A Diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the diver. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, and, for covert military use by frogmen or observation of underwater life, to eliminate the bubbles produced by an open circuit system. A diving rebreather is generally understood to be a portable unit carried by the user, and is therefore a type of self-contained underwater breathing apparatus (scuba). A semi-closed rebreather carried by the diver may also be known as a gas extender. The same technology on a submersible or surface installation is more likely to be referred to as a life-support system.
Diving rebreather technology may be used where breathing gas supply is limited, or where the breathing gas is specially enriched or contains expensive components, such as helium diluent. Diving rebreathers have applications for primary and emergency gas supply. Similar technology is used in life-support systems in submarines, submersibles, underwater and surface saturation habitats, and in gas reclaim systems used to recover the large volumes of helium used in saturation diving.
The recycling of breathing gas comes at the cost of technological complexity and additional hazards, which depend on the specific application and type of rebreather used. Mass and bulk may be greater or less than equivalent open circuit scuba depending on circumstances. Electronically controlled diving rebreathers may automatically maintain a partial pressure of oxygen between programmable upper and lower limits, or set points, and be integrated with decompression computers to monitor the decompression status of the diver and record the dive profile.
Applications
Diving rebreathers are generally used for scuba applications, where the amount of breathing gas carried by the diver is limited, but are also occasionally used as gas extenders for surface-supplied diving and as bailout systems for scuba or surface-supplied diving. Gas reclaim systems used for deep heliox diving use similar technology to rebreathers, as do saturation diving life support systems, but in these applications the gas recycling equipment is not carried by the diver. Atmospheric diving suits also carry rebreather technology to recycle breathing gas as part of the life-support system.
Rebreathers are usually more complex to use than open circuit scuba, and have more potential points of failure, so acceptably safe use requires a greater level of skill, attention and situational awareness, which is usually derived from understanding the systems, diligent maintenance and overlearning the practical skills of operation and fault recovery. 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.
Semi-closed rebreather technology is also used in diver carried surface supplied gas extenders, mainly to reduce helium use. Some units also function as an emergency gas supply using on-board bailout cylinders: The US Navy MK29 rebreather can extend the duration of the Flyaway Mixed Gas System diving operations by five times while retaining the original mixed-gas storage footprint on the support ship. The Soviet IDA-72 semi-closed rebreather has a scrubber endurance of 4 hours on surface supply, and bailout endurance at 200m of 40 minutes on on-board gas. The US Navy Mark V Mod 1 heliox mixed gas helmet has a scrubber canister mounted on the back of the helmet and an inlet gas injection system which recirculates the breathing gas through the scrubber to remove carbon dioxide and thereby conserve helium. The injector nozzle would blow 11 times the volume of the injected gas through the scrubber.
History
The first attempts at making practical rebreathers were simple oxygen rebreathers, when advances in industrial metalworking made high-pressure gas storage cylinders possible. From 1878 on they were used for work in unbreathable atmospheres in industry and firefighting, at high altitude, for escape from submarines; and occasionally for swimming underwater; but the usual way to work underwater was in standard diving dress, breathing open circuit surface-supplied air.