Thermal Finite Element Simulation of Dry Block Calibrator Based on Multi-fixed-Point Cells

Abstract

On-site thermometer calibration by the temperature scale transfer technique based on fixed points can effectively improve the calibration level of thermometers in large-diameter hot water pipes. Calibrations of contact thermometers in large-diameter hot water pipes are often carried out using dry block calibrators. Axial temperature gradients and the drift of reference thermometers are the main sources of error for the use of the traditional dry block calibrators. To overcome these disadvantages, a new dry block calibrator with small multi-fixed-point cells was designed in this work to improve the homogeneity of the inner temperature field and calibrate the reference thermometers in situ. The calibrator was based on small multi-fixed points including the melting point of Hg (234.28 K), the melting point of Ga (302.91 K), the melting point of H2O (273.15 K) and was developed for use in the temperature range from − 50 °C up to 100 °C. The calibrator was designed using thermal Finite Element simulations carried out with COMSOL Multiphysics and has an ectopic multi-fixed-point structure. The melting process of the multi-fixed points was simulated using the effective heat capacity method, and the fraction of the liquid phase for the fixed-point material was obtained at different phase-transition radius values and temperature plateaus of multiple fixed-points cells were obtained at different dT values. The simulation explored the influence of different heating temperatures on the temperature plateaus of multiple fixed-points cells, and the functional relationship between the heating temperature and the temperature plateaus of multiple fixed-points cells was obtained, calculating the error between temperature plateaus and thermometer at different heating power, providing a technical basis for the subsequent prototype experiments and the validation of the prototype design through experimental measurements in future work.