Dewar flask

A dewar is a mirror-coated, double-walled, evacuated glass or stainless steel container. It is also used in Thermos-/Isolierkannen as in special laboratory containers.

The dewar is used to the good thermal insulation of the material stored therein against the environment. In it cold or hot substances, usually liquids stored. In everyday life, we often find it in commercial jugs, for example, where coffee is kept hot.

It is named after the Scottish physicist Sir James Dewar, the vacuum vessels in 1874 used the first time and in 1893 mirrored glass vessels introduced as transport vessels for liquefied gases.

In his textbook " Physical demonstrations " also described Adolf Ferdinand Weinhold 1881 laboratory uses a vacuum jacket bottle.

Conventional Dewar vessels inside a wet chemical silver plated double wall glass, which is evacuated and - most centrally below a - abbrechempfindlichen - has meltdown glass nipples were originally turned wooden cups with Filzbettung and wooden lid almost exclusively in the laboratory in use. Later on cork ring in a cylindrical tin can. Already in the 1920s, such Thermoses arrived in Europe about the hike in use, partial ( lower half only) or completely in metal housing with cork or screw cap plus drinking cup made ​​of bakelite. Large distribution they found in 1950 as a supply while skiing and later increasingly in a plastic casing, the for thorough cleaning often can be screwed themselves. The closure is done with Gummiquetschschlauch by rotary knob or lever pivot. Own them is the risk of fracture, both by a blow to the outer glass wall, through a mixing spoon, as well as by the inertial force of the filling on the inside part, especially for a side kick.

After use in laboratory technology Dewar came from double-walled stainless steel plate about 1980 in the mountain sports trade. They are joined by arc welding in a vacuum at the discharge edge, no additional housing need more, possibly models himself a little, but are highly resistant to breakage. The mass production in the Far East lowered the price, so they already prevailed in the sports sector in 1990 against Dewar made ​​of glass. Positively affects the metals for relatively low thermal conductivity ( only relevant in the area of ​​the neck and closure ) of NiRo steel from. The closure has been greatly enhanced: In a relatively wide neck opening clamping screw sits a plastic plug with internal mechanics, whose top flush central push button locks lower on pressure, a valve head down from a fine silicone lip seal lifts and pouring allowed through an annular gap in the stopper. A second press dismisses the knob back up and closes the bottle, the ( double-walled plastic and NiRo ) drinking cups can again be screwed. That the plug must not be removed for pouring, offers the advantage of easier handling in tent, vehicle or on the mountain, he is not lost and does nothing wet and saves the contents of the bottle quite a bit of heat loss.

Operation

A dewar reduces the three possible heat transfer processes of heat conduction, radiation and convection. The heat conduction. Both by the choice of the material affected (glass has a very low thermal conductivity) as well as by the shape of the vessel Since the inner portion of the vessel is only connected over the top of the outer, heat over a relatively long distance to be transmitted, which limits the conduction of heat. The heat transfer by radiation is reduced by a silvering of the container walls. The evacuation prevents the transport of heat by convection.

Application Examples

  • Bottles and jugs with cap or lid for longer hot or cold holding drinks (coffee / iced coffee / tea / iced tea ) - vulgo Thermos Bottle ( Brand Name )
  • Hemispherical shell for ice cubes, cylindrical wine cooler for a liter bottle
  • Outdoors (no parking space ) also to protect fingers during handling before burning - cups of double-walled stainless steel
  • Storage of liquid nitrogen (see cryostat )
  • Application in a cold trap to condense as a gaseous substance by cooling
  • Self-heating test to determine the degree of rotting compost composting
  • Calorimeter
  • Storage of dry ice

Alternative to vacuum and silvering as insulation measures on vessels are: PET Bottle PU -hard foam covered in plastic housing, warm plate and cover made of plastic with foam core in hospitals, coolbox, internally coated aluminum canteen (military ) with felt case, the moistened can provide evaporative cooling, capped boxes made of polystyrene foam (possibly gray: with Alubeimischung ) for food, dry ice or medical laboratory samples, aluminum foil around the tapered, externally ribbed waffle bag of ready-made ice cream (about: Cornetto ), wrinkled multilayer aluminum foil packaging of Kebap, cooler bag with rigid foam core for camping or as a multilayer alubedampfte Platiktasche ( - bag ) for the purchase of frozen products ( alternative: wrapping with newspaper ), glass fiber and glass fabric by heating mantle for spherical flask in the chemistry laboratory, one-sided aluminized polyester film as emergency blanket and extra-light aluminum sleeping bag (1972 ), multi-layer composite films for space applications, outside polished silver- fall with the felt inside over coffee pot ( table culture around 1990 ).

250 liter Dewartanks for the transport of helium

Deutsches Museum Vacuum flask Digital Object Identifier
233915
de