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A vacuum flask, also called a thermos, is a storage vessel which provides thermal insulation by interposing a partial vacuum between the contents and the ambient environment. The evacuated region of the partial vacuum removes material that could serve as a heat conductor or carrier, enabling the flask to keep its contents hotter or cooler than its surroundings. Vacuum flasks are commonly used as insulated shipping containers.

The vacuum flask was invented by Scottish physicist and chemist Sir James Dewar in 1892 and is sometimes referred to as a Dewar flask after its inventor. Dewar came from a small village in fife called kincardine where there is now a street named after him. The first vacuum flasks for commercial use were made in 1904 when a Germanmarker company, Thermos GmbH, was formed. Thermos, their tradename for their flasks, remains a registered trademark in some countries but was declared a genericized trademark in the U.S. in 1963 as it is colloquially synonymous with vacuum flasks in general.

Theory of operation

A practical vacuum flask is a bottle made of glass, metal, or plastic with hollow walls; the narrow region between the inner and outer wall is evacuated of air. It can also be considered to be two thin-walled bottles nested one inside the other and sealed together at their necks.

Using vacuum as an insulator avoids heat transfer by conduction or convection. Radiative heat loss can be minimized by applying a reflective coating to surfaces: Dewar used silver.

The contents of the flask reach thermal equilibrium with the inner wall; the wall is thin, with low thermal capacity, so does not exchange much heat with the contents, affecting their temperature little. At the temperatures for which vacuum flasks are used (usually below the boiling point of water), and with the use of reflective coatings, there is little infrared (radiative) transfer.

The flask must, in practice, have an opening for contents to be added and removed. A vacuum cannot be maintained at the opening; therefore, a stopper made of insulating material must be used, originally cork, later plastics. Inevitably, most heat loss takes place through this stopper.

Purpose and uses

Food and drink

Vacuum flasks are used to maintain their contents (often but not always liquid) at a temperature higher or lower than ambient temperature, while retaining the ambient pressure of approximately 1 Atmosphere (14.7 Psi). Domestically and in the food industry, they are often used to keep food and drink either cold or hot. A typical domestic vacuum flask will keep liquid cool for about 24 hours, and warm for up to 8.

Laboratory and industry

In laboratories and industry, vacuum flasks are often used to store liquids which become gaseous at well below ambient temperature, such as oxygen and 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 so that a narrow unstoppered opening, or a stoppered opening protected by a pressure relief valve, is necessary to prevent pressure from building up and shattering the flask. The excellent insulation of the Dewar flask results in a very slow "boil", and thus the contents remain liquid for a long time without the need for expensive refrigeration equipment.


Several applications rely on the use of double Dewar flasks, such as NMR and MRI machines. These flasks have two vacuum sections. The flasks contain liquid helium in the inside flask and liquid nitrogen in the outer flask, with one vacuum section in between. The loss of expensive helium is limited in this way.

Other improvements to the Dewar flask include the vapor-cooled radiation shield and the vapor-cooled neck. These both help to reduce evaporation from the flask.

Gallery of vacuum flasks

Image:Us000872795-002-Cropped.PNG|Sketch of vacuum flask, from a U.S. patent, 1907

Image:Liquid nitrogen tank.JPG|A large stainless steel Dewar flask of liquid nitrogen, used to supply a cryogenic freezer (for storing laboratory samples at about -197°C).


Further reading

  • Burger, R., , "Double walled vessel with a space for a vacuum between the walls", December 3, 1907.

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