Vacuum flask

A vacuum flask (also known as a Dewar flask, Dewar bottle or thermos) is an insulating storage vessel that slows the speed at which its contents change in temperature. It greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings by trying to be as adiabatic as possible.

Invented by James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which significantly reduces heat transfer by conduction or convection. When used to hold cold liquids, this also virtually eliminates condensation on the outside of the flask.

Vacuum flasks are used domestically to keep contents inside hot or cold for extended periods of time. They are also used for thermal cooking. Vacuum flasks are also used for many purposes in industry.

History

The vacuum flask was invented by Scottish scientist James Dewar in 1892. He created it while studying very cold temperatures, a field called cryogenics. Dewar built a flask with two layers and removed the air between them, which helped keep the contents at a steady temperature. He did not patent his invention, so others could use new materials like glass and aluminium to improve it.

Soon after, two German glassblowers, Reinhold Burger and Albert Aschenbrenner, found that this flask could keep drinks cold or hot for longer. They made a stronger version for everyday use and named it Thermos. Although Dewar tried to claim ownership, he lost a court case to them. The design was later improved by Gustav Robert Paalen, who made many different types for homes and sold them widely. The American Thermos Bottle Company began making these flasks in large numbers in Norwich, CT, making them cheaper and more available. Over time, many companies made similar products to keep drinks hot or cold, such as for coffee or camping trips.

The name "thermos" became commonly used for all vacuum flasks. As of 2023^[update], Thermos and THERMOS are still trademarks in some countries, including the United States, but the lowercase "thermos" was declared a genericized trademark in the United States in 1963.

Design

The vacuum flask is made of two containers, one inside the other and connected at the top. The space between them has very little air, which helps keep the contents hot or cold for longer. Heat can still escape through the opening at the top, so flasks are usually made from metal, special glass, foam, or plastic and closed with cork or plastic lids. They are often used to keep food or drinks warm or cool during travel.

Very large flasks sometimes need extra support for the inner container, using special pieces called spacers. These can affect how well the flask keeps things hot or cold. Some special flasks used in machines like NMR and MRI have two layers of vacuum to protect very cold liquids. Other designs also help reduce evaporation from the flask.

Research and industry

See also: Cryogenic storage dewar

In labs and factories, vacuum flasks are used to store very cold liquids, like liquid nitrogen, which helps keep things frozen for experiments. These flasks can also hold gases that are very cold, such as oxygen and nitrogen. Because they are well-insulated, the liquid inside stays cold for a long time without needing extra cooling.

Vacuum flasks are also used to keep precise electrical equipment at the right temperature. They help make sure that special tools used to measure electricity stay accurate. One famous example is a device made by Guildline Instruments in Canada, which used a vacuum flask to keep its tools working perfectly.

These flasks are even used in space! NASA used them to store rocket fuel in the 1960s and 1970s. They are also used to protect medicines, like vaccines, from getting too warm during shipping. In the oil and gas industry, they help keep sensitive tools working in very hot places.

Safety

Vacuum flasks can sometimes break apart if not handled carefully. Glass ones, in particular, might shatter without warning. Small scratches or chips can make this more likely, especially when the flask is filled quickly with very hot or very cold liquid. To help prevent this, it is important to prepare glass flasks properly before use.

When storing very cold materials, some vacuum flasks are pressurized and could burst if special safety parts are not used. Also, because the inside and outside of the flask can heat up or cool down at different speeds, this can sometimes cause the flask to break apart. Special joints are often used in some flasks to help prevent this problem.