Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases.
The volume of gas used is reduced compared to that of open circuit; therefore, a smaller cylinder or cylinders, may be used for an equivalent dive duration.
Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers.
The first open-circuit scuba system was devised in 1925 by Yves Le Prieur in France.
Inspired by the simple apparatus of Maurice Fernez and the freedom it allowed the diver, he conceived an idea to make it free of the tube to the surface pump by using Michelin cylinders as the air supply, containing three litres of air compressed to 150 kilograms per square centimetre (2,100 psi; 150 bar).
Other equipment includes a mask to improve underwater vision, exposure protection, equipment to control buoyancy, and equipment related to the specific circumstances and purpose of the dive.
Scuba divers are trained in the procedures and skills appropriate to their level of certification by instructors affiliated to the diver certification organisations which issue these certifications.
Fernez's goggles didn't allow a dive deeper than ten metres due to "mask squeeze", so, in 1933, Le Prieur replaced all the Fernez equipment (goggles, noseclip and valve) by a full face mask, directly supplied with constant flow air from the cylinder.
In 1942, during the German occupation of France, Jacques-Yves Cousteau and Émile Gagnan designed the first successful and safe open-circuit scuba, known as the Aqua-Lung.
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The "Fernez-Le Prieur" diving apparatus was demonstrated at the swimming pool of Tourelles in Paris in 1926.
The unit consisted of a cylinder of compressed air carried on the back of the diver, connected to a pressure regulator designed by Le Prieur adjusted manually by the diver, with two gauges, one for tank pressure and one for output (supply) pressure.
These include standard operating procedures for using the equipment and dealing with the general hazards of the underwater environment, and emergency procedures for self-help and assistance of a similarly equipped diver experiencing problems.