Reading time: about 9 minutes · Category: Technical Articles and Training
Introduction
In laser metal cutting, the assist gas plays a central role in cutting speed, precision and quality. To supply these gases at stable pressure and high purity, a Dewar is used: a vacuum-insulated cryogenic vessel that stores liquefied gases such as liquid nitrogen, liquid oxygen and liquid argon and delivers them to the machine.
In this article, Asim Electronic explains what a Dewar is, how it works, which types exist, how a nitrogen Dewar differs from an oxygen Dewar, and which Dewar is right for a laser cutting machine.
What Is a Dewar? (Dewar Tank)
A Dewar is a double-wall cryogenic vessel whose inter-wall space is evacuated to minimize heat ingress into the liquid inside. Liquefied gases are stored at very low temperatures — about −183 °C for liquid oxygen, −186 °C for liquid argon and −196 °C for liquid nitrogen. Vacuum insulation greatly reduces evaporation, but no Dewar eliminates it completely; that is why Dewars have a safety relief valve to vent excess pressure safely.
Dewar features
- Stores nitrogen, oxygen, argon and other liquefied gases
- Supplies gas at stable pressure
- Less gas loss than constantly swapping cylinders
- Suitable for supplying gas to fiber laser cutting machines
- Liquid or gas withdrawal (depending on the model)
How Dewars Are Used with Laser Machines
On laser cutting machines, the Dewar supplies the assist gas needed for cutting:
- Cutting stainless steel with nitrogen
- Cutting iron and carbon steel with oxygen
- Cutting aluminum with nitrogen
- Stable-pressure assist gas in high-consumption workshops
Types of Dewars
Dewars are classified in two ways:
By gas
- Nitrogen Dewar (LN₂)
- Oxygen Dewar (LOX)
- Argon Dewar (LAr)
By construction
- Non-pressurized Dewar: smaller vessels, mostly for laboratories and carrying small amounts of liquid nitrogen. Boil-off gas vents freely.
- Pressurized Dewar (Cryogenic Liquid Cylinder): upright, wheeled industrial vessels with a safety relief valve, pressure gauge and liquid and gas withdrawal valves. Dewars used with laser machines are of this type.
In the Iranian market both types are called "Dewar", but to supply gas to a laser machine you need the pressurized type.
1. Nitrogen Dewar | LN₂ Dewar
A nitrogen Dewar stores liquid nitrogen at about −196 °C and is the most common Dewar type in laser workshops, because cutting stainless steel and aluminum with nitrogen uses a large volume of gas.
Nitrogen Dewar applications
- Cutting stainless steel (304/316)
- Cutting aluminum and brass
- Cutting without burning or edge oxidation
Advantages
- Clean, bright cut edge with no oxide layer
- Suitable for parts that are welded or painted after cutting
- Large gas volume without frequent cylinder changes
2. Oxygen Dewar | LOX Dewar
An oxygen Dewar stores liquid oxygen at about −183 °C. During cutting, oxygen reacts exothermically with the metal and helps melt it faster.
Oxygen Dewar applications
- Cutting iron and black (mild) steel sheet
- Cutting thick carbon steel plate
Advantages
- Can cut thicker carbon steel
- Lower pressure and gas consumption than nitrogen cutting
Safety note: vessels, valves and fittings used for oxygen must be perfectly clean, free of oil and grease, and rated for oxygen service. Do not use a vessel that has been in oxygen service for another gas without consulting a specialist.
3. Argon Dewar | LAr Dewar
Liquid argon is stored at about −186 °C. Argon is an inert gas, used mainly as a shielding gas in welding (including laser welding) and in some special cutting applications such as titanium.
Nitrogen Dewar vs. Oxygen Dewar
| Feature | Nitrogen Dewar | Oxygen Dewar |
|---|---|---|
| Gas | Liquid nitrogen | Liquid oxygen |
| Temperature | about −196 °C | about −183 °C |
| Laser application | Stainless steel and aluminum | Iron and carbon steel |
| Edge quality | Clean, oxide-free | Oxidized edge |
| Cutting pressure | High | Lower |
| Gas consumption | High | Lower |
| Reaction with metal | Inert | Reactive, exothermic |
Dewar Capacity
Non-pressurized laboratory Dewars usually have small capacities (from a few liters up to about 50 liters). Industrial pressurized Dewars used with laser machines are larger; for example, the Asim Electronic 210-liter Dewar is designed to supply gas to laser cutting workshops. Choose the capacity based on your daily gas consumption and the interval between refills.
Dewar Construction
- Stainless steel inner vessel
Holds the liquefied gas and withstands very low temperatures. - Vacuum space and insulation
Reduces heat transfer and therefore evaporation. - Outer jacket
Protects the inner vessel and insulation. - Valves and safety equipment
Liquid withdrawal valve, gas withdrawal valve, safety relief valve and pressure gauge.
Dewar Wall Thickness: The Spec You Should Always Ask About
Industrial Dewars have two walls, and they do different jobs:
- Inner vessel: holds the liquefied gas and withstands the vessel's working pressure. Its thickness is directly linked to the vessel's safety margin against pressure.
- Outer jacket: maintains the vacuum space and protects the inner vessel against impact, deformation and damage during handling.
In the Iranian market, Dewars with 304 stainless steel walls are sold in 2 + 2, 3 + 3 and 4 + 4 mm thicknesses. From the outside they look almost identical, and the difference only shows up in the specifications or by measurement — but a thinner inner vessel means a smaller safety margin against pressure and less resistance to impact and wear.
The Asim Electronic 210-liter Dewar uses 4 mm for both walls, both 304 stainless steel — the thickest common option on the market.
Questions to ask the seller before buying a Dewar
- How thick are the inner vessel and the outer jacket, each one separately?
- What material are both walls made of? (304 stainless steel or another sheet)
- What is the vessel's working pressure?
- Which brand are the safety relief valve and other valves?
- What does the warranty cover, and are spare parts available?
Wall thickness is not the only safety factor — weld quality, the condition of the vacuum insulation and a working safety relief valve matter just as much. But a seller who won't state the wall thicknesses clearly should raise questions.
Dewar vs. Gas Cylinder
A cylinder stores gas in compressed form, whereas a Dewar stores it as a liquid. Each liter of liquefied gas produces a large volume of gas when it evaporates, so one Dewar replaces many cylinders and is more economical for workshops with high gas consumption. For example, a 210-liter Dewar filled with liquid oxygen holds as much gas as about 22 oxygen cylinders of 50 liters.
How to Choose a Dewar for a Laser Machine
- ✔ Gas type: stainless steel and aluminum → nitrogen; iron → oxygen.
- ✔ Vessel working pressure: must match the gas pressure your laser machine needs and the sheet thicknesses you cut.
- ✔ Capacity: based on daily consumption and refill interval.
- ✔ Wall thickness and material: ask for the thickness of each wall separately; 4 + 4 mm 304 stainless steel is the thickest common option.
- ✔ Valve and safety equipment quality: the safety relief valve, pressure gauge and withdrawal valves are critical Dewar parts.
- ✔ Purity of the supplied gas: gas purity depends on the gas supplier, not the vessel; use high-purity nitrogen for nitrogen cutting.
- ✔ Warranty and after-sales service
Benefits of Dewars in the Laser Industry
- Lower cost than using many cylinders
- Uninterrupted gas supply
- Stable gas pressure
- Less workshop space
- More consistent cut quality
Dewar FAQ
What is a Dewar?
A Dewar is a double-wall, vacuum-insulated cryogenic vessel for storing liquefied gases such as nitrogen, oxygen and argon at very low temperatures.
What is the difference between a nitrogen Dewar and an oxygen Dewar?
The overall construction is similar, but a vessel and fittings used for oxygen must be rated for oxygen service and completely free of oil. In laser cutting, nitrogen is used for stainless steel and aluminum and oxygen for carbon steel.
Which Dewar is right for a laser cutting machine?
An industrial pressurized Dewar sized to your daily consumption, such as a 210-liter Dewar with both liquid and gas withdrawal.
How thick should Dewar walls be?
Dewars on the market come with 2 + 2, 3 + 3 and 4 + 4 mm 304 stainless steel walls. The inner vessel holds the pressure, so a thicker wall gives a larger safety margin; 4 + 4 mm is the thickest common option.
How many oxygen cylinders is one 210-liter Dewar equal to?
About 22 oxygen cylinders of 50 liters.
Is a Dewar the same as a gas cylinder?
No. A cylinder holds compressed gas, while a Dewar stores gas as a very cold liquid and holds far more gas.
Conclusion
A Dewar is a key part of the assist-gas supply for laser machines. A nitrogen Dewar gives the best edge quality when cutting stainless steel and aluminum, while an oxygen Dewar suits cutting iron and carbon steel. If edge quality matters most, nitrogen is the better choice; if cutting thick iron plate is the priority, oxygen is more suitable.