Small copper fixed-point cells of the hybrid type to be used in place of normal larger cells

Two small cells for the realization of the fixed-point of copper were constructed and investigated at INRIM. They are of the same hybrid design generally adopted for the eutectic high temperature fixed-point cells, namely a structure with a sacrificial graphite sleeve and a layer of flexible carbon-carbon composite sheet (C/C sheet). Because of the largely different design with respect to the cells normally adopted for the construction of pure metal fixed-points, they were compared and characterized with respect to the normal cells used at INRIM for the ITS-90 realisation. Two different furnaces were used to compare hybrid and normal cells. One of the hybrid cells was also used in different configurations, i.e., without the C/C sheet and with two layers of sheet. The cells were compared with different operative conditions, i.e., temperature settings of the furnaces for inducing the freeze, and repeatability and reproducibility were investigated. Freezing temperature and shape of the plateaux obtained in the different conditions were analyzed. As expected the duration of the plateaux obtained with the hybrid cells is considerably shorter than with the normal cell, but this does not affect the results in terms of freezing temperature. Measurements with the modified cell showed that the use of a double C/C sheet may improve both repeatability and reproducibility of the plateaux.