The duration of open-circuit dives is shorter than a rebreather dive, in proportion to the weight and bulk of the set. It can be uneconomic when used with expensive gas mixes such as heliox and trimix. Most divers use standard air (i.e. 21% Oxygen / 79% Nitrogen). The cylinder is nearly always worn on the back. "Twin sets" with two backpack cylinders were much more common in the 1960s than now; although twin cylinders (aka "doubles") are commonly used by technical divers for the increased duration and redundancy they provide. Submarine Products sold a sport air scuba with 3 backpack cylinders. Sometimes cave divers have cylinders slung at their sides instead.
See diving cylinder for more information about the cylinders and how they are arranged.
Newspapers and television news often describe open circuit scuba wrongly as "oxygen" equipment, probably by false analogy from airplane pilots` oxygen cylinders. Until Enriched Air Nitrox was widely accepted in the late 1990s, almost all sport scuba used simple compressed air. This allowed the scuba industry in the U.S. to bypass being supervised by the Food and Drug Administration (FDA), which defines non-air gas mixtures intended to prevent or treat diseases, as "drugs." Exotic gas mixtures presently used in scuba are intended to prevent decompression illness in diving, but officially, the FDA appears to continue to believe that scuba divers all use compressed air.
At partial pressures over about 1.6 atmospheres, oxygen becomes toxic. Open-circuit scuba sets may supply various breathing gases; but rarely pure oxygen, except during decompression stops in technical diving.
Some divers use Enriched Air Nitrox, which has a higher percentage of oxygen, usually 32% or 36% (EAN32 and EAN36, respectively). This lets them stay underwater longer, because less nitrogen is absorbed into the body`s tissues. The most common Nitrox blending method needs an oxygen service tank, which is a tank that has had any non-oxygen-compatible grease or rubber removed, by cleaning and replacing parts.
Constant flow scuba sets do not have a demand regulator; the breathing gas flows at a constant rate unless the diver switches it on and off by hand. They run out of air quicker than aqualungs. There were attempts at designing and using these before 1939, for diving and for industrial use. Examples were "Ohgushi`s Peerless Respirator", and Commandant le Prieur`s breathing sets: see Timeline of underwater tech
With a demand regulator
This type of set consists of one or more diving cylinders containing breathing gas at high pressure (typically 200-300 Bar) connected to a diving regulator. The regulator supplies the diver with as much of the gas as needed, at a pressure suitable for breathing at the depth of the diver.
Colloquially this type of breathing set is sometimes (depending on the country of the English speaker) often called an aqualung; however, the word "Aqua-Lung" (note spelling) is a tradename protected by the Cousteau-Gagnan patent. The trademark is now owned by U.S. Divers.