The Complete Handbook of Thermoses

Chapter 1: Understanding Thermoses

A thermos, in its most common professional form, is a double-walled container with a vacuum between the walls, designed to minimize the transfer of heat between its contents and the surrounding environment. The principle is not new - it was first commercialized by Reinhold Burger and Albert Aschenbrenner in 1904 - but the underlying physics and the practical implications of that physics remain poorly understood by most buyers and even some long-time users. To select, use, and maintain a thermos correctly, you need to understand not just what it is, but how it fights the three mechanisms of heat transfer: conduction, convection, and radiation.

Heat moves from hot to cold. If you pour hot coffee into a thermos, the coffee will attempt to heat the air inside the container, which will try to heat the inner wall, which will then attempt to heat the outer wall, which will finally heat the room. The goal of a thermos is to raise the resistance along each of those steps. The most significant barrier is the vacuum itself. A vacuum contains no particles, and without particles, heat cannot be transferred by conduction or convection directly across that space. A liquid or gas that is not there cannot carry the heat. This single principle is what separates a true thermos from an insulated mug. An insulated mug uses foam or air pockets to slow heat transfer; a thermos tries to eliminate the medium entirely.

Conduction is the transfer of heat