Supercooling Effect in Miniature Gallium Phase Transition Cell

Miniature gallium fixed-point cells are widely used as temperature standards and, because of their near-ambient transition temperature (29.7646 degrees C), are used in a variety of applications for the accurate in situ calibration of thermometers traceable to international system of units. A peculiarity of gallium, compared to other metal fixed-points, is its high degree of supercooling which can limit its possible applications. However, in literature, different authors observed the suppression of supercooling in case of small gallium cells. The reason for this is not completely clear. In this article, the supercooling effect was investigated in the specific case of a miniature gallium cells designed for space applications. The results show that the degree of supercooling is function of the thermal history of the cell. As the temperature of the metal, in the liquid state, increased, a linear dependence between the overheating of the sample and the supercooling effect was observed. This relationship is particularly important when the cell must be used to perform subsequent calibration cycles. An understanding of the factors that govern the supercooling, as proposed in this study, could help to develop ways to control it, in the design of calibration systems, and procedures to be used in different applications.