The anatomy of a stamp
Every DOT-spec compressed-gas cylinder carries a set of permanent markings that tell you what it is, who made it, when, and what's been done to it since. The markings are stamped into the metal at manufacture and added to over the cylinder's service life. The rules live in 49 CFR §178.35 for general marking requirements and in each individual spec section (§178.36 through §178.71) for spec-specific details.
On a typical seamless steel or aluminum cylinder you'll find the markings in two places: the upper shoulder area near the valve (manufacturer details, spec, service pressure) and the body or shoulder circumference (subsequent requalification stamps). Composite cylinders may carry markings on a label rather than stamped into the structure.
DOT specification
The first marking is the cylinder's DOT specification, which tells you how the cylinder was built, what materials, and what service pressures it supports. Common specs you'll see:
| Spec | Construction | Common use |
|---|---|---|
| 3A | Seamless steel, single-piece drawn | Industrial gas, older inventory |
| 3AA | Seamless heat-treated alloy steel | Industrial gas, breathing air, the most common high-pressure spec |
| 3AL | Seamless aluminum alloy | SCUBA, SCBA, medical oxygen |
| 3HT | Seamless steel, higher service pressure than 3AA | SCBA, aircraft service |
| 3T | Tube trailer cylinders | Bulk gas transport |
| 4B / 4BA / 4BW | Welded steel, lower service pressure | Propane, refrigerants, low-pressure gases |
| SP-XXXXX | Special Permit cylinder, custom construction or service rules | Whatever the SP authorizes |
An SP cylinder is marked with both its underlying spec reference (often a 3-series number) and the SP number. The SP itself defines the requalification rules, not the underlying spec.
Service pressure
Right after the spec is the service pressure in psi. "3AA 2400" means a 3AA cylinder with a marked service pressure of 2400 psi. The service pressure is the maximum the cylinder is rated to be filled to under normal conditions (without the plus mark, which authorizes 10% above).
Some common service-pressure values you'll see: 2015 psi (medical oxygen E-tank), 2216 psi (SCUBA aluminum), 3000 psi (high-pressure SCUBA, SCBA aluminum), 4500 psi (high-pressure SCBA steel/3AA), and 240 psi or 300 psi (low-pressure 4B propane). The number is part of the cylinder's identity. A 3AA 2400 isn't the same cylinder as a 3AA 3000.
Manufacturer and serial
After the spec and service pressure comes the manufacturer's mark and the cylinder's serial number. The manufacturer's mark is a code registered with PHMSA that uniquely identifies the shop or factory that made the cylinder. The serial number is unique within that manufacturer's output.
Together, the manufacturer code plus the serial number is the cylinder's permanent identity. This pair never changes. It's the field your records system should anchor on, not the customer's preferred name or the asset tag stuck on the side.
Manufacture date
The original manufacture date is stamped at production and never changes. Format is typically MM-YYYY or MM-YY with a separator dot or dash. So a cylinder stamped 04-89 was manufactured in April 1989.
The manufacture date matters for two reasons. First, it sets the start of the cylinder's requalification cycle: the first hydrostatic test is due no later than the spec's first-test interval (commonly 5 years). Second, it determines whether the cylinder falls into any date-specific regulatory bucket. The big one for aluminum: 6351-T6 cylinders made roughly 1972 through 1988 require eddy current examination at requalification per §180.209(m), as discussed in the eddy current article.
Plus (+) and star (★) marks
Two marks may be added to the cylinder during requalification, both of which extend the cylinder's capability:
- Plus (+) authorizes filling to 110% of the marked service pressure, per §173.302a. Only DOT 3A and 3AA cylinders can carry the plus mark.
- Star (★) authorizes a 10-year retest interval instead of the standard 5-year cycle, per §180.209(g). Only DOT 3A and 3AA cylinders that already qualify for plus and meet the additional service requirements can carry the star.
The marks are stamped near the existing markings on the cylinder shoulder, applied at requalification by the RIN holder who tested it. They are removed (typically by stamping over with an "X" or by re-marking per §180.213) when a subsequent requalification finds the cylinder no longer meets the qualifying criteria. Full coverage in the plus and star marks article.
Requalification stamps
Each time a cylinder passes requalification, the RIN holder adds a new stamp showing:
- The requalifier's RIN (a letter and four digits, e.g. M791)
- The requalification date, typically MM-YY format
- Plus and/or star marks if earned at this requalification
Stamps accumulate over the cylinder's service life. A 30-year-old SCBA cylinder may have five or six requalification stamps stamped sequentially around the shoulder. The most recent stamp tells you when the cylinder was last tested and by whom. The cylinder is in service until the next requalification due date.
Missing or illegible markings
Markings that have worn off, been painted over, or are no longer legible are a problem. Per §180.205, a cylinder must carry the required markings to be requalified. A cylinder whose spec, service pressure, manufacturer code, serial number, or manufacture date is unreadable cannot be returned to service through standard requalification. The cylinder is condemned.
Missing requalification stamps are a different problem. A stamp from 15 years ago that's been buffed off paint is fine, as long as the original manufacturer markings are still legible and a current valid requalification stamp is present. If the most recent stamp is unreadable, the cylinder is treated as if it has no current requalification and must be tested before further use.
Some cylinder shops repaint cylinders aggressively, which can fill stamp grooves and make markings hard to read. The original stamp is still there, just buried under paint. Cleaning carefully (without grinding away metal) can restore legibility. If the stamp itself is worn into the metal and gone, the cylinder is gone too.
