Aqua-LungAfter Cousteau himself, who had coined the word, the spelling was originally Aqua-Lung. See Jacques-Yves Cousteau & Frédéric Dumas, Le Monde du silence, Éditions de Paris, Paris, 1953, Dépôt légal 1er Trimestre 1954 - Édition N° 228 – Impression N° 741 (in French) was the first open-circuit, self-contained underwater breathing apparatus (or "scuba") to achieve worldwide popularity and commercial success. This class of equipment is now commonly referred to as a twin-hose diving regulator,Both regulators—the one from 1860 invented by Benoît Rouquayrol and the twin-hose Cousteau-type invented in 1943 by Gagnan and Cousteau—received, among some others, the name of régulateur (French for "regulator"). For the 1860 régulateur see the page of the Rouquayrol-Denayrouze apparatus in the Musée du Scaphandre website (a diving museum in Espalion, south of France) . For the word régulateur as used by Cousteau himself just check page 8 in the first French edition of Cousteau's book The Silent World: Jacques-Yves Cousteau & Frédéric Dumas, Le Monde du silence, Éditions de Paris, Paris, 1953, Dépôt légal 1er Trimestre 1954 – Édition N° 228 – Impression N° 741 (in French). or demand valve. The Aqua-Lung was invented in France during the winter of 1942–1943 by two Frenchmen: engineer Émile Gagnan and Jacques Cousteau, who was a Naval Lieutenant (). It allowed Cousteau and Gagnan to film and explore underwater more easily.
The invention revolutionized autonomous underwater diving by providing a compact, reliable system capable of a greater depth range and endurance than its precursors, and was a major factor influencing the development of recreational scuba diving after WWII.
The twin-hose Aqua-Lung demand regulator is the foundation of all scuba regulators. A diaphragm is used to control a valve to deliver the breathing gas to the diver on demand, at ambient water pressure. However, the layout has changed to a single hose system, where the second stage is split from the first stage along with the exhaust valve, as they must be kept at the same depth, and repositioned at the diver's mouth, eliminating the need for the exhaust hose, and allowing the use of a more rugged, smaller bore hose for the intermediate pressure gas supply to the second stage valve at the mouthpiece, but increasing the load on the diver's jaw and releasing bubbles nearer the eyes and ears.
Mechanism
The Aqua-Lung is a self-contained open-circuit demand system, which means that breathing gas is provided from high-pressure storage carried by to the diver on demand, when the diver inhales and reduces the pressure in the supply hose, subsequently the flow is shut off when not required. Once breathed, the exhaled gas is vented to the surroundings. Scuba systems invented before the Aqua-Lung were mostly closed circuit rebreather equipment, in which breathing gas flows through an ambient pressure hose to the diver, and exhaled gas is returned through a scrubber which removes carbon dioxide, to a counter-lung reservoir. Some fresh gas is added to maintain the oxygen content and is then circulated back to the diver again in a closed loop. In open circuit free-flow systems, the air is supplied at an approximately constant rate, and the diver uses only a relatively small part of the passing gas.
The original Aqua-Lung regulator was a single stage unit, packaged in a circular brass housing mounted on the cylinder valve behind the diver's neck. High pressure gas flows into the regulator from the cylinder valve outlet, and is blocked by the demand valve. When the diver consumes ambient pressure gas, the pressure falls in the low pressure chamber and the diaphragm deforms inwards pushing against the valve lifter. This opens the high pressure valve permitting gas to flow past the valve seat into the low pressure chamber. When the diver stops inhaling, pressure in the low pressure chambers quickly rises until the diaphragm returns to its neutral position and no longer presses on the valve lifter, shutting off the flow until the next breath is taken. On a single stage regulator, the flow rate through the demand valve orifice will vary depending on cylinder pressure for the same opening size, and the opening force required will vary depending on the inlet pressure and orifice area, together making the delivery rate vary as the pressure in the cylinder changes. Flow rate is also affected by downstream pressure, which varies with depth, so the effort of breathing could vary considerably during a dive, even without taking diver attitude into account.
Later models included a first stage regulator to provide air to the demand valve at a lower pressure, compensated for depth, which allowed finer control and greater sensitivity to small pressure differences over the second stage diaphragm, but less sensitivity to cylinder pressure variation during the dive, both desirable features. Both the first and second stage valve mechanisms of the regulator are packaged in the circular brass housing mounted on the cylinder valve behind the diver's neck. High pressure gas flows into the regulator from the cylinder valve outlet, and is blocked by the first stage valve. The first stage reduces cylinder pressure to an interstage pressure, a fairly constant few bars higher than the ambient pressure, set by an adjustable spring preloading the diaphragm, The interstage breathing gas is then reduced to ambient pressure by the second stage.
The first stage diaphragm is a spring-loaded flexible cover to the interstage pressure chamber. When the diver consumes gas from the second stage, the pressure falls in the interstage chamber and the diaphragm deforms inwards pushing against the valve lifter. This opens the high pressure valve permitting gas to flow past the valve seat into the interstage chamber. When the diver stops inhaling, pressure in the low pressure chambers quickly rises until the diaphragm returns to its neutral position and no longer presses on the valve lifter, shutting off the flow until the next breath is taken.
The second, or demand valve stage, keeps the gas in the interstage chamber until it is opened by a reduction in pressure in the low-pressure chamber. It reduces the pressure of the interstage air supply to very nearly ambient pressure when the diver inhales the air in the low pressure chamber and the larger, more sensitive, low pressure diaphragm deflects inwards to push against the lever operating the second stage valve. When the diver stops inhaling, the flow continues only until the pressure in the low pressure chamber balances the ambient water pressure on the outside of the low pressure diaphragm.
To prevent free-flow or excessive exhaust back-pressure, the exhaust valve must be at the same depth as the diaphragm, and the only reliable place to do this is in the same housing. The air flows through a pair of large bore corrugated rubber hoses to and from the mouthpiece. The supply hose is connected to one side of the regulator body and supplies air to the mouthpiece through a non-return valve, and the exhaled air is returned to the regulator housing on the outside of the diaphragm, also through a non-return valve on the other side of the mouthpiece and through another non-return exhaust valve in the regulator housing. The non-return valves fitted to each of the breathing hoses where they connect to the mouthpiece prevent any water that gets into the mouthpiece from going into the inhalation hose, and ensures that once it is blown into the exhalation hose that it cannot flow back. This slightly increases the flow resistance of air and the work of breathing, but makes the regulator easier to clear, particularly when the diver does not have enough air in their lungs to blast clear with a sharp exhalation. By rolling in the right direction in a horizontal position, the trapped water will flow by gravity into the exhaust hose, and when the mouthpiece is shallower than the diaphragm, the regulator will tend to free-flow when out of the mouth, which can also be used to purge the inhalation hose.
Ideally the delivered pressure is equal to the resting pressure in the diver's lungs as this is what human lungs are adapted to breathe. With a twin hose regulator behind the diver at shoulder level, the delivered pressure changes with diver orientation. if the diver rolls on his or her back the released air pressure is higher than in the lungs. Divers learned to restrict flow by using their tongue to close the mouthpiece. When the cylinder pressure was running low and air demand effort rising, a roll to the right side made breathing easier. Raising the mouthpiece above the regulator increases the delivered pressure of gas at the mouth and increases exhaust resistance, and lowering the mouthpiece reduces delivered pressure and increases inhalation resistance.
Invention and patent An earlier underwater breathing regulator, known as the régulateur, was invented in France in 1860 by Benoît Rouquayrol. He first conceived it as a device to help assist in escaping from flooded mines. The Rouquayrol regulator was adapted to diving in 1864, when Rouquayrol met the lieutenant de vaisseau Auguste Denayrouze. The Rouquayrol-Denayrouze apparatus went into mass production and commercialization on 28 August 1865, when the French Navy Minister ordered the first units.
After 1884, several companies and entrepreneurs bought or inherited the patent and produced it until 1965. In 1942, during the German occupation of France, the patent was held by the Bernard Piel Company (Établissements Bernard Piel).List of French companies which produced the Rouquayrol and Denayroze patents (Association les pieds lourds website, in French). One of their apparatuses went to Émile Gagnan, an engineer employed by the Air Liquide company. Gagnan miniaturized and adapted it to gas generators in response to a fuel shortage, which 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 Commander Yves le Prieur,Jacques-Yves Cousteau with Frédéric Dumas, The Silent World (London: Hamish Hamilton, 1953). so he introduced Cousteau to Gagnan in December 1942. On Cousteau's initiative, the Gagnan regulator was modified for use in diving. Cousteau and Gagnan were issued a patent some weeks later in 1943.The Musée du Scaphandre website (a diving museum in Espalion, south of France) mentions how Gagnan and Cousteau adapted a Rouquayrol-Denayrouze apparatus by means of the Air Liquide company (in French). After the war, in 1946, both men founded La Spirotechnique as a division of Air Liquide in order to mass-produce and sell their invention, this time under a new 1945 patent, and known as CG45 ("C" for Cousteau, "G" for Gagnan and "45" for 1945). This same CG45 regulator, produced for more than ten years and commercialized in France as of 1946, was the first to actually be called the "Aqua-Lung". In France, the terms scaphandre autonome ("autonomous diving set"), scaphandre Cousteau-Gagnan ("Cousteau-Gagnan diving set"), or CG45 were meaningful enough for commercialization, but to sell his invention in English-speaking countries, Cousteau needed an appealing name following English language standards. He then coined the trade name Aqua-Lung.