Dewar vessels are containers that use thermal insulation to keep their contents insulated from the external environment. As a result, they substantially increase the time their contents can maintain their temperature relative to the external environment. They are also commonly referred to as thermos flasks, after their first commercial name at the beginning of the 20th century. Dewar flasks consist of two containers, one inside the other, that are joined at the neck of the device. The section between the two containers is partially deprived of air, creating an almost vacuum environment, which minimises heat conduction. It is important to emphasise that the vacuum environment does not come into contact with the contents of the Dewar's goblet, but only allows its temperature to be maintained.
Functioning of Dewar vessels
The vacuum environment cannot conduct heat by conduction or convection, but only by radiation. It is therefore necessary to use a reflective coating to minimise radiation loss. This is why Dewar originally used silver. Because the thin walls of the inner section of the thermos have low heat capacity, they cannot exchange much heat with the contents. Consequently, the contents will not change their temperature substantially over time. The point where most of the heat loss is concentrated is the cap. This must therefore be made of insulating material. It was originally made of cork in the first models, whereas today it is made of plastic.
Dewar jars are also extremely common in their everyday use in the food industry. They are generally used to maintain the temperature of a beverage outside the home. They can maintain a cold drink for 24 hours, and a hot one for 8. They are, however, also used in the laboratory environment. For example, they are used as containers for substances that maintain their liquid state at particular temperatures, and which would become gaseous at room temperature. These include oxygen and nitrogen. A special feature of the Dewar vessels used in the laboratory is their management of the pressure inside the container. If an increase in temperature were to bring the material contained in the vessels to the boil, this would create an increase in pressure and a consequent risk of explosion. For this reason, Dewar's vessels are not pressurised. In addition, their cap allows the evaporating material to escape.