Definition
A double-walled container with a vacuum between the inner and outer walls, used to store liquids at temperatures very different from the surroundings. The vacuum gap dramatically reduces heat transfer, allowing extremely cold liquids (such as liquid oxygen or liquid nitrogen) to remain in liquid form for extended periods.
Plain English
A special insulated container that keeps very cold liquids cold, or very hot liquids hot, for a long time. It works because the empty space between its two walls blocks heat from getting in or out.
Context Anchor
Seen in aircraft oxygen-system servicing and in descriptions of liquid-oxygen storage equipment.
Derivation
Named after Sir James Dewar, the Scottish chemist who invented this type of vacuum-insulated container in 1892 while studying very cold liquefied gases. A common household thermos is the everyday version of the same design.
Why Pilots Care
Proper handling of Dewar flasks containing liquid oxygen is critical to prevent rapid expansion, fire hazards, and pressure-related incidents in aviation oxygen systems.
Analogy
It works on the same principle as a household thermos flask — same idea, just built to handle much colder contents.
Intuition Check
Do not picture an ordinary bottle or small fuel flask. A Dewar flask is a vacuum-insulated container designed for very cold liquids.
Example Sentence 1
The aircraft's liquid oxygen is stored in a Dewar flask mounted in the equipment bay.
Example Sentence 2
Before flight, the crew checked the Dewar flask levels to ensure sufficient oxygen supply for high-altitude operations.