Why the test exists

A refrigerant cylinder is a pressure vessel that may sit in service for decades. Metal fatigues, threads wear, and corrosion can weaken the cylinder wall. The DOT hydrostatic test is a periodic retest that verifies the cylinder can still hold its rated pressure safely. A cylinder that fails the test must be condemned and removed from service.

The Universal 608 exam asks at least one question about reading the hydrostatic test stamp on a refrigerant cylinder. The question is straightforward if you know what to look for.

Where the stamp is

On most refrigerant cylinders, the hydrostatic test date is stamped on the cylinder shoulder (the curved upper portion above the body, below the valve). The stamp reads in a month-year format:

• 7-22 means July 2022.
• 04-19 means April 2019.
• 12-03 means December 2003.

Some cylinders use a two-digit year (common in older cylinders), others use a four-digit year (more recent). The format is consistent across DOT-3AA, DOT-3AL, and DOT-4BA cylinders.

The retest interval

For DOT-3AA cylinders, the retest interval is 5 years (or 10 years if the cylinder has a 10-year stamped rating under specific exemption rules). For DOT-4BA welded cylinders, the retest interval is 5 years. For non-refillable cylinders (DOT-39), no retest is required — they are one-trip containers.

A cylinder stamped 7-22 (July 2022) is in test through July 2027 for a 5-year retest cycle. After July 2027, the cylinder is out of test and may not be filled. It may be transported to a testing facility for retesting, but it may not be filled with refrigerant until it has been retested and re-stamped.

Reading the complete stamp

The cylinder shoulder carries several stamped markings. Learning to identify each one:

• DOT specification (DOT-3AA, DOT-4BA, etc.) — the cylinder type.
• Service pressure (e.g., 1800 PSIG for DOT-3AA at 1800 psi working pressure) — the cylinder's rated pressure.
• Serial number — unique to the cylinder, used for tracking.
• Manufacturer symbol — the registered mark of the cylinder manufacturer.
• Test date (MM-YY or MM-YYYY format) — the most recent hydrostatic test.
• Water capacity (WC 47.6 LB or similar) — the weight of water the cylinder holds when full, used to calculate overfill limits.
• Tare weight (TW 28.5 LB or similar) — the empty weight of the cylinder, used for overfill calculation and refrigerant tracking.

The exam sometimes asks: "Given a cylinder stamped 3AA 1800 PSIG with a test date of 04-19, is the cylinder in test as of July 2025?" The answer is no — the test date is April 2019, the 5-year window closes in April 2024, and as of July 2025 the cylinder is out of test.

What to do when a cylinder is out of test

When a cylinder reaches its hydrostatic test expiration, the technician has three options:

1. Send the cylinder to a DOT-approved retest facility. The facility performs the hydrostatic test, re-stamps the cylinder with the new test date, and returns it for service.
2. Continue using the cylinder for recovery only, if the refrigerant can be recovered within a reasonable period and the cylinder is not used to transport refrigerant over public roads in commerce. This is a gray area — many technicians retire out-of-test cylinders as a matter of policy.
3. Condemn the cylinder. A cylinder that fails the retest, or that is damaged, must be condemned — cut in half or otherwise rendered unusable — to prevent future service.

Most refrigerant wholesalers accept out-of-test cylinders for trade-in or send them to retest facilities. Some large contractors run their own retest programs under DOT oversight.

Common exam traps

1. Believing that all cylinders have the same retest interval. DOT-3AA is 5 years (with a 10-year option for specific cylinders). DOT-4BA is 5 years. Some specialty cylinders have different intervals under specific exemptions. The default assumption is 5 years.

2. Forgetting that the test date is the original test plus the interval, not the original test alone. A 7-22 cylinder is in test until 7-27, not just during 7-22.

3. Assuming a cylinder with no visible stamp is in test. If the stamp is missing or illegible, the cylinder is presumed to be out of test until documentation proves otherwise. Most technicians retire such cylinders.

4. Believing a non-refillable cylinder (DOT-39) needs retesting. Non-refillable cylinders are one-trip and cannot be retested.

A practical field note

Check the test date on every cylinder before you put refrigerant in. If the cylinder is out of test, the recovery is illegal under DOT regulations — even if the recovery itself is required by EPA Section 608. EPA and DOT enforce jointly in some cases, and a cylinder out of test is a visible violation that often draws attention during an inspection.

The Universal exam rewards the candidate who can read a stamp and do the math in their head. Practice on real cylinders in your shop, and the test will feel routine.