Nonflammable Liquids for Cryostats

Several very serious accidents in laboratories have resulted from the use of flammable liquids, such as volatile petroleum distillates, or toluene, to form the bath of a cryostat. In some instances explosions have occurred, and burning liquid has been thrown upon the operator. The purpose of the investigation has been to find liquids that will not burn, that have very low freezing points, and that are otherwise suitable for use as cryostat liquids. The materials tried were chlorine and bromine derivatives of methane, ethane, and ethylene, and mixtures of these derivatives. By mixing the substances in suitable proportions it is usually possible to obtain mixtures having lower freezing points than those of any of the constituents. The eutectic compositions and temperatures of mixtures of two, three, and four components have been determined, and some mixtures of five components have been investigated. Some of the mixtures became very viscous at temperatures near their freezing points, and the limits of usefulness of these mixtures were determined by their viscosities rather than their freezing points. For this reason the viscosities of some of the mixtures were measured, at various temperatures. The corrosiveness of the liquids to metals was also studied. Liquids that will not burn and are suitable for use at temperatures down to about— 150° C. are described. Apparatus is described for the determination of freezing points at low temperatures, and for the determination of viscosity at low temperatures, with special means of avoiding the presence of solid particles in the liquid.