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how it works

When the respective phase transformation temperature is exceeded, so in case of melting or freezing, latent heat is either consumed or produced. Thus, for several minutes, the fixed-point temperature is established inside the cell. While the sensing element of the miniature fixed-point thermometer is in close thermal contact with the fixed-point temperature, a plateau section within the thermometer signal becomes apparent. The initial point of freezing of some high-purity substances is delayed. In many cases, such kind of supercooling can amount to more than 10K. The plateau signal thus reduced will be less appropriate for recalibration than the signal generated during the melting process (Fig. 4).


Fig. 4 emf signal of a type-S miniature fixed-point thermocouple during the melting and freezing of the integrated fixed-point substance (aluminium). In particular the melting phase can easily be recognized. Before freezing, the aluminium supercooles down to 5K.


By means of special software, such signals can be analysed automatically. The main result is an intersection point of two approximated lines (g1: signal section before phase transformation; g2: plateau section). This intersection point is the calibration point (Fig. 5).


Fig. 5 Detailed signal section of Fig. 4 after analysis. The two approximated lines, g1 and g2, form a calibration point at the melting temperature of aluminium (5863µV at 660.32°C).

To optimise miniature fixed-point thermocouples, a number of FEM model calculations were carried out at the Ilmenau University of Technology. In case of a simplified thermometer structure with an integrated aluminium fixed point, a simulation of a melting process and the corresponding emf signal are shown (see Animation).

Animation To view the full size click here.