page_title: History of scuba diving
section_title: Open circuit
hierarchical_section_title: History of scuba diving / First successful scuba equipment / Open circuit
context_page_description: The history of scuba diving is closely linked with the history of scuba equipment. By the turn of the twentieth century, two basic architectures for underwater breathing apparatus had been pioneered; open-circuit surface supplied equipment where the diver's exhaled gas is vented directly into the water, and closed-circuit breathing apparatus where the diver's carbon dioxide is filtered from the exhaled breathing gas, which is then recirculated, and more gas added to replenish the oxygen content. Closed circuit equipment was more easily adapted to scuba in the absence of reliable, portable, and economical high pressure gas storage vessels. By the mid-twentieth century, high pressure cylinders were available and two systems for scuba had emerged: open-circuit scuba where the diver's exhaled breath is vented directly into the water, and closed-circuit scuba where the carbon dioxide is removed from the diver's exhaled breath which has oxygen added and is recirculated. Oxygen rebreathers are severely depth limited due to oxygen toxicity risk, which increases with depth, and the available systems for mixed gas rebreathers were fairly bulky and designed for use with diving helmets.
context_section_description: The first systems that became widely popular with recreational divers were open circuit demand scuba. They were safer than early rebreather systems, less expensive to operate, and allowed dives to greater depths.
  An important step for the development of open circuit scuba technology was the invention of the demand regulator, a mechanism that conserves breathing gas supply by providing flow only when the outlet pressure is reduced when the diver inhales. In 1864, the French engineers Auguste Denayrouze and Benoît Rouquayrol designed and patented their "Rouquayrol-Denayrouze diving suit" after adapting a pressure regulator and developing it for underwater use. This would be the first diving suit that could automatically supply air to the diver on demand by adjusting the flow of air from the tank to meet the diver's breathing and pressure requirements. The system still had to use surface supply to provide useful endurance, as the pressure cylinder provided was only a low capacity accumulator.
  The first open-circuit scuba system was devised in 1925 by Yves Le Prieur in France. Inspired by the simple ambient pressure supply valve apparatus of Maurice Fernez and the freedom it allowed the diver, he made it independent of surface supply by using three litre Michelin cylinders containing air compressed to 150 kilograms per square centimetre (2,100 psi; 150 bar). 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 which was adjusted manually by the diver, with two gauges, one for tank pressure and one for output (supply) pressure. Air was supplied continuously through the mouthpiece and exhausted through a short tube fitted with a non-return exhaust valve as in the Fernez design. The continuous flow of air and the consequent low endurance of the apparatus limited the practical use of LePrieur's device.
  Fernez had previously invented a noseclip, a mouthpiece fitted with a one-way valve for exhalation and diving goggles, and Le Prieur just added a manually-controlled regulator and a compressed air cylinder to those elements. Fernez's goggles did not cover the nose, which allows the diver to equalise the internal pressure by letting air flow into the goggles through the nose, so they did not allow a dive deeper than ten metres due to "mask squeeze". In 1933, Le Prieur replaced the goggles, noseclip and valve) by a full face mask, directly supplied with constant flow air from the cylinder. Le Prieur's design was the first autonomous breathing device used by the first scuba diving clubs in history – Racleurs de fond founded by Glenn Orr in California in 1933, and Club des sous-l'eau founded by Le Prieur himself in Paris in 1935.
  In 1942, during the German occupation of France, Jacques-Yves Cousteau and Émile Gagnan designed the first reliable and commercially successful open-circuit scuba, known as the Aqua-Lung. Their system combined an improved demand regulator with high-pressure air tanks. Émile Gagnan, an engineer employed by the Air Liquide company, miniaturized and adapted the regulator manufactured for use with gas generators in response to the constant fuel shortage that was a consequence of German requisitioning. Gagnan's boss, Henri Melchior, knew that his son-in-law Jacques-Yves Cousteau was looking for an automatic demand regulator to increase the useful endurance of the underwater breathing apparatus invented by Le Prieur, so he introduced Cousteau to Gagnan in December 1942. On Cousteau's initiative, Gagnan's regulator was adapted to diving, and the new Cousteau-Gagnan patent was registered some weeks later in 1943.
caption: The Rouquayrol-Denayrouze apparatus was the first regulator to be mass-produced (from 1865 to 1965). In this picture the air reservoir presents its surface-supplied configuration.
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