Although strictly speaking the scuba set is only the diving equipment that is required for providing breathing gas to the diver, general usage includes the harness or rigging by which it is carried and those accessories which are integral parts of the harness and breathing apparatus assembly, such as a jacket or wing style buoyancy compensator and instruments mounted in a combined housing with the pressure gauge.

Scuba provides the diver with the advantages of mobility and horizontal range far beyond the reach of an umbilical hose attached to surface-supplied diving equipment (SSDE).

[6] Unlike other modes of diving, which rely either on breath-hold or on breathing gas supplied under pressure from the surface, scuba divers carry their own source of breathing gas, usually filtered compressed air,[7] allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold.

Their roles include direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations.

Holding the breath at constant depth for short periods with a normal lung volume is generally harmless, providing there is sufficient ventilation on average to prevent carbon dioxide buildup, and is done as a standard practice by underwater photographers to avoid startling their subjects.

Holding the breath during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a gas supply malfunction until it is too late to remedy.

The use of a low density inert gas, typically helium, in the breathing mixture can reduce this problem, as well as diluting the narcotic effects of the other gases.

Most open-circuit scuba sets have a demand regulator to control the supply of breathing gas, and most rebreathers have a constant-flow injector, or an electronically controlled injector to supply fresh gas, but also usually have an automatic diluent valve (ADV), which functions in the same way as a demand valve, to maintain the loop volume during descent.

At one time a firm called Submarine Products sold a sport air scuba set with three manifolded back-mounted cylinders.

[citation needed] Cave and wreck penetration divers sometimes carry cylinders attached at their sides instead, allowing them to swim through more confined spaces.

Examples were Charles Condert's dress in the US (as of 1831), and Yves le Prieur's hand-controlled supply valve in France (as of 1926); see Timeline of diving technology.

[13] 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 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.

[21] The Russian Kriolang (from Greek cryo- (= "frost" taken to mean "cold") + English "lung") was copied from Jordan Klein's "Mako" cryogenic open-circuit scuba.

The main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, increased work of breathing at depth, reliability of gas mixture control, and the requirement to be able to safely bail out at any point of the dive.

A drop tank is usually rigged to be carried in a sling mount, and has a regulator with a pressure gauge fitted and is deposited at a suitable place along the guideline, usually clipped to the line to ensure that it can be found again.

[39] They generally rely on an over-centre lever action to provide tensioning and locking, which may be modified by length adjustment slots and secondary security fastening such as velcro to hold the free end in place.

The most successful examples have been for military rebreathers, where there has been space in a rigid housing to fit the breathing hoses, mask or DSV, and simple harness straps inside the casing when the counterlungs are empty.

A more complex but still minimalist system is a webbing harness with shoulder straps, waist belt and crotch strap, supporting a variety of sliders and D-rings for attachment of cylinders and accessories, with or without integrated weighting or separate weight belts, and with or without a back mounted buoyancy compensator, which may be attached to the harness, or directly to the diver.

Sling rigging includes a bolt snap at the shoulder and one near the base of the cylinder, which are clipped to the main scuba harness, which may be either back or sidemount arrangement with D-rings or rails as attachment points.

The sidemount cylinder rigging may omit the shoulder clip, and has a bungee loop on the harness to secure and confine the top end of the set.

Recreational divers commonly mount the SPG in a rubber or plastic console along with other instruments like a compass, depth gauge and/or dive computer.

Mouthpiece retaining straps have been shown in navy experience over several years to be effective at protecting the airway in an unconscious rebreather diver as an alternative to a full-face mask.

[13] It can also be used to hold a primary demand valve in the same way, which will keep it in close proximity to the face if accidentally dropped or dislodged, making it often possible to recover hands-free.

These allow a wider range of comfortable hose routings, particularly useful for stage and side-mount cylinders, but add another potential point of failure at the swivel joint.

The Dräger P-port system with a bayonet connector is used on Dräger Panorama masks, The Ocean Reef mask has a screw in connection, and the Kirby-Morgan 48 SuperMask uses a clip-on "pod" system A diffuser is a component fitted over the exhaust outlet to break up the exhaled gas into bubbles small enough not to be seen above the surface the water, and make less noise (see acoustic signature).

The filter stones were mounted on a hinged arm to float 1 to 2 feet (30 to 60 cm) above the diver, to set up a depth-pressure-differential suction effect to counteract the extra exhalation pressure needed to breathe out through the diffuser.

[citation needed] A large number of cylinders, hoses and fittings passing through the water tends to increase hydrodynamic drag, reducing swimming efficiency.

Closed circuit equipment was more easily adapted to scuba in the absence of reliable, portable, and economical high pressure gas storage vessels.

[55] The first commercially practical scuba rebreather was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London.