Welding Reference Library · Gases & Cylinder Safety

Cylinder Markings, Labels and CGA Valve Connections

How to read the DOT stamps on a gas cylinder shoulder, why the label—not the paint color—identifies the gas, and the CGA valve connection numbers for common welding gases.

Ref WG-040249 CFR 178.3549 CFR 180.20949 CFR 173.302a29 CFR 1910.253(b)(1)(ii)CGA C-7CGA V-1

Safety-critical sheet, not yet signed off by an expert reviewer. Use it to understand the topic, then take every value and decision from the governing standard, the manufacturer’s instructions and a qualified person.

What it is

Every DOT-specification cylinder is permanently stamped on the shoulder or neck with its specification, service pressure, serial number, manufacturer and test history (49 CFR Part 178). The contents are identified by a label (CGA C-7 format) and by the valve outlet connection, which CGA V-1 standardizes so that a regulator for one gas family will not fit another. OSHA requires cylinders to be legibly marked with the contents (29 CFR 1910.253(b)(1)(ii)). Cylinder color is not standardized for identification in the US and must not be relied on.

How to identify — shoulder stamps

Marking	Meaning
DOT-3AA2015 (example)	DOT specification (3AA steel) followed by service pressure in psig
DOT-3AL	Seamless aluminum cylinder
DOT-8 / DOT-8AL	Acetylene cylinder with porous filler
Serial number and owner/maker symbol	Traceability to manufacturer or owner
Month–year with inspector mark	Original test date, then each requalification date
+ (after the test date)	Authorized to be filled up to 10% over the marked service pressure (when the conditions of 49 CFR 173.302a(b) are met)
★ (star after the test date)	Qualifies for an extended requalification interval under 49 CFR 180.209(b)

CGA outlet connections for common welding gases (CGA V-1)

CGA No.	Typical gases
540	Oxygen
510	Acetylene and propane (POL style)
580	Argon, helium, nitrogen and argon-based inert mixes
320	Carbon dioxide and many CO2-containing mixes
350	Hydrogen and other flammable gases

Key points

Requalification (hydrostatic or other approved method) intervals depend on the cylinder specification and service; check the table in 49 CFR 180.209. An out-of-test cylinder may not be refilled.
Acetylene cylinders are requalified differently from high-pressure cylinders (shell and filler inspection).
Mix connections are set by the mix composition—confirm the CGA number on the valve and the label before fitting a regulator.
Never use adapters to fit a regulator to a different CGA connection.
Fuel-gas hose and regulator outlet fittings typically use left-hand threads (notched nuts); oxygen uses right-hand threads.

Common mistakes

Identifying gas by cylinder color.
Filling or accepting a cylinder past its requalification date.
Using an argon regulator on CO2 with an adapter, which can freeze and fail.

Governing standards: 49 CFR 178.35; 49 CFR 180.209; 49 CFR 173.302a; 29 CFR 1910.253(b)(1)(ii); CGA C-7; CGA V-1.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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