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Q & A: Characteristics of Dewars

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Most recent answer: 06/15/2010
Q:
What are the characteristics of a dewar?
- Vivien
A:
Hi Vivien,

A dewar, named after Sir James Dewar, a Scottish physicist who invented it, is a double-walled container with a vacuum between the walls. The walls are made of or are coated with a highly reflective material. This container is designed to be a good thermal insulator for keeping the contents hot or cold. A thermos is an example of a dewar for keeping coffee hot, and scientists and engineers use them often to keep liquid nitrogen cold so that it can be used in Physics Van demonstrations. There's a picture of one holding liquid nitrogen on our web site .

The dewar covers all the bases in order to stop heat energy from flowing into or out of it.

1) Conduction: Dewars have two walls and they are not in contact, so there is no heat conduction (except perhaps at the lip where the walls are joined together to hold the dewar in one piece.

2) Convection: There is a vacuum between the two walls and so flowing air or other fluid cannot convey heat energy from one place to another. Cheap plastic thermoses may have double walls but with styrofoam or polyester fill or something else as an insulator. Styrofoam insulates because it keeps the air inside from flowing, transferring heat by convection. These aren't technically called Dewars, however, because they lack a vacuum.

3) Radiation: Here's the really clever part. A shiny surface reflects back all the light you shine on it (in this case, the infrared part of the spectrum is the important part). But more importantly, it also emits infrared radiation at a much lower rate than an object which is colored, say, black. The reason for this is that an object which is in equilibrium with a surrounding light radiation field emits and abosorbs radiation at the same rate. A black object absorbs everything, and so a warm black object must re-radiate the heat away. On the flip side, a shiny object absorbs almost nothing, and so it also emits almost nothing when warm. So the walls are made of shiny material so they will not radiate their energy away with infrared light.

Tom J.

(published on 10/22/2007)

Follow-Up #1: Why doesn't a Dewar flask collapse?

Q:
Why doesn't the walls of dewar collapse due to vaccum in between them?
- Indrajit
Delhi, India
A:
There is certainly atmospheric pressure that tends to squish things that are under vacuum.  However if you design a Dewar flask correctly, using straightforward engineering principles, the strength of the materials is  enough to withstand the forces.

LeeH

(published on 06/15/2010)

Follow-up on this answer.