Scuba divers carry tanks on their backs that are filled with compressed air or other breathable gas. These air supplies allow divers to remain underwater for an hour or more. There are several different types of scuba tanks, also known as cylinders, but they should all have certain basic features and be designed to serve the same purpose---allowing you to breathe underwater.
Features
All scuba tanks have a valve that air flows through to the diver's regulator, or breathing device. An O-ring around the metal valve forms an airtight seal with the metal of the regulator. You can turn the tank's airflow on or off by twisting a knob at the side of the valve. Most tanks also have a rubber boot to protect the base and allow them to stand more easily. All tanks must have inspection decals indicating the date of their most recent visual and hydrostatic inspections. Tanks filled with specialized breathing gases, such as enriched air nitrox or trimix, must be labeled appropriately.
History
In 1865, Benoit Rouquayrol and Auguste Denayrouze invented the first tank that was able to hold compressed air, according to MarineBio.org. Their tank could hold compressed air at pressures up to 250 to 350 psi. In 1933, Yves Le Prieur developed a tank that could hold compressed air at 1,500 psi. Le Prieur's tank was also the first to deliver air to the diver only when he inhaled.
Types
Scuba divers use several different types of tanks. Most divers use tanks made of steel or aluminum. There are generally more aluminum tanks sold at dive shops than steel tanks. Steel tanks tend to cost more than aluminum tanks, but they also last longer if properly maintained.
The carrying capacity of scuba tanks also varies. Some tanks, for example, can hold compressed air pressurized up to 3,000 psi, while others can hold up to 3,500 psi. A tank that can hold up to 3,000 psi typically holds about 80 cubic feet of air.
Most tanks have only one valve. Tanks designed for cave diving or technical diving, however, may have Y-pillar valves with two output valves, which allow divers to attach two regulators to the same tank.
Considerations
If you wish to purchase a scuba tank, first decide whether to buy a steel or aluminum tank. Steel tanks cost much more than aluminum tanks, but they weigh less and take up slightly less space. Steel tanks are less buoyant than aluminum tanks, meaning you need less weight on your weight belt to compensate for the tank. Steel tanks do not dent as easily as the softer aluminum tanks, but they tend to rust much more easily.
Many scuba divers choose to rent tanks for each dive rather than purchasing their own tanks. Tanks require a sizable investment and serious maintenance and upkeep. If you own a tank yourself, you must take responsibility for having it inspected regularly. If you travel to most of your dive destinations, it might not be practical or possible to bring your own tank with you.
Warning
Scuba tanks that have sustained damage or dents might not be able to hold highly pressurized air. Damaged tanks might explode under the pressure of compressed air. To avoid accidents with scuba tanks, you must have your tank visually inspected every year. Your tank must also pass a hydrostatic test every five years.
When you rent a tank, check its inspection labels to make sure it was visually inspected in the last year and hydrostatically tested in the last five years.
Visually inspect the tank's O-ring before each dive. Check for cracks or tears in the O-ring, and replace it if you notice any damage. Set your tank on its side when you leave it unattended.
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