Cryogenic storage dewar

A cryogenic storage dewar (or simply dewar) is a special kind of vacuum flask used to store very cold liquids called cryogens, such as liquid nitrogen or liquid helium. These liquids have boiling points much lower than normal room temperature, so they need special containers to stay cold.

The dewar is named after James Dewar, the inventor who created it for his own scientific work. It is widely used in fields like low-temperature physics and chemistry, where very cold temperatures are needed for experiments.

Dewars come in many sizes and shapes. They can look like open buckets, flasks with loose stoppers, or even self-pressurising tanks. No matter what they look like, all dewars have walls made from two or more layers with a high vacuum between them. This vacuum acts as a shield, keeping the cold inside and preventing the liquid from boiling away too quickly. Special care is taken in designing dewars to safely handle the gas that is released as the liquid slowly turns to gas.

Design

A cryogenic storage dewar is a special container used to keep very cold liquids, like liquid nitrogen or liquid helium. Simple dewars let the gas escape through an open top or a loose stopper. More advanced dewars keep the gas above the liquid under pressure, which helps the liquid stay cold longer. Safety valves let out extra gas to prevent pressure from getting too high. Dewars are made to stay strong and not break easily, since these cold liquids are expensive and some, like helium, are not easy to find.

Depending on the design, there are different ways to pour the liquid from a dewar. Simple ones can be tilted to pour from the neck, while others use the pressure of the gas to push the liquid up through a pipe to the neck.

Safety

Keeping very cold liquids safe is very important. Even though dewars are designed to keep things cold, the liquid will slowly turn into gas. In open-top dewars, this gas just goes into the air around it. But in sealed dewars, the gas can build up a lot of pressure, which can be dangerous. To prevent this, dewars have special pressure-relief valves that let the gas escape if the pressure gets too high.

If a dewar is left open, air can get into the very cold part and freeze, which can block the opening and cause pressure to build up. That’s why it’s important to keep dewars closed when not in use.

The gas from a dewar can also take away the oxygen in the air around it, which can be dangerous for anyone nearby. That’s why people are taught to always keep dewars in places with good airflow and to let out any extra gas before moving them in places like elevators.