A diving regulator is a pressure regulator used in a scuba set that supplies the diver with breathing gas at ambient pressure from one or more diving cylinders. The gas may be air or one of a variety of specially blended breathing gases. A gas pressure regulator has one or more valves in series, which let the gas out of a gas cylinder in a controlled way, lowering air pressure at each stage.
The terms "regulator" and "demand valve" are often used interchangeably, but a demand valve is the part of a regulator that delivers gas to the diver's mouth in a regulator with more than one stage.
For the history of the diving regulator, see Timeline of diving technology.
The parts of a regulator are described in downstream order as following the gas flow from the cylinder to its final use.
In an open-circuit scuba set, the first-stage of the regulator has an A-clamp, also known as a yoke, or a DIN fitting to connect it to the pillar valve of the diving cylinder. Yoke valves are the most common type by far; it clamps an open hole on the regulator against an open hole on the cylinder. The user loosely screws the clamp in place and once the cylinder valve is opened, gas pressure completes the seal along with an O-ring. The diver must take care not to screw the yoke down too tightly, or it may prove impossible to remove without tools. The DIN fitting is a type of direct screw-in connection to the cylinder. While less common worldwide, the DIN system has the advantage of withstanding greater pressure, permitting the use of high-pressure steel cylinders. DIN fittings are the standard in much of central Europe.
Most yoke-type valves are of the K-valve type, which is a simple on-off valve. In the mid-1960s, J-valves were widespread. J-valves contain a spring-operated shutoff that is triggered when tank pressure falls to 300-500 psi, causing breathing resistance and warning the diver that he or she is dangerously low on air. The reserve air is released by pulling a reserve lever on the valve. J-valves fell out of favor with the introduction of pressure gauges, which allow divers to keep track of their air underwater, especially as the valve-type are subject to accidental release of reserve air and increase the cost and servicing of the valve.
With the piston-type first stage, the piston is rigid and acts directly on the seat of the valve. When the pressure in the medium pressure drops because the diver has used gas from a second stage valve, the piston lifts off the valve seat and slides towards the medium pressure chamber. This brings high pressure gas into the medium pressure chamber until the pressure in the chamber has risen enough to push the piston back onto the seat and close the valve.
The diaphragm is a flexible cover to the medium-pressure chamber. When the diver consumes gas from a medium-pressure second stage, the pressure falls in the medium-pressure chamber and the diaphragm collapses inwards pushing against the valve lifter. This opens the valve letting high-pressure gas pass the valve seat into the medium-pressure chamber. When the pressure in the medium-pressure chambers rises, the diaphragm inflates outwards reducing the force on the valve lifter, letting the spring behind the valve close it.
The modern trend of using more plastics, instead of metals, within the regulators encourages freezing because it insulates the inside of a cold regulator from the warmer surrounding water. Environmental sealing of the ambient pressure chamber and teflon coatings around springs are used to reduce the risk of freezing inside the regulator.
The demand valve was invented in 1865 in France, and forgotten in the next few years, and was not invented again until the late 1930s.
The demand valve has a chamber, which in normal use contains breathing gas at ambient pressure. A valve which supplies medium pressure gas can vent into the chamber. Either a mouthpiece or a fullface mask is connected to the chamber, for the diver to breathe from. On one side of the chamber is a flexible diaphragm to control the operation of the valve.
When the diver tries to breathe in, the inhalation lowers the pressure inside the chamber, which moves the diaphragm inwards operating a system of levers. This operates against the closing spring and lifts the valve off its seat, opening the valve and releasing gas into the chamber. The medium pressure gas, at about 10 bar/140 psi over ambient pressure, expands, reducing its pressure to ambient pressure, blowing out any water in the chamber and supplying the diver with gas to breathe. When the chamber is full and the lowering of pressure has been reversed, the diaphragm expands outwards to its normal position to close the medium pressure valve when the diver stops breathing in.
When the diver exhales, one-way valves, made from a flexible and air-tight material, flex outwards under the pressure of the exhalation allowing gas to escape from the chamber. They close making a seal when the exhalation stops and the pressure inside the chamber reduces to ambient pressure.
The diaphragm is protected by being covered by a second chamber, which the outside water can enter freely through large holes or slits.
Some passive semi-closed circuit rebreathers use a form of demand valve, which senses the volume of the loop and injects more gas when the volume falls below a certain level.
Most modern demand valves use a downstream rather than an upstream valve mechanism. In a downstream valve, the moving part of the valve opens in the downstream direction and is kept closed by a spring. In an upstream valve, the moving part works against the pressure and opens in an upstream direction. If the first stage jams open and the medium pressure system over-pressurises, the second stage downstream valve opens automatically resulting in a "freeflow". With an upstream valve, the result of over-pressurisation may be a ruptured hose or the failure of another second stage valve such as one that inflates a buoyancy device.
Often a branch tube goes off without going through any pressure-reducing valve stages, to a pressure gauge.
In some rebreathers, e.g. the Siebe Gorman Salvus, the oxygen cylinder has two first stages in parallel. One is constant flow; the other is a plain on-off valve called a bypass; both feed into the same exit pipe which feeds the breathing bag. In the Salvus there is no second stage and the gas is turned on and off at the cylinder. Some simple oxygen rebreathers had no constant-flow valve, but only the bypass, and the diver had to operate the valve at intervals to refill the breathing bag as he used the oxygen.
With active semi-closed circuit rebreathers, the diver installs one of a number of different sized orifices in the valve before the dive. For safety reasons these should be chosen to provide more gas than the diver needs, to avoid hypoxia.
Before 1939, diving and industrial open-circuit breathing sets with constant-flow regulators were designed and made, but did not get into general use due to excessively short dive duration for its weight. Design complications resulted from the need to put the second-stage on/off valve where it could be easily operated by the diver. Examples were:-
In Europe and the USA, as officially made, regulators were always fastened to the cylinder with an A-clamp.
This type of regulator has two wide corrugated breathing tubes. The second tube was for exhalation; it was not for rebreathing but to keep the air inside the breathing tube at the same pressure as the water outside the regulator diaphragm. This second breathing tube returns the exhaled air to the regulator on the wet side of the diaphragm, where it is released through a duck's-beak-shaped rubber one-way valve, and comes out of the holes in the wet-side cover. Nearly always in the mouthpiece assembly there are one-way valves to stop air or water going from the mouthpiece into the inhaling tube or from the exhaling tube into the mouthpiece.
In Cousteau's first aqualung as first made, there was no second tube and the exhaled breath exited to the outside through a one-way valve at the mouthpiece. It worked out of water, but when he tested the aqualung in the river Marne air escaped from the regulator before it could be breathed when the mouthpiece was above the regulator. After that, he had the second breathing tube fitted.
Even with both tubes fitted, raising the mouthpiece above the regulator increases the flow of gas and lowering the mouthpiece increases breathing resistance. As a result, many aqualung divers, when they were snorkeling on the surface to save air while reaching the dive site, put the loop of hoses under an arm to avoid the mouthpiece floating up causing free flow.
Divers had to carry more weight underwater to compensate for the buoyancy of the air in the hoses. An advantage with this type of regulator is that the bubbles leave the regulator behind the diver's head, increasing visibility, and not interfering with underwater photography. They have been superseded by the single hose regulator and became obsolete for most diving in the 1980s.
The original Cousteau twin-hose diving regulators could deliver about 140 litres of air per minute, and that was officially thought to be adequate; but divers sometimes needed a faster rate, and had to learn not to "beat the lung", i.e. to try to breathe faster than the regulator could supply. Between 1948 and 1952 Ted Eldred designed his Porpoise air scuba to supply 300 litres/minute if the diver need to breathe that fast, and that soon became British and Australian naval standard.
Some modern twin-hose regulators have one or more low-pressure ports that branch off between the two valve stages, as direct feeds, as described under #Two stage, single hose below.
Someone made a twin-hose type regulator where the energy released as the air expands from cylinder pressure to the surrounding pressure as the diver breathes in, is not thrown away but used to power a propeller.
In least one version of Russian twin-hose aqualung, the regulator did not have an A-clamp but screwed into a large socket on the cylinder manifold; that manifold was thin, and meandered somewhat. It had two cylinders and a pressure gauge. There is suspicion that those Russian aqualungs started as a factory-made improved descendant of an aqualung home-made by British sport divers and obtained unofficially by a Russian and taken to Russia.
With regulators that are used as breathing sources, at least one low-pressure hose connects to a demand valve. Some low-pressure hoses connect to the diving suit inflation valve and the buoyancy compensator inflation valves: these low-pressure hoses are called direct feeds.
This type of second stage is called demand valve or DV. It is fed by a medium pressure hose from the first-stage. It works as described in the #Types of last stage section above. When the diver breathes out, the air goes to the dry side of the diaphragm, and is released to the outside through (usually two) one-way valves. It has a purge button, which the diver can press to depress the diaphragm to make gas flow to blow water out of the mouthpiece (or for other purposes such as filling a lifting bag).
The demand valve could be a hybrid DV and buoyancy compensator inflation valve. Both types are sometimes called alternate air sources, and more confusingly a DV on a regulator connected to a separate independent diving cylinder would also be called an "alternate air source".
There have been at least two cases of a single-hose-type demand regulator last stage built into a circular fullface mask so that the mask's big circular front window plus the flexible rubber seal joining it to its frame, was a very big and thus very sensitive regulator diaphragm:-
In the United States Military, scuba regulators must adhere to performance specifications as outlined by the Mil-R-24169B which was based on equipment performance until recently.
Various breathing machines have been developed and used for assessment of breathing apparatus performance. ANSTI has developed a testing machine that measures the inhalation and exhalation effort in using a regulator; publishing results of the performance of regulators in the ANSTI test machine has resulted in big performance improvements.
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