Scuba divers carry their supply of breathing air in an aluminum or steel tank. Divers strap their tanks into the back of their buoyancy compensator vest, or BC, and carry them like backpacks. Scuba tanks carry enough compressed air to allow divers to breathe underwater for an hour or more.
History
According to MarineBio.org, Benoit Rouquoayrol and Auguste Denayrouse pioneered the first tank for diving in 1865. Their tank could carry compressed air at a low pressure of about 250 to 350 psi. This tank model received air from the surface through a long hose, keeping divers tethered to the surface. In 1933, Yves Le Prieur developed a tank that could hold air at a pressure of 1,500 psi, allowing the diver to remain underwater for longer without having to breathe surface air from a hose. Modern scuba tanks can hold compressed air at a pressure of 3,000 to 3,500 psi.
Tank Parts
The scuba tank consists of a steel or aluminum canister and a control valve. The control valve covers the top of the tank. A knob on the side of the valve controls the flow of air in and out of the valve. When you turn the knob to turn on the air, a piston inside the valve moves into the open position to allow air to pass out of the tank. Divers attach their regulator’s first stage to the tank’s valve. A small O-ring enables the metal valve to form an airtight seal with the first stage of the regulator. Once you have attached the regulator to the tank’s valve, you can turn on the air and the air will not escape until you inhale through your regulator’s mouthpiece.
Types of Breathing Gas
Divers fill their tanks with several different types of breathing gas. Most recreational, open water divers breathe normal air, which contains about 21 percent oxygen and 78 percent nitrogen as well as traces of inert gases such as argon. Divers with specialized training can breathe nitrox, a gas mixture that contains a slightly higher percentage of oxygen and a slightly lower percentage of nitrogen compared to normal air. The lower percentage of nitrogen in nitrox reduces the amount of nitrogen that passes into the body’s tissues during a dive. This allows divers to make longer dives with shorter decompression stops and shorter surface intervals between dives.
Trimix breathing gas contains oxygen, nitrogen and helium. While nitrogen and oxygen can become toxic at depths of 100 feet or more, helium does not become toxic at increased depths. Thus, divers breathing trimix can make longer and deeper dives with less risk of nitrogen narcosis or oxygen toxicity. Divers must have specialized training to use trimix.
Labels
Tanks filled with any breathing gas besides normal air must have clear labels indicating what mixture the tank contains. Nitrox-filled tanks must have a large green and yellow label bearing the words “Enriched Air Nitrox.” Tanks filled with trimix must have a trimix label that indicates what percentage of oxygen and what percentage of helium the mixture contains. Clear labels or stickers should indicate the month and year of a tank’s most recent hydrostatic test and visual inspection.
Testing Requirements
Because scuba tanks contain compressed air at high pressures, damaged or poorly constructed tanks can pose a safety risk. When you rent a scuba tank, check the labels to make sure it has been tested within the last five years and visually inspected within the last year. The United States Department of Transportation requires that scuba tanks pass a hydrostatic test every five years and a visual inspection each year. In hydrostatic testing, trained technicians determine whether a tank can safely hold the pressures it has been designed to hold. In visual inspections, professionals remove the tank’s valve and look inside the tank for signs of corrosion or other damage.
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