This work investigates the effect of heating techniques on the realization of the ITS-90 fixed points above room temperature. For that purpose, LNE has constructed a new apparatus to realize the indium fixed point under adiabatic conditions using the "calorimetric" method. The adiabatic condition, in general, is established by maintaining a temperature difference between the fixed-point cell and its surroundings that is as small as possible. In this work, the indium fixed-point cell is located within thermally controlled heat shields whose walls also contain indium. Thus, the shields themselves are also indium cells. The experiments realizing the melting and freezing temperatures of indium using the calorimetric method are described. The results revealed the existence of thermal effects in the realization of the indium fixed-point cell by the conventional "continuous heat flux" method. The advantages of the "cell-withincell" technique are presented.
Laboratoire national de métrologie et d'essais has recently constructed a new device to realize the indium fixed point adiabatically. In parallel, a numerical heat transfer model has been developed as an aid to understanding its thermal behavior. This transient axially symmetric two-dimensional (2D) model simulates the melting process using the apparent specific heat method; the effects of mixing and convection within the liquid phase of indium are not taken into account. The thermal parameters, the nonuniformity of the furnace, and the thermal control of the surroundings were assessed with the aim of reducing parasitic heat exchanges. The results of the modeling are in good agreement with the measurements and clarify the parasitic heat flux observed during the phase transition. This article describes the model and the first results obtained. The model is a helpful tool in evaluating future technical improvements of the enclosure used to realize the indium fixed point.
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