Dewar Flasks: The Science of Keeping Materials Cold

Have you ever wondered how a container can keep liquid nitrogen at around -196°C for hours? A Dewar flask makes this possible by minimizing heat entering the container. Invented by Scottish scientist James Dewar in 1892, it remains widely used in laboratories and industry. A Dewar flask has two walls with a vacuum between them to reduce heat transfer. Its inner surfaces may also have a reflective coating that helps keep heat out.

Depending on its design and materials, a Dewar flask can handle temperatures ranging from around -270°C to 200°C. It comes in different sizes, from 250 mL to 5 L. In this blog, we’ll explore how a Dewar flask works and why laboratories and industries use it.​

​What Is a Dewar Flask and How Does It Work?

A Dewar flask is a double-walled container that keeps its contents at a steady temperature by slowing heat transfer. Scottish scientist James Dewar developed it around 1892 while studying extremely cold liquefied gases. Also called a vacuum flask, it is a standard tool for storing cryogenic liquids such as liquid nitrogen.

The idea behind it is simple. Heat always moves from a warmer place to a cooler one, and a Dewar flask makes that trip very hard with three main parts:

• Two walls: The inner wall holds the liquid, and the outer wall faces the room.
• A vacuum gap: Air is removed from the space between the walls, leaving almost nothing to carry heat across.
• A silvered surface: A shiny silver coating on the inner surfaces reflects heat instead of letting it pass through.

The Science of Vacuum Insulation in Dewar Flasks

Air is a poor conductor of heat, but moving air can transfer heat through convection. A vacuum slows heat transfer by removing most of the air between two surfaces. In a Dewar flask, the gap between the two walls is evacuated to create a high vacuum. This greatly reduces conduction and stops convection in the gap. However, heat can still transfer by radiation, which reflective surfaces minimize.

The vacuum is not the whole story, though. The neck is the one place where the two walls join, so heat can still creep in through it. Designers keep this joint as small as possible to limit the loss. Good insulation also slows evaporation. A cryogenic liquid such as liquid nitrogen boils as it warms. The stronger the vacuum, the slower the boil-off, so that the flask can hold its contents much longer.

​Key Design Features That Make Dewar Flasks Effective

Here are several design choices that work together to make a Dewar flask effective:

• Double-walled body: Two separate walls keep the contents isolated from outside air.
• Silvered inner surface: A reflective coating bounces radiant heat back.
• Few contact points: Fewer places where the walls touch means less heat conduction.
• Gas management: A loosely fitted or vented stopper lets boiled-off gas escape, which prevents dangerous pressure buildup.
• Different configurations: Dewars come as open buckets, loose-stopper flasks, or self-pressurizing tanks.
• Size range: Capacities run from 250 mL to 5 L.

Dewar Flasks vs. Thermos (Vacuum) Flasks

A Dewar flask and a Thermos bottle work on the same principle: a vacuum between two walls slows heat transfer. In fact, “Thermos” is a brand name for the everyday vacuum flask. The difference lies in what each one is built to do.​

​

​Where Are Dewar Flasks Used in Laboratories and Industry?

Dewar flasks are best known for storing and dispensing cryogenic liquids such as liquid nitrogen, oxygen, helium, and hydrogen. In research laboratories, they flash-freeze samples and support low-temperature physics and chemistry experiments. Scientists also rely on them to study superconductivity.

Their use goes beyond the lab. In the oil and gas industry, they protect electronic components inside wireline logging tools from high temperatures. In medicine, they help keep embryos at very low temperatures during fertility treatments. Portable Dewar flasks with handles and wheels make it easier to move cryogenic liquids around factories and laboratories. The same technology also powers everyday vacuum flasks that keep drinks hot or cold for hours.

Factors Affecting Dewar Flask Performance

Not every Dewar flask performs the same way. Several factors determine how long it holds its temperature.

• Vacuum quality: If the vacuum between the walls weakens or the walls get damaged, more heat can enter the flask.
• Silvering: A smooth, intact reflective coating blocks radiant heat better.
• Wall material: Glass, borosilicate glass, and stainless steel each have their own safe temperature range.

1. Glass: -150°C to -50°C.
2. Borosilicate glass: -270°C to 150°C.
3. Stainless steel: -190°C to 200°C.
4. Cryogenic Dewar flask: -200°C to -160°C

• Neck and stopper: Heat can enter through the neck, where the two walls join. Cryogenic flasks also need a vented stopper to let gas escape safely.
• Size: Larger flasks usually retain heat or cold for longer because they have less surface area relative to their volume.
• Handling: Keep the flask upright and handle it carefully to prevent breakage, especially if it’s glass.
• Surroundings: High room temperatures and direct sunlight can affect how well the flask maintains its contents’ temperature.

Choosing the Right Dewar Flask

A Dewar flask keeps things cold in three ways: a double wall, a vacuum gap, and a silvered surface that blocks conduction, convection, and radiation. This simple design makes it necessary for storing liquids like liquid nitrogen and helium in laboratories and industry.

Need a Dewar flask for your lab or industrial project? Techinstro offers Dewar flasks from 250 mL to 5 L. We can arrange custom sizes and specifications to meet your requirements. Explore our website to view the available models or request a bulk quotation.