CO2 Tank Certification: Five-Year Rule, Star Stamp, and Costs

A CO2 tank must pass hydrostatic testing every five years to stay legal for refilling and transport. The rule comes from the U.S. Department of Transportation under Title 49 of the Code of Federal Regulations, and the actual testing has to be done by a facility that holds a Requalifier Identification Number (RIN) issued by the Pipeline and Hazardous Materials Safety Administration. Miss the deadline and no gas supplier will touch the tank; move it on public roads and it becomes a federal hazmat violation.

Finding the Test Date on Your Tank

The date you need is stamped into the metal near the valve at the top of the cylinder. Look for a month and year, with a small four-character code arranged in a square pattern between them. That code is the RIN of the facility that last tested the tank, and the month-year is when they did it.¹ If the tank has been tested more than once, you’ll see several date stamps clustered together; the most recent one is what counts.

A tank stamped 06 21 was tested in June 2021 and comes due at the end of June 2026. You can bring it in anytime during or before that month.²

While you’re up there, note the DOT specification. DOT-3AL means seamless aluminum; DOT-3AA means seamless steel.³⁴ The testing facility needs that to set the correct test pressure.

The Five-Year Rule and the Star-Stamp Trap

DOT-3AL and DOT-3AA cylinders in CO2 service get requalified every five years.⁵ That’s the number to remember for beverage, aquarium, welding, and paintball tanks alike.

You may have read that a five-point star stamped after the test date extends the interval to ten years. It does for some compressed gases, but not for CO2. The regulation specifically excludes carbon dioxide, fire extinguisher, and several other industrial gas cylinders from the extended interval.⁵ A star on a CO2 tank does not buy you more time.

Tanks That Follow Different Rules

Small CO2 cartridges, the kind used in bike inflators and single-use consumer products, are exempt from the volumetric expansion part of the test if they measure no more than two inches in outside diameter and less than two feet long.⁵ SodaStream-style beverage cylinders operate under separate DOT special permits tied to their exchange programs.

Carbon fiber composite cylinders, marked DOT-CFFC, are a different animal. They have a hard 15-year service life from date of manufacture, with PHMSA-approved extensions possible up to 30 years total.⁶ Passing a hydro test won’t override that cap. If you own a composite tank, the manufacture date matters as much as the last test date.

Getting the Tank Ready

Bring the tank empty, or close to it. Some shops will recover leftover gas for a small fee, but a full cylinder complicates handling. Pull off any plastic boot, vinyl sleeve, sticker, or aftermarket handle before you go. The regulations require removal of coatings or attachments that would block a visual inspection of the cylinder wall, and doing that yourself avoids extra charges.²

Have the serial number and DOT specification written down. Most facilities also want your name and a phone number for pickup.

What the Test Actually Involves

The standard method is the water jacket test. The technician seals the cylinder inside a water-filled chamber, then pressurizes it beyond its normal working limit. The walls stretch, pushing water out of the chamber into a measuring tube; that reading is total expansion. The pressure is released, and the technician measures how much the cylinder springs back.⁷

The difference is permanent expansion. If it exceeds 10 percent of total expansion, the cylinder fails.⁵ Metal that stays stretched is metal that’s fatiguing.

A visual inspection, inside and outside, is part of every test. Technicians look for corrosion, pitting, dents, cracks, and thread damage against Compressed Gas Association standards.² A cylinder can pass the pressure portion and still get rejected on visual grounds. Tanks that have been dropped, dragged on concrete, or stored damp are the usual casualties.

Where to Take It and What It Costs

Only facilities holding a valid RIN from PHMSA can legally perform the test and apply a new date stamp. PHMSA runs an online locator where you can search by city, state, or zip code and see which cylinder types and methods each facility is authorized for.⁸⁹

Fire equipment companies, industrial gas and welding suppliers, and homebrew stores are the usual public-facing options. Turnaround runs three to seven business days. Fees typically land in the $25 to $45 range depending on cylinder size and location. You’ll get a receipt, and the new stamp on the tank itself is permanent proof.

Reading the New Stamp

A passing tank comes back with a fresh mark on the shoulder: month, the facility’s RIN in a small square, then year. A September 2026 test by facility RIN “A123” reads as 9, the RIN characters in the corners of a square, then 26.¹

A plus sign after the date means the cylinder is approved for filling to 10 percent above its marked service pressure. The stamp has to be permanent, cut into the metal or onto a permanently affixed metal plate; anything you can peel or rub off doesn’t qualify.¹

If the Tank Fails

A failed cylinder gets condemned. The facility stamps a row of Xs across the DOT specification and service pressure markings, or stamps “CONDEMNED” on the shoulder or neck. You can also ask them to drill through the wall so the tank physically can’t hold pressure again.²

Condemned means condemned. The cylinder can’t be refilled, re-stamped, or returned to pressurized service. Steel and aluminum tanks can usually go to scrap metal recycling; check with a hazardous waste facility or your gas supplier, since some run take-back programs. Never toss a cylinder in curbside trash or recycling.

What Happens If You Skip It

Filling, transporting, or offering to transport an expired CO2 cylinder is a federal hazmat violation. Under 49 U.S.C. 5123, a knowing violation can draw civil penalties up to $75,000 per violation, rising to $175,000 if the violation causes death, serious injury, or substantial property destruction. Each day the offense continues counts as a separate violation.¹⁰

The heavy end of that scale is aimed at businesses. For an individual owner, the routine consequence is being turned away at the fill station. For a business running draft beverage lines or any CO2-dependent process, a rejected tank is downtime. Scheduling a retest a month or two before the deadline is the simplest way around it.

1
eCFR. 49 CFR 180.213 – Requalification Markings
2
eCFR. 49 CFR 180.205 – General Requirements for Requalification of Specification Cylinders
3
eCFR. 49 CFR 178.46 – Specification 3AL Seamless Aluminum Cylinders
4
eCFR. 49 CFR 178.37 – Specification 3AA and 3AAX Seamless Steel Cylinders
5
eCFR. 49 CFR 180.209 – Requirements for Requalification of Specification Cylinders
6
Pipeline and Hazardous Materials Safety Administration. DOT-CFFC Basic Requirements
7
Pipeline and Hazardous Materials Safety Administration. Letter of Interpretation 13-0071
8
Pipeline and Hazardous Materials Safety Administration. Cylinder Requalifiers
9
Pipeline and Hazardous Materials Safety Administration. RIN/VIN Locator Tool
10
Office of the Law Revision Counsel. 49 USC 5123 – Civil Penalty