment to the solidified sample prior to determining the triple point temperature was found to decrease the width of the melting curve. Based on melting curves acquired following this thermal treatment, the triple point temperature of SF₆ was found to be 223.556 47(53) K. The temperature scale constructed using SF₆ was found to be consistent with that generated using mercury within a maximum deviation of 70 K from 13.8033 K to 273.16 K.

For over a century the Type S (Pt-10%Rh) and R (Pt-13%Rh) thermocouples have been used as primary or secondary reference thermometers. However, the measurement uncertainties of both types at the silver point (~962 °C) are typically limited to about 0.5 °C, due to drift caused by crystallographic ordering between 200 °C and 500 °C and rhodium oxidation between 500 °C and 900 °C. Although both processes can be reversed using an 1100 °C anneal, regular annealing can be inconvenient or impracticable. This paper follows up a prior study indicating that an alternative noble-metal thermocouple comprised of Pt-20%Rh and Pt is relatively insensitive to both drift mechanisms. Four thermocouples, assembled using wire from four different manufacturers, were evaluated using a gradient-furnace and homogeneity scanner. Measurements were also made using a salt-bath (200 °C to 500 °C) and fixed points between the indium and silver points. The experiments indicate that individual thermocouples are stable at the silver point to within 0.18 °C for periods of up to 100 h and all four thermocouples have emf versus temperature characteristics within 0.3 °C of each other. This intrinsic stability and similarity, coupled with cost, assembly and use conditions identical to a Type R or S thermocouple make the Pt-20%Rh versus Pt thermocouple an attractive alternative. Recommended annealing procedures enabling the greatest stability are also given.

Measurements of the refractive indices of helium and argon using a quasi-spherical microwave resonator and upgraded experimental protocol are reported at the temperatures of the triple points of water and xenon. The results at the triple point of water are used to determine the compressibility of the resonating cavity, which is bounded by the copper shell of the resonator. This experimentally-determined compressibility is consistent with the literature value for copper, but with much smaller measurement uncertainty. The measured compressibility is extrapolated to the triple point of xenon, and combined with the refractive index results at that temperature to determine the thermodynamic accuracy of the International Temperature Scale of 1990 (ITS-90): mK at ITS-90 temperature K, corresponding to a xenon triple point thermodynamic temperature of K. The experimental compressibility is further extrapolated to the triple points of argon, oxygen and neon, and used to re-analyze earlier refractive index gas thermometry measurements made using the same resonator, yielding updated values with reduced uncertainties: mK at T₉₀  =  83.8058 K, mK at T₉₀  =  54.3584 K, and mK at T₉₀  =  24.5561 K. The ITS-90 thermodynamic accuracy results of the present refractive index gas thermometry study agree with those previously reported by acoustic gas thermometry and dielectric constant gas thermometry.

Equations are derived for propagating the uncertainties contributing to the blackbody corrections for the calibration of low-temperature radiation thermometers operating over a spectral range near the 8 µm–14 µm band. These equations include the full Planck treatment of spectral radiance, which is necessary at these long wavelengths, and account for the thermometer bandwidth. The influence quantities include the source emissivity, reference temperature, reflected ambient radiation, detector temperature, and spectral responsivity. Various cases are considered when using either a blackbody cavity or a flat-plate calibrator as the calibration source, and when using a contact thermometer or a radiation thermometer as the reference. Total uncertainties tend to be smallest when a radiation thermometer is used as the reference as, in this case, the influence of the source emissivity is minimised.

In the paper, an in-house W/Re thermocouple was calibrated at the eutectic fixed points of Pd-C (1492 °C), Pt-C (1738 °C) and Ru-C (1953 °C). To correct the temperature error caused by the thermal conduction of the thermocouple sheath, a linear extrapolation method was developed to determine the emf of the thermocouple based on the melting and freezing values at the eutectic fixed points realized with different offset furnace temperatures. In a simpler approach, the average of the melting and freezing values was also used. The results showed that the temperatures corresponding to the emfs derived from the linear extrapolation and the average method agreed (1–2) °C with the standard reference values of type-C thermocouple in the temperature range studied.

The determination of the differences (T–T₉₀) between the thermodynamic temperature T and the international temperature scale of 1990 (ITS-90) T₉₀, is important for an evaluation of the approximation to T by T₉₀. Such evaluations are necessary for the potential revision of ITS-90. A number of efforts have been devoted to the determination of (T–T₉₀) by the acoustic gas thermometry (AGT) using spherical or quasi-spherical resonators. We report in this paper a new study in the temperature range from 234 K to 303 K using AGT in argon with a cylindrical resonator. Piezo-electric acoustic transducers were used to measure the acoustic resonant frequencies. The resonant frequencies of the transverse magnetic microwave modes of the cavity were measured using straight probe antennas. This work further illustrates the high performance of the microwave resonant procedure to measure the thermal expansion of a cylindrical cavity at different temperatures and pressures. The (T–T₉₀) measurements, with standard uncertainties in the range from 0.5 mK to 0.8 mK, agree well with the existing data from AGT with spherical or quasi-spherical resonators. We believe that cylindrical acoustic gas thermometry, with an uncertainty comparable with that using spherical or quasi-spherical resonators but simpler mechanical assembly, has the potential for accurate measurements of (T–T₉₀) at higher temperatures.