Dewar Flasks

(named after J. Dewar), vessels with double walls, between which a vacuum is created (not less than 1.33 millinewtons/m² [10^-5 mm Hg]) that ensures a high thermal insulation of the substance inside the flask. The transfer of heat in Dewar flasks occurs in effect only as a result of radiation and heat conductivity along the walls. In order to diminish radiation, the vacuum casing is silver-plated. Dewar flasks are used chiefly for the long-term preservation of easily evaporating liquefied gases and in those cases when a good thermal insulation of a substance is necessary. Small flasks for laboratory purposes are made of highly durable glass, while larger flasks, used for the storage and transport of liquefied gases (such as helium), are made of metal. When the flasks are in use, the instructions for accident prevention must be strictly observed. The commonly used Thermos bottle is related to the Dewar flask.