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This page describes products and general use; it is not medical advice.
Anyone receiving, storing or handling an oxygen cylinder in a clinic, hospital or lab can read the permanent markings on it and tell them apart from the paper label. Oxygen cylinder markings are two different things. The stamp is permanent metal marking that U.S. Department of Transportation (DOT) rules require on specification cylinders. The label is the hazard communication marking applied to the cylinder. Color is a third, separate matter.
Who uses this: clinic and lab staff who receive cylinders, buyers checking what a supplier delivers, and students learning how to read cylinder markings.
What the oxygen cylinder stamp and label are
Under 49 CFR 178.35(f)(4), the markings on a DOT specification cylinder must be “stamped plainly and permanently on the shoulder, top head, or neck,” and each marking must be at least 0.25 inch “or as space permits” (178.35(f)(5)). The stamp is made at manufacture and gains new dates each time the cylinder is requalified. The label is different: it is the hazard communication label under OSHA’s Hazard Communication Standard, a paper or applied marking rather than a stamp. For how label and color relate, see compressed gas cylinder color codes.
Reading a DOT stamp field by field
The regulation fixes the order of the original markings. 49 CFR 178.35(f)(1) lists them as follows.
| Field | What the rule says | Position |
|---|---|---|
| DOT specification and service pressure | “The DOT specification marking must appear first, followed immediately by the service pressure.” Example in the rule: DOT-3A1800. | First |
| Serial number | Placed “just below or immediately following” the specification marking. | After or below the specification |
| Manufacturer symbol (letters) | Placed just below, immediately before or following the serial number; the symbol must be registered with the Associate Administrator and duplications are not authorized. | Beside the serial number |
| Inspector’s mark and test date | The inspector’s official mark and date of test (such as 5-95 for May 1995) go near the serial number, placed so that dates of subsequent tests can be easily added. | Near the serial number |
The regulation’s own worked example is DOT-3A1800 1234 XY AB 5-95: specification DOT-3A, service pressure 1800, serial number 1234, then the symbol and inspector mark, then a May 1995 test date. The rule allows other sequences only when a lack of space makes them necessary, so a stamp that departs from this order is not automatically wrong.
DOT 3AA vs DOT 3AL: what the specification letters mean
For oxygen, the two specifications most often seen on shoulder stamps are DOT-3AA (steel) and DOT-3AL (aluminum). The definitions come from the cylinder specifications in 49 CFR Part 178; the intervals come from Table 1 of 49 CFR 180.209.
| Marking | Definition | Requalification interval options (180.209, Table 1) |
|---|---|---|
| DOT-3AA (178.37) | Seamless steel cylinder, nominal water capacity not over 1,000 pounds, service pressure of at least 150 psig. | 5, 10, or 12 years |
| DOT-3AL (178.46) | Seamless aluminum cylinder, maximum water capacity of 1000 pounds, minimum service pressure of 150 psig; aluminum alloy similar to 6061, 6061-T6 temper; welding or brazing prohibited. | 5 or 12 years |
The longer intervals apply only when the conditions in the rule are met. For the 3AL 12-year option, 180.209(j)(2)(i) ties it to cylinders used as fire extinguishers under 173.309(a), so it does not apply to oxygen service. Scheduling details are covered in the cylinder hydrostatic testing requirements guide.
Retest dates, the RIN square, the star and the plus sign
Requalification adds a second kind of marking. Under 49 CFR 180.213(d), a cylinder that passes requalification is marked with the requalifier identification number (RIN) “set in a square pattern, between the month and year of the requalification date.” The first character of the RIN goes in the upper left corner of the square, and the remaining characters follow clockwise. The rule’s example is a cylinder requalified in September 2006 by a person holding RIN A123: month, then the RIN square, then the two-digit year.
The rule also lets a symbol replace the “X” in that layout:
- Five-point star: replaces the “X” to show the 10-year volumetric expansion test for cylinders conforming to 180.209(b). Among the conditions in 180.209(b)(1), oxygen is on the listed gases and the cylinder must be “stamped with a five-pointed star at least one-fourth of an inch high immediately following the test date.”
- Plus sign (+): replaces the “X” to show special filling limits under 173.302a(b).
Requalification markings go on the metal of the upper end of the cylinder; “marking on the cylinder sidewall is prohibited” unless the specification authorizes it, and marking may be by stamping, engraving, scribing, or a label embedded in epoxy (180.213). Oxygen-specific note: 180.209(m) requires aluminum alloy 6351-T6 cylinders in oxygen service (as well as SCUBA and SCBA) to be requalified with eddy current and visual inspection on a five-year period.
What the paper label must carry
The stamp does not tell you the hazards. Under OSHA’s Hazard Communication Standard, 29 CFR 1910.1200(f)(1), a label needs: product identifier, signal word, hazard statements, pictograms, precautionary statements, and the name, U.S. address and U.S. telephone number of the manufacturer, importer or other responsible party. A stamp with a valid test date does not replace this label, and a label does not show the test date. Related checks are in the compressed gas cylinder inspection checklist.
Safe handling basics
- Check that the stamp is legible: 49 CFR 180.213(b)(1) requires requalification and specification markings to be “legible so as to be readily visible at all times.”
- Read the most recent test date on the stamp, then check the label for the product identifier and hazard statements.
- Use the storage rules in compressed gas cylinder storage requirements and the parts overview in oxygen cylinder parts.
Common mistakes
- Treating color as the identifier. Check the label, not the paint; see the color-code guide above.
- Reading the first date as the only date. The rule is built so that dates of subsequent tests are added to the stamp (178.35(f)(1)(iv)); the most recent requalification date is the one that matters for scheduling.
- Assuming a star appears on every cylinder. It appears only where the 10-year conditions in 180.209(b) are met.
- Confusing the RIN with the manufacturer symbol. The symbol sits beside the serial number; the RIN sits in the square between month and year.
FAQ
What does DOT 3AA mean on an oxygen cylinder?
It identifies a seamless steel cylinder of not over 1,000 pounds nominal water capacity with a service pressure of at least 150 psig (49 CFR 178.37).
What does DOT 3AL mean?
It identifies a seamless aluminum cylinder of up to 1000 pounds water capacity and a minimum service pressure of 150 psig, made from an alloy similar to 6061 in the T6 temper (49 CFR 178.46).
What is DOT cylinder requalification?
It is the periodic retesting of a specification cylinder under 49 CFR Part 180, after which the requalifier’s RIN and the date are marked in the square layout described above.
Is oxygen cylinder hydrostatic testing the same as an inspection?
No. Hydrostatic testing is one requalification method; details are in the hydrostatic testing guide. For 6351-T6 aluminum cylinders in oxygen service, 180.209(m) specifies eddy current and visual inspection.
Sources
- 49 CFR 178.35, General requirements for specification cylinders, https://www.law.cornell.edu/cfr/text/49/178.35 (checked 2026-10-30)
- 49 CFR 178.37, Specification 3AA, https://www.law.cornell.edu/cfr/text/49/178.37 (checked 2026-10-30)
- 49 CFR 178.46, Specification 3AL, https://www.law.cornell.edu/cfr/text/49/178.46 (checked 2026-10-30)
- 49 CFR 180.209, Requalification of specification cylinders, https://www.law.cornell.edu/cfr/text/49/180.209 (checked 2026-10-30)
- 49 CFR 180.213, Requalification markings, https://www.law.cornell.edu/cfr/text/49/180.213 (checked 2026-10-30)
- 29 CFR 1910.1200(f)(1), OSHA Hazard Communication, https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200 (checked 2026-10-30)
