Rebreather diving

Rebreather diving is practiced by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable.

Fault tolerant design can make a rebreather less likely to fail in a way that immediately endangers the user, and reduces the task loading on the diver which in turn may lower the risk of operator error.

The saving is proportional to the ambient pressure, so is greater for deeper dives, and is particularly significant when expensive mixtures containing helium are used as the inert gas diluent.

In normal use at constant depth, only oxygen is consumed: small volumes of inert gases are lost during any one dive, due mainly to venting of the gas as it expands on ascent.

[5][1] The deep sector set-point is chosen to minimise decompression obligation while also maintaining a low risk of oxygen toxicity over the expected dive duration.

There is little that can go wrong with the function other than flooding, leaking, running out of gas, and scrubber breakthrough, all of which are obvious to the user, and there is no risk of decompression sickness, so emergency free ascent to the surface is always an option in open water.

This system provides a fairly stable oxygen fraction which is a reasonable approximation of open circuit for decompression and maximum operating depth purposes.

This is very similar to the demand controlled SCR in effect on the oxygen fraction of the loop gas, which remains nearly constant at all depths where the compensation is linear, and for aerobic levels of exercise.

[5][1] The manually controlled closed circuit rebreather (MCCCR or MCCR) relies on the attention, knowledge and skill of the diver to maintain the gas mixture at the desired composition.

It relies on electrochemical sensors and electronic monitoring instruments to provide the diver with the information required to make the necessary decisions and take the correct actions to control the gas mixture.

An alternative mode of failure is one in which the injection valves are kept open, resulting in an increasingly hyperoxic gas mix in the loop, which may pose the danger of oxygen toxicity.

Mitigation: The diver will usually be made aware of flooding by increased breathing resistance, water noise, or carbon dioxide buildup, and sometimes by buoyancy loss.

[citation needed] The percentage of deaths that involve the use of a rebreather among US and Canadian residents increased from approximately 1 to 5% of the total diving fatalities collected by the Divers Alert Network from 1998 through 2004.

Electronically controlled rebreathers may have an automatic sensor check routine which compares the readings from all the cells using the diluent and pure oxygen as calibration gases.

Partial pressure of oxygen is of critical importance on CCRs and is monitored at frequent intervals, particularly at the start of the dive, during descent, where transient increases due to compression may occur, and during ascent, where the risk of hypoxia is highest.

More than half of the divers in a survey had experienced at least one caustic cocktail, an event that renders the rebreather unfit for further use until it has been cleaned and repacked, and therefore urgently requires bailout.

EUF Certification International is an independent body for global verification of rebreather training agencies based in Europe and associated with the Austrian Standards organisation.

[58][59] This apparatus was first used under operational conditions in 1880 by Alexander Lambert, the lead diver on the Severn Tunnel construction project, who was able to travel 1,000 feet (300 m) in the darkness to close several submerged sluice doors in the tunnel; this had defeated the best efforts of standard divers due to extremely long distance, along which their air supply hoses became fouled on submerged debris, and the strong water currents in the workings.

[58][60] Fleuss continually improved his apparatus, adding a demand regulator[clarification needed] and tanks capable of holding greater amounts of oxygen at higher pressure.

Flow rate of the injector nozzle was nominally 0.5 cubic foot per minute at 100 psi above ambient pressure, which would blow 11 times the volume of the injected gas through the scrubber.

[73] Rebreather Forum 2.0 was held in Redondo Beach, California, in September 1996, and the proceedings were published by the PADI subsidiary Diving Science and Technology (DSAT).

[73] At this time only the US Navy 0.7atm constant PO2 decompression tables had been validated for nitrox and heliox, and it was not clear whether reprogramming the existing algorithms to use the PO2 provided by the rebreathers would be effective.

Some recommendations published by the forum included third party testing for quality assurance, the use of full-face masks or mouthpiece retaining straps, adherence to the buddy system, and a maximum PO2 setpoint of 1.3 atm.

About a year later (1997), Ambient Pressure Diving in the UK started selling the Inspiration electronically controlled closed circuit rebreather, followed by the KISS from Jetsam Technologies.

[73] By 1998 French cave diver Olivier Isler used a fully redundant RI 2000 semi-closed rebreather in the Doux de Coly resurgence, France.

Tactical divers (frogmen) are the main users of oxygen rebreathers, and for deeper work such as clearance diving, when mobility and a low magnetic and acoustic signature are important, mixed gas SCRs and CCRs may be used.

[75] A major advantage of closed-circuit rebreathers for biological research diving is the absence of noise and visual disturbance due to the bubbles of exhaled gas, which is known to have disruptive effects on animal behaviour.

[75] Developments in electronically controlled closed circuit mixed gas rebreathers have extended the range of ambient pressure scientific diving since the mid 1990s.

To a large degree this implies a high level of competence in operating the rebreather equipment, so the diver has sufficient excess capacity to allow the scientific task to be carried out successfully without compromising safety.

The use of institutionally approved checklists for equipment preparation and dive planning has been recommended as a way to reduce error and ensure that all essential steps are followed.