Diving rebreather
A diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's exhaled breath to permit the rebreathing 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. A semi-closed rebreather carried by the diver may also be known as a gas extender. The same technology on a submersible, underwater habitat, 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. There are also use cases where the noise of open circuit systems is undesirable, such as certain wildlife photography.
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.
Semi-closed rebreathers are also used as bailout sets for saturation divers diving from closed bells, as they are capable of longer endurance than open circuit bailout sets, and are constrained by the requirement of fitting through the bell lower hatch while on the diver.
The DIVEX COBRA is an emergency rebreather system intended for use as a bailout set for saturation diving to a maximum of 450 msw. It is CE certified to NORSOK U101 and EN14143. Work of breathing is claimed to be low. The semi-closed rebreather is purely mechanical in operation, has an endurance of 45 minutes using self contained gas supplied from twin cylinders mounted in the casing, and is activated by a single turn of a control valve.
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.The Italian Decima Flottiglia MAS, the first unit of combat frogmen, was founded in 1938 and went into action in 1940. WWII saw a great expansion of military-related use of rebreather diving. During and after WWII, needs arose in the armed forces to dive deeper than allowed by pure oxygen. That prompted, at least in Britain, design of simple constant-flow "mixture rebreather" variants of some of their diving oxygen rebreathers : SCMBA from the SCBA, and CDMBA from the Siebe Gorman CDBA, by adding an extra gas supply cylinder. Before a dive with such a set, the diver had to know the maximum or working depth of his dive, and how fast his body used his oxygen supply, and from those to calculate what to set his rebreather's gas flow rate to.
During this long period before the modern age of automatic sport nitrox rebreathers, there were some sport oxygen diving clubs, mostly in the USA.
Eventually the Cold War ended and in 1989 the Communist Bloc collapsed, and as a result the perceived risk of sabotage attacks by combat divers dwindled, and Western armed forces had less reason to requisition civilian rebreather patents, and automatic and semi-automatic recreational diving rebreathers with ppO2 sensors started to appear.
General concept
As a person breathes, the body consumes oxygen and produces carbon dioxide. Base metabolism requires about 0.25 L/min of oxygen from a breathing rate of about 6 L/min, and a fit person working hard may ventilate at a rate of 95 L/min but will only metabolise about 4 L/min of oxygen The oxygen metabolised is generally about 4% to 5% of the inspired volume at normal atmospheric pressure, or about 20% of the available oxygen in the air at sea level. Exhaled air at sea level contains roughly 13.5% to 16% oxygen.The situation is even more wasteful of oxygen when the oxygen fraction of the breathing gas is higher, and in underwater diving, the compression of breathing gas due to depth makes the recirculation of exhaled gas even more desirable, as an even larger proportion of open circuit gas is wasted. Continued rebreathing of the same gas will deplete the oxygen to a level which will no longer support consciousness, and eventually life, so gas containing oxygen must be added to the recycled breathing gas to maintain the required concentration of oxygen.
However, if this is done without removing the carbon dioxide, it will rapidly build up in the recycled gas, resulting almost immediately in mild respiratory distress, and rapidly developing into further stages of hypercapnia, or carbon dioxide toxicity.
A high ventilation rate is usually necessary to eliminate the metabolic product carbon dioxide. The breathing reflex is triggered by carbon dioxide concentration in the blood, not by the oxygen concentration, so even a small buildup of carbon dioxide in the inhaled gas quickly becomes intolerable; if a person tries to directly rebreathe their exhaled breathing gas, they will soon feel an acute sense of suffocation, so rebreathers must chemically remove the carbon dioxide in a component known as a carbon dioxide scrubber.
By adding sufficient oxygen to compensate for the metabolic usage, removing the carbon dioxide, and rebreathing the gas, most of the volume is conserved. There will still be minor losses when gas must be vented as it expands during ascent, and additional gas will be needed to make up volume as the gas is compressed during descent.
| PO2 | Application and effect |
| <0.08 | Coma ultimately leading to death |
| 0.08-0.10 | Unconsciousness in most people |
| 0.09-0.10 | Serious signs/symptoms of hypoxia |
| 0.14-0.16 | Initial signs/symptoms of hypoxia |
| 0.21 | Normal environment oxygen |
| 0.35–0.40 | Normal saturation dive PO2 level |
| 0.50 | Threshold for whole-body effects; maximum saturation dive exposure |
| 1.0–1.20 | Common range for recreational closed circuit set point |
| 1.40 | Recommended limit for recreational open circuit bottom sector |
| 1.60 | NOAA limit for maximum exposure for a working diver Recreational/technical limit for decompression |
| 2.20 | Commercial/military "Sur-D" chamber surface decompression on 100% O2 at 12 msw |
| 2.40 | 40% O2 nitrox recompression treatment gas for use in the chamber at 50 msw |
| 2.80 | 100% O2 recompression treatment gas for use in the chamber at 18 msw |
| 3.00 | 50% O2 nitrox recompression treatment gas for use in the chamber at 50 msw |