Rebreather

A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing of the substantial unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. 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, while also eliminating the bubbles otherwise produced by an open circuit system. The latter advantage over other systems is useful for covert military operations by frogmen, as well as for undisturbed observation of underwater wildlife. A rebreather is generally understood to be a portable apparatus carried by the user. The same technology on a vehicle or non-mobile installation is more likely to be referred to as a life-support system.
Rebreather technology may be used where breathing gas supply is limited, such as underwater, in space, where the environment is toxic or hypoxic, mine rescue, high-altitude operations, or where the breathing gas is specially enriched or contains expensive components, such as helium diluent or anaesthetic gases.
Rebreathers are used in many environments: underwater, diving rebreathers are a type of self-contained underwater breathing apparatus which have provisions for both a primary and emergency gas supply. On land they are used in industrial applications where poisonous gases may be present or oxygen may be absent, firefighting, where firefighters may be required to operate in an atmosphere immediately dangerous to life and health for extended periods, in hospital anaesthesia breathing systems to supply controlled concentrations of anaesthetic gases to patients without contaminating the air that the staff breathe, and at high altitude, where the partial pressure of oxygen is low, for high altitude mountaineering. In aerospace there are applications in unpressurised aircraft and for high altitude parachute drops, and above the Earth's atmosphere, in space suits for extra-vehicular activity. Similar technology is used in life-support systems in submarines, submersibles, atmospheric diving suits, underwater and surface saturation habitats, spacecraft, and space stations, 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 specific hazards, some of which depend on the application and type of rebreather used. Mass and bulk may be greater or less than open circuit 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.

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 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 CO₂ concentration in the blood, not by the oxygen concentration, so even a small buildup of CO₂ 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 remove the CO₂ 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.

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

Endurance

The endurance of a rebreather, the duration for which it can be safely and comfortably used, is dependent on the oxygen supply at the oxygen consumption rate of the user, and the capacity of the scrubber to remove carbon dioxide at the rate it is produced by the user. These variables are closely linked, as the carbon dioxide is a product of metabolic oxygen consumption, though not the only product. This is independent of depth, except for work of breathing increase due to gas density increase.

Architecture

There are two basic arrangements controlling the flow of breathing gas inside the rebreather, known as the pendulum and loop systems.

Pendulum

In the pendulum configuration, the user inhales gas from the counterlung through a breathing hose, and exhaled gas returns to the counterlung by flowing back through the same hose. The scrubber is usually between the breathing hose and the counterlung bag, and gas flow is bi-directional. All of the flow passages between the user and the active absorbent in the scrubber are dead space – volume containing gas which is rebreathed without modification by the rebreather. The dead space increases as the absorbent is depleted. Breathing hose volume must be minimised to limit dead space.

Loop

In the loop configuration, the user inhales gas through one hose, and exhales through a second hose. Exhaled gas flows into the scrubber from one side, and exits at the other side. There may be one large counterlung, on either side of the scrubber, or two smaller counterlungs, one on each side of the scrubber. Flow is in one direction, enforced by non-return valves, which are usually in the breathing hoses where they join the mouthpiece. Only the flow passage in the mouthpiece before the split between inhalation and exhalation hoses is dead space, and this is not affected by hose volume.

Components

There are some components that are common to almost all personal portable rebreathers. These include the ambient pressure breathing volume components, usually called the breathing loop in a circulating flow rebreather, and the make-up gas supply and control system.

Counterlung

The counterlung is an airtight bag of strong flexible material that holds the volume of the exhaled gas until it is inhaled again. There may be a single counterlung, or one on each side of the scrubber, which allows a more even flow rate of gas through the scrubber, which can reduce work of breathing and improve scrubber efficiency by a more consistent.

Scrubber

The scrubber is a container filled with carbon dioxide absorbent material, mostly strong bases, through which the exhaled gas passes to remove the carbon dioxide. The absorbent may be granular or in the form of a moulded cartridge. Granular absorbent may be manufactured by breaking up lumps of lime and sorting the granules by size, or by moulding granules at a consistent size and shape. Gas flow through the scrubber may be in one direction in a loop rebreather, or both ways in a pendulum rebreather. The scrubber canister generally has an inlet on one side and an outlet on the other side.
A typical absorbent is soda lime, which is made up of calcium hydroxide Ca₂, and sodium hydroxide NaOH. The main component of soda lime is calcium hydroxide, which is relatively cheap and easily available. Other components may be present in the absorbent. Sodium hydroxide is added to accelerate the reaction with carbon dioxide. Other chemicals may be added to prevent unwanted decomposition products when used with standard halogenated inhalation anaesthetics. An indicator may be included to show when carbon dioxide has dissolved in the water of the soda lime and formed carbonic acid, changing the pH from basic to acid, as the change of colour shows that the absorbent has reached saturation with carbon dioxide and must be changed.
The carbon dioxide combines with water or water vapor to produce a weak carbonic acid: CO₂ + H₂O –> H₂CO₃. This reacts with the hydroxides to produce carbonates and water in an exothermic reaction. In the intermediate reaction, the carbonic acid reacts exothermically with sodium hydroxide to form sodium carbonate and water: H₂CO₃ + 2NaOH –> Na₂CO₃ + 2H₂O + heat. In the final reaction, the sodium carbonate reacts with the slaked lime to form calcium carbonate and sodium hydroxide: Na₂CO₃ + Ca₂ –> CaCO₃ + 2NaOH. The sodium hydroxide is then available again to react with more carbonic acid. of this absorbent can remove about of carbon dioxide at standard atmospheric pressure. This process also heats and humidifies the air, which is desirable for diving in cold water, or climbing at high altitudes, but not for working in hot environments.
Other reactions may be used in special circumstances. Lithium hydroxide and particularly lithium peroxide may be used where low mass is important, such as in space stations and space suits. Lithium peroxide also replenishes the oxygen during the scrubbing reaction.
Another method of carbon dioxide removal occasionally used in portable rebreathers is to freeze it out, which is possible in a cryogenic rebreather which uses liquid oxygen. The liquid oxygen absorbs heat from the carbon dioxide in a heat exchanger to convert the oxygen to gas, which is sufficient to freeze the carbon dioxide. This process also chills the gas, which is sometimes, but not always, desirable.