Diving regulator
A diving regulator or underwater diving regulator is a
The terms "regulator" and "demand valve" (DV) are often used interchangeably, but a demand valve is the final stage pressure-reduction regulator that delivers gas only while the diver is inhaling and reduces the gas pressure to approximately ambient. In single-hose demand regulators, the demand valve is either held in the diver's mouth by a mouthpiece or attached to the full-face mask or helmet. In twin-hose regulators the demand valve is included in the body of the regulator which is usually attached directly to the cylinder valve or manifold outlet, with a remote mouthpiece supplied at ambient pressure.
A pressure-reduction regulator is used to control the delivery pressure of the gas supplied to a free-flow helmet or full-face mask, in which the flow is continuous, to maintain the downstream pressure which is limited by the ambient pressure of the exhaust and the flow resistance of the delivery system (mainly the umbilical and exhaust valve) and not much influenced by the breathing of the diver.
The
Purpose
The diving regulator is a mechanism which reduces the pressure of the supply of breathing gas and provides it to the diver at approximately ambient pressure. The gas may be supplied on demand, when the diver inhales, or as a constant flow past the diver inside the helmet or mask, from which the diver uses what is necessary, while the remainder goes to waste.[2]: 49
The gas may be provided directly to the diver, or to a rebreather circuit, to make up for used gas and volume changes due to depth variations. Gas supply may be from a high-pressure scuba cylinder carried by the diver, or from a surface supply through a hose connected to a compressor or high pressure storage system.
Types
An open circuit demand valve provides gas flow only while the diver inhales, a free flow regulator provides a constant flow rate at the delivery pressure, reclaim and built-in-breathing-systems regulators allow exhaust outflow only during exhalation. Rebreathers use demand regulators to make up a volume deficit in the loop, and may use constant mass flow regulators to refresh the oxygen content of the loop gas mixture.
Open circuit demand valve
A demand valve detects the pressure drop when the diver starts inhaling and supplies the diver with a breath of gas at ambient pressure. When the diver stops inhaling, the demand valve closes to stop the flow. The demand valve has a chamber, which in normal use contains breathing gas at ambient pressure, which is connected to a bite-grip mouthpiece, a
This is done by a mechanical system linking the diaphragm to a valve which is opened to an extent proportional to the displacement of the diaphragm from the closed position. The pressure difference between the inside of the mouthpiece and the ambient pressure outside the diaphragm required to open the valve is known as the cracking pressure. This cracking pressure difference is usually negative relative to ambient, but may be slightly positive on a positive pressure regulator (a regulator that maintains a pressure inside the mouthpiece, mask or helmet, which is slightly greater than the ambient pressure). Once the valve has opened, gas flow should continue at the smallest stable pressure difference reasonably practicable while the diver inhales, and should stop as soon as gas flow stops. Several mechanisms have been devised to provide this function, some of them extremely simple and robust, and others somewhat more complex, but more sensitive to small pressure changes.[3]: 33 The diaphragm is protected by a cover with holes or slits through which outside water can enter freely. This cover reduces sensitivity of the diaphragm to water turbulence and dynamic pressure due to movement, which might otherwise trigger gas flow when it is not needed.
When the diver starts to inhale, the removal of gas from the casing lowers the pressure inside the chamber, and the external water pressure moves the diaphragm inwards operating a lever which lifts the valve off its seat, releasing gas into the chamber. The inter-stage gas, at about 8 to 10 bars (120 to 150 psi) over ambient pressure, expands through the valve orifice as its pressure is reduced to ambient and supplies the diver with more gas to breathe. When the diver stops inhaling the chamber fills until the external pressure is balanced, the diaphragm returns to its rest position and the lever releases the valve to be closed by the valve spring and gas flow stops.[3]
When the diver exhales, one-way valves made from a flexible air-tight material flex outwards under the pressure of the exhalation, letting gas escape from the chamber. They close, making a seal, when the exhalation stops and the pressure inside the chamber reduces to ambient pressure.[3]: 108
The vast majority of demand valves are used on open circuit breathing apparatus, which means that the exhaled gas is discharged into the surrounding environment and lost. Reclaim valves can be fitted to helmets to allow the used gas to be returned to the surface for reuse after removing the carbon dioxide and making up the oxygen. This process, referred to as "push-pull", is technologically complex and expensive and is only used for deep commercial diving on heliox mixtures, where the saving on helium compensates for the expense and complications of the system, and for diving in contaminated water, where the gas is not reclaimed, but the system reduces the risk of contaminated water leaking into the helmet through an exhaust valve.[4]
Open circuit free-flow regulator
These are generally used in surface supply diving with free-flow masks and helmets. They are usually a large high-flow rated industrial gas regulator that is manually controlled at the gas panel on the surface to the pressure required to provide the desired flow rate to the diver. Free flow is not normally used on scuba equipment as the high gas flow rates are inefficient and wasteful.
In constant-flow regulators the pressure regulator provides a constant reduced pressure, which provides gas flow to the diver, which may be to some extent controlled by an adjustable orifice controlled by the diver. These are the earliest type of breathing set flow control. The diver must physically open and close the adjustable supply valve to regulate flow. 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, and must flow at the rate required for peak inhalation. Before 1939, self contained diving and industrial open circuit breathing sets with constant-flow regulators were designed by
Reclaim regulators
The cost of breathing gas containing a high fraction of
Reclaim regulators are also sometimes used for hazmat diving to reduce the risk of backflow of contaminated water through the exhaust valves into the helmet. In this application there would not be an underpressure flood valve, but the pressure differences and the squeeze risk are relatively low.[8][4]: 109 The breathing gas in this application would usually be air and would not actually be recycled.
Built-in breathing systems
BIBS regulators for hyperbaric chambers have a two-stage system at the diver similar to reclaim helmets, though for this application the outlet regulator dumps the exhaled gas through an outlet hose to the atmosphere outside the chamber.[9]
These are systems used to supply breathing gas on demand in a chamber which is at a pressure greater than the ambient pressure outside the chamber.[10] The pressure difference between chamber and external ambient pressure makes it possible to exhaust the exhaled gas to the external environment, but the flow must be controlled so that only exhaled gas is vented through the system, and it does not drain the contents of the chamber to the outside. This is achieved by using a controlled exhaust valve which opens when a slight over-pressure relative to the chamber pressure on the exhaust diaphragm moves the valve mechanism against a spring. When this over-pressure is dissipated by the gas flowing out through the exhaust hose, the spring returns this valve to the closed position, cutting off further flow, and conserving the chamber atmosphere. A negative or zero pressure difference over the exhaust diaphragm will keep it closed. The exhaust diaphragm is exposed to the chamber pressure on one side, and exhaled gas pressure in the oro-nasal mask on the other side. The supply of gas for inhalation is through a demand valve which works on the same principles as a regular diving demand valve second stage. Like any other breathing apparatus, the dead space must be limited to minimise carbon dioxide buildup in the mask.
In some cases the outlet suction must be limited and a
The major application for this type of BIBS is supply of breathing gas with a different composition to the chamber atmosphere to occupants of a hyperbaric chamber where the chamber atmosphere is controlled, and contamination by the BIBS gas would be a problem.[10] This is common in therapeutic decompression, and hyperbaric oxygen therapy, where a higher partial pressure of oxygen in the chamber would constitute an unacceptable fire hazard, and would require frequent ventilation of the chamber to keep the partial pressure within acceptable limits. Frequent ventilation is noisy and expensive, but can be used in an emergency.[9]
Rebreather regulators
Rebreather systems used for diving recycle most of the breathing gas, but are not based on a demand valve system for their primary function. Instead, the
The
The bailout valve (BOV) is an open circuit demand valve built into a rebreather mouthpiece or other part of the breathing loop. It can be isolated while the diver is using the rebreather to recycle breathing gas, and opened, while at the same time isolating the breathing loop, when a problem causes the diver to bail out onto open circuit. The main distinguishing feature of the BOV is that the same mouthpiece is used for open and closed-circuit, and the diver does not have to shut the dive/surface valve (DSV), remove it from their mouth, and find and insert the bailout demand valve in order to bail out onto open circuit. Although costly, this reduction in critical steps makes the integrated BOV a significant safety advantage, particularly when there is a high partial pressure of carbon dioxide in the loop, as hypercapnia can make it difficult or impossible for the diver to hold their breath even for the short period required to swap mouthpieces.[14][15]
Constant mass flow addition valves are used to supply a constant mass flow of fresh gas to an active type semi-closed rebreather to replenish the gas used by the diver and to maintain an approximately constant composition of the loop mix. Two main types are used: the fixed orifice and the adjustable orifice (usually a needle valve). The constant mass flow valve is usually supplied by a gas regulator that is isolated from the ambient pressure so that it provides an absolute pressure regulated output (not compensated for ambient pressure). This limits the depth range in which constant mass flow is possible through the orifice, but provides a relatively predictable gas mixture in the breathing loop. An over-pressure relief valve in the first stage is used to protect the output hose. Unlike most other diving gas supply regulators, constant mass flow orifices do not control the downstream pressure, but they do regulate the flow rate.
Manual and electronically controlled addition valves are used on manual and electronically controlled closed circuit rebreathers (mCCR, eCCR) to add oxygen to the loop to maintain oxygen partial pressure set-point. A manually or electronically controlled valve is used to release oxygen from the outlet of a standard scuba regulator first stage into the breathing loop. An over-pressure relief valve on the first stage is necessary to protect the hose in case of first stage leaks. Strictly speaking, these are not pressure regulators, they are flow control valves.
History
The first recorded demand valve was
On 19 June 1838, in London, William Edward Newton filed a patent (no. 7695: "Diving apparatus") for a diaphragm-actuated, twin-hose demand valve for divers.[18] However, it is believed that Mr. Newton was merely filing a patent on behalf of Dr. Guillaumet.[19]
In 1860 a mining engineer from Espalion (France), Benoît Rouquayrol, invented a demand valve with an iron air reservoir to let miners breathe in flooded mines. He called his invention régulateur ('regulator'). In 1864 Rouquayrol met the French Imperial Navy officer Auguste Denayrouze and they worked together to adapt Rouquayrol's regulator to diving. The Rouquayrol-Denayrouze apparatus was mass-produced with some interruptions from 1864 to 1965.[20] As of 1865 it was acquired as a standard by the French Imperial Navy,[21] but never was entirely accepted by the French divers because of a lack of safety and autonomy.
In 1926
In 1937 and 1942 the French inventor,
It was not until December 1942 that the demand valve was developed to the form which gained widespread acceptance. This came about after French naval officer
The single hose regulator, with a mouth held demand valve supplied with low pressure gas from the cylinder valve mounted first stage, was invented by Australian Ted Eldred in the early 1950s in response to patent restrictions and stock shortages of the Cousteau-Gagnan apparatus in Australia. In 1951 E. R. Cross invented the "Sport Diver," one of the first American-made single-hose regulators. Cross' version is based on the oxygen system used by pilots. Other early single-hose regulators developed during the 1950s include Rose Aviation's "Little Rose Pro," the "Nemrod Snark" (from Spain), and the Sportsways "Waterlung," designed by diving pioneer Sam LeCocq in 1958. In France, in 1955, a patent was taken out by Bronnec & Gauthier for a single hose regulator, later produced as the Cristal Explorer.[25] The "Waterlung" would eventually become the first single-hose regulator to be widely adopted by the diving public. Over time, the convenience and performance of improved single hose regulators would make them the industry standard.[3]: 7 Performance still continues to be improved by small increments, and adaptations have been applied to rebreather technology.
The single hose regulator was later adapted for surface supplied diving in lightweight helmets and full-face masks in the tradition of the Rouquayrol-Denayrouze equipment to economise on gas usage. By 1969 Kirby-Morgan had developed a full-face mask - the KMB-8 Bandmask - using a single hose regulator. This was developed into the Kirby-Morgan SuperLite-17B by 1976,
Secondary (octopus) demand valves, submersible pressure gauges and low pressure inflator hoses were added to the first stage.[when?]
In 1994 a reclaim system was developed in a joint project by Kirby-Morgan and Divex to recover expensive helium mixes during deep operations.[26]
Mechanism and function
Both free-flow and demand regulators use mechanical feedback of the downstream pressure to control the opening of a valve which controls gas flow from the upstream, high-pressure side, to the downstream, low-pressure side of each stage.[28] Flow capacity must be sufficient to allow the downstream pressure to be maintained at maximum demand, and sensitivity must be appropriate to deliver maximum required flow rate with a small variation in downstream pressure, and for a large variation in supply pressure. Open circuit scuba regulators must also deliver against a variable ambient pressure. They must be robust and reliable, as they are life-support equipment which must function in the relatively hostile seawater environment.
Diving regulators use mechanically operated valves.[28] In most cases there is ambient pressure feedback to both first and second stage, except where this is avoided to allow constant mass flow through an orifice in a rebreather, which requires a constant upstream pressure.
The parts of a regulator are described here as the major functional groups in downstream order as following the gas flow from the diving cylinder to its final use.
Connection to the diving cylinder
The first-stage of the scuba regulator will usually be connected to the cylinder valve by one of two standard types of fittings. The CGA 850 connector, also known as an international connector, which uses a yoke clamp, or a DIN screw fitting. There are also European standards for scuba regulator connectors for gases other than air, and adapters to allow use of regulators with cylinder valves of a different connection type.
CGA 850 Yoke connectors (sometimes called A-clamps from their shape) are the most popular regulator connection in North America and several other countries. They clamp the high pressure inlet opening of the regulator against the outlet opening of the cylinder valve, and are sealed by an O-ring in a groove in the contact face of the cylinder valve. The user screws the clamp in place finger-tight to hold the metal surfaces of cylinder valve and regulator first stage in contact, compressing the o-ring between the radial faces of valve and regulator. When the valve is opened, gas pressure presses the O-ring against the outer cylindrical surface of the groove, completing the seal. The diver must take care not to screw the yoke down too tightly, or it may prove impossible to remove without tools. Conversely, failing to tighten sufficiently can lead to O-ring extrusion under pressure and a major loss of breathing gas. This can be a serious problem if it happens when the diver is at depth. Yoke fittings are rated up to a maximum of 240 bar working pressure.
The DIN fitting is a type of screw-in connection to the cylinder valve. The DIN system is less common worldwide, but has the advantage of withstanding greater pressure, up to 300 bar, allowing use of high-pressure steel cylinders. They are less susceptible to blowing the O-ring seal if banged against something while in use. DIN fittings are the standard in much of Europe and are available in most countries. The DIN fitting is considered more secure and therefore safer by many
Conversion kits
Several manufacturers market an otherwise identical first stage varying only in the choice of cylinder valve connection. In these cases it may be possible to buy original components to convert yoke to DIN and vice versa. The complexity of the conversion may vary, and parts are not usually interchangeable between manufacturers. The conversion of
Adaptors
Adaptors are available to allow connection of DIN regulators to yoke cylinder valves (A-clamp or yoke adaptor), and to connect yoke regulators to DIN cylinder valves.[29] There are two types of adaptors for DIN valves: plug adaptors and block adaptors. Plug adaptors are screwed into a 5-thread DIN valve socket, are rated for 232/240 bar, and can only be used with valves which are designed to accept them. These can be recognised by a dimple recess opposite to the outlet opening, used to locate the screw of an A-clamp. Block adaptors are generally rated for 200 bar, and can be used with almost any 200 bar 5-thread DIN valve. A-clamp or yoke adaptors comprise a yoke clamp with a DIN socket in line. They are slightly more vulnerable to O-ring extrusion than integral yoke clamps, due to greater leverage on the first stage regulator.
Single-hose demand regulators
Most contemporary diving regulators are single-hose two-stage demand regulators. They consist of a first-stage regulator and a second-stage demand valve connected by a low pressure hose to transfer breathing gas, and allow relative movement within the constraints of hose length and flexibility.
The first stage is mounted to the cylinder valve or manifold via one of the standard connectors (Yoke or DIN), and reduces cylinder pressure to an intermediate pressure, usually about 8 to 11 bars (120 to 160 psi) higher than the ambient pressure, also called interstage pressure, medium pressure or low pressure.[28]: 17–20
A balanced regulator first stage automatically keeps a constant pressure difference between the interstage pressure and the ambient pressure even as the tank pressure drops with consumption. The balanced regulator design allows the first stage orifice to be as large as needed without incurring performance degradation as a result of changing tank pressure.[28]: 17–20
The first stage regulator body generally has several low-pressure outlets (ports) for second-stage regulators and BCD and dry suit inflators, and one or more high-pressure outlets, which allow a submersible pressure gauge (SPG), gas-integrated diving computer or remote pressure tranducer to read the cylinder pressure. One low-pressure port with a larger bore may be designated for the primary second stage as it will give a higher flow at maximum demand for lower work of breathing.[2]: 50
The mechanism inside the first stage can be of the diaphragm or piston type, and can be balanced or unbalanced. Unbalanced regulators produce an interstage pressure which varies slightly as the cylinder pressure changes and to limit this variation the high-pressure orifice size is small, which decreases the maximum capacity of the regulator. A balanced regulator maintains a constant interstage pressure difference for all cylinder pressures.[28]: 17–20
The second stage, or demand valve reduces the pressure of the interstage air supply to ambient pressure on demand from the diver. The operation of the valve is triggered by a drop in downstream pressure as the diver breathes in. In an upstream valve, the valve is held closed by the interstage pressure and opens by moving into the flow of gas. They are often made as tilt-valves, which are mechanically extremely simple and reliable, but are not amenable to fine tuning.[3]: 14
Most modern demand valves use a downstream valve mechanism, where the valve poppet moves in the same direction as the flow of gas to open and is kept closed by a spring. The poppet is lifted away from the crown by a lever operated by the diaphragm.[3]: 13–15 Two patterns are commonly used. One is the classic push-pull arrangement, where the actuating lever goes onto the end of the valve shaft and is held on by a nut. Any deflection of the lever is converted to an axial pull on the valve shaft, lifting the seat off the crown and allowing air to flow.[3]: 13 The other is the barrel poppet arrangement, where the poppet is enclosed in a tube which crosses the regulator body and the lever operates through slots in the sides of the tube. The far end of the tube is accessible from the side of the casing and a spring tension adjustment screw may be fitted for limited diver control of the cracking pressure. This arrangement also allows relatively simple pressure balancing of the second stage.[3]: 14, 18
A downstream valve will function as an over-pressure valve when the inter-stage pressure is raised sufficiently to overcome the spring pre-load. If the first stage leaks and the inter-stage over-pressurizes, the second stage downstream valve opens automatically. If the leak is bad this could result in a "freeflow", but a slow leak will generally cause intermittent "popping" of the DV, as the pressure is released and slowly builds up again.[3]
If the first stage leaks and the inter-stage over-pressurizes, the second stage upstream valve will not release the excess pressure, This might hinder the supply of breathing gas and possibly result in a ruptured hose or the failure of another second stage valve, such as one that inflates a buoyancy device. When a second stage upstream valve is used a relief valve will be included by the manufacturer on the first stage regulator to protect the hose.[3]: 9
If a shut-off valve is fitted between the first and second stages, as is found on scuba bailout systems used for commercial diving and in some technical diving configurations, the demand valve will normally be isolated and unable to function as a relief valve. In this case an overpressure valve must be fitted to the first stage. They are available as aftermarket accessories which can be screwed into any available low pressure port on the first stage.[30]
Some demand valves use a small, sensitive pilot valve to control the opening of the main valve. The Poseidon Jetstream and Xstream and Oceanic Omega second stages are examples of this technology. They can produce very high flow rates for a small pressure differential, and particularly for a relatively small cracking pressure. They are generally more complicated and expensive to service.[3]: 16
Exhaled gas leaves the demand valve housing through one or two exhaust ports. Exhaust valves are necessary to prevent the diver inhaling water, and to allow a negative pressure difference to be induced over the diaphragm to operate the demand valve. The exhaust valves should operate at a very small positive pressure difference, and cause as little resistance to flow as reasonably possible, without being cumbersome and bulky. Elastomer mushroom valves serve the purpose adequately.[3]: 108 Where it is important to avoid leaks back into the regulator, such as when diving in contaminated water, a system of two sets of valves in series can reduce the risk of contamination. A more complex option which can be used for surface supplied helmets, is to use a reclaim exhaust system which uses a separate flow regulator to control the exhaust which is returned to the surface in a dedicated hose in the umbilical.[4]: 109 The exhaust manifold (exhaust tee, exhaust cover, whiskers) is the ducting that protects the exhaust valve(s) and diverts the exhaled air to the sides so that it does not bubble up in the diver's face and obscure the view.[3]: 33
A standard fitting on single-hose second stages, both mouth-held and built into a full-face mask or demand helmet, is the purge-button, which allows the diver to manually deflect the diaphragm to open the valve and cause air to flow into the casing. This is usually used to purge the casing or full-face mask of water if it has flooded. This will often happen if the second stage is dropped or removed from the mouth while under-water.
It may be desirable for the diver to have some manual control over the flow characteristics of the demand valve. The usual adjustable aspects are cracking pressure and the feedback from flow rate to internal pressure of the second stage housing. The inter-stage pressure of surface supplied demand breathing apparatus is controlled manually at the control panel, and does not automatically adjust to the ambient pressure in the way that most scuba first stages do, as this feature is controlled by feedback to the first stage from ambient pressure. This has the effect that the cracking pressure of a surface supplied demand valve will vary slightly with depth, so some manufacturers provide a manual adjustment knob on the side of the demand valve housing to adjust spring pressure on the downstream valve, which controls the cracking pressure. The knob is known to commercial divers as "dial-a-breath". A similar adjustment is provided on some high-end scuba demand valves, to allow the user to manually tune the breathing effort at depth[3]: 17
Scuba demand valves which are set to breathe lightly (low cracking pressure, and low work of breathing) may tend to free-flow relatively easily, particularly if the gas flow in the housing has been designed to assist in holding the valve open by reducing the internal pressure. The cracking pressure of a sensitive demand valve is often less than the hydrostatic pressure difference between the inside of an air-filled housing and the water below the diaphragm when the mouthpiece is pointed upwards. To avoid excessive loss of gas due to inadvertent activation of the valve when the DV is out of the diver's mouth, some second stages have a desensitising mechanism which causes some back-pressure in the housing, by impeding the flow or directing it against the inside of the diaphragm.[3]: 21
Twin-hose demand regulators
The "twin", "double" or "two" hose configuration of scuba demand valve was the first in general use.
The mechanism of the twin hose regulator is packaged in a usually circular metal housing mounted on the cylinder valve behind the diver's neck. The demand valve component of a two-stage twin hose regulator is thus mounted in the same housing as the first stage regulator, and in order to prevent free-flow, the exhaust valve must be located 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 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 usually through another non-return exhaust valve in the regulator housing - often a "duckbill" type.[36]
A non-return valve is usually fitted to the breathing hoses where they connect to the mouthpiece. This prevents 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, but makes the regulator easier to clear.[36]: 341
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. The mouthpiece can be purged by lifting it above the regulator (shallower), which will cause a free flow.[36]: 341 Twin hose regulators have been superseded almost completely by single hose regulators and became obsolete for most diving since the 1980s.[37] Raising the mouthpiece above the regulator increases the delivered pressure of gas and lowering the mouthpiece reduces delivered pressure and 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.
The original twin-hose regulators usually had no ports for accessories, though some had a high pressure port for a submersible pressure gauge. Some later models have one or more low-pressure ports between the stages, which can be used to supply direct feeds for suit or BC inflation and/or a secondary single-hose demand valve, and a high pressure port for a submersible pressure gauge.[36] The new Mistral is an exception as it is based on the Aqualung Titan first stage. which has the usual set of ports.[34]
Some early twin hose regulators were of single-stage design. The first stage functions in a way similar to the second stage of two-stage demand valves, but would be connected directly to the cylinder valve and reduced high pressure air from the cylinder directly to ambient pressure on demand. This could be done by using a longer lever and larger diameter diaphragm to control the valve movement, but there was a tendency for cracking pressure, and thus work of breathing, to vary as the cylinder pressure dropped.[36]
The twin-hose arrangement with a bite-grip
Performance
The breathing performance of regulators is a measure of the ability of a breathing gas regulator to meet the demands placed on it at varying ambient pressures and under varying breathing loads, for the range of breathing gases it may be expected to deliver. Performance is an important factor in design and selection of breathing regulators for any application, but particularly for underwater diving, as the range of ambient operating pressures and variety of breathing gases is broader in this application. It is desirable that breathing from a regulator requires low effort even when supplying large amounts of breathing gas as this is commonly the limiting factor for underwater exertion, and can be critical during diving emergencies. It is also preferable that the gas is delivered smoothly without any sudden changes in resistance while inhaling or exhaling. Although these factors may be judged subjectively, it is convenient to have a standard by which the many different types and manufactures of regulators may be compared.
The original Cousteau twin-hose diving regulators could deliver about 140
Various breathing machines have been developed and used for assessment of breathing apparatus performance.[39] ANSTI Test Systems Ltd (UK) has developed a testing machine that measures the inhalation and exhalation effort in using a regulator at all realistic water temperatures. Publishing results of the performance of regulators in the ANSTI test machine has resulted in big performance improvements.[40][41]
At higher gas densities associated with greater depth and pressure, breathing may be physiologically limited by the capacity of the diver to move gas through the breathing passages of the lungs against
Ergonomics
Several factors affect the comfort and effectiveness of diving regulators. Work of breathing has been mentioned, and can be critical to diver performance under high workload and when using dense gas at depth.
Mouth-held demand valves may exert forces on the teeth and jaws of the user that can lead to fatigue and pain, occasionally repetitive stress injury, and early rubber mouthpieces often caused an allergic reaction of contact surfaces in the mouth, which has been largely eliminated by the use of hypoallergenic silicone rubber. Various designs of mouthpiece have been developed to reduce this problem. The feel of some mouthpieces on the palate can induce a gag reflex in some divers, while in others it causes no discomfort. The style of the bite surfaces can influence comfort and various styles are available as aftermarket accessories. Personal testing is the usual way to identify what works best for the individual, and in some models the grip surfaces can be moulded to better fit the diver's bite. The lead of the low-pressure hose can also induce mouth loads when the hose is of an unsuitable length or is forced into small radius curves to reach the mouth. This can usually be avoided by careful adjuctment of hose lead and sometimes a different hose length.
Regulators supported by helmets and full-face masks eliminate the load on the lips, teeth and jaws, but add mechanical dead space, which can be reduced by using an orinasal inner mask to separate the breathing circuit from the rest of the interior air space. This can also help reduce fogging of the viewport, which can seriously restrict vision. Some fogging will still occur, and a means of defogging is necessary. The internal volume of a helmet or full-face mask may exert unbalanced buoyancy forces on the diver's neck, or if compensated by ballast, weight loads when out of the water. The material of some orinasal mask seals and full-face mask skirts can cause allergic reactions, but newer models tend to use hypoallegenic materials and are seldom a problem.
Malfunctions and failure modes
Most regulator malfunctions involve improper supply of breathing gas or water leaking into the gas supply. There are two major gas supply failure modes, where the regulator shuts off delivery, which is extremely rare, and free-flow, where the delivery will not stop and can quickly exhaust a scuba supply. Various lesser malfunctions mostly involve partial reductions in supply, non-catastrophic leaks, and ergonomic faults that make the regulator difficult, uncomfortable, or dangerous to use. Some malfunctions can be quickly and easily corrected by the user if they know what to do, others may require professional servicing, troubleshooting, or replacement of parts. Some may simply be the consequence of using it beyond its specified operating range.[2][43]
- Inlet filter blockage
- [43] The inlet to the first stage is usually protected by a filter to prevent corrosion products or other contaminants in the cylinder from getting into the fine between moving parts of the first and second stage and jamming them, either open or closed. If enough dirt gets into these filters, they themselves can be blocked sufficiently to reduce performance, but are unlikely to result in a total or sudden catastrophic failure. Sintered bronze filters can also gradually clog with corrosion products if they get wet. Inlet filter blockage will become more noticeable as the cylinder pressure drops or depth increases.
- Sticking valves
- The moving parts in first and second stages have fine tolerances in places, and some designs are more susceptible to contaminants causing friction between the moving parts. this may increase cracking pressure, reduce flow rate, increase work of breathing or induce free-flow, depending on what part is affected.
- Free-flow
- [43] Either of the stages may get stuck in the open position, causing a continuous flow of gas from the regulator known as a free-flow. This can be triggered by a range of causes, some of which can be easily remedied, others not. Possible causes include incorrect interstage pressure setting, incorrect second stage valve spring tension, damaged or sticking valve poppet, damaged valve seat, valve freezing, wrong sensitivity setting at the surface and in Poseidon servo-assisted second stages, low interstage pressure.
- Freezing
- [44] In cold conditions the cooling effect of gas expanding through a valve orifice may cool either first or second stage sufficiently to cause ice to form. External icing may lock up the spring and exposed moving parts of first or second stage, and freezing of moisture in the stored gas may cause icing on internal surfaces. Either may cause the moving parts of the affected stage to jam open or closed. If the valve freezes closed, it will usually defrost quite rapidly and start working again, and may freeze open soon after. Freezing open is more of a problem, as the valve will then free-flow and cool further in a positive feedback loop, which can normally only be stopped by closing the cylinder valve and waiting for the ice to thaw. If not stopped, the cylinder will rapidly be emptied.
- Intermediate pressure creep
- [43] This is a slow leak of the first stage valve, often caused by a worn, damaged or dirty valve seat. The effect is for the interstage pressure to rise until either the next breath is drawn, or the pressure exerts more force on the second stage valve than can be resisted by the spring, and the valve opens briefly, often with a popping sound, to relieve the pressure. the frequency of the popping pressure relief depends on the flow in the second stage, the back pressure, the second stage spring tension and the magnitude of the leak. It may range from occasional loud pops to a constant hiss.
- Gas leaks
- [3]: 185 A relatively common o-ring failure occurs when the yoke clamp seal extrudes due to insufficient clamp force or elastic deformation of the clamp by impact with the surroundings. Gas leaks can be caused by burst or leaky hoses, defective or blown o-rings, particularly in yoke connectors, loose connections, and several of the previously listed malfunctions. Low pressure inflation hoses may fail to connect properly, or the non-return valve may leak.
- Wet breathing
- [43] Wet breathing is caused by water getting into the regulator and compromising breathing comfort and safety. Water can leak into the second stage body through damaged soft parts like torn mouthpieces, damaged exhaust valves and perforated diaphragms, through cracked housings, or through poorly sealing or fouled exhaust valves.
- Excessive work of breathing