Scuba Tank

The high-pressure cylinder that stores a diver's breathing gas, most commonly an aluminum 80 filled with air or nitrox to about 207 bar.

The scuba tank is the container of everything you breathe on a dive: compressed air or another mix stored at pressures most divers never otherwise handle. The world's most common rental cylinder is the aluminum 80, holding about 11.1 liters of water volume and filled to 207 bar (3000 psi), which yields roughly 2200 surface liters of gas. Steel tanks pack similar gas into a smaller, negatively buoyant package that many cold water and technical divers prefer.

Buoyancy behavior is a real planning factor: an aluminum 80 swings from slightly negative when full to positively buoyant when nearly empty, gaining about 2 kg of lift as roughly 2.7 kg of gas is breathed away, which is why you weight yourself for the end of the dive. Tanks marked for nitrox need oxygen-clean service if they will ever see mixes handled at high oxygen fractions.

Cylinders live under a strict inspection regime: an annual visual inspection and a hydrostatic test, typically every 5 years in the US and at intervals set by local rules elsewhere. Store tanks with some positive pressure inside so moisture never enters.