Thermos Information
A vacuum flask, colloquially called a thermos after a genericized ubiquitous brand, 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, or Dewar bottle, after its inventor. The first vacuum flasks for commercial use were made in 1904 when a German company, Thermos GmbH, was formed. Thermos, their trademark 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.
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Principle of operation
A vacuum flask is a bottle made of metal, glass, foam, plastic with 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 in his earliest prototype. The contents of the flask reach thermal equilibrium with the inner wall; the wall is thin, with low thermal capacity, so it exchanges little heat with the contents and hence has little effect on their temperatures. 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. Dewar's original prototype used cork; later versions used various plastics. Inevitably, most heat loss or heat gain 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.
Some vacuum flasks include a fitted cup or two, for convenience of use with drinks, especially during traveling or family picnics. All designed and combined in one.
Laboratories and industry
See also: Dewar flaskIn 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 insulation of the vacuum flask results in a very slow "boil," and thus the contents remain liquid for a long time without the need for expensive refrigeration equipment.
Electronics Industry
Vacuum flasks have been used to house standard cells, and ovenized zener diodes, along with their printed circuit board, in precision voltage regulating devices used as electrical standards. The flask helped with controlling the zener temperature over a long time span, and was used to control the output voltage of the zener standard, due to temperature fluctuation, to within a few parts per million.
One notable use was by Guildline Instruments, of Canada, in their "Transvolt", model 9154B, saturated standard cell, which is an electrical voltage standard. Here, a silvered vacuum flask, that was encased in foam insulation, and using a large glass vacuum plug, held the saturated cell. The voltage output of the device was 1.018 volts, and was held to within a few parts per million.
NASA use
The principle of the vacuum flask makes it ideal for storing certain types of rocket fuels, and NASA used it extensively in the propellant tanks of the Saturn launch vehicles in the 1960s and 1970s.
Security threat
In December of 2010, airport security in Canada and the United States began subjecting insulated beverage containers to greater scrutiny, because of the possibility that they could be used to conceal explosives. A U.S. military official said that officials were trying to anticipate possible terror attacks.[1]
Modifications
Several applications rely on the use of double vacuum 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 vacuum flask include the vapor-cooled radiation shield and the vapor-cooled neck,[2] which both help to reduce evaporation from the flask.
Gallery
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Sketch of vacuum flask, from a U.S. patent, 1907.
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A large stainless steel vacuum flask of liquid nitrogen, used to supply a cryogenic freezer (for storing laboratory samples at about -197°C).
See also
References
- ^ CTV.ca News Staff (2010-12-24). "Canadian airports to check drink containers for bombs". http://www.ctv.ca/CTVNews/World/20101224/drink-containers-could-hide-explosives-tsa-101224/. Retrieved 2010-12-24.
- ^ http://www.cryogenicsociety.org/resources/cryo_central/history_of_cryogenics/
Further reading
- Burger, R., U.S. Patent 872,795, "Double walled vessel with a space for a vacuum between the walls," December 3, 1907.
- Sella, Andrea (August 2008). "Dewar's Flask". Chemistry World: 75. http://www.rsc.org/chemistryworld/Issues/2008/August/DewarsFlask.asp. Retrieved 2008-08-30. Thermos Flask
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