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How a Thermos Flask Works

Vacuum flasks and how they work : Thermos Flask technology explained

The thing about Thermos Flasks is that they seem to have an almost miraculous ability to insulate the hot or cold liquid inside against the outside environment. Heat won't leak out, or in, at least not much. Yet, when you look inside a Thermos flask, it's typically just a shiny glass inside. How can that insulate?!

First the facts: You can fill a Thermos flask with hot tea or coffee, and it stays hot. When you feel the outside of the flask, it's not hot. You can also fill a Thermos flask with something cold, such as liquid nitrogen, and it will be kept cold. The outside of the flask doesn't feel cold. How can that be?

Whatever's happening, there's a phenomenally good insulation between the inside of the Thermos flask and the outside. However, nothing has such good insulating properties. That's what's inbetween the inside layer and the outside layer: nothing. Space, like the stuff up in the sky.

Vacuum flaskHow it works: A Thermos flask has an inside which is double-glazed, but inbetween the two layers of glass there's... absolutely nothing. It's a bit like the vacuum of outer space or the vacuum inside a vacuum tube / valve. This vacuo is between the two layers of glass.

Here's a picture of a Thermos Flask glass which has had the misfortune to be smashed. It used to be possible to get replacement insides for Thermos flasks known as "fillers". The Thermos company are now (2011) claiming it's not practical to get these. However, if you wanted to, you could get replacement glass for a Thermos flask if you could source them in the Far East and get them shipped in polystyrene.

Now in the picture, you can see the glass is made up of two layers, effectively a thin inside bottle and a thin outside bottle. Vacuum flaskThe space inbetween is what provides the amazing insulation. It is empty of any air. It's got vacuum inbetween the glass.

Officially, heat leaks out of cups of tea on the table by three mechanisms: conduction, convection, and radiation. But with vacuum, there's no material to allow any conduction across the gap, and no fluid (such as air) to provide convection. As for radiation, the temperature difference isn't big, and heat-radiation tends to work best when things are shining bright like stars. Cups of tea, well they don't do that so much.

Shop ThermosA glass bottle inside a glass bottle, with vacuum in the gap between. That's how it's done. However, this won't stand up without some supports, so there are a few small spacers inbetween to keep the whole thing together. The spacers are small and don't allow much leakage.

So anyway, that's it. How a Thermos Flask works.

The same principle applies to trucks transporting liquid nitrogen, and to large flasks, but instead of glass, steel is used. It's the vacuum that provides the insulation, not the glass or steel.

Thermos TM is a trademark of the Thermos Flask company. The scientific term for a Thermos Flask is "Dewar".