Scuba tanks have evolved significantly since their appearance in the 19th century. Early scuba tanks, such as the one invented in 1865 by Benoit Rouquayrol and Auguste Denayrouse, carried compressed air with pressures of about 250 psi, meaning divers still had to rely on surface-supplied air, according to MarineBio.org. Modern scuba tanks can carry compressed air and other breathing gases at pressures up to 3,500 psi, allowing divers to dive deeper and longer than ever.
Canister
The scuba tank's largest part is the canister, commonly referred to as a cylinder. Most scuba tank canisters consist of aluminum or steel, both durable materials capable of holding air at high pressures. Steel cylinders are slightly denser than aluminum cylinders, making them useful for cold water divers who need to compensate for the excess buoyancy caused by an insulating drysuit.
Control Valve
A control valve on top of the scuba tank's canister controls the flow of air in and out of the tank. When you turn a knob on the side of the valve, you move a piston inside the valve into the open or closed position to turn the tank's air on or off. Once you have attached your regulator to the tank's valve, you can turn on the air and breathe through your mouthpiece.
O-Ring
An O-ring surrounds the opening on the tank's control valve. This O-ring, typically made of rubber, allows the tank's valve to form an airtight seal with the regulator's first stage. Tanks filled with high-oxygen gas mixtures or Trimix might require halocarbon O-rings to reduce the risk of fire. A small rubber dust cap attached to the tank's valve keeps the O-ring dry between dives. Inspect and replace O-rings regularly.
Labels
Tanks must undergo visual inspection at least once a year and hydrostatic testing every five years. All scuba tanks must bear clear labels indicating the month and year of the most recent visual inspection and hydrostatic test. Tanks filled with breathing gases other than air must have labels indicating their contents. Nitrox-filled tanks must have a large label reading, "Enriched Air Nitrox," for example, and Trimix-filled tanks must have a label indicating the percentage of oxygen and helium in the tank.
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