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 its pressure at each stage.

A diving regulator is such a regulator which is used in a scuba set and supplies the diver with breathing gas at ambient pressure from one or more diving cylinders. The gas may be air.

People often use the words "regulator" and "demand valve" interchangeably, but here a demand valve is the part of a regulator that delivers gas to the diver's mouth.

The parts of a regulator are described in downstream order as following the gas flow from the cylinder to its final use.

Fastening the Regulator to the Cylinder or Cylinder Block

In an open-circuit scuba set, the regulator, or its first-stage, has an A-clamp or a DIN fitting to connect it to the pillar valve of the diving cylinder

A-Clamp or Yoke

This is the traditional type. It clamps an open hole on the regulator against an open hole on the cylinder. This connection is made gas pressure tight with an O-ring.

A-clamp or yoke - the connection on the regulator surrounds the valve pillar and presses the output O-ring of the pillar valve against the input seat of the regulator. This type is simple, cheap and very widely used worldwide. It has a maximum pressure rating of 232 bar and the weakest part of the seal, the o-ring, is not well protected from over-pressurisation.

DIN Valve

A DIN fitting stage with 2 medium pressure and 1 high pressure hose. The DIN Fitting is a modern type of direct screw-in connection.

232 bar DIN (5-thread, metric M 25×2) - the regulator screws into the pillar valve trapping the O-ring securely. These are more reliable than A-clamps because the o-ring is well protected, but many countries do not use DIN fittings widely on compressors, or cylinders which have DIN fittings, so a European diver with a DIN system abroad in many places will need to take an adaptor.

300 bar DIN : (7-thread, metric M 25×2) - these are similar to 5-thread DIN fitting but are rated to 300 bar working pressures. The 300 bar pressures are common in European diving and in US cave diving, but their acceptance in U.S. sport diving has been hampered by the fact that United States Department of Transportation rules presently prohibit the transport of metal scuba cylinders on public roads with pressures above about 230 bar, even if the cylinders and air delivery systems have been rated for these pressures by the American agencies which oversee cylinder testing and equipment compatibility for SCUBA (OSHA and CGA).

Pressure Gauge

To monitor breathing gas pressure in the diving cylinder, a diving regulator usually has a high pressure hose leading to a contents gauge (also called pressure gauge). The port for this hose leaves the first-stage upstream of all pressure-reducing valves. The contents gauge is a pressure gauge measuring the gas pressure in the diving cylinders so the diver knows how much gas remains in the cylinder. It is also known as submersible pressure gauge or SPG. There are several types of contents gauge:

Standard Type

This is an analogue gauge that can be held in the palm of a hand and is connected to the first stage by a high pressure hose. It displays with a pointer moving over a dial. Sometimes they are fixed in a console, which is a plastic or rubber case that holds the air pressure gauge and also a depth gauge and/or a dive computer and/or a compass.

Button Gauges

These are coin-sized analogue gauges connected directly to the first stage. They are often used on decompression cylinders. Due to their small size, it can be difficult to read the gauge to a resolution of less than 20 bar / 300 psi.

Air Integrated Computers

Some dive computers are designed to measure and display and monitor pressure in the diving cylinder. This can be very beneficial to the diver, but if the dive computer fails, the diver can no longer monitor his or her gas reserves. Most divers using a gas-integrated computer will also have a standard air pressure gauge. The computer is either connected to the first stage by a high pressure hose, or has two parts, the pressure transducer on the first stage and the display at the wrist or console, which communicate by radio link; the signals are encoded to eliminate the risk of one diver's computer picking up a signal from another diver's transducer, or radio interference from other sources.

Mechanical Reserve Valves

In the past, some types of diving cylinder had a mechanical reserve valve that restricted air flow when the pressure was below 500 psi. Alerted to having a low gas supply the diver would pull a lever to open the reserve valve and surface using the reserve gas. These valves are known as "J valves" due to the letter J being next to that valve in the US Divers product catalog. Valves without the reserve lever are called "K valves" for the same reason; being the next item in the catalog they were denoted by the letter K. Modern divers using "J valves" dive with the reserve valve in the open position and depend on a contents gauge or computer to monitor gas supply.

First stage

The first stage of the regulator is usually designed to release the gas at a constant rate from the cylinder despite the pressure in the cylinder becoming less as the gas in the cylinder is used. The first stage takes gas from the diving cylinder at pressures of 200 - 300 bar (3000 - 4500 psi) and reduces its pressure to 10 bar (150 psi) higher than ambient pressure in the medium pressure hoses.

Types

The mechanism inside the first stage can be of the diaphragm type or the piston type. Both types can be balanced or unbalanced. A diaphragm first stage may be over-balanced as well. The performance of unbalanced regulators decreases as the cylinder pressure falls. A balanced regulator will keep the same ease of breathing at all depths and pressures. The over-balanced regulator will become easier to breath at depth.

Piston type

Piston-type first stages are simpler to make than the diaphragm type. They provide higher performance when breathed at depth. They need more careful maintenance because some of the internal moving parts are exposed to water and contaminants in the water.

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.

Diaphragm type

Diaphragm-type first stages are more complex and have more components than the piston type. They are more responsive; they provide gas when the diver uses little inhalation effort.

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.

Risk of the Regulator Becoming Blocked with Ice

As gas leaves the cylinder it decreases in pressure in the first stage, becoming very cold due to adiabatic expansion. Where the water temperature is less than 5°C any water inside the regulator may freeze, preventing the valve closing, causing a free-flow that can empty a full cylinder within a minute or two. Generally the water that freezes is in the ambient pressure chamber around a spring that keeps the valve open and not in moisture in the dry breathing gas from the cylinder.

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.

Types of Last Stage

Not Present

If there is only one stage, and that stage is constant flow, the gas must be turned on and off at the cylinder.

Manually operated valve

The diver uses a button or lever or knob to blow gas or air into a device, such as buoyancy compensators, drysuits, and many rebreathers. This type of valve is connected to the first stage with a medium pressure hose commonly called a "direct feed". (The valves on blowtorches are this type also.)

Demand Valve

A demand valve detects when the diver starts inhaling and supplies the diver with a breath of gas at ambient pressure.

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 it 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.

Valve Operated by a Solenoid

Fully closed circuit, electronic rebreathers have electronically controlled valves to inject fresh oxygen into the loop. The valve is opened with a solenoid in response to falling oxygen partial pressure detected by the electro-galvanic fuel cells that monitor the loop. These valves are connected to the first stage with a direct feed.

Arrangements of the Assembly of Valves

Often one first stage supplies in parallel two or more second stages of various types. Each of these second stages should be looked for below according to its type.

Often a branch tube goes off without going through any pressure-reducing valve stages, to a pressure gauge.

Types of Regulator

Constant flow

In constant-flow regulators the first stage is constant flow, and the second stage is a plain on/off valve. (In a blowtorch the first stage is fastened to the cylinder and the second stage is on the torch head.) They are the earliest type of breathing set regulator. They are used now in many rebreathers. The only control the diver has is to open or close the second stage. Constant flow valves in an open-circuit breathing set consume gas less economically than demand valve regulators because gas flows even when it is not needed.

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 need to put the second-stage on/off valve where it could be easily operated by the diver. Examples were:

* "Ohgushi's Peerless Respirator". The valve was operated by the diver's teeth.
* Commandant le Prieur's breathing sets: see Timeline of underwater technology. They were used for some sport diving on the French Riviera.

Twin-hose

This is the first type of scuba demand valve that got into general use. This type of regulator has two (or occasionally one or three) stages in series in a large circular valve assembly mounted on top of the cylinder pack. The last (or only) stage is the demand valve.

In Europe and the USA, as officially made, twin-hose 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 breathing out through; it was not for rebreathing but to keep the air inside the breathing tube at the same depth 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 OK on land, but when he tested the aqualung in the river Marne the air ran away 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 snorkelling 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 bulk of 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 become 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. In 1948 to 1952 Ted Eldred designed his Porpoise air scuba to supply 300 litres/minute if breathed from 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.

The twin-hose mouthpiece or fullface mask has reappeared in modern rebreathers, but as part of the breathing loop, not as part of a regulator.

Twin-hose - Home-made

In 1956 and for some years afterwards in Britain, factory-made aqualungs were very expensive, and many aqualungs of this type were made by sport divers in diving clubs' workshops, using miscellaneous industrial and war-surplus parts. One necessary raw material was a Calor Gas bottled butane gas regulator, whose 1950's version was like an aqualung regulator's second stage but operated constant-flow because its diaphragm was spring-loaded; conversion included changing the spring and making several big holes in the wet-side casing. The cylinder was often an ex-RAF pilot's oxygen cylinder; some of these cylinders were called tadpoles from their shape.

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.

Two Stage, Single Hose

Most modern scuba regulators are of this type. Its main components are: a first stage, from which one or more medium pressure hoses run to various equipment listed below. The first make of this sort of scuba was the Porpoise (make of scuba gear) which was made in Australia.

First Stage Valve

The first stage has a high-pressure "port" for the high-pressure hose to the pressure gauge. It has a number of "ports" for low-pressure hoses to carry gas to other components, which serve as second-stage valves of various sorts. All unused ports must be blanked off.

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 compensators inflation valves: these low-pressure hoses are called direct feeds.

Second Stage Valve

A second stage valve can be:

Direct Feed

A connection to inflate a buoyancy compensators or a drysuit, is manually operated by a button or lever or knob.

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).

Octopus

Sometimes (nowadays nearly always) a single-hose regulator has more than one demand valve (= DV). If the extra DV is simply a spare DV for use by the diver's buddy it is usually called an octopus. The medium pressure hose on the octopus is usually longer than the medium pressure hose on the DV that the diver uses.

The demand valve could be a hybrid DV and buoyancy compensators inflation valve. Both types are sometimes called alternate air sources, and more confusingly a DV on a regulator connected to a separate independent diving cylinders 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:

* A version of the Le Prieur breathing set. Yves Le Prieur patented it in 1946 and the patent was granted on 10 February 1947.
* Captain Trevor Hampton invented independently a similar regulator-mask in the 1950's and submitted it for patent, but the British Navy requisitioned the patent, but found no use for it and eventually released it, but by then the market had moved on and it was too late to make this regulator-mask in bulk for sale.

Dive/Surface Valve or Bailout Valve

A Dive/surface valve (DSV) or bailout valve (BOV) is a device in the mouthpiece on the loop of a rebreather which connects to a bailout demand valve and can be switched to provide gas from either the loop or the demand valve without the diver taking the mouthpiece from his or her mouth. An important safety device when carbon dioxide poisoning occurs.

Manufacturers

* AquaLung
* Apeks
* Atomic Aquatics
* Dive Rite
* Mares
* Oceanic
* Poseidon
* ScubaPro
* Sherwood
* Cressi Sub

Regulator Tips

The most important piece of gear you can buy. Literally, your underwater life support system. A regulator setup is comprised of several parts: the first-stage which takes air from your tank and delivers it to the second-stage that you breath from and the octopus, or safe-second, for air-sharing.

Here are our tips for choosing the best regulator for your needs: