Definition
A U.S. Department of Transportation specification covering seamless steel high-pressure cylinders used in aviation, primarily for storing breathing oxygen on aircraft. DOT 3HT cylinders are built to a defined wall thickness, working pressure, and material standard, are stamped with the 3HT marking, and must be hydrostatically retested every three years. They have a defined service life and a maximum allowable number of pressurization cycles.
Plain English
A type of steel oxygen bottle approved for use on aircraft. It is built to a strict government standard, marked with '3HT' on the bottle, and has to be pressure-tested on a set schedule to remain legal for use.
Context Anchor
Seen stamped on aircraft oxygen cylinders and in maintenance instructions for inspecting or servicing those cylinders.
Derivation
DOT stands for the U.S. Department of Transportation, which regulates the manufacture and testing of pressure cylinders that move on public roads. The '3HT' is simply the specification number — the 'HT' indicates high-tensile steel construction, which allows a thinner, lighter wall at high working pressures. Knowing this explains why these cylinders are popular in aviation: lighter than older steel bottles, but with strict retest rules because of the thinner walls.
Why Pilots Care
Confirms the cylinder is built and tested to withstand the pressures required for reliable supplemental oxygen supply without risk of rupture.
Grounding Statement
The DOT 3HT marking tells maintenance personnel which safety rules apply to that oxygen bottle.
Intuition Check
Do not read DOT 3HT as the kind of oxygen inside the bottle. It is the specification for the bottle itself.
Example Sentence 1
The technician checked the hydrostatic test date stamped on the DOT 3HT oxygen bottle before refilling it.
Example Sentence 2
Maintenance replaced the older cylinder with a new DOT 3HT unit after the hydrostatic test date expired.