SCUBA, or Self-Contained Underwater Breathing Apparatus, has been in use by recreational, exploration, scientific, and military divers since the advent of the technology by Cousteau and Gagnon after World War II. At the most fundamental level, a UBA, or underwater breathing apparatus, can be delineated into surface supplied, and self-contained units. Surface supplied divers make use of an umbilical to provide breathing gas to the diver, while SCUBA divers carry their life support equipment with them at all times. SCUBA systems themselves are further classified as open circuit or closed circuit. Open circuit dive equipment provides the diver breathing gas from a container of compressed gas (a SCUBA tank), and the diver exhales directly to the environment. While simple and easy to maintain, this system is inefficient as a large volume of inert gas is exhaled to the environment, and a substantial amount of oxygen goes unused. With a closed circuit breathing apparatus (commonly known as a rebreather), the exhaled gas is scavenged, scrubbed of carbon dioxide, enriched with oxygen, and then sent back to the diver in the breathing loop. The oxygen consumed by the diver via metabolic consumption is the only thing replaced to continue to sustain the diver. This system is inherently more efficient (perhaps as much as 40 to 50 times more efficient for a given gas supply) but also more complex, requiring constant monitoring of the gas in the system to ensure it is capable of sustaining life. 
Rebreathers have been in use since antiquity when man first discovered that by breathing in and out of a leather bag he could extend the time of a free dive. More sophisticated models were developed over 100 years ago to allow miners to escape from contaminated atmospheric conditions and used with varying degrees of success. During World War II rebreather technology was adapted to allow combat divers to work in enemy harbors without showing surface bubbles, which would give away their position. The challenges associated with rebreathers include the potential for hypercarbia, oxygen toxicity, and hypoxia. Prior to technically advanced devices, blackout due to the various gas problems was a common complication. In the 1990s, several commercial units became available on the civilian market, and this led to a significant increase in civilian diver use in the early 2000s. There are now many types of rebreathers available to meet diverse needs of different types of diving environments.
Rebreathers are divided into types based on how they perform the function of adding gas, specifically oxygen, to the breathing loop. There are several types: a semi-closed circuit with active or passive addition mechanisms and fully-closed circuit rebreathers with manual addition of oxygen or electronically controlled addition of oxygen. There are also hybrid units which incorporate more than one mechanism of oxygen addition.
Fundamental Rebreather Design
There are commonalities among rebreathers dictated by the physics that govern the function of the rebreather. Every rebreather has a scrubber of some design that is capable of removing exhaled carbon dioxide from the system and a counter lung, which is a flexible bag that the diver can breathe into and out of while underwater, as it is impossible to breathe into a rigid closed container. Rebreathers typically have one-way valves installed in the breathing loop to direct the flow of gas into and out of the unit, to prevent a diver from rebreathing gas that has not been scrubbed of carbon dioxide. All models of rebreathers currently available have a sensor system to monitor the partial pressure of oxygen to prevent the diver from breathing a hypoxic or hyperoxic gas mixture. Depending on the type of rebreather, they have one or more gas supply bottles to provide fresh gas to the diver. Some rebreathers simply bleed oxygen enriched mixtures into the counter lung, while others sense the amount of oxygen available to the diver and bleed pure oxygen into the system to make up for the oxygen-depleted by the metabolic needs of the diver. As both the diver’s lungs and the counter lung on the rebreather are subject to compression with descent due to the effects described by Boyles Law, all rebreathers have a mechanism to inject gas into the breathing loop to counter the effects of volume loss with increased pressure. Since pure oxygen becomes toxic at relatively shallow depths, rebreathers with pure oxygen supplies usually have a diluent gas to make up for volume loss without inducing oxygen toxicity. Various military organizations around the world make use of shallow water pure oxygen rebreathers with single gas supplies due to their relatively specific needs and advantages. Many of these rebreathers employ mechanisms such as retention straps to prevent loss of airway protection in the event of a hyperoxic seizure. 
Semi-Closed Circuit Rebreathers (SCR)
Semi-closed circuit systems add breathing gas to the loop constantly via a constant mass flow orifice and vent exhaled gas from the loop every few exhalations. By using this passive system that is typically controlled by the diver’s breathing, the fraction of inspired oxygen (FiO2) in the breathing loop is typically maintained within 2% to 3% of a pre-selected amount. The system is simple to use and highly reliable with early units being sold devoid of oxygen monitoring equipment. There are, however, notable limitations as the FiO2 is designed for a diver workload within a specific range, and hypoxia can occur with unexpectedly high workloads. As the most systems vent roughly one-fifth of each breath, the volume of the counter lung varies, and diver buoyancy is affected. Currently, this system is seldom used due to the availability of more advanced units with far greater depth and duration capability. Semi-closed units did have an advantage of simplicity in design and training that made them desirable to some divers in the early days of rebreather use in the nonmilitary community.
Closed-Circuit Rebreathers (CCR)
These units are available in many different designs, and they have several oxygen addition mechanisms. CCRs are usually delineated based on their oxygen addition mechanism. The most common mechanisms are the manually-closed circuit rebreather (mCCR), electronically-closed circuit rebreather (eCCR), and the hybrid-closed circuit rebreather (hCCR) which is a combination of the manually and electronically controlled rebreather. Each model has advantages and limitations.
Manual CCRs are quite common and rely upon the diver to maintain the partial pressure of oxygen in the breathing loop. In their most basic form, the diver simply must inject oxygen manually with the press of a button and monitor the partial loop pressure of oxygen on a series of sensors. If the diver fails to act with this system, no oxygen will be added, and the diver will eventually become hypoxic. Due to the dire consequences of failing to act, the diver is forced to pay close attention to the unit and most units appear to have a good safety record as a result. A more common variation of this unit is one where oxygen addition is manual via a button, and oxygen bleeds into the loop at a very slow rate as well which matches the diver’s resting metabolic rate. Because there is a small constant addition of oxygen, the diver may only need to add oxygen every 10 to 15 minutes when at rest, which decreases diver workload. The oxygen in this system is injected via a constant mass flow restricted orifice. Because the orifice is a sonic orifice, once drive pressure is set (typically about 160 psi), the mass of oxygen injected over any given period is constant, regardless of depth. This drive pressure also limits the depth of the unit, as the unit will stop injecting oxygen at the depth of the drive pressure, which typically varies from 250 feet to 360 feet of water. This form of passive constant oxygen injection combined with manual push button injection makes up the majority of mCCRs on the market currently.
Electronic closed circuit rebreathers inject oxygen via a magnetic solenoid and a computer which senses the amount of oxygen in the loop and then automatically injects a bolus of oxygen to make up for diver usage. The system has the advantage of being fully automated, depth unlimited as the oxygen first stage does not have to be preset at a specific pressure, and highly efficient in oxygen usage. Some argue that the fully automatic systems allow the diver to become complacent about monitoring his or her oxygen levels, and thereby delay time to detection of critical gas supply problems, and there is some evidence to support this theory, especially early in the introduction of eCCRs to the civilian market. Recent evidence suggests that the accident rate between the two is identical. What is not in debate is that complex electronics immersed in salt water have a failure rate higher than the simple mechanical push-button valves found on mCCRs, and that solenoids and computers require electricity to operate whereas mechanical valves do not. Of note, some divers do dive eCCRs manually, while using a slightly lower oxygen set point on the computer, which then serves as a “reserve parachute” and kicks in if the diver fails to add oxygen. This system presumes that it would be even less likely that a diver would become distracted at the same exact moment the electronics failed.
Some divers, who were committed to running eCCRs manually, subsequently added the constant mass flow orifice to their eCCRs, thereby greatly extending the time they have before they are required to add additional oxygen. Thus, the hybrid closed circuit rebreather (hCCR) was born. The hCCR is now available as a variant of several commercial eCCRs.
Rebreathers add complexity and potential for error over open circuit dive equipment, but they allow additional operational benefits to the diver such as decreased gas utilization, increased bottom time, and stealthy excursions. In the underwater exploration world, unexpected events often add time to a dive, sometimes at the maximum depth of a dive. Traditionally, this unexpected time has resulted in depletion of emergency gas supplies and resulted in both near misses and fatalities. A rebreather can substantially reduce the time pressure on the diver as it can literally extend dive time several hours if needed to allow for the diver to deal with problems like: being lost, clearing an entanglement, finding a lost buddy or dealing with unexpected environmental challenges. In reality, the use of mixed gas rebreathers has been documented to allow divers to deal with these problems, but are still associated with higher fatality rates than traditional open circuit SCUBA. This apparent disparity is most easily explained by the types of dives undertaken with this apparatus. Rebreather equipped divers are able to undertake dives that either would not have been possible on traditional open circuit SCUBA or would have been logistically challenging, requiring multiple set up dives to stage safety gas bottles prior to the dive. This may be somewhat analogous to the introduction of the jet aircraft: higher and faster flights became possible with cities connected in hours instead of days, but the crashes were more spectacular initially as well. Additional safety advances such as the introduction of CO2 monitors that are capable of reliable long-term function in a 100 percent humidity environment and scrubber bed monitoring technology may help reduce risks associated with rebreathers. Currently, rebreathers most certainly offer advantages in depth and duration over traditional open circuit equipment.
Divers using rebreathers are subject to additional risks for oxygen toxicity, hypoxia, and hypercarbia above and beyond those associated with traditional open circuit SCUBA.  Divers using rebreathers are also capable of getting all forms of decompression illness, including decompression sickness and overexpansion injuries, that an open circuit diver can get. Carbon dioxide absorbents are also limited in their ability to bind CO2 and this limitation also functionally limits the time the rebreather can be used without replenishment. The finite scrubbing capacity creates a situation where a diver may have gas, but the rebreather may no longer be able to effectively scrub carbon dioxide from the loop once the sorb is exhausted. While there are temperature sensors that can be used to monitor the effective life of a scrubber, they are imperfect and only an estimation of time remaining based on the current workload of the diver.  In addition, the material used for removing carbon dioxide from the breathing loop is strongly caustic (the pH can approach 14 as the material contains sodium hydroxide) and forms a strong base when mixed with water. While most modern commercial rebreathers have water traps of various design to help prevent this caustic material from being ingested or inhaled if the breathing loop is inadvertently flooded, this can not usually be completely prevented. If a diver inhales or ingests this caustic material (known as a "caustic cocktail" to divers), the mouth should be thoroughly irrigated with fresh water, and the diver should be evaluated for a caustic burn. The diver should never be instructed to drink acidic fluids as the resultant exothermic reaction can be clinically significant. Airway compromise can occur from ingestion of a caustic cocktail. Consultation of a toxicologist and or hyperbaric physician trained in undersea medicine is recommended.
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