Scuba Tanks

What a scuba tank is

A scuba tank is a seamless metal cylinder built to hold gas at very high pressure, usually between 200 and 300 bar (about 3,000 to 4,400 psi). It is fitted with a valve at the top where the regulator attaches. Most hold compressed air or nitrox; tanks used for higher oxygen mixes need special cleaning.

Aluminium and steel

Aluminium tanks are common in warm-water resorts and for rental. They resist corrosion well and are relatively cheap. Their main drawback is buoyancy: an aluminium tank becomes positively buoyant as it empties, often by around 2 kg (4 to 5 lb), so you need a little extra weight to hold your safety stop at the end of the dive.

Steel tanks are stronger, so their walls can be thinner for the same pressure. They are usually smaller and heavier for the gas they hold and stay negatively buoyant even when nearly empty, which lets many divers carry less lead. They need more care to prevent rust.

Sizes

In metric countries, tanks are described by their internal water volume in litres, such as 10, 12 or 15 litres. In the United States, tanks are described by how many cubic feet of gas they hold at their working pressure. The most common rental tank in the world is the aluminium 80: an 11.1-litre cylinder that holds about 77 cubic feet (about 2,200 litres) of air at 207 bar (3,000 psi).

To work out how much gas a tank holds, multiply its volume by its pressure. A 12-litre tank at 200 bar holds about 2,400 litres of surface-pressure gas.

Valves

Tank valves come in two main types that match the two regulator fittings. Yoke (also called INT or K-valve) valves have an O-ring on the face of the valve, and a yoke regulator clamps over it. DIN valves have a threaded opening that a DIN regulator screws into. Many valves accept either with a removable insert. Technical divers often use twin tanks joined by a manifold or carry extra cylinders slung at their sides.

Inspections and testing

Because a tank holds so much energy, it must be inspected and tested. Rules vary by country, but in general:

• A visual inspection inside and out, often every year, checks for corrosion, cracks and damage.
• A hydrostatic test every few years, every five years in the United States, checks that the tank still stretches and recovers safely under pressure.

Dive centres won't fill a tank that is out of date. Never fill a tank above its marked working pressure, keep a little pressure in it when stored so moisture can't get in, and secure tanks so they can't fall or roll.

Planning your gas

How long a tank lasts depends on its size, your depth and your breathing rate. Because of Boyle's law, you use gas faster the deeper you go. Most recreational divers plan to be back on the boat or shore with at least 50 bar (500 psi) left. Divers in overhead environments use stricter rules, such as the rule of thirds used in cave diving: one third of the gas to go in, one third to come out and one third kept in reserve.

Frequently asked questions