For some cold water diving applications the capacity to deliver high flow rates at low ambient temperatures without jamming due to regulator freezing is important.

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.

[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.

[11] When an externally vented BIBS is used at low chamber pressure, a vacuum assist may be necessary to keep the exhalation backpressure down to provide an acceptable work of breathing.

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.

The apparatus was demonstrated to and investigated by a committee of the French Academy of Sciences:[16][17] On 19 June 1838, in London, William Edward Newton filed a patent (no.

[5][22] In 1937 and 1942 the French inventor, Georges Commeinhes from Alsace, patented a diving demand valve supplied with air from two gas cylinders through a full-face mask.

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.

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.

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.

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.

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.

[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.

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.

[34][35] 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.

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 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.

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.

[44] Generally the water that freezes is in the ambient pressure chamber around a spring that keeps the valve open and not moisture in the breathing gas from the cylinder, but that is also possible if the air is not adequately filtered.

Although it presents the diver with an imminent "out of air" crisis, this failure mode lets gas escape directly into the water without inflating buoyancy devices.

There are a few models, notably those made by Poseidon Diving Systems AB, but historically also from other manufacturers, which have side exhausts and work equally well in either orientation.

The origins of the secondary demand valve are obscure, and it may have been independently invented several times, but it was used by Dave Woodward at UNEXSO around 1965–6 to support the freedive attempts of Jacques Mayol.

[54] In the most common recreational configuration, divers wear the secondary demand valve on the right side, ready for rapid deployment if the buddy runs out of breathing gas.

These comparisons do not apply with the long hose and necklace or with BCD inflator integrated systems, or with DVs with side exhaust which work upside down.

[58][59] Hoses may be fitted to low pressure ports of the regulator first stage to provide gas for inflating buoyancy compensators and/or dry suits.

These instruments would otherwise be carried somewhere else such as strapped to the wrist or forearm or in a pocket and are only regulator accessories for convenience of transport and access, and at greater risk of damage during handling.

In the normally closed condition when not mounted, this valve prevents ingress of water and other contaminants to the first stage interior which could be caused by negligent handling of the equipment or by accident.

[65][66] Surface supplied divers operating for long periods in cold water, or using helium based breathing gas mixtures, commonly use a hot-water suit to maintain body temperature.

[67][68] Standard air regulators are considered to be suitable for nitrox mixtures containing 40% or less oxygen by volume, both by NOAA, which conducted extensive testing to verify this, and by most recreational diving agencies.