A Dewar flask is a glass vessel designed to provide very good thermal insulation. For instance, when filled with a hot liquid, the vessel will not allow the heat to easily escape, and the liquid will stay hot for far longer than in a typical container. The Dewar flask was named after its inventor Scottish physicist Sir James Dewar (1842–1923).

In form, a Dewar flask is a glass or metal bottle, with a double-layer construction; in fact, it can be considered to be two thin-walled bottles nested one inside the other, and sealed together at the neck. The narrow space between is evacuated almost entirely of air; the near vacuum prevents conduction and convection of heat.

The inner surfaces of the outer bottle, and the outer surface of the inner bottle, have a metallic or similar reflective coating to prevent heat from being transmitted via radiation. This is so effective that what little heat transport does occur is almost entirely via the neck and stopper; insulating materials such as cork are often used for the stopper.

A very common use of the Dewar flask in laboratories is the storage of liquid nitrogen; in this case, the leakage of heat into the extremely cold interior of the bottle results in a slow "boiling-off" of the liquid (a pressure relief valve is provided to prevent pressure from building up). The excellent insulation of the Dewar flask results in a very slow "boil" and thus the nitrogen lasts a long time without the need for expensive refrigeration equipment.

A "Thermos bottle" (trademarked), or vacuum flask (generic) used to keep coffee or other beverages hot, is usually based on the design of a Dewar flask; since glass dewar flasks are fragile and somewhat expensive to fabricate, other constructions based on plastic or insulated metal are also popular.

See also

• Cryogenics
• Dewar tube