How to Read a Cylinder Requalification Stamp

Why this matters

A refillable gas cylinder is a pressure vessel that spends its life being filled, emptied, dropped onto truck beds and left in hot vans. The steel or aluminum thins with corrosion, takes dents and gouges, and fatigues. Periodic requalification is how anyone finds out whether a cylinder that held its rated pressure when it was made still can. The date of the last one is stamped into the cylinder's shoulder, and reading it takes about a minute.

That minute matters most for HVAC and refrigeration shops, because a recovery cylinder is filled by the tech, on site, under pressure from a recovery machine. If that cylinder's requalification has lapsed, the shop is the one filling an overdue pressure vessel, then carrying it down the highway. A nitrogen cylinder from a supplier is usually somebody else's problem until the shop owns its own cylinders, at which point it is the shop's problem too.

Before you handle a cylinder you are inspecting

A charged cylinder holds enough stored energy to become a projectile if its valve is sheared off. Keep the valve protection cap on any high-pressure cylinder (nitrogen, argon, oxygen) whenever it is not connected to a regulator, and keep every cylinder secured upright with a chain or strap while you read it. Do not strike, grind or file the shoulder to make a faint stamp legible, and never apply heat; both can weaken the wall or the stamp area. If a cylinder shows a bulge, fire damage (blistered or burned paint, a melted valve handwheel or relief device) or arc burns from welding, stop reading and set it aside tagged as do not fill; a cylinder in that condition goes to a requalifier for condemnation or disposal, not back into service. Transport rules for loading cylinders are covered in Loading the Van So Chemicals, Cylinders and Batteries Ride Safely.

Step 1: Find the specification marking

Every specification cylinder carries its specification stamped near the top, starting with "DOT" (or "ICC" on very old cylinders, or "TC" for a Canadian specification). Examples you will meet:

• DOT-3AA2015 or DOT-3AA2265: a seamless steel high-pressure cylinder, with the number after the letters being the service pressure in psi. This is the typical nitrogen cylinder.
• DOT-4BA400 or DOT-4BW400: a welded or brazed low-pressure steel cylinder with a 400 psi service pressure. This is the typical refillable refrigerant or recovery cylinder.
• DOT-39: a non-refillable cylinder. Disposable refrigerant jugs are DOT-39.

Why first: the specification decides everything that follows, including whether the cylinder may be refilled at all and what its requalification interval is. A DOT-39 cylinder may not be refilled under federal hazardous materials rules, whatever is stamped near it, so a disposable jug saved as a spare recovery tank is out at this step.

What breaks if you skip it: you read a date correctly on a cylinder that was never allowed to be refilled, or apply a nitrogen cylinder's interval to a recovery cylinder.

Step 2: Find the original manufacture date

Near the specification you will find a serial number, the maker's mark, and the month and year of the original test at manufacture, written as two-digit month and two-digit year (for example "03 19" for March 2019), usually beside an inspector's mark. Low-pressure cylinders also carry WC (water capacity, in pounds) and TW (tare weight, the empty weight in pounds). You need the tare weight every time you fill a recovery cylinder by weight, and the water capacity to know the fill limit.

Why: if no requalification date appears anywhere on the cylinder, the manufacture date is the last test, and the interval runs from it.

Step 3: Find the most recent requalification mark

Each requalification adds a new mark, usually stamped below or beside the earlier ones, carrying the month and year and the identification of the facility that did it. On cylinders requalified in recent decades that identification is the Requalifier Identification Number (RIN) issued by PHMSA, the federal agency that administers the hazardous materials regulations; older marks may carry an independent inspector's symbol instead. On some cylinders the RIN's characters are arranged around the date in a small square pattern, so the date can look scattered at first glance.

  Shoulder stamps, illustrative recovery cylinder

  DOT-4BW400      SN 1234567     [MAKER MARK]
  WC 47.7   TW 27.5
  03 19 [INSPECTOR]        made March 2019
  04 A123 24               requalified April 2024

Read every date on the shoulder and take the latest one. A cylinder can carry three or four marks, and the one nearest the specification is often the oldest.

Two symbols that change how a high-pressure cylinder's date is read:

• A plus sign after the original test date on a DOT-3A or 3AA cylinder means it may be filled to 10 percent above its marked service pressure, under the conditions 49 CFR 173.302a lists.
• A five-pointed star after a requalification date on a DOT-3A or 3AA cylinder means it qualified for a 10-year interval instead of 5, under the conditions 49 CFR 180.209 lists. No star, five years.

Step 4: Work out the due date from the right interval

The requalification interval is set by specification and service in the table at 49 CFR 180.209. Two common cases:

• DOT-3A and 3AA high-pressure cylinders (nitrogen): generally 5 years from the last requalification, or 10 years with the star mark.
• Refrigerant recovery cylinders (DOT-4BA and 4BW): commonly on a 5-year cycle. The table gives some 4B-family cylinders longer intervals in particular services, so read the row that matches your cylinder and its use rather than assuming.

Add the interval to the month and year of the latest mark. A recovery cylinder requalified in April 2024 on a 5-year interval is due in April 2029.

Why it matters: under 49 CFR 180.205, a cylinder that is overdue for requalification may not be filled and offered for transportation. A cylinder filled before its due date may generally stay in service until it is emptied, but it may not be refilled until it is requalified. For a recovery cylinder, "filled" includes recovering refrigerant into it on a job.

Step 5: Check that the cylinder suits the gas

The service pressure on the specification has to suit what goes in. For refrigerant recovery, the common requirement is a cylinder whose service pressure is at least what the refrigerant's pressure at a high storage temperature calls for, and for R-410A that is commonly stated as a 400 psi cylinder. A recovery machine's or the refrigerant reclaimer's instructions will say what is acceptable. Recovery cylinders also have an 80 percent liquid fill limit by weight, which is why the tare weight from Step 2 matters.

Step 6: Route the overdue cylinder to someone who can requalify it

A shop cannot requalify its own cylinders. Requalification is done by a facility holding a RIN from PHMSA, typically by hydrostatic testing (filling with water under pressure and measuring how much the cylinder expands and recovers) plus internal and external visual inspection, with the outcome being a new stamp or condemnation. In practice, gas suppliers handle requalification of cylinders in their exchange programs, and refrigerant reclaimers or cylinder testing facilities handle shop-owned recovery cylinders. Empty, out-of-service cylinders that will not be requalified go out through the route in Disposing of Empty Gas and Refrigerant Cylinders.

Worked example: an October 2026 cylinder audit

An HVAC shop with five trucks audits every cylinder in the shop in October 2026. Four of them tell the story.

Cylinder 1, recovery, DOT-4BW400. Made 03 19, requalified 04 24. Five-year cycle, due April 2029. Clean shoulder, no damage. In service. The audit sheet records the due date so the shop can send it out early in 2029 rather than finding out on a job.

Cylinder 2, nitrogen, DOT-3AA2265, shop-owned. Made 08 09, requalified 07 14 and 06 19, no star on either mark. The latest mark is June 2019; five years later is June 2024. October 2026 is 2 years and 4 months past due. The cylinder is nearly full: the supplier's tag shows it was filled in May 2024, one month before the due date. It may be used until empty but not refilled. The shop marks it "use to empty, then requalify" and puts it on the supplier's next pickup list; the supplier will either requalify it or swap it for an in-date exchange cylinder.

Cylinder 3, recovery, DOT-4BA300. Made 05 13, no requalification mark anywhere. Five years from May 2013 is May 2018, so it has been overdue for 8 years and 5 months. It is also a 300 psi cylinder in a shop whose systems are mostly R-410A. It is empty. Out of service at once, tagged do not fill, and sent to the reclaimer for requalification and reassignment to a lower-pressure refrigerant, or for retirement. It would fail the shop's rule at Step 4 alone, and Step 5 would have caught it even if it were in date.

Cylinder 4, a DOT-39 disposable jug with a homemade recovery adapter on it, found behind a truck seat. Out at Step 1. Refilling it was never permitted, and it is the kind of improvised cylinder that ruptures. It is kept upright and out of the sun, then emptied with the shop's recovery equipment, by a certified tech in safety glasses and refrigerant-rated gloves, into a separate in-date recovery cylinder tagged as unknown contents so it does not contaminate a clean one, and retired through the disposal route.

What changes the answer. If Cylinder 2 had carried a star after its 06 19 mark, its interval would have been 10 years and it would be in date until June 2029. Nothing else on the cylinder would look different, which is exactly why the star is worth knowing.

How to verify you got this right

• Re-read the latest date, not the first. For each cylinder, write down every date stamped on the shoulder and circle the most recent. The audit is wrong if any due date was computed from a manufacture date on a cylinder that has a later mark.
• Check the interval against the table, not memory. Pull the current 49 CFR 180.209 table once a year and confirm the intervals on your audit sheet still match it.
• Put the due dates where they will be seen. A due date on a spreadsheet nobody opens is not a control; a due date on a tag on the cylinder, checked when it is loaded onto the truck, is.

References

• 49 CFR 180.205 (general requirements for requalification) and 180.209 (requalification intervals by specification); 49 CFR 173.302a for the plus mark
• PHMSA cylinder requalification program and Requalifier Identification Numbers
• See related: Disposing of Empty Gas and Refrigerant Cylinders; Loading the Van So Chemicals, Cylinders and Batteries Ride Safely
• See related: The Cylinder That Was Not What the Label Said; Counterfeit and Contaminated Refrigerant