Question: What are rebreathers more complex and expensive than?
About the article "Scuba diving", section "Rebreather".

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When compared with open circuit scuba, rebreathers have some disadvantages, including expense, complexity of operation and maintenance, and more critical paths to failure.
A malfunctioning rebreather can supply a gas mixture which contains too little oxygen to sustain life, or it may allow carbon dioxide to build up to dangerous levels.
Some rebreather designers try to solve these problems by monitoring the system with electronics, sensors and alarm systems.
These are expensive and susceptible to failure, improper configuration and misuse.
Oxygen rebreathers (simple closed circuit) are limited to a shallow depth range of approximately 6 m, beyond which the risk of acute oxygen toxicity rises to unacceptable levels very rapidly.
Semi-closed circuit rebreathers are less efficient than closed circuit, and are more mechanically complex than open circuit or closed circuit oxygen rebreathers.
Closed circuit rebreathers are yet more mechanically complex, and generally rely on electronic instruments and control systems to monitor and maintain a safe breathing gas mixture.
This makes them more expensive to produce, more complex to maintain and test, and sensitive to getting their circuitry wet.
Depending on the complexity of the rebreather, there are more failure modes than for open circuit scuba, and several of these failure modes are not easily recognised by the diver without technological intervention.
A major disadvantage of a rebreather is that, due to a failure, gas may continue to be available for breathing, but the mixture provided may not support life, and this may not be apparent to the user.
With open circuit, this type of failure can only occur if the diver selects an unsuitable gas, and the most common type of open circuit failure, the lack of gas supply, is immediately obvious, and corrective steps like changing to an alternative supply would be taken immediately.
The bailout requirement of rebreather diving can sometimes also require a rebreather diver to carry almost as much bulk of cylinders as an open-circuit diver so the diver can complete the necessary decompression stops if the rebreather fails completely.
Some rebreather divers prefer not to carry enough bailout for a safe ascent breathing open circuit, but instead rely on the rebreather, believing that an irrecoverable rebreather failure is very unlikely.
This practice is known as "alpinism" or "alpinist diving" and is generally maligned due to the perceived extremely high risk of death if the rebreather fails. 
Closed-circuit or semi-closed circuit rebreather systems allow recycling of exhaled gases.
This reduces the volume of gas used, so that a smaller cylinder, or cylinders, than open-circuit scuba may be used for the equivalent dive duration, and giving the ability to spend far more time underwater compared to open circuit for the same gas consumption.
Rebreathers also produce far less bubble volume and less noise than scuba, which makes them attractive to military, scientific and media divers.
They also have a larger number of critical failure modes, are more expensive and require more training to use at a reasonable level of safety. 
A rebreather recirculates the breathing gas already used by the diver after replacing oxygen used by the diver and removing the carbon dioxide metabolic product.
Rebreather diving is used by recreational, military and scientific divers where it can have advantages over open circuit scuba.
Since 80% or more of the oxygen remains in normal exhaled gas, and is thus wasted, rebreathers use gas very economically, making longer dives possible and special mixes cheaper to use at the cost of more complicated technology and more possible failure points.
More stringent and specific training and greater experience is required to compensate for the higher risk involved.
There are two main variants of rebreather — semi-closed circuit rebreathers, and fully closed circuit rebreathers, which include the subvariant of oxygen rebreathers.
The rebreather's economic use of gas, typically of oxygen per minute, allows dives of much longer duration for an equivalent gas supply than is possible with open circuit equipment where gas consumption may be ten times higher.
Oxygen rebreathers have a maximum safe operating depth of around , but several types of fully closed circuit rebreathers, when using a helium-based diluent, can be used deeper than .
The main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, increased work of breathing at depth, reliability of gas mixture control, and the requirement to be able to safely bail out at any point of the dive.
Rebreathers are generally used for scuba applications, but are also occasionally used for bailout systems for surface supplied diving.
The possible endurance of a rebreather dive is longer than an open-circuit dive, for similar weight and bulk of the set, once a practical lower limit for rebreather size has been exceeded, and a rebreather can be more economical when used with expensive gas mixes such as heliox and trimix, but this may require a lot of diving before the break-even point is reached, due to the high initial and running costs of most rebreathers, and this point will be reached sooner for deep dives where the gas saving is more pronounced. 
As pure oxygen is toxic when inhaled at pressure, recreational diver certification agencies limit oxygen decompression to a maximum depth of 6 meters (20 ft) and this restriction has been extended to oxygen rebreathers; In the past they have been used deeper (up to 20 m) 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 fsw (7.5 msw) for 240 minutes.
Maximum working limit 50 fsw (15 msw) for 10 minutes.
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. 
Less common are closed circuit (CCR) and semi-closed (SCR) rebreathers, which unlike open-circuit sets that vent off all exhaled gases, process all or part of each exhaled breath for re-use by removing the carbon dioxide and replacing the oxygen used by the diver.
Rebreathers release little or no gas bubbles into the water, and use much less stored gas volume, for an equivalent depth and time because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications.
Rebreathers are more complex and more expensive than open-circuit scuba, and special training and correct maintenance are required for them to be safely used, due to the larger variety of potential failure modes.
In a closed-circuit rebreather the oxygen partial pressure in the rebreather is controlled, so it can be maintained at a safe continuous maximum, which reduces the inert gas (nitrogen and/or helium) partial pressure in the breathing loop.
Minimising the inert gas loading of the diver's tissues for a given dive profile reduces the decompression obligation.
This requires continuous monitoring of actual partial pressures with time and for maximum effectiveness requires real-time computer processing by the diver's decompression computer.
Decompression can be much reduced compared to fixed ratio gas mixes used in other scuba systems and, as a result, divers can stay down longer or require less time to decompress.
A semi-closed circuit rebreather injects a constant mass flow of a fixed breathing gas mixture into the breathing loop, or replaces a specific percentage of the respired volume, so the partial pressure of oxygen at any time during the dive depends on the diver's oxygen consumption and/or breathing rate.
Planning decompression requirements requires a more conservative approach for a SCR than for a CCR, but decompression computers with a real time oxygen partial pressure input can optimise decompression for these systems.
Because rebreathers produce very few bubbles, they do not disturb marine life or make a diver’s presence known at the surface; this is useful for underwater photography, and for covert work.