Scuba cylinders: materials, valves and the rules behind them.

Steel or aluminium

Scuba cylinders are made of steel or aluminium alloy. The usual comparisons come from retail and magazine guides, which we could see only as search summaries, so the points below are those sources' descriptions rather than confirmed facts:

• Aluminium cylinders become positively buoyant as they empty, while steel cylinders stay negatively buoyant or close to neutral, so steel usually needs less added weight.
• Aluminium dents more easily. Steel is tougher but can rust inside.
• A steel cylinder can weigh less than an aluminium one of similar capacity, because its walls can be thinner.

How a cylinder's buoyancy changes a diver's weighting is worked out in training; see weights for the systems.

DIN and yoke valves

There are two ways to connect a regulator to a cylinder valve. Retail guides, again seen as search summaries, describe them this way:

DIN

The regulator's first stage screws into the valve, and the sealing O-ring sits on the regulator. Many technical divers and European divers prefer it.

Yoke (international)

The first stage clamps over the valve, and the O-ring sits in the valve. It is mostly seen in the US.

Convertible valves exist that take either. As one maker example of ratings, Mares' regulator manual rates its DIN 477/50 screw connection to 300 bar and its international yoke (CGA 850) connection to 232 bar. That is the maker's statement for its own fittings.

The 6351 aluminium alloy

One piece of history still affects how aluminium cylinders are inspected. Some older cylinders were made from aluminium alloy 6351-T6, which US federal rules single out for checks for sustained load cracking.

• Luxfer, a cylinder maker, says it made 6351-alloy cylinders in the US from 1972 to mid-1988 and in England from 1958 to 1995, then moved to its own 6061 alloy, which it says is not prone to sustained load cracking. Luxfer says the original test date stamped on the crown shows which alloy period a cylinder belongs to.
• DAN's Alert Diver (Q4 2017), in an article by Mark A. Gresham, president of the inspector-training organisation PSI-PCI, says 6351-T6 cylinders stopped being produced in 1989 and that 31 ruptures are known among more than 50 million cylinders made.

The two disagree on when production ended: Luxfer gives mid-1988 for its US output and 1995 for England, while the DAN article gives 1989 in general. The same DAN article calls blanket refusals to fill 6351 or older cylinders arbitrary and says US rules let cylinders that pass the required inspections and tests stay in service.

US federal rule 49 CFR 180.209(m) sets an extra eddy current check for 6351-T6 aluminium cylinders in scuba service, and Luxfer sets a shorter interval of its own. Both are set out on cylinder inspection and testing.

Who regulates cylinders

Cylinders are regulated as transportable pressure vessels, so the rules come from transport and gas-cylinder bodies.

Transport Canada adds that a cylinder may only be filled while it is within its requalification period and service life. The intervals each country sets are compared on cylinder inspection and testing, and the cylinder test date checker is a companion tool.

Markings and labels

Cylinders carry stamped markings. BSAC, relaying the UK test body IDEST, says a hydraulic test is stamped on the shoulder with the test centre, year and month, and a quadrant label shows when the next inspection is due. A cylinder used for nitrox carries extra labelling; SDI (2012) describes a band, usually yellow with green lettering, and a tag showing the oxygen content and maximum operating depth. See nitrox equipment. For flying with a cylinder, see travelling with dive gear.