My first CO2 tank cost thirty dollars from a man clearing out a garage, and then it cost me a Saturday. The gas shop turned it away at the counter — not because it hissed or looked beaten up, but because of a shallow stamp near the shoulder that read 04 09. The cylinder was years past due for its pressure test, and no fill station was going to put gas in it. Nothing was wrong with the tank. What was wrong was my ability to read it.
The four lines stamped into the shoulder
A gas cylinder carries its own paperwork punched into the metal below the valve, and once you can read it you can price a used tank in ten seconds. On the aluminum cylinders that dominate homebrewing, the top line is specification and service pressure: DOT-3AL 1800 means a cylinder built to the Department of Transportation's 3AL aluminum standard, rated for 1800 psi. Under it sit the maker's mark and a serial number, then TW and a figure — the tare weight, what the cylinder and valve weigh with no gas in them. Last is a date, month and year, usually beside a short code identifying whoever ran the test.
That date is the whole ballgame on a secondhand tank. The other stamps describe something that will never change; the date tells you where the cylinder sits in a cycle that repeats for as long as anyone keeps filling it.
Five years, and no extension for carbon dioxide
Cylinders have to be requalified on a schedule, and the schedule lives in a table in the federal hazardous materials rules. In 49 CFR 180.209, the row for 3AL cylinders lists a requalification period of five or twelve years, with the twelve-year option carrying a footnote worth reading twice: it does not apply to cylinders used for carbon dioxide service. Whatever a seller says about a decade between tests, a homebrew CO2 tank is on a five-year clock.
Nobody at the counter can wave this off for you, which is why the answer there is a flat no rather than a shrug. A requalification costs a modest amount at a welding or fire-equipment supplier — worth paying on a good tank, worth subtracting from a stranger's asking price.
Aluminum, steel, and what five pounds means
Homebrew tanks come in five and ten pound sizes, and that number is the weight of the gas, not the tank. A five pound aluminum cylinder tares around eight and a half pounds, so a full one lands near fourteen. Aluminum is the common choice because it is lighter and cannot rust from the inside. Steel tanks turn up cheaper used and work fine, but interior rust is a failure mode nobody can see from outside, and a rough exterior hints at how the thing was stored.
Five pounds is the sweet spot for a home setup: several five-gallon batches carbonated and pushed before a refill, most of a season of kegging instead of bottling, small enough to sit in a chest freezer beside a keg, light enough to carry to the shop without resenting the trip. Ten pounds cuts the refill trips at the price of a tank you stop wanting to carry.
Why the big gauge stays near full and then quits
The high-pressure gauge on a CO2 regulator does not measure how much gas you have left, and every new kegger gets fooled by this once. Inside the cylinder the carbon dioxide is mostly liquid, and a liquid in a closed vessel sits at its own vapor pressure — for carbon dioxide, in the neighborhood of 838 psi at room temperature, a figure you will find on any supplier's safety data sheet. Draw gas off and more liquid boils to replace it, holding the pressure steady. The needle stays put through most of the tank's useful life, then falls off a cliff in the last stretch once the liquid is gone.
The gauge is still worth having: a needle well below that vapor pressure in a normal room means the liquid is spent. But the honest measurement is a bathroom scale. Weigh the tank, subtract the tare weight stamped on the shoulder, and what is left is pounds of CO2. I write that figure on masking tape stuck to the tank, and I have not been surprised mid-pour since.
The regulator parts that actually matter
A regulator has one job: take whatever the cylinder is doing and hand you a steady dozen psi — the number carbonation depends on. Buy two gauges — one for cylinder pressure, one for what goes out to the keg — because a single-gauge unit shows your output while leaving you blind to the tank. Then look for a shutoff on the outlet, so you can isolate a keg without spinning the adjustment screw, and a check valve so nothing travels back up the gas line. Sticky liquid inside a used gas line is what a missing check valve looks like after the fact.
Skip the multi-body manifolds until you actually run more than a keg or two. The one part I would not economize on is the connection to the tank itself, which comes down to a fifty-cent piece of nylon.
The soapy-water check that pays for itself
A slow leak costs money and beer, and it always surfaces on a weekend when nothing is open. Mix a squirt of dish soap into a cup of water, open the tank, and paint it onto every joint with a finger: cylinder to regulator, regulator to line, line to disconnect, keg posts and lid. Growing bubbles mean gas is escaping. A minute of that has caught three leaks I would otherwise have found as an empty tank.
Repeat it after any change to the system, and close the cylinder valve when you are done for the day — a five pound tank can empty itself through a failed seal overnight.
Where the tank lives between brew days
Keep the cylinder upright and keep it from falling. That is most of gas safety in one sentence: on its side a tank can feed liquid rather than gas into the regulator, and a heavy cylinder toppling onto its valve is the one genuinely dangerous thing in a kegging setup. A strap, a bungee to a shelf leg, or a milk crate trapping the base all count. Keep it away from heaters and out of a hot trunk, since pressure climbs with heat.
The room matters too. Carbon dioxide is about one and a half times as dense as air, so a large release pools low and displaces the air you were planning to breathe. In a basement or garage that is a non-issue; in a sealed closet the size of a phone booth it is worth a thought. My tank sits in the open rather than shut in with the kegerator.
What I would buy again
A used five pound aluminum 3AL cylinder with a test date inside the last two years, a dual-gauge regulator with an outlet shutoff and an inline check valve, a handful of nylon washers, and a bathroom scale I already owned. That combination has run my kegs for years and the only recurring cost is gas. When I look at a listing now, I ask for a photo of the shoulder stamps before anything else — if the date is current, the rest is negotiable, and if it is not, I offer the price of a fill and a requalification and let the seller decide. Either way I know what I am buying, which is the whole difference between my second tank and my first.