Editing Vacuum flask (section)

== Design ==
[[File:Termos, Thermofix original - Nordiska museet - NMA.0097258.jpg|thumb|upright|1930s "Thermofix" vacuum flask]]
[[file:Thermal dosirak case.jpg|thumb|A low, wide opening design]]

The vacuum flask consists of two vessels, one placed within the other and joined at the neck. The gap between the two vessels is partially evacuated of air, creating a partial-[[vacuum]] which reduces heat [[heat conduction|conduction]] or [[convection]]. Heat transfer by [[thermal radiation]] may be minimized by [[silvering]] flask surfaces facing the gap but can become problematic if the flask's contents or surroundings are very hot; hence vacuum flasks usually hold contents below the boiling point of water. Most heat transfer occurs through the neck and opening of the flask, where there is no vacuum. Vacuum flasks are usually made of [[metal]], [[borosilicate glass]], [[foam]] or [[plastic]] and have their opening [[bung|stoppered]] with [[cork (material)|cork]] or polyethylene plastic. Vacuum flasks are often used as [[insulated shipping container]]s.

Extremely large or long vacuum flasks sometimes cannot fully support the inner flask from the neck alone, so additional support is provided by ''spacers'' between the interior and exterior shell. These spacers act as a thermal bridge and partially reduce the insulating properties of the flask around the area where the spacer contacts the interior surface.

Several technological applications, such as [[NMR]] and [[MRI]] machines, rely on the use of double vacuum flasks. These flasks have two vacuum sections. The inner flask contains liquid [[helium]] and the outer flask contains liquid nitrogen, with one vacuum section in between. The loss of precious helium is limited in this way.

Other improvements to the vacuum flask include the ''vapour-cooled radiation shield'' and the ''vapour-cooled neck'',<ref>{{cite web |url=http://www.cryogenicsociety.org/resources/cryo_central/history_of_cryogenics/ |title=History of Cryogenics: A Cryo Central resource from the CSA |publisher=Cryogenicsociety.org |date=2008-04-18 |access-date=2012-11-29 |archive-date=2018-03-27 |archive-url=https://web.archive.org/web/20180327202832/https://www.cryogenicsociety.org/resources/cryo_central/history_of_cryogenics/ |url-status=dead }}</ref> both of which help to reduce evaporation from the flask.