Editing Scuba set (section)

====Open circuit demand scuba====
{{main|Diving regulator}}
This system consists of one or more [[diving cylinder]]s containing [[breathing gas]] at high pressure, typically {{convert|200|–|300|bar|psi|sigfig=2}}, connected to a [[diving regulator]]. The demand regulator automatically supplies the diver with as much gas as is needed at the ambient pressure.

This type of breathing set is sometimes called an ''aqualung''. The word ''[[Aqua-Lung]]'', which first appeared in the [[Jacques-Yves Cousteau|Cousteau]]-[[Gagnan]] [[patent]], is a [[trademark]], currently owned by [[Aqua Lung/La Spirotechnique]].<ref name="justia" />

=====Twin-hose demand regulator=====
{{main|Diving regulator#Twin-hose open-circuit demand scuba regulators}}
[[Image:Aqualung old type.jpg|thumb|Classic twin-hose Cousteau-type aqualung]]
This is the first type of diving demand valve to come into general use, and the one that can be seen in classic 1960s television scuba adventures, such as [[Sea Hunt]]. They were often use with manifolded twin cylinders.

All the stages of this type of regulator are in a large valve assembly mounted directly to the cylinder valve or manifold, behind the diver's neck. Two large bore corrugated rubber [[Breathing tube (in breathing apparatus)|breathing hoses]] connect the regulator with the mouthpiece, one for supply and one for exhaust. The exhaust hose is used to return the exhaled air to the regulator, to avoid pressure differences due to depth variation between the exhaust valve and final stage [[diaphragm (mechanical device)|diaphragm]], which would cause a free-flow of gas, or extra resistance to breathing, depending on the diver's orientation in the water. In modern single-hose sets this problem is avoided by moving the second-stage regulator to the diver's [[Mouthpiece (scuba)|mouthpiece]]. The twin-hose regulators came with a mouthpiece as standard, but a [[full-face diving mask]] was an option.{{citation needed|date=December 2016}}

=====Single-hose regulator=====
{{main|Diving regulator#Single hose two-stage open-circuit demand regulators}}
[[Image:Aqua lung.jpg|thumbnail|A single-hose regulator with 2nd stage, gauges, BC attachment, and dry suit hose mounted on a cylinder]]
Most modern open-circuit scuba sets have a [[diving regulator]] consisting of a first-stage pressure-reducing valve connected to the [[diving cylinder]]'s output valve or manifold. This regulator reduces the pressure from the cylinder, which may be up to {{convert|300|bar|psi}}, to a lower pressure, generally between about 9 and 11&nbsp;bar above the ambient pressure. A low-pressure hose links this with the second-stage regulator, or "demand valve", which is mounted on the mouthpiece. Exhalation occurs through a rubber one-way mushroom valve in the chamber of the demand valve, directly into the water quite close to the diver's mouth. Some early single hose scuba sets used full-face masks instead of a mouthpiece, such as those made by Desco<ref name=Desco /> and Scott Aviation<ref name="Scott Aviation" /> (who continue to make breathing units of this configuration for use by [[firefighters]]).

Modern regulators typically feature high-pressure ports for pressure sensors of dive-computers and submersible pressure gauges, and additional low-pressure ports for hoses for inflation of dry suits and BC devices.<ref name="Harlow" />

=====Primary demand valve=====
<!--target for redirect [[Primary demand valve]] -->
The primary demand valve is the one the diver usually breathes from. There may be a secondary demand valve on the same regulator, or on a different first stage connected to the same scuba set. Additional scuba sets used for bailout, stages, decompression, or sidemount diving usually only have one second stage, which for that set is the primary by default.

=====Secondary demand valve =====
{{see also|Diving regulator#Secondary demand valve (Octopus)}}
[[File:Plongee-StabilisateurDorsal 20090220 PlaqueLacasse.jpeg|thumb|Scuba harness with backplate and back mounted "wing" buoyancy compensator
{{ordered list | type=decimal
| Regulator first stage
| Cylinder valve
| Shoulder straps
| Buoyancy compensator bladder
| Buoyancy compensator relief and lower manual dump valve
| DV/Regulator second stages (primary and “octopus”)
| Console (submersible pressure gauge, depth gauge & compass)
| Dry-suit inflator hose
| Backplate
| Buoyancy compensator inflator hose and inflation valve
| Buoyancy compensator mouthpiece and manual dump valve
| Crotch strap
| Waist strap
}}
]]

Most recreational scuba sets have a backup second-stage demand valve on a separate hose, a configuration called a "secondary", or "octopus" demand valve, "alternate air source", "safe secondary" or "safe-second". This arrangement was intended to reduce the problems of buddy breathing from a single demand valve and has become the standard in recreational diving. By providing a second demand valve the need to alternately breathe off the same mouthpiece when sharing air is eliminated. This reduces the stress on divers who are already in a stressful situation, and this in turn reduces air consumption during the rescue and frees the donor's hand.{{citation needed|date=November 2011}}

Some diver training agencies recommend that a diver routinely offer their primary demand valve to a diver requesting to share air, and then switch to their own secondary demand valve.<ref name="Jablonski 2006" /> The idea behind this technique is that the primary demand valve is known to be working, and the diver donating the gas is less likely to be stressed or have a high carbon dioxide level, so has more time to sort out their own equipment after temporarily suspending the ability to breathe. In many instances, panicked divers have grabbed the primary regulators out of the mouths of other divers,{{citation needed|date=November 2011}} so changing to the backup as a routine reduces stress when it is necessary in an emergency.

In [[technical diving]] donation of the primary demand valve is commonly the standard procedure, and the primary is connected to the first stage by a long hose, typically around 2&nbsp;m, to allow gas sharing while swimming in single file in a narrow space as might be required in a cave or wreck. In this configuration the secondary is generally held under the chin by a loose bungee loop around the neck, supplied by a shorter hose, and is intended for backup use by the diver donating gas.<ref name="Jablonski 2006" /> The backup regulator is usually carried in the diver's chest area where it can be easily seen and accessed for emergency use. It may be worn secured by a breakaway clip on the [[Buoyancy compensator (diving)|buoyancy compensator]], plugged into a soft friction socket attached to the harness, secured by sliding a loop of the hose into the shoulder strap cover of a jacket style BC, or suspended under the chin on a break-away bungee loop known as a necklace. These methods also keep the secondary from dangling below the diver and being contaminated by debris or snagging on the surroundings. Some divers store it in a BC pocket, but this reduces availability in an emergency.

Occasionally, the secondary second-stage is combined with the inflation and exhaust valve assembly of the buoyancy compensator device. This combination eliminates the need for a separate low pressure hose for the BC, though the low pressure hose connector for combined use must have a larger bore than for standard BC inflation hoses, because it will need to deliver a higher flow rate if it is used for breathing.{{citation needed|date=December 2016}} This combination unit is carried in the position where the inflator unit would normally hang on the left side of the chest. With integrated DV/BC inflator designs, the secondary demand valve is at the end of the shorter BC inflation hose, and the donor must retain access to it for buoyancy control, so donation of the primary regulator to help another diver is essential with this configuration.<ref name="Air 2" />
The secondary demand valve is often partially yellow in color, and may use a yellow hose, for high visibility, and as an indication that it is an emergency or backup device.

When a side-mount configuration is used, the usefulness of a secondary demand valve is greatly reduced, as each cylinder will have a regulator and the one not in use is available as a backup. This configuration also allows the entire cylinder to be handed off to the receiver, so a long hose is also less likely to be needed.

Some diving instructors continue to teach buddy-breathing from a single demand valve as an obsolescent but still occasionally useful technique, learned in addition to the use of the backup DV, since availability of two second stages per diver is now assumed as standard in recreational scuba.{{citation needed|date=June 2012}}