Compressed air tanks attached to regulators allow scuba divers to breathe underwater for an hour or longer. Divers wear their tanks on their backs, strapped into their buoyancy compensator devices. Scuba tanks have evolved and improved since their beginnings in the 19th century.
History
Benoit Rouquayrol and Auguste Denayrouse developed the first compressed air tank for divers in 1865, according to MarineBio.org. Their low-pressure tank could only support air at pressures of 250 to 350 psi. The tank required surface-supplied air. In 1933, Yves Le Prieur invented a tank that could carry air at pressures up to 1,500 psi, allowing divers to breathe for longer without having to connect to a surface-supplied air hose. Jacques-Yves Cousteau and Emile Gagnan invented a self-contained tank and regulator device, later known as the Aqua Lung, in 1942. Modern scuba tanks hold compressed air at pressures of 3,000 to 3,500 psi, allowing for longer, freer dives than early tanks could support.
Tank Basics
Modern scuba tanks are large steel or aluminum canisters with a control valve at the top. You can control the flow of air out of the tank by turning a knob on the side of the control valve. The knob controls a piston inside the valve that can move into the open or closed position. When you turn on the air, the piston moves into the open position, allowing air to pass freely out of the tank. The first stage of a diver's regulator attaches to the tank's valve, forming an airtight seal with a small rubber O-ring in the valve. Once you have secured the regulator onto the tank, air will only escape from the tank when you inhale through your mouthpiece.
Breathing Gases
Dive shops fill tanks with a variety of different breathing gases. Recreational divers simply breathe compressed air. Normal air contains 21 percent oxygen, 78 percent nitrogen and traces of inert gases. You can earn specialized certifications to breathe different gas mixtures, such as Nitrox or Trimix. Nitrox breathing gas contains a lower percentage of nitrogen than normal air, allowing divers to remain underwater for longer with less nitrogen buildup in the tissues. The higher percentage of oxygen in Nitrox puts divers at greater risk of oxygen toxicity, however. Trimix contains helium, a gas that, unlike nitrogen and oxygen, does not become toxic to divers at depths.
Tank Labels
Scuba tanks must have clear labels that indicate the contents of the tank, as well as the dates of the tank's most recent inspections. Tanks that contain breathing gases other than normal air must have stickers that indicate the type of gas in the tank. A large green and yellow sticker reading "Enriched Air Nitrox" indicates a Nitrox-filled tank, for example. Trimix-gilled tanks must have a detailed label that lists the percentages of oxygen and helium in the mixture. Inspection labels must prove that the tank has undergone a hydrostatic test within the last five years and a visual inspection within the last year.
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