Diving rebreather - Misplaced Pages

(Redirected from Bailout valve (rebreather)) Closed or semi-closed circuit scuba This article is about the equipment. For the activity and procedures, see Rebreather diving.

Diving rebreather
A fully closed circuit electronic rebreather (AP Diving Inspiration)
Acronym	CCUBA (closed circuit underwater breathing apparatus); CCR (closed circuit rebreather), SCR (semi-closed rebreather)
Uses	Breathing set
Related items	Rebreather

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

Main article: Rebreather diving

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.

(Draeger and Mark V Helium helmet)

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 (= what is now called "nitrox"): SCMBA from the SCBA (Swimmer Canoeist's Breathing Apparatus), 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.

This section needs expansion. You can help by adding to it. (October 2022)

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 (CO₂). 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.

Effects of different levels of oxygen partial pressure
PO₂ (bar)	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 (normal environment oxygen in some very high altitude areas)
0.21	Normal environment oxygen (sea level air)
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 (meters of sea water)
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

Design constraints

The widest variety of rebreather types is used in diving, as the physical and physiological consequences of breathing under pressure complicate the requirements, and a large range of engineering options are available depending on the specific application and available budget. A diving rebreather is safety-critical life-support equipment – some modes of failure can kill the diver without warning, others can require immediate appropriate response for survival.

General operational requirements include:

waterproof and corrosion resistant construction
reasonably close to neutrally buoyant after ballasting
acceptably streamlined, to minimize added swimming resistance
low work of breathing in all diver attitudes and over the full operating depth range
the unit should not adversely affect the diver's trim and balance
easy and quick release of harness and unaided removal of the unit from the diver
accessibility of control and adjustment components
unambiguous feedback to the diver of critical information
no critical single-point failure modes – The user should be able to deal with any single reasonably foreseeable failure without outside help

Special applications may also require:

low noise signal
low emission of bubbles/small bubbles
low electromagnetic signature
rugged construction
light weight in air
minimal additional task-loading for normal operation

Oxygen rebreathers

As pure oxygen is toxic when inhaled at pressure, recreational diver certification agencies limit oxygen decompression to a maximum depth of 6 metres (20 ft) and this restriction has been extended to oxygen rebreathers; In the past they have been used deeper (up to 20 metres (66 ft)) but such dives were more risky than what is now considered acceptable. Oxygen rebreathers are also sometimes used when decompressing from a deep open-circuit dive, as breathing pure oxygen helps the nitrogen diffuse out of the body tissues more rapidly, and the use of a rebreather may be more convenient for long decompression stops.

US Navy restrictions on oxygen rebreather use:

Normal working limit 25 feet (7.6 m) for 240 minutes. (P_O₂ = 1.76 bar)
Maximum working limit 50 feet (15 m) for 10 minutes. (P_O₂ = 2.5 bar)

Oxygen rebreathers are no longer commonly used in recreational diving because of the depth limit imposed by oxygen toxicity, but are extensively used for military attack swimmer applications where greater depth is not required, due to their simplicity, light weight and compact size.

Mixed gas rebreathers

Semi-closed circuit rebreathers (SCRs) used for diving may use active or passive gas addition, and the gas addition systems may be depth compensated. They use a mixed supply gas with a higher oxygen fraction than the steady state loop gas mixture. Usually only one gas mixture is used, but it is possible to switch gas mixtures during a dive to extend the available depth range of some SCRs.

Operational scope and restrictions of SCRs:

Non-depth compensated passive addition SCRs reduce the safe range of operating depths in inverse proportion to gas endurance extension. This can be compensated by gas switching, at the expense of complexity and increased number of potential failure points.
Constant mass flow SCRs provide a gas mixture which is not consistent over variation in diver exertion. This also limits safe operating depth range unless gas composition is monitored, also at the expense of increased complexity and additional potential failure points.
Demand controlled active gas addition provides reliable gas mixtures throughout the potential operating depth range, and do not require oxygen monitoring, but at the cost of more mechanical complexity.
Depth compensated passive addition provides reliable gas mixture over the potential operating depth range, which is only slightly reduced from the open circuit operational range for the gas in use at the cost of more mechanical complexity.

Closed circuit diving rebreathers may be manually or electronically controlled, and use both pure oxygen and a breathable mixed gas diluent.

Operational scope and restrictions of CCRs: Closed circuit rebreathers are mainly restricted by physiological limitations on the diver, such as maximum operating depth of the diluent mix while remaining breathable up to the surface, though this can be worked around by switching diluent. Work of breathing at depth can be a constraint, as there is a point where the breathing effort required to counter metabolic carbon dioxide production rate exceeds the work capacity of the diver, after which hypercapnia increases and distress followed by loss of consciousness and death is inevitable. Work of breathing is affected by gas density, so use of a low density helium rich diluent can increase depth range at acceptable work of breathing for a given configuration. WoB is also increased by turbulent flow, which is affected by flow velocity (Reynolds number). To some extent work of breathing can be reduced or limited by breathing circuit design, but there are physiological limits too, and the work of circulating the gas through the breathing loop and scrubber can be a large part of the total work of breathing.

Recreational rebreathers

Some recreational diver certification agencies distinguish a class of rebreather which they deem suitable for recreational diving. These rebreathers are unsuitable for decompression diving, and when electronically controlled, will not allow the diver to do dives with obligatory decompression, thereby allowing an immediate ascent at any point of the planned dive without undue risk of developing symptomatic decompression sickness. This limitation reduces the necessity to carry offboard bailout gas, and the need for the skills to bail out with a staged decompression obligation. This class of rebreather diving provides an opportunity to sell training and certification which omits a large part of the more complex and difficult skills, and reduces the amount of equipment that the diver needs to carry. PADI criteria for "R" class rebreathers include electronic prompts for pre-dive checks, automatic setpoint control, status warnings, a heads up display for warnings, a bailout valve, pre-packed scrubber canisters and a system for estimating scrubber duration. While these constraints do make the recreational class of rebreather inherently less hazardous, they do not reduce the risk to the same level as open circuit equipment for the same dive profile.

Atmospheric diving suits

Main article: Atmospheric diving suit

An atmospheric diving suit is a small one-man articulated submersible of roughly anthropomorphic form, with limb joints which allow articulation under external pressure while maintaining an internal pressure of one atmosphere. Breathing gas supply could be surface supplied by umbilical, but would then have to be exhausted back to the surface to maintain internal pressure below the external ambient pressure, which is possible but presents pressure-hull breach hazards if the umbilical hoses are damaged, or from a rebreather system built into the suit. As there is a similar problem in venting excess gas, the simple and efficient solution is to make up oxygen as it is consumed and scrub out the carbon dioxide, with no change to the inert gas component, which simply recirculates. In effect, a simple closed circuit oxygen rebreather arrangement used as a life-support system. Since there is usually an adequate power supply for other services, powered circulation through the scrubber should not normally be an issue for normal service, and is more comfortable for the operator, as it keeps the face area clear and facilitates voice communication. As the internal pressure is maintained at one atmosphere, there is no risk of acute oxygen toxicity. Endurance depends on the scrubber capacity and oxygen supply. Circulation through the scrubber could be powered by the diver's breathing, and this is an option for an emergency backup rebreather, which may also be fitted to the suit. A breathing driven system requires reduction of mechanical dead space by using a mouthpiece and counterlung to form a closed loop.

Architecture

Essential components

Breathing gas passage configuration

There are two basic gas passage configurations: The loop and the pendulum.

The loop configuration uses a one directional circulation of the breathing gas which on exhalation leaves the mouthpiece, passes through a non-return valve into the exhalation hose, and then through the counterlung and scrubber, to return to the mouthpiece through the inhalation hose and another non-return valve when the diver inhales.

The pendulum configuration uses a two-directional flow. Exhaled gas flows from the mouthpiece through a single hose to the scrubber, into the counterlung, and on inhalation the gas is drawn back through the scrubber and the same hose back to the mouthpiece. The pendulum system is structurally simpler, but inherently contains a larger dead space of unscrubbed gas in the combined exhalation and inhalation tube, which is rebreathed. There are conflicting requirements for minimising the volume of dead space while minimising the flow resistance of the breathing passages.

Counterlung configuration

A pendulum rebreather only has one counterlung, on the far side of the scrubber from the single breathing hose. The diver blows exhaled gas through the scrubber, then sucks it back during inhalation. Gas flow rate through the scrubber is forced by the breathing rate of the diver.

A single counterlung in a loop rebreather can be an exhalation or inhalation counterlung, or fitted between split scrubber canisters. If it is an exhalation counterlung it is inflated on exhalation, but no gas flows through the scrubber until inhalation starts, at which point the diver sucks the gas through at a rate forced by inhalation rate. If it is an inhalation counterlung, the diver must blow gas through the scrubber during exhalation, but inhales from the full inhalation counterlung, with no further flow through the scrubber. If it is between split scrubbers the diver must blow the gas through the exhalation scrubber during exhalation, and suck it through the inhalation scrubber. In all these cases there is no buffer, and peak flow rates are relatively high, which means peak flow resistance is relatively high and may be in one half of the breathing cycle or split between both halves, analogous to the pendulum configuration, but without the large dead space.

A twin counterlung rebreather has two breathing bags, so the exhaled gas inflates the exhalation counterlung while starting to pass through the scrubber and starting to inflate the inhalation counterlung. By the time the diver starts to inhale, the inhalation counterlung has built up a volume buffer, so there is less flow resistance as the gas continues to flow through the scrubber during inhalation at a slower rate than if there were only one counterlung. This decreases work of breathing, and also increases dwell time of the gas in the scrubber, as it flows through the scrubber during both exhalation and inhalation. Most mixed gas diving rebreathers use this arrangement.

This section needs expansion with: diagram of twin scrubber single counterlung arrangement from "A digital tour of the KISS Spirit and Sidewinder rebreathers". Divetech Grand Cayman·. 13 June 2020. Retrieved 31 May 2024 – via YouTube.. You can help by adding to it. (May 2024)

General arrangement

Many rebreathers have their main components in a hard casing for support, protection and/or streamlining. This casing must be sufficiently vented and drained to let surrounding water or air in and out freely to allow for volume changes as the counterlung inflates and deflates, and to prevent trapping large volumes of buoyant air as the diver submerges, and of water as the diver emerges into air.

The components may be mounted on a frame or inside a casing to hold them together. Sometimes the structure of the scrubber canister forms part of the framework, particularly in side-mount configuration. Position of most parts is not critical to function, but the counterlungs must be positioned so that their centroid of volume is at a similar depth to the centroid of the diver's lungs at most times while underwater, and the breathing tubes to the mouthpiece should not encumber the diver more than necessary, and allow free movement of the head as much as possible.

Early oxygen rebreathers were often built without frame or casing, and relied on the harness and a strong counterlung to hold the components together.

The parts of a diving rebreather (counterlung, absorbent canister, gas cylinder(s), tubes and hoses linking them), can be arranged on the wearer's body in four basic ways, with the position of the counterlung having a major effect on work of breathing.

Back-mounted rebreathers

Back mount is common on the more bulky and heavier units. This is good for support of the weight out of the water, and keeps the front of the diver clear for working underwater. Back mount usually uses back or over the shoulder counterlungs, which have a centroid above the lung in most common orientations of the diver, resulting in slight negative pressure breathing.

Chest-mounted rebreathers

Chest mount is fairly common for military oxygen rebreathers, which are usually relatively compact and light. It allows easy reach of the components underwater, and leaves the back free for other equipment for amphibious operations. The rebreather can be unclipped from a common harness without disturbing the load on the back. Front mounted counterlungs have a centroid which is generally slightly below the lung centroid, and result in slight positive pressure breathing for most common orientations of the diver.

Side-mounted rebreathers

System variants

Rebreathers can be primarily categorised as diving rebreathers, intended for hyperbaric use, and other rebreathers used at pressures from slightly more than normal atmospheric pressure at sea level to significantly lower ambient pressure at high altitudes and in space. Diving rebreathers must often deal with the complications of avoiding hyperbaric oxygen toxicity, while normobaric and hypobaric applications can use the relatively trivially simple oxygen rebreather technology, where there is no requirement to monitor oxygen partial pressure during use providing the ambient pressure is sufficient.

Mixed gas rebreathers

All rebreathers other than oxygen rebreathers may be considered mixed gas rebreathers. These can be divided into semi-closed circuit, where the supply gas is a breathable mixture containing oxygen and inert diluents, usually nitrogen and helium, and which is replenished by adding more of the mixture as the oxygen is used up, sufficient to maintain a breathable partial pressure of oxygen in the loop, and closed circuit rebreathers, where two parallel gas supplies are used: the diluent, to provide the bulk of the gas, and which is recycled, and oxygen, which is metabolically expended.

Semi-closed circuit rebreathers

These are almost exclusively used for underwater diving, as they are bulkier, heavier, and more complex than closed circuit oxygen rebreathers. Military and recreational divers use these because they provide better underwater duration than open circuit, have a deeper maximum operating depth than oxygen rebreathers and can be fairly simple and cheap. They do not rely on electronics for control of gas composition, but may use electronic monitoring for improved safety and more efficient decompression. An alternative term for this technology is "gas extender".

Semi-closed circuit equipment generally supplies one breathing gas such as air, nitrox or trimix at a time. The gas is injected into the loop at a constant rate to replenish oxygen consumed from the loop by the diver. Excess gas must be constantly vented from the loop in small volumes to make space for fresh, oxygen-rich gas. As the oxygen in the vented gas cannot be separated from the inert gas, semi-closed circuit is wasteful of both oxygen and inert components.

A gas mix which has a maximum operating depth that is safe for the depth of the dive being planned, and which will provide a breathable mixture at the surface must be used, or it will be necessary to change mixtures during the dive.

As the amount of oxygen required by the diver increases with work rate, the gas injection rate must be carefully chosen and controlled to prevent unconsciousness in the diver due to hypoxia. A higher gas addition rate reduces the likelihood of hypoxia but wastes more gas.

Passive addition semi-closed circuit

This type of rebreather works on the principle of adding fresh gas to compensate for reduced volume in the breathing circuit. A portion of the respired gas is discharged that is in some way proportional to oxygen consumption. Generally it is a fixed volumetric fraction of the respiratory flow, but more complex systems have been developed which exhaust a close approximation of a ratio to the surface respiratory flow rate. These are described as depth compensated or partially depth compensated systems. Gas addition is triggered by low counterlung volume which activates a demand valve.

The simple case of a fixed ratio discharge can be achieved by concentric bellows counterlungs, where the exhaled gas expands both the counterlungs, and while the larger volume outer bellows discharges back to the loop when the diver inhales the next breath, the inner bellows discharges its contents to the surroundings, using non return valves to ensure a one-directional flow. The amount processed during each breath depends on the tidal volume of that breath.

Towards the end of inhalation the bellows bottoms out and activates an addition valve, in much the way that a regulator diaphragm activates the demand valve, to make up the gas discharged by the inner bellows. This type of rebreather therefore tends to operate at a minimal volume.

The fixed ratio systems usually discharge between 10% (1/10) and 25% (1/4) of the volume of each breath overboard. As a result, gas endurance is from 10 times to four times that of open circuit, and depends on breathing rate and depth in the same way as for open circuit. Oxygen fraction in the loop depends on the discharge ratio, and to a lesser extent on the breathing rate and work rate of the diver. As some gas is recycled after breathing, the oxygen fraction will always be lower than that of the make-up gas, but can closely approximate the make-up gas after a loop flush, so the gas is generally chosen to be breathable at maximum depth, which allows it to be used for open circuit bailout. The loop gas oxygen fraction will increase with depth, as the mass rate of metabolic oxygen use remains almost constant with a change in depth. This is the opposite tendency of what is done in a closed circuit rebreather, where the oxygen partial pressure is controlled to be more or less the same within limits throughout the dive. The fixed ratio system has been used in the DC55 and Halcyon RB80 rebreathers. Passive addition rebreathers with small discharge ratios may become hypoxic near the surface when moderate or low oxygen fraction supply gas is used, making it necessary to switch gases between deep and shallow diving.

The depth compensating systems discharge a portion of the diver's tidal volume which varies in inverse proportion to the absolute pressure. At the surface they generally discharge between 20% (1/5) and 33% (1/3) of each breath, but that decreases with depth, to keep the oxygen fraction in the loop approximately constant and reduce gas consumption. A fully depth compensated system will discharge a volume of gas, inversely proportional to pressure, so that the volume discharged at 90m depth (10 bar absolute pressure) will be 10% of the surface discharge. This system will provide an approximately fixed oxygen fraction regardless of depth, when used with the same make-up gas, because the effective mass discharge remains constant.

Partially depth compensating systems are part way between the fixed ratio and the depth compensating systems. They provide a high discharge ratio near the surface, but the discharge ratio is not fixed either as a proportion of respired volume or mass. Gas oxygen fraction is more difficult to calculate, but will be somewhere between the limiting values for fixed ratio and fully compensated systems. The Halcyon PVR-BASC uses a variable volume inner bellows system to compensate for depth.

Active addition semi-closed circuit

An active addition system adds feed gas to the breathing circuit and excess gas is dumped to the environment. These rebreathers tend to operate near maximum volume.

Constant mass flow gas addition

The most common system of active addition of make-up gas in semi-closed rebreathers is by use of a constant mass flow injector, also known as choked flow. This is easily achieved by using a sonic orifice, as provided the pressure drop over the orifice is sufficient to ensure sonic flow, the mass flow for a specific gas will be independent of the downstream pressure. The mass flow through a sonic orifice is a function of the upstream pressure and the gas mixture, so the upstream pressure must remain constant for the working depth range of the rebreather to provide a reliably predictable mixture in the breathing circuit, and a modified regulator is used which is not affected by changes in ambient pressure. Gas addition is independent of oxygen use, and the gas fraction in the loop is strongly dependent on exertion of the diver – it is possible to dangerously deplete the oxygen by excessive physical exertion.

Demand controlled gas addition

Only one model using this gas mixture control principle has been marketed. This is the Interspiro DCSC. The principle of operation is to add a mass of oxygen that is proportional to the volume of each breath. This approach is based on the assumption that the volumetric breathing rate of a diver is directly proportional to metabolic oxygen consumption, which experimental evidence indicates is close enough to work.

The fresh gas addition is made by controlling the pressure in a dosage chamber proportional to the counterlung bellows volume. The dosage chamber is filled with fresh gas to a pressure proportional to bellows volume, with the highest pressure when the bellows is in the empty position. When the bellows fills during exhalation, the gas is released from the dosage chamber into the breathing circuit, proportional to the volume in the bellows during exhalation, and is fully released when the bellows is full. Excess gas is dumped to the environment through the overpressure valve after the bellows is full.

The result is the addition of a mass of gas proportional to ventilation volume, and the oxygen fraction is stable over the normal range of exertion.

The volume of the dosage chamber is matched to a specific supply gas mixture, and is changed when the gas is changed. The DCSC uses two standard mixtures of nitrox: 28% and 46%.

Closed circuit rebreathers

Oxygen rebreathers

This is the earliest type of rebreather and was commonly used by navies and for mine rescue from the early twentieth century. Oxygen rebreathers can be remarkably simple designs, and they were invented before open-circuit scuba. They only supply oxygen, so there is no requirement to control the gas mixture other than purging before use and removing the carbon dioxide.

Oxygen feed options

In some rebreathers, e.g. the Siebe Gorman Salvus, the oxygen cylinder has two oxygen supply mechanisms in parallel. One is constant flow, and the other is a manual on-off valve called a bypass valve. Both feed into the counterlung. There is no necessity for a second stage and the gas can be turned on and off at the cylinder valve.

Others, such as the USN Mk25 UBA, are supplied automatically via a demand valve on the counterlung, which will add gas at any time that the counterlung is emptied and the diver continues to inhale. Oxygen can also be added manually by a button which activates the demand valve, equivalent to the purge button on an open-circuit demand valve.

Some simple oxygen rebreathers had no automatic supply system, only the manual feed valve, and the diver had to operate the valve at intervals to refill the breathing bag as the volume of oxygen decreased below a comfortable level. This is task loading, but the diver cannot remain unaware of the need to top up. Control of the volume in the loop would also control buoyancy.

Closed circuit mixed gas rebreathers

Military, photographic, and recreational divers use closed circuit rebreathers because they allow long dives and produce no bubbles. Closed circuit rebreathers supply two breathing gases to the loop: one is pure oxygen and the other is a diluent or diluting gas such as air, nitrox, heliox or trimix.

A major function of the closed circuit rebreather is to control the oxygen partial pressure in the loop and to warn the diver if it becomes dangerously low or high. Too low a concentration of oxygen results in hypoxia leading to unconsciousness and ultimately death. Too high a concentration of oxygen results in hyperoxia, leading to oxygen toxicity, a condition causing convulsions which can make the diver lose the mouthpiece when they occur underwater, and can lead to drowning. The monitoring system uses oxygen-sensitive electro-galvanic fuel cells to measure the partial pressure of oxygen in the loop. The partial pressure of oxygen in the loop can generally be controlled within reasonable tolerance of a fixed value. This set point is chosen to provide an acceptable risk of both long-term and acute oxygen toxicity, while minimizing the decompression requirements for the planned dive profile.

The gas mixture is controlled by the diver in manually controlled closed circuit rebreathers. The diver can manually control the mixture by adding diluent gas or oxygen. Adding diluent can prevent the loop gas mixture becoming too oxygen rich, and adding oxygen is done to increase oxygen concentration.

In fully automatic closed-circuit systems, an electronically controlled solenoid valve injects oxygen into the loop when the control system detects that the partial pressure of oxygen in the loop has fallen below the required level. Electronically controlled CCRs can be switched to manual control in the event of some control system failures.

Addition of gas to compensate for compression during descent is usually done by an automatic diluent valve.

Standard diving dress rebreathers

In 1912 the German firm Drägerwerk of Lübeck introduced a version of standard diving dress using a gas supply from an oxygen rebreather and no surface supply. The system used a copper diving helmet and standard heavy diving suit with a back-mounted set of cylinders and scrubber. The breathing gas was circulated by using an injector system in the loop powered by the added gas. This was developed further with the Modell 1915 "Bubikopf" helmet and the DM20 oxygen rebreather system for depths up to 20 m, and the DM40 mixed gas rebreather which used an oxygen cylinder and an air cylinder for the gas supply, producing a nitrox mixture, for depths up to 40 m.

The US Navy developed a variant of the Mark V system for heliox diving. These were successfully used during the rescue of the crew and salvage of the USS Squalus in 1939. The US Navy Mark V Mod 1 heliox mixed gas helmet is based on the standard Mark V Helmet, with 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 helium helmet uses the same breastplate as a standard Mark V except that the locking mechanism is relocated to the front, there is no spitcock, there is an additional electrical connection for heated underwear, and on later versions a two or three-stage exhaust valve was fitted to reduce the risk of flooding the scrubber. The gas supply at the diver was controlled by two valves. The "Hoke valve" controlled flow through the injector to the "aspirator" which circulated gas from the helmet through the scrubber, and the main control valve used for bailout to open circuit, flushing the helmet, and for extra gas when working hard or descending. 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.

Both these systems were semi-closed and did not monitor partial pressures of oxygen. They both used an injector system to recirculate the breathing gas and did not increase work of breathing.

Rebreathers using an absorbent that releases oxygen

There have been a few rebreather designs (e.g. the Oxylite) which had an absorbent canister filled with potassium superoxide, which gives off oxygen as it absorbs carbon dioxide: 4KO₂ + 2CO₂ = 2K₂CO₃ + 3O₂; it had a very small oxygen cylinder to fill the loop at the start of the dive. This system is dangerous because of the explosively hot reaction that happens if water gets on the potassium superoxide. The Russian IDA71 military and naval rebreather was designed to be run in this mode or as an ordinary rebreather.

Tests on the IDA71 at the United States Navy Experimental Diving Unit in Panama City, Florida showed that the IDA71 could give significantly longer dive time with superoxide in one of the canisters than without.

This technology may be applied to both oxygen and mixed gas rebreathers, and can be used for diving and other applications.

Rebreathers which use liquid oxygen

A liquid oxygen supply can be used for oxygen or mixed gas rebreathers. If used underwater, the liquid-oxygen container must be well insulated against heat transfer from the water. Industrial sets of this type may not be suitable for diving, and diving sets of this type may not be suitable for use out of water due to conflicting heat transfer requirements. The set's liquid oxygen tank must be filled immediately before use.

Cryogenic rebreather

A cryogenic rebreather removes the carbon dioxide by freezing it out in a "snow box" by the low temperature produced as liquid oxygen evaporates to replace the oxygen used.

A cryogenic rebreather prototype called the S-1000 was built by Sub-Marine Systems Corporation. It had a duration of 6 hours and a maximum dive depth of 200 metres (660 ft). Its ppO₂ could be set to anything from 0.2 to 2 bars (3 to 30 psi) without electronics, by controlling the temperature of the liquid oxygen, thus controlling the equilibrium pressure of oxygen gas above the liquid. The diluent could be either nitrogen or helium depending on the depth of the dive. The partial pressure of oxygen was controlled by temperature, which was controlled by controlling the pressure at which liquid nitrogen was allowed to boil, which was controlled by an adjustable pressure relief valve. No control valves other than the nitrogen pressure relief valve were required. Low temperature was also used to freeze out up to 230 grams of carbon dioxide per hour from the loop, corresponding to an oxygen consumption of 2 litres per minute as carbon dioxide will freeze out of the gaseous state at -43.3 °C or below. If oxygen was consumed faster due to a high workload, a regular scrubber was needed. No electronics were needed as everything followed the setting of the nitrogen release pressure from the cooling unit, and the refrigeration by evaporation of liquid nitrogen maintained a steady temperature until the liquid nitrogen was exhausted. The loop gas flow was passed through a counterflow heat exchanger, which re-heated the gas returning to the diver by chilling the gas headed for the snow box (the cryogenic scrubber). The first prototype, the S-600G, was completed and shallow-water tested in October 1967. The S1000 was announced in 1969, but the systems were never marketed.

Cryogenic rebreathers were widely used in Soviet oceanography in the period 1980 to 1990.

Components and subsystems

User respiratory interface

The diver breathes from the rebreather circuit through a bite-grip mouthpiece or an oro-nasal mask which may be part of a full-face mask or diving helmet. The mouthpiece is connected to the rest of the rebreather by flexible breathing hoses. The mouthpiece of a diving rebreather will usually include a shutoff valve, and may incorporate a dive/surface valve or a bailout valve or both. On loop-configured rebreathers, the mouthpiece is usually the place where the non-return valves for the loop are fitted.

Dive/surface valve

The dive/surface valve (DSV) is a valve on the mouthpiece which can switch between the loop and ambient surroundings. It is used to close the loop at the surface to allow the diver to breathe atmospheric air, and may also be used underwater to isolate the loop so that it will not flood if the mouthpiece is taken out of the mouth.

Bailout valve

A dive/surface valve which can be switched to close the loop and simultaneously open a connection to an open circuit demand valve is known as a bailout valve (BOV), as its function is to switch over to open circuit bailout without having to remove the mouthpiece. An important safety device when carbon dioxide poisoning occurs.

Mouthpiece retaining strap

A mouthpiece retaining strap (MRS) is an item of safety equipment which is a mandatory design feature for rebreathers sold in the EU and UK, following European rebreather standard EN14143:2013. 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. The arrangement is required to be adjustable or self adjusting, to hold the mouthpiece firmly and comfortably in the user's mouth, and to maintain a seal. The MRS also reduces stress on the jaw during the dive.

Breathing hoses

Flexible corrugated synthetic rubber hoses are used to connect the mouthpiece to the rest of the breathing circuit, as these allow free movement of the diver's head. These hoses are corrugated to allow greater flexibility while retaining a high resistance to collapse. The hoses are designed to provide low resistance to flow of the breathing gas. A single breathing hose is used for pendulum (push-pull) configuration, and two hoses for a one-way loop configuration. Hose weights may be used to reduce excessive buoyancy.

Counterlungs

The counterlung is a part of the loop which is designed to change in volume by the same amount as the user's tidal volume when breathing. This lets the loop expand and contract when the user breathes, letting the total volume of gas in the lungs and the loop remain constant throughout the breathing cycle. The volume of the counterlung should allow for the maximum likely breath volume of a user, but does not generally need to match the vital capacity of all possible users.

Underwater, the position of the counterlung – on the chest, over the shoulders, or on the back – has an effect on the hydrostatic work of breathing. This is due to the pressure difference between the counterlung and the diver's lung caused by the vertical distance between the two.

Recreational, technical and many professional divers will spend most of their time underwater swimming face down and trimmed horizontally. Counterlungs should function well with low work of breathing in this position, and with the diver upright.

Front mounted: When horizontal they are under greater hydrostatic pressure than the diver's lungs. Easier to inhale, harder to exhale.
Back mounted: When horizontal they are under less hydrostatic pressure than the diver's lungs. The amount varies, as some are closer to the back than others. Harder to inhale, easier to exhale.
Over the shoulder: The hydrostatic pressure will vary depending on how much gas is in the counterlungs, and increases as the volume increases and the lowest part of the gas space moves downward. The resistive work of breathing often negates the gains of good positioning close to the lung centroid.

The design of the counterlungs can also affect the swimming diver's streamlining due to location and shape of the counterlungs, if they are not in a casing.

A rebreather which uses rubber counterlungs which are not in an enclosed casing should be sheltered from sunlight when not in use, to prevent the rubber from perishing due to ultraviolet light.

Concentric bellows counterlungs

Most passive addition semi-closed diving rebreathers control the gas mixture by removing a fixed volumetric proportion of the exhaled gas, and replacing it with fresh feed gas from a demand valve, which is triggered by low volume of the counterlung.

This is done by using concentric bellows counterlungs – the counterlung is configured as a bellows with a rigid top and bottom, and has a flexible corrugated membrane forming the side walls. There is a second, smaller bellows inside, also connected to the rigid top and bottom surfaces of the counterlung, so that as the rigid surfaces move towards and away from each other, the volumes of the inner and outer bellows change in the same proportion.

The exhaled gas expands the counterlungs, and some of it flows into the inner bellows. On inhalation, the diver only breathes from the outer counterlung – return flow from the inner bellows is blocked by a non-return valve. The inner bellows also connects to another non-return valve opening to the outside environment, and thus the gas from the inner bellows is dumped from the circuit in a fixed proportion of the volume of the inhaled breath. If the counterlung volume is reduced sufficiently for the rigid cover to activate the feed gas demand valve, gas will be added until the diver finishes that inhalation.

Carbon dioxide scrubber

Further information: Carbon dioxide scrubber

The exhaled gases are directed through the chemical scrubber, a canister full of a suitable carbon dioxide absorbent such as a form of soda lime, which removes the carbon dioxide from the gas mixture and leaves the oxygen and other gases available for re-breathing.

Some of the absorbent chemicals are produced in granular format for diving applications, such as Atrasorb Dive, Sofnolime, Dragersorb, or Sodasorb. Other systems use a prepackaged Reactive Plastic Curtain (RPC) based cartridge: The term Reactive Plastic Curtain was originally used to describe Micropore's absorbent curtains for emergency submarine use by the US Navy, and more recently RPC has been used to refer to their Reactive Plastic Cartridges, which are claimed to provide better and more reliable performance than the same volume of granular absorbent material.

The carbon dioxide passing through the scrubber absorbent is removed when it reacts with the absorbent in the canister; this chemical reaction is exothermic. Most of this reaction occurs along a "front" which is a region across the flow of gas through the soda-lime in the canister. This front moves through the scrubber canister, from the gas input end to the gas output end, as the reaction consumes the active ingredients. This front would be a zone with a thickness depending on the grain size, reactivity, and gas flow velocity because the carbon dioxide in the gas going through the canister needs time to reach the surface of a grain of absorbent, and then time to penetrate to the interior of each grain of absorbent as the outside of the grain becomes exhausted. Eventually gas with remaining carbon dioxide will reach the far end of the canister and "breakthrough" will occur. After this the carbon dioxide content of the scrubbed gas will tend to rise as the effectiveness of the scrubber falls until it becomes noticeable to the user, then unbreathable.

In rebreather diving, the typical effective endurance of the scrubber will be half an hour to several hours of breathing, depending on the grain size distribution and composition of the absorbent, the ambient temperature, the size of the canister, the dwell time of the gas in the absorbent material, and the production of carbon dioxide by the diver.

Scrubber design and size

Scrubber design and size is a compromise between bulk, cost of consumables, and work of breathing. Bulk affects the size of the unit and the amount of ballast weight needed, which affect the logistics of the dive. Work of breathing can be safety critical at greater depths, where it can become a significant part of the available aerobic work capacity of the diver, and can be overwhelming when it exceeds the diver's limit.

Single or multiple scrubbers

The usual arrangement is a single scrubber, but configurations with two scrubbers have been used, such as the IDA71, which has the scrubbers mounted in parallel (for some applications one of them may be filled with a superoxide type absorbent, which generates oxygen to replace the carbon dioxide) and the KISS Sidewinder, which has the scrubbers in series, with a single back mounted counterlung between the scrubbers so that transverse buoyancy shifts do not occur during the breathing cycle.

Axial or radial flow

The scrubber gas flow path may be axial, where the gas flows in at one end and out at the other, or radial, where the gas flows from the centre of the scrubber to the periphery (usually) or vice versa. The flow path should be of consistent length to minimise early breakthrough of some parts of the scrubber, which constrains radial designs to circular cylinders of variable length to diameter ratio, and axial scrubbers to approximately constant sectional shape along the flow (length) axis.

This section needs expansion with: Cross-flow scrubbers, as used in KISS Sidewinder 2. You can help by adding to it. (December 2024)

Grain size and size distribution

The distribution of grain sizes of the sorb affects the porosity of the packed canister. a wider distribution of grain size gives a low porosity. Porosity ranges from about 32% for uniformly sized (well sorted) approximately spherical grains down to less than 12% for a poorly sorted grain sizes with a large standard deviation, where the smaller granules occupy much of the space between the larger granules. A low porosity requires higher flow velocity for the same volume flow rate in the same scrubber canister. High flow rate produces high frictional resistance and low residence time (dwell time). The high resistance causes high work of breathing, and the low residence time makes breakthrough of carbon dioxide from the far side of the sorb occur sooner, i.e; shorter canister duration.

Effect of temperature on scrubber endurance

The scrubber absorbent (sorb) reaction rate is reduced at lower temperatures due to lower kinetic energy of gas molecules reducing the mean time before contact with the reactive material (Arrhenius equation). The sorb reaction releases heat, and cold water surroundings absorb heat through the canister walls, so the reaction front moves from the inlet end to outlet end, heating the sorb, and the heat is lost through the walls, which are around the sides in axial flow canisters. Carbon dioxide gets further through the colder parts of the sorb before it is absorbed, so tends to break through along the walls first. Breakthrough occurs in practice at about 50% of the theoretical canister endurance in 1.7°C water. This effect can be reduced by insulating the canister walls where they are in contact with absorbent material

Gas venting – Overpressure valve and diffuser

During ascent the gas in the breathing circuit will expand, and must have some way of escape before the pressure difference causes injury to the diver or damage to the loop. The simplest way to do this is for the diver to allow excess gas to escape around the mouthpiece or through the nose, but a simple overpressure valve is reliable and can be adjusted to control the permitted overpressure. The overpressure valve is typically mounted on the counterlung and in military diving rebreathers it may be fitted with a diffuser, which helps to conceal the diver's presence by masking the release of bubbles, by breaking them up to sizes which are less easily detected. A diffuser also reduces bubble noise.

Loop drainage

Many rebreathers have "water traps" in the counterlungs or scrubber casing, to stop large volumes of water from entering the scrubber media if the diver removes the mouthpiece underwater without closing the valve, or if the diver's lips get slack and let water leak in. Some rebreathers have manual pumps to remove water from the water traps, and a few of the passive addition SCRs automatically pump water out along with the gas during the exhaust stroke of the bellows counterlung. Others use internal pressure to expel water through the manually overridden dump valve when it is in a low position.

Gas sources

A rebreather must have a source of oxygen to replenish that which is consumed by the diver. Depending on the rebreather design variant, the oxygen source will either be pure oxygen or a breathing gas mixture, which is almost always stored in a gas cylinder. In a few cases oxygen is supplied as liquid oxygen or from a chemical reaction.

Diluent gas

Pure oxygen is not considered to be safe for recreational diving deeper than 6 meters, so closed circuit rebreathers for deeper use also have a cylinder of diluent gas. This diluent cylinder may be filled with compressed air or another diving gas mix such as nitrox, trimix, or heliox. The diluent reduces the percentage of oxygen breathed and increases the maximum operating depth of the rebreather. The diluent is not normally an oxygen-free gas, such as pure nitrogen or helium, and is breathable so it may be used in an emergency either to flush the loop with breathable gas of a known composition or as a bailout gas. Diluent gas is commonly referred to as diluent, dilutant, or just "dil" by divers. Diluent gas composition also affects gas density, and thereby the work of breathing at depth.

Gas addition valves

Gas must be added to the breathing loop if the volume gets too small or if it is necessary to change the gas composition.

Automatic diluent valve (ADV)

This has a similar function to an open circuit demand valve., and in many cases uses the mechanism from a commonly available open circuit demand valve. It adds gas to the circuit if the volume in the circuit is too low. The mechanism is either operated by a dedicated diaphragm like in a scuba second stage, or may be operated by the top of a bellows type counterlung reaching the bottom of its travel.

Manual addition

Closed circuit rebreathers usually allow the diver to add gas manually. In oxygen rebreathers this is just oxygen, but mixed gas rebreathers usually have a separate manual addition valve for oxygen and diluent, as either might be required to correct the composition of the loop mixture, either as the standard operating method for manually controlled CCRs, or as a backup system on electronically controlled CCRs. The manual diluent addition is sometimes by a purge button on the ADV.

Constant mass flow

Passive addition

In passive addition semi-closed rebreathers, gas is usually added by a demand type valve actuated by the bellows counterlung when the bellows is empty. This is the same actuation condition as the automatic diluent valve of any rebreather, but the actual trigger mechanism is slightly different. A passive addition rebreather of this type does not need a separate ADV as the passive addition valve already serves this function, though for engineering redundancy two such demand valves may be fitted, which operate simultaneously.

Electronically controlled (solenoid valves)

Electronically controlled closed circuit mixed gas rebreathers may have part of the oxygen feed provided by a constant mass flow orifice, but the fine control of partial pressure is done by solenoid operated valves actuated by the control circuits. Timed opening of the solenoid valve will be triggered when the oxygen partial pressure in the loop mix drops below the lower set-point.

If the constant mass flow orifice is compromised and does not deliver the correct flow, the control circuit will compensate by firing the solenoid valve more often.

Off-board gas

On some technical diving rebreathers it is possible to connect an alternative gas supply into the rebreather, usually using a wet quick-connect system. This is usually a feature of bailout rebreathers and other side-mounted rebreathers, where the rebreather unit is intentionally kept as compact as possible, and the gas supply may be slung on the other side of the diver for convenience and balance. This facility also allows all of the gas carried by a diver to be potentially supplied via a rebreather.

Bailout gas

Bailout gas and bailout procedure are closely linked. The procedure must be appropriate for the gas supply configuration. Initial bailout to open circuit is often the first step, even when a bailout rebreather is carried, as it is simple and robust, and some time is needed to get the bailout rebreather ready for use. Bailout gas supply must be sufficient for safe return to the surface from any point in the planned dive, including any required decompression, so it is not unusual for two bailout cylinders to be carried, and the diluent cylinder to be used as the first bailout to get to a depth where the other gas can be used. On a deep dive, or a long penetration, open circuit bailout can easily be heavier and more bulky than the rebreather, and for some dives a bailout rebreather is a more practical option.

Control of the breathing gas mix

Narked at 90 Ltd – Deep Pursuit Advanced electronic rebreather controller

The fundamental requirements for the control of the gas mixture in the breathing circuit for any rebreather application are that the carbon dioxide is removed, and kept at a tolerable level, and that the partial pressure of oxygen is kept within safe limits. For rebreathers which are used at normobaric or hypobaric pressures, this only requires that there is sufficient oxygen, which is easily achieved in an oxygen rebreather. Hyperbaric applications, as in diving, also require that the maximum partial pressure of oxygen is limited, to avoid oxygen toxicity, which is technically a more complex process, and may require dilution of the oxygen with metabolically inert gas.

If not enough oxygen is added, the concentration of oxygen in the loop may be too low to support life. In humans, the urge to breathe is normally caused by a build-up of carbon dioxide in the blood, rather than lack of oxygen. Hypoxia can cause blackout with little or no warning, followed by death.

The method used for controlling the range of oxygen partial pressure in the breathing loop depends on the type of rebreather.

In an oxygen rebreather, once the loop has been thoroughly flushed, the mixture is effectively static at 100% oxygen, and the partial pressure is a function only of ambient pressure.
In a semi-closed rebreather the loop mix depends on a combination of factors:

the type of gas addition system and its setting, combined with the gas mixture in use, which control the rate of oxygen added.
work rate, and therefore the oxygen consumption rate, which controls the rate of oxygen depletion, and therefore the resulting oxygen fraction.
ambient pressure, as partial pressure varies in proportion to ambient pressure and oxygen fraction.

In manually controlled closed circuit rebreathers (MCCCR), also known as diver-controlled closed-circuit rebreathers (DCCCR), the diver monitors the loop mix using one or more oxygen sensors, and controls the gas mixture and volume in the loop by injecting the appropriate available gases to the loop and by venting the loop. In this application the diver needs to know the partial pressure of oxygen in the loop and correct it as it drifts away from the set point. A common method for increasing the time between corrections is to use a constant mass flow orifice set to the diver's relaxed diving metabolic oxygen consumption rate to add oxygen at a rate that is unlikely to increase the partial pressure at a constant depth.
Most electronically controlled closed-circuit rebreathers (ECCCR) have electro-galvanic oxygen sensors and electronic control circuits, which monitor the ppO₂, injecting more oxygen if necessary and issuing an audible, visual and/or vibratory warning to the diver if the ppO₂ reaches dangerously high or low levels.

The volume in the loop is usually controlled by a pressure or volume triggered automatic diluent valve, and an overpressure relief valve. The automatic diluent valve works on the same principle as a demand valve to add diluent when the pressure in the loop is reduced below ambient pressure, such as during descent or if gas is lost from the loop. The set may also have a manual addition valve, sometimes called a bypass. In some early oxygen rebreathers the user had to manually open and close the valve to the oxygen cylinder to refill the counterlung each time the volume got low.

Instrumentation and displays

Instrumentation may vary from the minimal depth, time and remaining gas pressure necessary for a closed circuit oxygen rebreather or semi-closed nitrox rebreather to redundant electronic controllers with multiple oxygen sensors, redundant integrated decompression computers, carbon dioxide monitoring sensors and a head-up display of warning and alarm lights with a sound and vibration alarm.

Alarms for malfunctions

Alarms may be provided for a few malfunctions. The alarms are electronically controlled and may rely on input from a sensor and processing by the control circuitry. These may include:

Failure of the control system.
Failure of one or more sensors.
Low partial pressure of oxygen in the loop.
High partial pressure of oxygen in the loop.
Gas other than pure oxygen in the oxygen supply system. (unusual)
High carbon dioxide levels in the loop. (unusual)
Impending scrubber breakthrough (unusual)

Alarm displays:

Visible (digital screen displays, flashing LEDs)
Audible (buzzer or tone generator)
Tactile (Vibrations)
Control panel displays (usually with digital readout of the value and status of the measured parameter, often with blinking or flashing display)
Head-up displays (usually a colour coded LED display, sometimes providing more information by the rate of flashing.)

If a rebreather alarm goes off there is a high probability that the gas mixture is deviating from the set mixture. There is a high risk that the gas in the rebreather loop will soon be unsuitable to support consciousness. A good general response is to add diluent gas to the loop as this is known to be breathable. This will also reduce carbon dioxide concentration if that is high.

Ascending while breathing off the loop without identifying the problem may increase risk of a hypoxia blackout.
If the partial pressure of oxygen is not known the rebreather can not be trusted to be breathable, and the diver should immediately bailout to open circuit to reduce the risk of losing consciousness without warning

Work of breathing

Work of breathing is the effort required to breathe. Part of the work of breathing is due to inherent physiological factors, part is due to the mechanics of the external breathing apparatus, and part is due to the characteristics of the breathing gas. A high work of breathing may result in carbon dioxide buildup in the diver, and reduces the diver's ability to produce useful physical effort. In extreme cases work of breathing may exceed the aerobic work capacity of the diver, with fatal consequences.

Work of breathing of a rebreather has two main components: Resistive work of breathing is due to the flow restriction of the gas passages causing resistance to flow of the breathing gas, and exists in all applications where there is no externally powered ventilation. Hydrostatic work of breathing is only applicable to diving applications, and is due to difference in pressure between the lungs of the diver and the counterlungs of the rebreather. This pressure difference is generally due to a difference in hydrostatic pressure caused by a difference in depth between lung and counterlung, but can be modified by ballasting the moving side of a bellows counterlung.

Resistive work of breathing is the sum of all the restrictions to flow due to bends, corrugations, changes of flow direction, valve cracking pressures, flow through scrubber media, etc., and the resistance to flow of the gas, due to inertia and viscosity, which are influenced by density, which is a function of molecular weight and pressure. Rebreather design can limit the mechanical aspects of flow resistance, particularly by the design of the scrubber, counterlungs and breathing hoses. Diving rebreathers are influenced by the variations of work of breathing due to gas mixture choice and depth. Helium content reduces work of breathing, and increased depth increases work of breathing. Work of breathing can also be increased by excessive wetness of the scrubber media, usually a consequence of a leak in the breathing loop, or by using a grain size of absorbent that is too small.

The semi-closed rebreather systems developed by Drägerwerk in the early 20th century as a scuba gas supply for Standard diving dress, using oxygen or nitrox, and the US Navy Mark V Heliox helmet developed in the 1930s for deep diving, circulated the breathing gas through the helmet and scrubber by using an injector system where the added gas entrained the loop gas and produced a stream of scrubbed gas past the diver inside the helmet, which eliminated external dead space and resistive work of breathing, but was not suitable for high breathing rates.

Safety

Hazards

Some of the hazards are due to the way the equipment works, while others are related to the environment in which the equipment is used.

Hypoxia

Hypoxia can occur in any rebreather which contains enough inert gas to allow breathing without triggering automatic gas addition.

In an oxygen rebreather this can occur if the loop is not sufficiently purged at the start of use. Purging should be done while breathing off the unit so that the inert gas from the user's lungs is also removed from the system.

Hyperoxia

A dangerously high partial pressure of oxygen can occur in the breathing loop for several reasons:

Descent below the maximum operating depth with an oxygen rebreather or a semi-closed rebreather.
Failure to correctly maintain the loop mixture within tolerance of the set point. This may be due to:
- Oxygen sensor malfunction: If the cell fails current limited, it will register a partial pressure lower than reality, and the control system may attempt to correct by continuous injection of oxygen.
- Voting logic error Where there are three of more oxygen cells, in the system, the voting logic will assume that the two with most similar outputs are correct. This may not be the case – there have been cases where two cells with almost identical history have failed in the same way at the same time, and the voting logic has dismissed the one remaining correctly functioning cell, with fatal consequences.
- Power supply malfunction
- Use of a diluent with too high oxygen fraction for the planned depth in a CCR. In this case a diluent flush will not produce a breathable gas in the loop.

This section needs expansion. You can help by adding to it. (May 2021)

Carbon dioxide buildup

Carbon dioxide buildup will occur if the scrubber medium is absent, badly packed, inadequate or exhausted. The normal human body is fairly sensitive to carbon dioxide partial pressure, and a buildup will be noticed by the user. However, there is not often much that can be done to rectify the problem except changing to another breathing gas supply until the scrubber can be repacked. Continued use of a rebreather with an ineffective scrubber is not possible for very long, as the levels will become toxic and the user will experience extreme respiratory distress, followed by loss of consciousness and death. The rate at which these problems develop depends on the volume of the circuit and the metabolic rate of the user.

Excessive work of breathing

Carbon dioxide buildup can also occur when a combination of exertion and work of breathing exceeds the capacity of the user. If this occurs where the user cannot reduce exertion sufficiently, it may be impossible to correct. In this case it is not the scrubber that fails to remove carbon dioxide, but the inability of the diver to circulate gas efficiently through the scrubber against the frictional resistance of the circuit causing the problem. This is more likely to occur with diving rebreathers at depths where the density of the breathing gas is severely elevated, or when water in the scrubber obstructs gas flow.

Fire hazards of high concentration of oxygen

High partial pressures of oxygen greatly increase fire hazard, and many materials which are self-extinguishing in atmospheric air will burn continuously in a high oxygen concentration. This is more of a risk for terrestrial applications such as rescue and firefighting than for diving, where the ignition risk is relatively low.

Caustic cocktail

Loop flooding that reaches the scrubber can cause a "caustic cocktail" when the alkaline components of carbon dioxide absorbent materials are mixed with the water. This mixture is caustic and can cause chemical burns to the mucosa and skin. The mixture is generally a liquid or watery slurry with a chalky and bitter taste, which should prompt the diver to switch to an alternative source of breathing gas and immediately rinse their mouth out well with water. Some modern diving rebreather absorbents are designed not to produce "caustic cocktail" if they get wet.

Failure modes

Diving rebreathers are susceptible to some failure modes which cannot occur in other breathing apparatus.

Scrubber failure

The term "breakthrough" means the failure of the scrubber to continue removing sufficient carbon dioxide from the gas circulating in the loop. This will inevitably happen if the scrubber is used too long, but can happen prematurely in some circumstances. There are several ways that the scrubber may fail or become less efficient:

Complete consumption of the active ingredient in a "general breakthrough". Depending on scrubber design and diver workload, this may be gradual, allowing the diver to become aware of the problem in time to make a controlled bailout to open circuit, or relatively sudden, triggering an urgent or emergency response.
Bypassing the absorbent. The absorbent granules must be packed closely so that all exhaled gas comes into contact with the surface of soda lime and the canister is designed to avoid any spaces or gaps between the absorbent granules or between the granules and the canister walls that would let gas bypass contact with the absorbent. If any of the seals, such as O-rings, or spacers that prevent bypassing of the scrubber, are not present or not fitted properly, or if the scrubber canister has been incorrectly packed or fitted, it may allow the exhaled gas to bypass the absorbent, and the scrubber will be less effective. This failure mode is also called "tunneling" when absorbent settles to form void spaces inside the canister. Bypass will cause an unexpected early breakthrough.
When the gas mix is under pressure at depth, the gas molecules are more densely packed, and the carbon dioxide molecules' mean path between collisions is shorter, so they are not so free to move around to reach the absorbent surface, and require a longer dwell time. Because of this effect, the scrubber must be bigger for deep diving than is needed for a shallow-water, industrial or high altitude rebreather.
Carbon dioxide absorbent when mixed with water from a leak, can produce a caustic cocktail. The excessive wetting of the sorb also reduces the rate of carbon dioxide removal and can cause premature breakthrough even if no caustic liquid reaches the diver. Work of breathing may also increase. Many modern diving rebreather absorbents are designed not to produce this caustic fluid if they get wet.
In below-freezing surface conditions while preparing for diving, wet scrubber chemicals can freeze while there is a pause in the exothermic reaction of taking up the carbon dioxide, thus preventing carbon dioxide from reaching the scrubber material, and slowing the reaction when used again.

Flooding of the loop

Flooding of the breathing loop can occur due to a leak at a low point in the loop where internal gas pressure is less than the external water pressure. One of the more common ways this can happen is if the mouthpiece is dislodged or removed from the diver's mouth without first closing the dive/surface valve or switching to bailout. This can happen due to accidental impact or through momentary inattention. Depending on the layout of the loop and the attitude of the rebreather in the water, the amount of water ingress can vary, as can the distance it travels into the air passages of the breathing loop. In some models of rebreather a moderate amount of water will be trapped at a low point in a counterlung or the scrubber housing, and prevented from reaching the absorbent in the scrubber. Some rebreathers have a system to expel water trapped in this way, either automatically through the vent valve, such as in the Halcyon RB80 and the Interspiro DCSC, or manually by using a small pump.

Gas leakage

There are several places on a rebreather where gas leakage can cause problems. Leakage can occur from the high and intermediate pressure components, and from the loop, at pressure slightly above ambient. The effects on system integrity depend on severity of the leak. If only small volumes of gas are lost the leak may be tolerable for the rest of the dive, but a leak may become more severe, depending on the cause, and may in some cases deteriorate catastrophically.

This section needs expansion. You can help by adding to it. (May 2021)

Oxygen monitoring failure

Failure of electro-galvanic oxygen sensors.
Failure of voting logic
Failure of display

Oxygen monitoring failure can lead to incorrect partial pressure of oxygen in the breathing gas. The consequences can include hypoxia, hyperoxia, and incorrect decompression information, all three of which are potentially life-threatening.

This section needs expansion. You can help by adding to it. (May 2021)

Gas injection system failure

Constant mass flow orifice blockage:– In a CCR, blockage of a CMF oxygen injection orifice will increase the frequency of manual or solenoid valve injection, which is an inconvenience rather than an emergency. In active addition SCRs the unnoticed failure of gas injection will lead to the mix becoming hypoxic. If there is instrumentation monitoring the partial pressure of oxygen in the loop, the diver can compensate by manual injection or forcing automatic injection via the ADV by dumping gas into the environment by exhaling through the nose.
Injector control circuit malfunction:– If the control circuit for oxygen injection fails, the usual mode of failure results in the oxygen injection valves being closed. Unless action is taken, the breathing gas will become hypoxic with potentially fatal consequences. 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. Two basic approaches for preventing loss of availability are possible. Either a redundant independent control system may be used, or the risk of the single system failing may be accepted, and the diver takes the responsibility for manual gas mixture control in the event of failure. Both methods depend on continued reliable oxygen monitoring. Most (possibly all) electronically controlled CCRs have manual injection override. If the electronic injection fails, the user can take manual control of the gas mixture provided that the oxygen monitoring is still reliably functioning. Alarms are usually provided to warn the diver of failure.
Automatic diluent valve malfunction:– The ADV is the same technology as an open circuit demand valve, and as such is generally very reliable if maintained correctly. Two failure modes are possible, Free flow, where the valve sticks open, and the less likely failure of the valve to open. As diluent is usually chosen to be breathable at all or most depths of the planned dive, this is not usually immediately dangerous, but a free flow will use up the diluent rapidly and unless rectified soon the diver will have to abort the dive and bail out. There may be a manual diluent valve which the diver can use to add gas if the valve fails closed.

Scrubber monitoring

The methods available for monitoring the condition of the scrubber and predicting and identifying imminent breakthrough include:

Dive planning and scheduled replacement. Divers are trained to monitor and plan the exposure time of the absorbent material in the scrubber and replace it within the recommended time limit. This method is necessarily very conservative, as actual carbon dioxide produced during a dive is not accurately predictable and is not measured. Manufacturers recommendations for replacement periods tend to allow for worst cases to reduce risk, and this is relatively uneconomical in absorbent usage.
An indicator dye that changes colour when the active ingredient is consumed may be included in the absorbent. For example, a rebreather absorbent called "Protosorb" supplied by Siebe Gorman had a red indicator dye, which was said to go white when the absorbent was exhausted. With a transparent canister, this may show the position of the reaction front. This is useful where the canister is visible to the user, which is seldom possible on diving equipment, where the canister is often inside the counterlung or a back mounted casing. Colour indicating dye was removed from US Navy fleet use in 1996 when it was suspected of releasing chemicals into the circuit.
Temperature monitoring. As the reaction between carbon dioxide and soda lime is exothermic, temperature sensors along the length of the scrubber can be used to measure the position of the reaction front and therefore the estimated remaining life of the scrubber.
While carbon dioxide gas sensors exist, they are not useful as a tool for predicting remaining scrubber endurance as they measure the carbon dioxide in the scrubbed gas, and the onset of scrubber break through generally occurs quite rapidly. Such systems are fitted as a safety device to warn divers to bail off the loop immediately.

Fault tolerant design

Manufacturing and operation standards

Diving rebreathers must comply with EN 14143: Respiratory equipment - Self-contained re-breathing diving apparatus to be legally manufactured of marketed in the European Union. European Standard EN 14143 specifies performance requirements intended to ensure safe operation of a diving rebreather. Maximum depths are specified at 6m for oxygen rebreathers, 40m for nitrox rebreathers and 100m for rebreathers using helium based breathing gases. Water temperature range is from 4°C to 34°C unless the manufacturer specifies a wider range.

Diving rebreathers fall under the European Personal Protective Equipment Directive, which requires certification of product quality assurance and tested product performance by a neutral accredited third party. This is a costly and time consuming process, which can inhibit innovation, but is also recognised as an important influence on product safety and manufacturer liability limitation.

This section needs expansion. You can help by adding to it. (December 2024)

Operation

Main article: Rebreather diving

Rebreathers are 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.

Technological innovations

Rebreather technology has advanced considerably, often driven by the growing market in recreational diving equipment, particularly in underwater cave exploration. Innovations include:

Bailout valves – a device in the mouthpiece of the loop which connects to a bailout demand valve and can be switched to provide gas from either the loop or the demand valve without the diver taking the mouthpiece from their mouth. An important safety device when carbon dioxide poisoning occurs.
Closed circuit bailout.
Active and passive oxygen sensor validation.
Hyperoxic linearity test.
Integrated decompression computers – input to a dive computer from the oxygen sensors of the rebreather allow divers to take advantage of the actual partial pressure of oxygen to generate an optimised schedule for decompression.
Gas integrated decompression computers – these allow divers to take advantage of the actual gas mixture, as measured by one or more oxygen cells in real time, to generate a schedule for decompression in real time.
Carbon dioxide scrubber life monitoring systems – temperature sensors monitor the progress of the reaction of the soda lime and provide an indication of when the scrubber will be exhausted.
Carbon dioxide level monitoring systems – Gas sensing cell and interpretive electronics which detect the concentration of carbon dioxide in the rebreather loop downstream from the scrubber.
Multiple set-points automatically selected by depth – Electronic rebreather control systems can be programmed to change set-point above and below selectable limiting depths to limit oxygen exposure during the working dive, but increase the limit during decompression above the limiting depth to accelerate decompression.
Automated pre-dive checklists and systems checks.
Head-up displays for status and alarms.
Data logging.
Sidemount rebreathers

Active and passive oxygen sensor validation

Accurate and reliable oxygen partial pressure measurement is one of the most problematic factors in rebreather diving safety. Control systems using this data have developed to the extent that they are robust and reliable, and the use of an independent backup improves the reliability to about as good as for any other component. The weakest point is the sensors, which are prone to several modes of failure, some of which are relatively insidious as the cell may pass a normobaric calibration and fail when the partial pressure is near the high end of the acceptable working range, which is also the range in which constant partial pressure diving has the maximum benefit. When it has been possible to infer the cause, the leading cause of rebreather fatalities is hypoxia, at approximately 17%, with hyperoxia assumed in an additional 4% of cases. If these trends extend into the range of indeterminate cases, it is possible that inappropriate oxygen content is involved in 30% of rebreather fatalities.

The standard method for improving reliability of oxygen monitoring has been multiple redundancy – the use of 3 or more sensors – and using the multiple data inputs with a voting logic system to try to identify failure of a sensor in time to make a controlled and safe termination of the dive. Voting logic normally assumes that if one sensor produces a reading significantly differing from two or more others when exposed to the same environment, the outlier is faulty, and the input of the others is assumed accurate. Unfortunately this is not always the case, and there have been cases where the outlier sensor was most correct. It has been shown that the reliability of this system is lower than originally expected due to a lack of sufficient statistical independence of the three sensors, and that outcomes are not symmetrical – the effects of faulty low or high partial pressure readings are also depth dependent.

If a sensor gives relatively static output with little response to variations in depth and temperature, and changes in gas composition due to use, gas addition, incomplete mixing or loop turbulence, it is likely that the sensor may not be responding correctly, and when two sensors follow a similar pattern of response this is a warning that both may be defective. Algorithms that track sensor output against expected output taking known changes into account can indicate reliability of the sensors. This method of monitoring sensors is known as passive sensor validation (PSV), can be used to improve reliability of sensor integrity assessment, and can be used in the control system to make more reliable decisions on which sensors are most likely to be giving trustworthy output in comparison with voting logic based only on calibration values for the sensors. PSV is an improvement on simple voting logic but is still susceptible to errors related to statistical independence of components.

Early work on design of an automatic sensor validation system, in which the rebreather control system would periodically inject gas of known composition onto the oxygen sensors during the dive and use the output to determine the viability of the sensor response with greater precision and accuracy than a human diver, was started in 2002, and further developed to be used on the Poseidon/Cis-Lunar MK-VI rebreather. This "Active Sensor Validation" (ASV) system has been refined over thousands of hours of field test diving in varied conditions

The ASV system has become more sophisticated than the manual implementation in the Cis-Lunar MK-5P. It involves more than comparing the measured PO₂ value from the sensor with the calculated PO₂ of the diluent at the current depth. In the implementation in the Poseidon rebreathers the computer automatically injects either diluent or oxygen directly onto a single primary oxygen sensor every five minutes during a dive. The algorithm takes into account current depth, FO₂ of the injected gas, ambient temperature, duration of gas injection, and calibration values for the sensor for that dive to predict how the sensor should respond over the next few seconds after each gas injection, and compares that with the measured results to produce a confidence level for correct sensor performance.

This type of sensor validation test can identify several modes of failure by the ways the measured values deviate from expected values with variations of calculated partial pressure of the test gas, and is capable of detecting failures due to incorrect temperature readings, incorrect input of the FO₂ of the diluent condensation on the oxygen sensor, a defective oxygen sensor, validation gas supply failure and other reasons that would not be detected by voting logic.

Hyperoxic linearity test

The oxygen sensors for most rebreathers are calibrated at the surface before the dive using air or 100% oxygen at normal atmospheric pressure. These are reliable calibration points but the range of operational partial pressures may extend beyond these calibration points, and if the sensors are calibrated for a linear response between these conditions and the response is extrapolated, for set points above 1 bar, which is standard practice, the control system must operate outside of the range for which response is known to be linear. One of the most common modes of failure is for a cell to become current-limited as it ages. The internal impedance changes as the anode is consumed by the reaction which produces the output current, and the response becomes non-linear at higher oxygen partial pressures. The signal may indicate a lower partial pressure and does not increase proportionately as oxygen is added, leading to a loop oxygen partial pressure that may increase to dangerous levels without warning. A way of validating the sensors at high partial pressures is to expose the sensor to higher PO₂ than the upper set point by exposing it to pure oxygen at a depth of 6 m, for a PO₂ of 1.6 bar during the dive, or at 1.6 bar or more in a calibration pressure pot. Both these methods are cumbersome and the in-water method may cause spiking of the PO₂ during descent. A variation of the ASV system using oxygen, called a hyperoxic linearity test (HLT), uses oxygen as the flushing gas at 6 m, which can check that the sensor is linear to 1.6 bar PO₂, and if it fails, the set point can be automatically reduced to within the linear range established during calibration. A single sensor with PSV and ASV has been shown to be more reliable than three sensors with conventional voting logic. The effectiveness of cell validation algorithms is expected to improve with the acquisition of more field data gathered by the rebreather control systems.

Carbon dioxide monitoring

Hypercapnia has been identified as one of the most prevalent factors in rebreather diving fatalities. This is generally a consequence of scrubber failure to remove carbon dioxide as fast as it is produced, which may be caused by any one or a combination of spent, wet, or inadequately packed, absorbent material, incorrectly designed or assembled canisters, mismatch of absorbent and canister design, or absorbent used beyond its operational range. Higher carbon dioxide partial pressure in the loop leads to higher levels of carbon dioxide in the blood and tissue, which can have a range of symptoms including respiratory distress, increased susceptibility to CNS oxygen toxicity, disorientation, and loss of consciousness.

Most rebreather designs have relied on very conservative time-based limits for absorbent duration based on experimental testing, using cold conditions and high workloads and high depth pressures. The usually unnecessarily high conservatism encourages divers to stretch the absorbent duration, which works well enough until it doesn't, often without warning, which can have serious consequences. A more sophisticated method is to base absorbent duration limits on metabolic oxygen consumption, as a proxy for metabolic carbon dioxide production, which is reasonably stable for most people most of the time, and can compensate fairly well for variations in exertion and base metabolism, but does not compensate reliably for depth and pressure effects on absorbent function.

A more direct and empirical approach is to take advantage of the production of heat and rise in temperature of the active zone of the absorbent in the scrubber. More carbon dioxide is absorbed by the first zone of relatively unused absorbent that it reaches as the breathing gas passes through the scrubber, and this relatively active zone progresses through the canister as the zone first reached by the gas is exhausted, and more reaction occurs further along. This reaction front is at a higher temperature than the spent absorbent, and the absorbent not yet exposed to high carbon dioxide levels, and the front progresses along the scrubber until part of it reaches the end of the absorbent, and unscrubbed gas breaks through to the other side of the loop, after which there is a fairly constant and irreversible increase in inspired carbon dioxide. Some rebreather manufacturers have developed linear temperature probes which identify the position of the reactive front, allowing the user to estimate the remaining duration of the canister.

None of these methods can detect canister bypass and they have little ability to identify completely spent absorbent, channeling, badly packed, or inappropriate absorbent material, but this can be done by a direct measurement of carbon dioxide partial pressure in the inhalation side of the loop.

Research and development of carbon dioxide sensors goes back at least as far as the early 1990s when Teledyne Analytical Instruments and Cis-Lunar Development Laboratories worked on a sensor for the Cis-Lunar MK-III rebreather, which was accurate in laboratory conditions but in the field susceptible to high humidity and condensation causing unreliable readings, which was a recurring problem with real-time carbon dioxide measurement. High pressures also caused problems for depth compensation. In 2009 VR Technologies released a commercial CO₂ sensor using hydrophobic membranes to keep the sensors dry without excessively reducing gas flow to the sensors.

Since then other manufacturers have introduced their products to the market but they have not gained widespread use. They are relatively expensive, give unreliable readings in some circumstances, can only detect failure of the scrubber, and do not predict remaining duration. A combination of temperature measurement and post scrubber CO₂ measurement can give both prediction and failure warning, for increased cost and complexity.

Placement of the sensor in the loop can affect sensitivity to actual CO₂ content of inspired gas. Measuring gas in the mouthpiece has problems due to dead space, and mounting in the inhalation hose near the mouthpiece makes the sensor sensitive to small leaks in the inhalation check valve, while also able to detect high CO₂ due to major check valve leaks which would cause a big increase in dead space, which would not be detected if the sensor is further upstream in the loop.

Furthermore, increased levels of CO₂ in inspired gas is only one cause of hypercapnia. It is also affected by work of breathing, diver fitness, respiratory ventilation patterns, and other behavioural, physiological, and mechanical factors. A better option would be to measure both inhaled and exhaled CO₂ levels, and this would require sensors that are fast and reliable in wet conditions, and reasonably inexpensive

Automated pre-dive checklists

Following the strong endorsement by Rebreather Forum 3 of the use of written checklists to improve safety, Cis-Lunar Development Laboratories programmed an electronic pre-dive checklist into their MK-5P rebreather operating system, as a way to prevent the user from neglecting to carry out the recommended checks before use. This was considered successful and implemented on later generations in the Poseidon MK-VI and SE7EN rebreathers, and developed to include robust internal diagnostics for the core electronic components and software, and automatic calibration of the oxygen sensor cells at normobaric pressures. Failure to complete the full checklist results in a range of alarms if the user attempts to dive with the unit. While not entirely foolproof – Oxygen cells are not calibrated at hyperbaric working pressures – a number of safety critical errors will be picked up and the diver made aware of them. The software also logs the steps and data from the pre-dive check and this has been valuable for accident analysis. The pre-dive checks also take less time and require no paper or user logging effort. This system has been shown to reduce risk and has been adopted by several manufacturers.

Head-up displays

Closed circuit bailout

A major logistical problem for long and deep rebreather dives is the volume of bailout equipment that must be carried to allow a safe return to the surface from any point of the dive after irrecoverable failure of the primary system. The open circuit option can become extremely bulky and awkward to manage, and while more compact and efficient, the rebreather option has its own set of logistic challenges.

One of the main design challenges in developing a closed circuit bailout system for rebreathers is to maintain the bailout set in a condition ready for use at all depths. This implies breathable gas for the depth, though not necessarily optimised, as the mix can be brought to set point quite rabidly after bailout, and a gas volume that does not vary excessively, so that buoyancy control is not unduly complicated. The bulk of the system must be manageable, and the bailout set mouthpiece must be easily accessible, but secure. Since bailout rebreathers are most likely to be used on dives with large decompression obligations, the switch to bailout must be accommodated by the decompression management system. If real-time monitoring of oxygen partial pressure is included in decompression computation, it must be possible to transfer this facility between units, without compromising their independence. Task-loading of the diver in managing the two loops must not be excessive, as the diver is recognised as the least reliable aspect of the operation, and may be under significant stress when bailout becomes necessary.

Data logging

Main article: Data logger

Data logged from rebreather dives is useful for accident analysis, testing and development of rebreathers, and for diver educational purposes. Dive profile logging by integrated decompression computers is also of value for research into effectiveness of decompression schedules. Aggregation of such data can provide insights into diving patterns across the population of users and help in analysing risk.

The control systems of electronic rebreathers have continued to increase in processing and storage capacity, and in parallel, their capacity for capturing data at increased granularity and precision has increased. In 1994 the Cis-Lunar Mk-IV data logging system recorded data at several hundred points per hour of dive time, and by 1997 the Cis-Lunar Mk-5P was logging over a thousand points per hour. By 2007 the Poseidon MK-VI Discovery was logging between 15,000 and 25,000 points per hour, and in 2016 the Poseidon SE7EN recorded more than double that quantity, in alignment with the recommendations of Rebreather Forum 3, which states:

The forum recommends that all rebreathers incorporate data-logging systems that record functional parameters relevant to the particular unit and dive data and that allow download of these data. Diagnostic reconstruction of dives with as many relevant parameters as possible is the goal of this initiative. An ideal goal would be to incorporate redundancy in data-logging systems and, as much as practical, to standardize the data to be collected

Some of the logged data is specific to the rebreather model, and is not appropriate for general analysis, but some data is useful for external analysis of user population and diving practices which could improve understanding of behaviour and safety analysis.

Manufacturers and models

Oxygen rebreather manufacturers

Aqua Lung/La Spirotechnique – French company manufacturing breathing apparatus and diving equipment
- FROGS (Full Range Oxygen Gas System) – Closed circuit oxygen diving rebreather – a model of closed circuit oxygen rebreather for intensive shallow water work and clandestine special forces operations made by AquaLung, which has been used in France since October 2002. The unit can be worn on the chest, or with an adaptor frame, on the back. The scrubber has an endurance of about 4 hours at 4°C and respiratory minute volume of 40 litres per minute, and a 2.1 litre 207 bar cylinder. It is manufactured in non-magnetic and magnetic versions and can use either 2.6kg of granular sorb or a moulded carbon dioxide absorbent insert.
Dräger – German manufacturer of breathing equipment
- LAR-5 – Military oxygen tebreather by Drägerwerk
- LAR-6 – Military oxygen rebreather by Drägerwerk
- LAR-V – Military oxygen rebreather by Drägerwerk
Lambertsen Amphibious Respiratory Unit – Early closed circuit oxygen diving rebreather
Porpoise – Australian scuba manufacturer
- Porpoise (rebreather) – Australian oxygen rebreather – Ted Eldred's oxygen rebreather.
Siebe Gorman – British manufacturer of diving equipment and salvage contractor
- Mark IV Amphibian – British military oxygen rebreather
- CDBA – Type of diving rebreather used by the Royal Navy
- Davis Submerged Escape Apparatus – Early submarine escape oxygen rebreather also used for shallow water diving. – One of the first rebreathers to be produced in quantity.
- Salvus – Industrial rescue and shallow water oxygen rebreather
- The "Universal" rebreather was a long-dive derivative of the Davis Submerged Escape Apparatus, intended to be used with the Sladen Suit.
IDA71 – Russian military rebreather for underwater and high altitude use
SDBA – Special duty oxygen breathing apparatus, a military rebreather. – A type of frogman's oxygen rebreather. It has a nitrox variant called ONBA.

Mixed gas rebreather manufacturers

AP Diving – British manufacturer of underwater diving equipment
- Inspiration series – one of the first electronic closed circuit rebreathers to be mass produced for the recreational market.
Aqua Lung/La Spirotechnique – French company manufacturing breathing apparatus and diving equipment
- DC55
BioMarine – American manufacturer of diving rebreathers
- BioMarine CCR 1000
- BioMarine Mk-15 military rebreather
- BioMarine Mk-16 military rebreather
Carleton Life Support – Subsidiary of Cobham based in Davenport, Iowa
- Siva – Range of military rebreathers military rebreather, and
- Viper – Electronically-controlled closed circuit mixed gas military rebreather
- Viper E – made by Carleton and Juergensen Defense Corporation
- Carleton CDBA – Military rebreather by Cobham plc – Clearance Diver's Breathing Apparatus.
Cis-Lunar – Manufacturer of electronically controlled closed-circuit rebreathers for scuba diving
Divex – Scottish provider of diving equipment and related services.
- Clearance Divers' Life Support Equipment (CDLSE) – An electronic closed circuit rebreather allowing diving to 60 metres (200 ft).
Dräger – German manufacturer of breathing equipment
- Dräger Dolphin – Semi-closed circuit recreational diving rebreather
- Dräger Ray – Semi-closed circuit diving rebreather
Halcyon Dive Systems – American manufacturer of diving equipment
- Halcyon PVR-BASC – Semi-closed circuit depth compensated passive addition diving rebreather
- Halcyon RB80 – Non-depth-compensated passive addition semi-closed circuit rebreather
- Halcyon Symbios chest mount eCCR
IDA71 – Russian military rebreather for underwater and high altitude use
Interspiro DCSC – Military semi-closed circuit passive addition diving rebreather
Jetsam Technologies – Manufacturer of manual closed circuit rebreathers – Manufacturer of KISS brand rebreathers
- KISS – Manufacturer of recretional/technical mixed gas rebreathers – line of manually operated closed circuit rebreathers originally designed by Gordon Smith . and manufactured by Jetsam Technologies, which was later acquired by Darkwater Group (XDEEP and SEAL Drysuits)
  - KISS Classic – Back mount manually controlled closed circuit rebreather – Back mount mCCR
  - Kiss Spirit – Lightweight back mount manually controlled closed circuit rebreather – lightweight back mount mCCR
  - KISS Sidewinder – Symmetrical side mount manually controlled closed circuit rebreather – Lightweight sidemount mCCR
  - KISS Sidekick – Side mount manually controlled closed circuit or semi-closed circuit rebreather – Sidemount SCR/mCCR
  - KISS GEM – Back mount semi-closed circuit rebreather – Compact back mount SCR
JJ CCR – A technical diving rebreather built to allow mounting of large cylinders to enable carrying larger quantities of bailout gas on the rebreather frame.
Divesoft Liberty rebreathers Back and sidemount mixed gas technical diving rebreathers.
Mark 29 Underwater Breathing Apparatus
Poseidon Diving Systems – Swedish manufacturer of diving equipment
- Poseidon MkVI – the world's first fully automatic closed circuit rebreather for recreational use, based on the Cis-Lunar MK5 design and further developed into,
- Poseidon SE7EN.
Prism 2 – Notable for a radial scrubber and high-current oxygen cells from the Navy MK15 unit enabling an analogue gauge to read the oxygen levels.
ScubaForce
- SF2 (rebreather) – Back or sidemount ECCR with bellows counterlung.
Siebe Gorman – British manufacturer of diving equipment and salvage contractor
- Siebe Gorman CDBA – Type of diving rebreather used by the Royal Navy – also CDMBA, SCBA, SCMBA, UBA
- A type introduced in 1999 in the British Navy, being an update of the BioMarine/Carleton MK16:

Some military rebreathers (for example the US Navy MK-25 and the MK-16 mixed-gas rebreather), and the Phibian CCS50 and CCS100 rebreathers, were developed by Oceanic.
The current US Navy Mark 16 Mod 2 (Explosive Ordnance Disposal) and Mark 16 Mod 3 (Naval Special Warfare) units use the Juergensen Defense Corporation Mark V Control System.
The Orca ECR is a CCR design that has both carbon dioxide and oxygen monitoring
The Megalodon
The rEvo III
The O₂ptima CM

IDA-71 with lid of casing opened showing interior
IDA-71 mask, DSV and breathing hoses
Inspiration with casing open showing interior
Liberty closed circuit rebreathers
rEvo rebreather back view, right side

References

"JFD | COBRA (Compact bailout rebreathing apparatus)". www.jfdglobal.com. Retrieved 25 October 2022.
Crawford, J. (2016). "8.5.1 Helium recovery systems". Offshore Installation Practice (revised ed.). Butterworth-Heinemann. pp. 150–155. ISBN 9781483163192.
Thornton, Michael Albert (December 2000). A Survey and Engineering Design of Atmospheric Diving Suits (PDF) (Report). Texas A&M University. Archived (PDF) from the original on 2023-03-19. Retrieved 2023-09-15.
^ Sieber, Arne; Schuster, Andreas; Reif, Sebastian; Kessler, Michael; Lucyshyn, Thomas; Buzzacott, Peter; Enoksson, Peter (2013). "Compact Recreational Rebreather With Innovative Gas Sensing Concept and Low Work of Breathing Design". Marine Technology Society Journal. 47 (27): 27–41. doi:10.4031/MTSJ.47.6.5..
^ Šimánek, Jakub (2 February 2021). "Building A Fault Tolerant Rebreather: Our Path to Simplicity". In Depth. Retrieved 12 February 2021.
^ "MK 29 Underwater Breathing Apparatus (UBA) Project Team extends Navy Mobile Diving and Salvage Capabilities" (PDF). www.secnav.navy.mil. Retrieved 25 October 2022.
"IDA-72 (ИДА-72)". www.therebreathersite.nl. J.W. Bech. Retrieved 25 October 2022.
^ "Commercial Dive Gear: Diving Helmets: DESCO 29019D Mark V Diving Helmet". Milwaukee, Wisconsin: DESCO Corporation. Retrieved 17 January 2019.
^ "12". US Navy Diving Manual Revision 1 Navsea-0994-LP001-9020 (PDF). Vol. 2. Washington DC: Navy Department. July 1981. Archived (PDF) from the original on July 2, 2019.
^ NOAA Diving Program (U.S.) (28 Feb 2001). Joiner, James T. (ed.). NOAA Diving Manual, Diving for Science and Technology (4th ed.). Silver Spring, Maryland: National Oceanic and Atmospheric Administration, Office of Oceanic and Atmospheric Research, National Undersea Research Program. ISBN 978-0-941332-70-5. CD-ROM prepared and distributed by the National Technical Information Service (NTIS)in partnership with NOAA and Best Publishing Company
P.S.Dhami; G.Chopra; H.N. Shrivastava (2015). A Textbook of Biology. Jalandhar, Punjab: Pradeep Publications. pp. V/101.
^ US Navy Diving Manual 2016, Chapter 15 - Electronically Controlled Closed-Circuit Underwater Breathing Apparatus (EC-UBA) Diving, Section 15-2 Principles of operation.
^ Walker, J R. III; Murphy-Lavoie, Heather M. (11 January 2021). "Diving Rebreathers". Treasure Island, FL: StatPearls Publishing. PMID 29494076. Retrieved 11 May 2021.
James W. Miller, ed. (1979). "Fig 2.4". NOAA Diving Manual (2nd ed.). Washington, DC.: US Dept of Commerce - National Oceanographic and Atmospheric Administration. pp. 2–7.
^ US Navy (2006). "Chapter 19: Closed-Circuit Oxygen UBA Diving". US Navy Diving Manual, 6th revision. United States: US Naval Sea Systems Command. p. 19–9. Archived from the original on 2008-05-02. Retrieved 2008-06-15.
^ Parker, Martin (Nov 2012). "Rebreather user manual" (PDF). www.apdiving.com. Ambient Pressure Diving Ltd. Retrieved 11 May 2021.
^ Mitchell, S.J.; Cronje, F.; Meintjies, W.A.J.; Britz, H.C. (2007). "Fatal respiratory failure during a technical rebreather dive at extreme pressure". Aviat Space Environ Med. 78 (2): 81–86. PMID 17310877.
^ Heinerth, Jill (10 March 2019). "Diving Sidemount Rebreathers". www.youtube.com. Retrieved 24 October 2022.
"Rebreather Courses: PADI Type R and Type T Rebreathers: What is a PADI Type R Rebreather?". www.idcphuket.com. Retrieved 25 October 2022.
^ Odom, J. (August 1999). An Introduction to Semi-Closed Circuit Rebreathers: The Dräger Series of Recreational Rebreathers (PDF) (Revision 4a ed.). Technical Diving International, 1995. p. 14.
Menduno, Michael (8 January 2014). "Rise of the Recreational Rebreather". Diver. Retrieved 25 October 2022.
Douglas, Eric (31 March 2014). "Ask an Expert: Is the New Breed of Rebreathers Really Recreational?". Scuba Diving. Retrieved 25 October 2022.
^ "KISS Sidewinder". www.kissrebreathers.com. Retrieved 31 May 2024.
"What is a "Rebreather"?". bishopmuseum.org. Archived from the original on 2019-06-11.
Elliott, David (1997). "Some limitations of semi-closed rebreathers". South Pacific Underwater Medicine Society Journal. 27 (1). ISSN 0813-1988. OCLC 16986801.
^ Larsson, Åke (15 July 2002). "Le Spirotechnique DC55". Teknosofen.com. Retrieved 31 July 2013.
Daucherty, RL; Franzini, JB (1977). Fluid Mechanics with Engineering Applications (7th ed.). Kogakusha: McGraw-Hill. pp. 257–261. ISBN 0-07-085144-1.
^ Larsson, A. (2000). "The Interspiro DCSC". Retrieved 30 April 2013.
Older, P. (1969). "Theoretical Considerations in the Design of Closed Circuit Oxygen Rebreathing Equipment". Royal Australian Navy, School of Underwater Medicine. RANSUM-4-69.
Davis, R.H. (1955). Deep Diving and Submarine Operations (6th ed.). Tolworth, Surbiton, Surrey: Siebe Gorman & Company Ltd. p. 693.
Shreeves, K; Richardson, D (2006). Lang, M.A.; Smith, N.E. (eds.). Mixed-Gas Closed-Circuit Rebreathers: An Overview of Use in Sport Diving and Application to Deep Scientific Diving. Proceedings of Advanced Scientific Diving Workshop (Report). Washington, DC: Smithsonian Institution. OCLC 70691158.
^ "Rebreathers guide for beginners". www.apdiving.com. Retrieved 11 May 2021.
Dekker, David L. "Diving apparatus 'Modell 1912' Draegerwerk Lübeck, helmet with 'lock system'". Chronology of Diving in Holland: 1889. Draegerwerk Lübeck. www.divinghelmet.nl. Retrieved 17 September 2016.
^ Kelley, J.S.; Herron, J.M.; Dean, W.W.; Sundstrom, E.B. (1968). Mechanical and Operational Tests of a Russian 'Superoxide' Rebreather. US Navy Experimental Diving Unit Technical Report (Report). Vol. NEDU-Evaluation-11-68.
Fischel, H. (1970). "Closed circuit cryogenic SCUBA". Equipment for the Working Diver - 1970 Symposium. Washington, DC: Marine Technology Society: 229–244.
Cushman, L. (1979) . "Cryogenic Rebreather". Skin Diver Magazine: 29–31, 85–87 – via Aqua Corps magazine, N7, 28. Also available online via the Rebreather Site
^ Bech, JW. "S-600 G and SS100 cryogenic rebreather". therebreathersite.nl. Retrieved 28 May 2019.
"Popular mechanics (ru), №7(81) June 2009". Retrieved 2009-07-17.
"Sportsmen-podvodnik journal, 1977" (PDF). Retrieved 2008-07-17.
^ "OC – DSV – BOV – FFM page". www.therebreathersite.nl. 8 November 2010. Retrieved 2010-12-29.
Haynes, P; Menduno, M; Toomer, P (21 March 2023). "Rebreather Training Council. Mouthpiece Retraining Strap Safety Guidance Notice, Issue" (PDF). rebreathertrainingcouncil.org. Retrieved 3 April 2024.
^ "Back Mounted Counterlungs: User Instruction Manual Issue 5" (PDF). www.apdiving.com. Ambient Pressure Diving Ltd. Retrieved 6 May 2021.
Reynolds, Glen Harlan (December 2006). "Seeking New Depths". Popular Mechanics. 183 (12): 58.
Norfleet, W & Horn, W (2003). Carbon Dioxide Scrubbing Capabilities of Two New Non-Powered Technologies. US Naval Submarine Medical Research Center Technical Report (Report). Vol. NSMRL-TR-1228.
"Micropore Brochure". www.apollomilitary.com. Retrieved 6 May 2021.
Bulman, Jake; Coffield, Skanda (27 October 2022). "Keep It Simple Sidewinder". InDepth. Global Underwater Explorers. Retrieved 31 May 2024.
^ Clarke, John (20–22 April 2023). Demystifying scrubbers. Rebreather Forum 4. Valetta, Malta. Retrieved 29 April 2024.
Chapple, JCB; Eaton, David J. "Development of the Canadian Underwater Mine Apparatus and the CUMA Mine Countermeasures dive system". Defence R&D Canada Technical Report (DCIEM 92–06). Defence R&D Canada. section 1.2.a
"Scuba Force Bellow Part Counterlung for SF2 Rebreather". www.divestock.com. Retrieved 6 May 2021.
"Counterlungs". flexccr.com. Retrieved 6 May 2021.
"Design Specifics". Halcyon Dive Systems. Retrieved 19 December 2016.
Rhea, David (2 February 2021). "The RB80 Semi-closed Rebreather: A Successful Exploration Tool". gue.com. In Depth. Retrieved 16 February 2021.
^ "Rubicon Shop – Choosing an SF2 Rebreather". rubicondiving.com. Retrieved 5 May 2021.
Šimánekk, Jakub (10 June 2020). "Using a Bailout Rebreather". www.tdisdi.com. Retrieved 7 February 2024.
^ Mitchell, Simon (April 2015). "Operational Aspects of Technical Diving". www.youtube.com. DAN Southern Africa. Event occurs at 48:00. Retrieved 28 September 2021.
"Deep Life Design Team: databases and analysis of rebreather accident data". Deeplife.co.uk. Retrieved 31 July 2013.
"Going deep". divingheritage.com. Retrieved 2 July 2019.
Raymaekers, Paul (18 October 2010). "Understanding oxygen sensors and why NOT change them all at the same time" (PDF). www.revo-rebreathers.com. Retrieved 28 September 2021.
Lillo RS, Ruby A, Gummin DD, Porter WR, Caldwell JM (March 1996). "Chemical safety of U.S. Navy Fleet soda lime". Undersea and Hyperbaric Medicine Journal. 23 (1): 43–53. PMID 8653065.
^ Warkander, Dan E. (2007). "Development of a scrubber gauge for closed-circuit diving". Undersea and Hyperbaric Medicine Abstract. 34.
"Ambient Pressure Diving Ltd". apdiving.com. Archived from the original on 2013-11-06.
González, Oscar; Shrikumar, H.; Stankovic, John. A; Ramamritham, Krithi (1997). Adaptive Fault Tolerance and Graceful Degradation Under Dynamic Hard Real-Time Scheduling. Computer Science Department Faculty Publication Series. 188. (Report). University of Massachusetts - Amherst.
"EN 14143:2013(Main) Respiratory equipment - Self-contained re-breathing diving apparatus". standards.iteh.ai. 10 July 2013. Retrieved 17 December 2024.
Quirion, Florine (8 May 2023). "The Love-Hate Relationship Between CCR & CE Marking". indepthmag.com. InDEPTH Magazine. Retrieved 17 December 2024.
^ Pyle, Richard L (2016). "Rebreather Evolution in the Foreseeable Future". In Pollock, N.W.; Sellers, S.H.; Godfrey, J.M. (eds.). Rebreathers and Scientific Diving (PDF). Proceedings of NPS/NOAA/DAN/AAUS Workshop, June 16–19, 2015. Durham, NC. pp. 40–65. ISBN 978-0-9800423-9-9.
Staff. "Equipements des Commandos Marine" (in French). Archived from the original on 26 May 2013. Retrieved 11 October 2013.
"Military Rebreathers". www.opstechnologies.com. Retrieved 4 May 2021.
"F.R.O.G.S. Rebreather" (PDF). www.ihchytech.com. Retrieved 4 May 2021.
Williams, Des. "Ted Eldred's Porpoise Oxygen Rebreather 1946". Historical Diving Times, No. 38 Winter 2006. Historical Diving Society. pp. 5–8. Retrieved 12 December 2016 – via www.therebreathersite.nl.
https://www.apdiving.com/shop/en_gb/rebreather.html Archived 2021-07-09 at the Wayback Machine AP Diving rebreathers
Juergensen, Kevin (16 Jul 1998). "The History of BioMarine" (PDF). www.therebreathersite.nl. Retrieved 27 July 2021.
Historical Diving Times #42 Summer 2007, p27
https://www.jfdglobal.com/products/defence-divers-equipment/underwater-life-support-systems/stealth-cdlse-mk2-ed/ images
"Halcyon Dive Systems". Retrieved 4 October 2024.
^ Menduno, Michael (13 February 2024). "Sneak Preview: Halcyon Symbios Chest Mount and KISS Sidewinder 2". indepthmag.com. InDepthh Magazine. Retrieved 17 December 2024.
"KISS Closed Circuit Rebreather". Archived from the original on 2008-09-19. Retrieved 2013-10-09.
^ "KISS Rebreathers". www.kissrebreathers.com. Retrieved 18 December 2024.
"User manual, CCR Liberty, Manual version: 2.17 CU HW rev 1.0, HS HW rev 3.0, FW 2.17". www.divesoft.com. Retrieved 23 March 2024.
"BioMarine/Carleton MK16 and Royal Navy CDBA". www.cybermaps.co.uk. Archived from the original on 2008-02-19. Retrieved 19 February 2008.
"Carbon Dioxide Monitoring". Lungfish Dive Systems.

Works cited

US Navy (1 December 2016). U.S. Navy Diving Manual Revision 7 SS521-AG-PRO-010 0910-LP-115-1921 (PDF). Washington, DC.: US Naval Sea Systems Command.

External links

Media related to Diving rebreathers at Wikimedia Commons

Breathing apparatus

High altitude breathing apparatus
Aviation breathing apparatus Emergency oxygen system Mountaineering breathing apparatus

Occupational breathing apparatus

Respirator
Regulated by NIOSH and others	Gas mask Canister Supplied-air respirator Self-contained breathing apparatus Self-contained self-rescue device Particulate respirator N95 respirator Dust mask Powered air-purifying respirator Chemical cartridge Facepieces Smoke hood Elastomeric respirator
Regulations	42 CFR 84 (United States) ANSI K13.1-1973 EN 149/14683/143 (Europe) GB2626 (China), and others

Medical breathing apparatus

Underwater breathing apparatus
Open-circuit scuba Diving helmet Diving rebreather Full-face diving mask Surface-supplied diving equipment Snorkel Helicopter escape set Submarine escape set Atmospheric diving suit Gas extender Helium reclaim system

User respiratory interface
Breathing mask Full facepiece Half mask orinasal mask

General

Category
Commons
Glossary

Underwater diving

Diving equipment

Basic equipment

Breathing gas

Buoyancy and
trim equipment

Decompression
equipment

Diving suit

Helmets
and masks

Instrumentation

Mobility
equipment

Safety
equipment

Underwater
breathing
apparatus

Open-circuit scuba	Scuba set Bailout bottle Decompression cylinder Independent doubles Manifolded twin set Scuba manifold Pony bottle Scuba configuration Sidemount Sling cylinder
Diving rebreathers	Carbon dioxide scrubber Carleton CDBA Clearance Divers Life Support Equipment Cryogenic rebreather CUMA DSEA Dolphin Halcyon PVR-BASC Halcyon RB80 IDA71 Interspiro DCSC LAR-5 LAR-6 LAR-V LARU Mark IV Amphibian Porpoise Ray Siebe Gorman CDBA Salvus Siva
Surface-supplied diving equipment	Air line Diver's umbilical Diving air compressor Gas panel Hookah Scuba replacement Snuba Standard diving dress

Diving
equipment
manufacturers

Diving support equipment

Access equipment

Breathing gas
handling

Decompression
equipment

Platforms

Underwater habitat	Aquarius Reef Base Continental Shelf Station Two Helgoland Habitat Scott Carpenter Space Analog Station SEALAB Tektite habitat

Remotely operated
underwater vehicles

Safety equipment

General

Freediving

Activities	Aquathlon Apnoea finswimming Freediving Haenyeo Pearl hunting Ama Snorkeling Spearfishing Underwater football Underwater hockey Underwater rugby Underwater target shooting
Competitions	Nordic Deep Vertical Blue Disciplines Constant weight (CWT) Constant weight bi-fins (CWTB) Constant weight without fins (CNF) Dynamic apnea (DYN) Dynamic apnea without fins (DNF) Free immersion (FIM) No-limits apnea (NLT) Static apnea (STA) Skandalopetra diving Variable weight apnea (VWT) Variable weight apnea without fins
Equipment	Diving mask Diving suit Hawaiian sling Polespear Snorkel (swimming) Speargun Swimfins Monofin Water polo cap
Freedivers	Deborah Andollo Simone Arrigoni Peppo Biscarini Michael Board Sara Campbell Derya Can Göçen Goran Čolak Carlos Coste Robert Croft Mandy-Rae Cruickshank Yasemin Dalkılıç Leonardo D'Imporzano Flavia Eberhard Şahika Ercümen Emma Farrell Francisco Ferreras Pierre Frolla Flavia Eberhard Mehgan Heaney-Grier Elisabeth Kristoffersen Andriy Yevhenovych Khvetkevych Loïc Leferme Enzo Maiorca Jacques Mayol Audrey Mestre Karol Meyer Kate Middleton Stéphane Mifsud Alexey Molchanov Natalia Molchanova Dave Mullins Patrick Musimu Guillaume Néry Herbert Nitsch Umberto Pelizzari Liv Philip Annelie Pompe Stig Severinsen Tom Sietas Aharon Solomons Martin Štěpánek Walter Steyn Tanya Streeter William Trubridge Devrim Cenk Ulusoy Fatma Uruk Danai Varveri Alessia Zecchini Nataliia Zharkova
Hazards	Barotrauma Drowning Freediving blackout Deep-water blackout Shallow-water blackout Hypercapnia Hypothermia
Historical	Ama Octopus wrestling Swimming at the 1900 Summer Olympics – Men's underwater swimming
Organisations	AIDA International Scuba Schools International Australian Underwater Federation British Freediving Association Confédération Mondiale des Activités Subaquatiques Fédération Française d'Études et de Sports Sous-Marins Performance Freediving International

Professional diving

Occupations

Military
diving

Military
diving
units

Underwater
work

Salvage diving	SS Egypt Kronan La Belle SS Laurentic RMS Lusitania Mars Mary Rose USS Monitor HMS Royal George Vasa

Diving
contractors

Tools and
equipment

Underwater
weapons

Underwater firearm	Gyrojet Mk 1 Underwater Defense Gun Powerhead Underwater pistols Heckler & Koch P11 SPP-1 underwater pistol Underwater revolvers AAI underwater revolver Underwater rifles ADS amphibious rifle APS underwater rifle ASM-DT amphibious rifle QBS-06

Recreational diving

Specialties	Altitude diving Cave diving Deep diving Ice diving Muck diving Open-water diving Rebreather diving Sidemount diving Solo diving Technical diving Underwater photography Wreck diving
Diver organisations	British Sub-Aqua Club (BSAC) Cave Divers Association of Australia (CDAA) Cave Diving Group (CDG) Comhairle Fo-Thuinn (CFT) Confédération Mondiale des Activités Subaquatiques (CMAS) Federación Española de Actividades Subacuáticas (FEDAS) Fédération Française d'Études et de Sports Sous-Marins (FFESSM) International Association for Handicapped Divers (IAHD) Quintana Roo Speleological Survey (QRSS) Woodville Karst Plain Project (WKPP)
Diving tourism industry	Dive center Diving in East Timor Diving in the Maldives Environmental impact of recreational diving Scuba diving tourism Scuba diving in the Cayman Islands Shark tourism Sinking ships for wreck diving sites Underwater diving on Guam
Diving events and festivals	Diversnight Underwater Bike Race

Diving safety

Diving
hazards

Consequences	Barotrauma Decompression sickness Drowning Hypothermia Hypoxia Hypercapnia Hyperthermia Non-freezing cold injury

Diving
procedures

Risk
management

Checklist
Hazard identification and risk assessment
Hyperbaric evacuation and rescue
Risk control
- Hierarchy of hazard controls
Incident pit
Lockout–tagout
Permit To Work
Redundancy
Safety data sheet
Situation awareness

Diving team	Bellman Chamber operator Diver medical technician Diver's attendant Diving supervisor Diving systems technician Gas man Life support technician Stand-by diver
Equipment safety	Breathing gas quality Testing and inspection of diving cylinders Hydrostatic test Sustained load cracking Diving regulator Breathing performance of regulators
Occupational safety and health	Association of Diving Contractors International International Marine Contractors Association Code of practice Contingency plan Diving regulations Emergency response plan Diving safety officer Diving superintendent Diving supervisor Operations manual Standard operating procedure

Diving medicine

Diving
disorders

Pressure
related

Oxygen	Freediving blackout Hyperoxia Hypoxia Oxygen toxicity
Inert gases	Avascular necrosis Decompression sickness Dysbaric osteonecrosis Inner ear decompression sickness Isobaric counterdiffusion Taravana High-pressure nervous syndrome Hydrogen narcosis Nitrogen narcosis
Carbon dioxide	Hypercapnia Hypocapnia
Breathing gas contaminants	Carbon monoxide poisoning

Immersion
related

Treatment

Personnel

Screening

Research

Researchers in
diving physiology
and medicine

Diving medical
research
organisations

Law
Civil liability in recreational diving Diving regulations Duty of care List of legislation regulating underwater diving Investigation of diving accidents Convention on the Protection of the Underwater Cultural Heritage

History of underwater diving

Archeological
sites

Underwater art
and artists

Engineers
and inventors

Historical
equipment

Diver propulsion vehicles	Advanced SEAL Delivery System Cosmos CE2F series Dry Combat Submersible Human torpedo Motorised Submersible Canoe Necker Nymph R-2 Mala-class swimmer delivery vehicle SEAL Delivery Vehicle Shallow Water Combat Submersible Siluro San Bartolomeo Welfreighter Wet Nellie

Military and
covert operations

Scientific projects

Awards and events

Incidents

Dive boat incidents	Sinking of MV Conception
Diver rescues	Alpazat cave rescue Tham Luang cave rescue
Early diving	John Day (carpenter) Charles Spalding Ebenezer Watson
Freediving fatalities	Stephen Keenan Loïc Leferme Audrey Mestre Nicholas Mevoli Natalia Molchanova
Offshore diving incidents	Byford Dolphin diving bell accident Drill Master diving accident Star Canopus diving accident Stena Seaspread diving accident Venture One diving accident Waage Drill II diving accident Wildrake diving accident
Professional diving fatalities	Roger Baldwin John Bennett Victor F. Guiel Jr. Francis P. Hammerberg Craig M. Hoffman Peter Henry Michael Holmes Johnson Sea Link accident Edwin Clayton Link Gerard Anthony Prangley Per Skipnes Robert John Smyth Albert D. Stover Richard A. Walker Lothar Michael Ward Joachim Wendler Bradley Westell Arne Zetterström
Scuba diving fatalities	1973 Mount Gambier cave diving accident Ricardo Armbruster Allan Bridge David Bright Berry L. Cannon Cotton Coulson Cláudio Coutinho E. Yale Dawson Deon Dreyer Milan Dufek Sheck Exley Maurice Fargues Fernando Garfella Palmer Guy Garman Steve Irwin death Jim Jones Henry Way Kendall Artur Kozłowski Yuri Lipski Kirsty MacColl Linnea Mills Agnes Milowka François de Roubaix Chris and Chrissy Rouse Dave Shaw Wesley C. Skiles Dewey Smith Rob Stewart Esbjörn Svensson Josef Velek

Publications

Manuals	NOAA Diving Manual U.S. Navy Diving Manual Basic Cave Diving: A Blueprint for Survival Underwater Handbook Bennett and Elliott's physiology and medicine of diving Encyclopedia of Recreational Diving The new science of skin and scuba diving Professional Diver's Handbook Basic Scuba
Standards and Codes of Practice	Code of Practice for Scientific Diving (UNESCO) DIN 7876 IMCA Code of Practice for Offshore Diving ISO 24801 Recreational diving services — Requirements for the training of recreational scuba divers
General non-fiction	The Darkness Beckons Goldfinder The Last Dive Shadow Divers The Silent World: A Story of Undersea Discovery and Adventure
Research	List of Divers Alert Network publications
Dive guides

Training and registration

Diver
training

Competence and assessment
Diver training standard
Diving instructor
Diving school
Occupational diver training
Recreational diver training
- Introductory diving
- ISO training standards
Teaching method

Skills	Combat sidestroke Diver navigation Diver trim Ear clearing Frenzel maneuver Valsalva maneuver Finning techniques Scuba skills Buddy breathing Low impact diving Diamond Reef System Surface-supplied diving skills Underwater searches

Recreational
scuba
certification
levels

Core diving skills	Advanced Open Water Diver Autonomous diver CMAS* scuba diver CMAS** scuba diver Introductory diving Low Impact Diver Master Scuba Diver Open Water Diver Supervised diver
Leadership skills	Dive leader Divemaster Diving instructor Master Instructor
Specialist skills	Rescue Diver Solo diver

Diver training
certification
and registration
organisations

Commercial diver
certification
authorities

Commercial diving
schools

Free-diving
certification
agencies

AIDA International (AIDA)
Confédération Mondiale des Activités Subaquatiques (CMAS)
Performance Freediving International (PI)
Scuba Schools International (SSI)

Recreational
scuba
certification
agencies

American Canadian Underwater Certifications (ACUC)
American Nitrox Divers International (ANDI)
Association nationale des moniteurs de plongée (ANMP)
British Sub-Aqua Club (BSAC)
Comhairle Fo-Thuinn (CFT)
Confédération Mondiale des Activités Subaquatiques (CMAS)
Federación Española de Actividades Subacuáticas (FEDAS)
Fédération Française d'Études et de Sports Sous-Marins (FFESSM)
Federazione Italiana Attività Subacquee (FIAS)
Global Underwater Explorers (GUE)
International Association for Handicapped Divers (IAHD)
International Association of Nitrox and Technical Divers (IANTD)
International Life Saving Federation (ILS)
Israeli Diving Federation (TIDF)
National Academy of Scuba Educators (NASE)
National Association of Underwater Instructors (NAUI)
Nederlandse Onderwatersport Bond (NOB)
Professional Association of Diving Instructors (PADI)
Professional Diving Instructors Corporation (PDIC)
Professional Technical and Recreational Diving (ProTec)
Rebreather Association of International Divers (RAID)
Sub-Aqua Association (SAA)
Scuba Diving International (SDI)
Scuba Educators International (SEI)
Scottish Sub Aqua Club (ScotSAC)
Scuba Schools International (SSI)
Türkiye Sualtı Sporları Federasyonu (TSSF)
United Diving Instructors (UDI)
YMCA SCUBA Program

Scientific diver
certification
authorities

Technical diver
certification
agencies

American Nitrox Divers International (ANDI)
British Sub-Aqua Club (BSAC)
Confédération Mondiale des Activités Subaquatiques (CMAS)
Diving Science and Technology (DSAT)
Federazione Italiana Attività Subacquee (FIAS)
International Association of Nitrox and Technical Divers (IANTD)
Professional Association of Diving Instructors (PADI)
Professional Diving Instructors Corporation (PDIC)
Professional Technical and Recreational Diving (ProTec)
Rebreather Association of International Divers (RAID)
Trimix Scuba Association (TSA)
Technical Extended Range (TXR)

Cave diving	Cave Divers Association of Australia (CDAA) Cave Diving Group (CDG) Global Underwater Explorers (GUE) National Speleological Society#Cave Diving Group (CDG) National Association of Underwater Instructors (NAUI) Technical Diving International (TDI)

Military diver
training centres

Military diver
training courses

United States Marine Corps Combatant Diver Course

Underwater sports

Surface snorkeling	Finswimming
Snorkeling/breath-hold	Spearfishing Underwater football Underwater hockey Australia Turkey Underwater rugby Colombia United States Underwater target shooting
Breath-hold	Aquathlon Apnoea finswimming Freediving
Open Circuit Scuba	Immersion finswimming Sport diving Underwater cycling Underwater orienteering Underwater photography
Rebreather	Underwater photography
Sports governing organisations and federations	International AIDA International Confédération Mondiale des Activités Subaquatiques) National AIDA Hellas Australian Underwater Federation British Freediving Association British Octopush Association British Underwater Sports Association Comhairle Fo-Thuinn Federación Española de Actividades Subacuáticas Fédération Française d'Études et de Sports Sous-Marins South African Underwater Sports Federation Türkiye Sualtı Sporları Federasyonu Underwater Society of America)
Competitions	14th CMAS Underwater Photography World Championship Underwater Hockey World Championships Underwater Orienteering World Championships Underwater Rugby World Championships

Underwater divers

Pioneers of diving	Eduard Admetlla i Lázaro Aquanaut Mary Bonnin Amelia Behrens-Furniss James F. Cahill Jacques Cousteau Billy Deans Dottie Frazier Trevor Hampton Hans Hass Hannes Keller Dick Rutkowski Teseo Tesei Arne Zetterström
Underwater scientists archaeologists and environmentalists	Michael Arbuthnot Robert Ballard George Bass Mensun Bound Louis Boutan Jeffrey Bozanic Hugh Bradner Cathy Church Eugenie Clark James P. Delgado Sylvia Earle John Christopher Fine George R. Fischer Anders Franzén Honor Frost Fernando Garfella Palmer David Gibbins Graham Jessop Swietenia Puspa Lestari Pilar Luna Robert F. Marx Anna Marguerite McCann Innes McCartney Charles T. Meide Mark M. Newell Lyuba Ognenova-Marinova John Peter Oleson Mendel L. Peterson Richard Pyle Andreas Rechnitzer William R. Royal Margaret Rule Gunter Schöbel Stephanie Schwabe Myriam Seco E. Lee Spence Robert Sténuit Peter Throckmorton Cristina Zenato
Scuba record holders	Pascal Bernabé Jim Bowden Mark Ellyatt Sheck Exley Nuno Gomes Claudia Serpieri Krzysztof Starnawski
Underwater filmmakers and presenters	Samir Alhafith David Attenborough Ramón Bravo Jean-Michel Cousteau Richie Kohler Paul Rose Andy Torbet Ivan Tors Andrew Wight James Cameron
Underwater photographers	Doug Allan Tamara Benitez Georges Beuchat Adrian Biddle Jonathan Bird Eric Cheng Neville Coleman Jacques Cousteau John D. Craig Ben Cropp Bernard Delemotte David Doubilet Candice Farmer John Christopher Fine Rodney Fox Ric Frazier Stephen Frink Peter Gimbel Monty Halls Hans Hass Henry Way Kendall Rudie Kuiter Joseph B. MacInnis Luis Marden Agnes Milowka Noel Monkman Pete Oxford Steve Parish Zale Parry Pierre Petit Leni Riefenstahl Peter Scoones Brian Skerry Wesley C. Skiles E. Lee Spence Philippe Tailliez Ron Taylor Valerie Taylor Albert Tillman John Veltri Stan Waterman Michele Westmorland John Ernest Williamson J. Lamar Worzel
Underwater explorers	Caves Graham Balcombe Sheck Exley Martyn Farr Jochen Hasenmayer Jill Heinerth Jarrod Jablonski William Hogarth Main Tom Mount Jack Sheppard Bill Stone Reefs Arthur C. Clarke Wrecks Leigh Bishop John Chatterton Clive Cussler Bill Nagle Valerie van Heest Aristotelis Zervoudis
Aquanauts	Andrew Abercromby Joseph M. Acaba Clayton Anderson Richard R. Arnold Serena Auñón-Chancellor Michael Barratt (astronaut) Robert A. Barth Robert L. Behnken Randolph Bresnik Timothy J. Broderick Justin Brown Berry L. Cannon Scott Carpenter Gregory Chamitoff Steve Chappell Catherine Coleman Robin Cook Craig B. Cooper Fabien Cousteau Philippe Cousteau Timothy Creamer Jonathan Dory Pedro Duque Sylvia Earle Jeanette Epps Sheck Exley Albert Falco Andrew J. Feustel Michael Fincke Satoshi Furukawa Ronald J. Garan Jr. Michael L. Gernhardt Christopher E. Gerty David Gruber Chris Hadfield Jeremy Hansen José M. Hernández John Herrington Paul Hill Akihiko Hoshide Mark Hulsbeck Emma Hwang Norishige Kanai Les Kaufman Scott Kelly Karen Kohanowich Timothy Kopra Dominic Landucci Jon Lindbergh Kjell N. Lindgren Michael López-Alegría Joseph B. MacInnis Sandra Magnus Thomas Marshburn Matthias Maurer K. Megan McArthur Craig McKinley Jessica Meir Simone Melchior Dorothy Metcalf-Lindenburger Andreas Mogensen Karen Nyberg John D. Olivas Takuya Onishi Luca Parmitano Nicholas Patrick Tim Peake Thomas Pesquet Marc Reagan Garrett Reisman Kathleen Rubins Dick Rutkowski Tara Ruttley David Saint-Jacques Josef Schmid Robert Sheats Dewey Smith Steve Squyres Heidemarie Stefanyshyn-Piper Robert Sténuit Hervé Stevenin Nicole Stott James Talacek Daniel M. Tani Robert Thirsk Bill Todd Mark T. Vande Hei Koichi Wakata Rex J. Walheim Shannon Walker John Morgan Wells Joachim Wendler Douglas H. Wheelock Peggy Whitson Dafydd Williams Jeffrey Williams Sunita Williams Reid Wiseman Kimiya Yui
Writers and journalists	Michael C. Barnette Victor Berge Philippe Diolé Gary Gentile Bret Gilliam Bob Halstead Hillary Hauser Trevor Jackson Steve Lewis John Mattera
Rescuers	Craig Challen Richard Harris Rick Stanton John Volanthen
Frogmen	Lionel Crabb Ian Edward Fraser Sydney Knowles James Joseph Magennis
Commercial salvors	Keith Jessop

Science of underwater diving

List of researchers in underwater diving

Diving
physics

Diving
physiology

Decompression theory	Decompression models: Bühlmann decompression algorithm Haldane's decompression model Reduced gradient bubble model Thalmann algorithm Thermodynamic model of decompression Varying Permeability Model Equivalent air depth Oxygen window Physiology of decompression

Diving
environments

Underwater exploration
- Deep-sea exploration

Classification	List of diving environments by type Altitude diving Benign water diving Confined water diving Deep diving Inland diving Inshore diving Muck diving Night diving Open-water diving Black-water diving Blue-water diving Penetration diving Cave diving Torricellian chamber Ice diving Wreck diving Recreational dive sites Underwater environment Underwater diving environment
Impact	Environmental impact of recreational diving Low impact diving

Other

Deep-submergence
vehicle

Submarine rescue

Deep-submergence rescue vehicle	LR5 LR7 MSM-1 Mystic-class deep-submergence rescue vehicle DSRV-1 Mystic DSRV-2 Avalon NATO Submarine Rescue System Priz-class deep-submergence rescue vehicle Russian deep submergence rescue vehicle AS-28 Russian submarine AS-34 ASRV Remora SRV-300 Submarine Rescue Diving Recompression System Type 7103 DSRV URF (Swedish Navy)

Submarine escape

Escape set	Davis Submerged Escape Apparatus Momsen lung Steinke hood Submarine Escape Immersion Equipment

Special
interest
groups

Neutral buoyancy
facilities for
Astronaut training

Other

Category
Commons
Glossary
Indexes: Dive sites
Divers
Diving
Outline
Portal

Categories: