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).

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).

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).