Rebreather
Description
A Rebreather is a type of breathing equipment that provides an oxygen-based breathing gas and recycles exhaled gases. This recycling reduces the volume of breathing gas used making a rebreather a lightweight and compact machine for supplying breathing gas for long durations in environments where humans cannot safely breathe from the atmosphere.
Rebreather technology is used in many environments:
- underwater - where it is known as "closed circuit SCUBA" as opposed to Aqua-Lung-type equipment, which is known as "open circuit SCUBA".
- mine rescue - where poisonous gases may be present or oxygen may be absent.
- space suits - outer space is a near vacuum where there is not enough oxygen to support life.
- hospital anesthesia breathing systems - to supply controlled proportions of gases to patients without altering the atmosphere the staff breathe.
- submarines and hyperbaric oxygen therapy chambers - where the gas in the habitat must remain safe. The atmosphere must contain enough oxygen to support life but not enough to provoke fires. Poisonous carbon dioxide must be removed.
The rebreather takes advantage of the fact that an inhalation typically contains twenty times more gas than the body's metabolism consumes. The rebreather is designed to recycle the useful portion of the exhaled gas for further inhalation. The body also produces, as part of the metabolic process, carbon dioxide, which is poisonous and therefore must be removed from the breathing gas before it is re-inhaled.
Rebreathers capture the exhaled gas, remove carbon dioxide from it and inject supplemental oxygen back into the gas mixture so that the mixture, which is about to be returned to the diver to breathe, is able to support life and the physical activity of the diver.
History of rebreathers
It is possible that Cornelius Drebbel accidentally made a crude rebreather around 1620 in England. He made an early oar-powered submarine. Records show that, to re-oxygenate the air inside it, he likely generated oxygen by heating saltpeter (sodium or potassium nitrate) in a metal pan to make it emit oxygen. That would turn the saltpeter into sodium or potassium oxide or hydroxide, which would tend to absorb carbon dioxide from the air around. That may explain how Drebbel's men were not affected by carbon dioxide build-up as much as would be expected. If so, he made a crude rebreather nearly three centuries before Fluess and Davis [1] (http://www.dutchsubmarines.com/specials/special_drebbel.htm).
The first certainly known closed circuit breathing device using stored oxygen and absorption of carbon dioxide by an absorbent (here caustic soda) was invented by Henry Fluess in 1879 to rescue mineworkers who were trapped by water.
The Davis Escape Set was the first rebreather which was practical for use and produced in quantity. It was designed about 1900 in Britain for escape from sunken submarines. Various industrial oxygen rebreathers Siebe Gorman Salvus and the Siebe Gorman Proto (http://www.therebreathersite.nl/Zuurstofrebreathers/English/photos_proto.htm) were descended from it. The Proto (distinguish from `Proton') was much used by firefighters.
The first known systematic use of rebreathers for diving was by Italian sport spearfishers in the 1930s; this practice came to the attention of the Italian Navy, which developed its frogman unit which had a big effect in World War II.
Diving rebreathers
The main advantage of the rebreather over other breathing equipment is the rebreather's economic use of gas. With the "open circuit" Aqua-Lung, an alternative form of SCUBA, the entire breath is expelled into the surrounding water when the diver exhales. So, long or deep dives using open circuit equipment require much more gas than when using a rebreather. This open circuit gas must be carried by the diver in heavy and bulky diving cylinders.
The economy of gas consumption is also useful when the gas being breathed is expensive, such as the helium in trimix or heliox gas mixes used in technical diving. Also, rebreathers produce many fewer bubbles than Aqua-Lungs, making military divers much less visible. Marine biology and underwater photography also become easier with no bubbles to alarm the fish being studied.
Parts of a rebreather
There are several different design variations of diving rebreather. All types have some form of "loop" that the diver inhales from and exhales into, a "counter lung" to hold gas when it is not in the diver's lungs, a carbon dioxide "scrubber" to remove that gas from the loop and a supply of an oxygen-rich gas, nearly always from a cylinder or cylinders, to inject into the loop.
In some early rebreathers the user had to switch the oxygen on and off by hand to refill the counter-lung each time. In others the oxygen flow is kept constant by a pressure-reducing flow valve like the valves on blowtorch cylinders; the set also has a manual on/off valve called a bypass. In some modern rebreathers, the pressure in the counter-lung controls the oxygen flow like the demand valve in open-circuit scuba. Many modern automatic rebreathers have ppO2 (= partial pressure of oxygen) sensors and onboard electronics which reads this sensor and the depth pressure and controls the gas flows accordingly.
The active ingredient of the scrubber is often soda lime. It is important that all gas moving through the loop passes through the soda lime so that its carbon dioxide is removed. At present, there is no effective technology for detecting the end of the life of the scrubber or a dangerous increase in the concentration of carbon dioxide. The diver must monitor the exposure of the scrubber and replace it when necessary. Carbon dioxide gas sensors do exist, but they are not sensitive enough to be used in a rebreather - the scrubber "break through" occurs quite suddenly and the diver shows symptoms before the sensor indicates a dangerous build-up of carbon dioxide. A main hazard with diving with early rebreathers was "caustic cocktail" caused by water entering the loop and dissolving absorbent; but many modern diving rebreather absorbents are designed not to produce "cocktail" if they get wet.
Even if a sensitive carbon dioxide sensor is developed, it may not be useful as the primary tool for monitoring scrubber life when underwater because rebreathers allow very long dives where long decompression stops may be needed: knowing that the rebreather will begin to deliver a poisonous breathing gas in five minutes may not be useful to a diver needing to carry out an hour or more of decompression stops.
Most of the variants of rebreather have some sort of "twin hose" mouthpiece where the direction of flow of gas through the loop is controlled by one-way valves. Some have a single "pendulum" hose, where the inhaled and exhaled passes through the same tube in opposite directions.
The mouthpiece often has a valve letting the diver take the mouthpiece from the mouth underwater without water entering the loop. Many rebreathers have "water traps", which prevent large volumes of water entering the loop if the diver removes the mouthpiece underwater without closing the valve, or if his lips get slack letting water leak in.
In many rebreathers the diver is able to control the gas mix in the loop manually by injecting each of the different available gases to the loop and by venting the loop. The loop often has a pressure relief valve preventing the "hamster cheek" effect on the diver caused by over-pressure of the loop.
The position of the "counter lungs", on the chest, over the shoulders or on the back, has an effect on the ease of breathing.
Main Rebreather design variants
Oxygen rebreather
This is the oldest type of rebreather and was commonly used by navies from the early twentieth century. The only gas the rebreather supplies is oxygen. As pure oxygen is toxic when inhaled at pressure, oxygen rebreathers are limited to a depth of 6 meters (20 feet). These are also sometimes used when decompressing from a deep open-circuit dive, as breathing pure oxygen makes the nitrogen diffuse out of the blood quicker.
Semi-closed circuit rebreather
Military and recreational divers use these because they provide good underwater duration with fairly simple and cheap equipment. Semi-closed circuit equipment generally supplies one breathing gas such as air, nitrox or trimix. The gas is injected at a constant rate. Excess gas is constantly vented from the loop in small volumes.
The diver must fill the cylinders with gas mix that has a maximum operating depth that is safe for the depth of the dive being planned. As the amount of oxygen required by the diver increases with work rate, the injection rate must be carefully chosen and controlled to prevent unconsciousness in the diver due to hypoxia.
Fully closed circuit rebreather
Military, photographic and recreational divers use these because they allow long dives and produce no bubbles. Closed circuit rebreathers generally supply two breathing gases to the loop: one is pure oxygen and the other is a diluent or diluting gas such as air, nitrox or trimix.
The major task of the fully closed circuit rebreather is to control the oxygen concentration, known as the oxygen partial pressure, in the loop and to warn the diver if it is becoming dangerously low or high. The concentration of oxygen in the loop depends on two factors: depth and the proportion of oxygen in the mix. Too low a concentration of oxygen results in hypoxia leading to sudden unconsciousness and ultimately death when the oxygen is exhausted. Too high a concentration of oxygen results in oxygen toxicity, a condition causing convulsions, which when they occur underwater can lead to drowning.
In fully closed-circuit systems there is a mechanism that injects oxygen into the loop when it detects that the partial pressure of oxygen in the loop has fallen below the required level. Often this mechanism is electrical and relies on oxygen sensitive electro-galvanic fuel cells called "ppO2 meters" to measure the concentration of oxygen in the loop.
The diver may be able to manually control the mixture by adding diluent gas or oxygen. Adding diluent can prevent the loop's gas mixture becoming too oxygen rich. Manually adding oxygen is risky as additional small volumes of oxygen in the loop can easily raise the partial pressure of oxygen to dangerous levels.
Many diver training organizations teach the "diluent flush" technique as a safe way to restore the mix in the loop to a level of oxygen that is neither hypoxic nor hyperoxic. It only works when partial pressure of oxygen in the diluent alone would not cause hypoxia or hyperoxia, such as when using a normoxic diluent and observing the diluent's maximum operating depth. The technique involves simultaneously venting the loop and injecting diluent. This has the effect of flushing out the old mix and replacing it with a known proportion of oxygen from the diluent.
Other designs
There have been a few rebreather designs (e.g. the Oxylite) which instead of an oxygen cylinder had an absorbent canister filled with potassium superoxide, which gives off oxygen as it absorbs carbon dioxide: 4KO2 + 2CO2 = 2K2CO3 + 3O2. 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.
There was a plan for a "cryogenic rebreather". It has a tank of liquid oxygen and no absorbent canister. The carbon dioxide is frozen out in a "snow box" by the cold produced as the liquid oxygen expands to gas as the oxygen is used.
In the Siebe Gorman Proto the absorbent was loose in the bottom of the breathing bag and not in a canister.
Rebreather risks
In addition to the other diving disorders suffered by divers, rebreather divers are also more susceptible to:
- Sudden blackout due to hypoxia caused by too low a partial pressure of oxygen in the loop.
- Seizures due to oxygen toxicity caused by too high a partial pressure of oxygen in the loop.
- Disorientation, panic, headache, and hyperventilation due to hypercapnia (= excess of carbon dioxide) caused by failure of the scrubber.
- "Caustic cocktail" in the loop if water comes into contact with the soda lime used in the carbon dioxide scrubber. The diver is normally alerted to this by a chalky taste in the mouth. A safe response is to bale out to "open circuit" and rinse the mouth out.
When compared with Aqua-Lungs, Rebreathers have some disadvantages including expense, difficulty of operation, unreliability and complexity of maintenance.
Some makes of rebreather
- Russian IDA71 military and naval rebreather
- Siebe Gorman CDBA and CDMBA, SCBA, SCMBA, UBA
- Siebe Gorman Salvus
External links
Diving rebreather manufacturers
- Ambient Pressure Diving (http://www.ambientpressurediving.com/) - maker of the Inspiration and Evolution rebreathers
- Drager (http://www.draeger.com) - maker of various semi-closed circuit rebreathers
- Halcyon (http://www.halcyon.net/index.shtml) - maker of the semi-closed circuit rebreather
- Jetsam (http://www.jetsam.ca/) - maker of the KISS rebreather
- Steam Machines (http://www.steammachines.com/)
Other information sources
- Richard Pyle's rebreather page (http://www.bishopmuseum.org/research/treks/palautz97/rb.html)
- Engineering aspects of Apollo (http://www.hq.nasa.gov/alsj/apollo.engin.html)
- The Rebreather Site (http://www.therebreathersite.nl) Long list of types of rebreathers (including nitrox) at "database on oxygen rebreathers".
- Diver Dave's site (http://www.nobubblediving.com/teardown.htm)
- Karl Kramer's site (http://mitglied.lycos.de/dg8fz/rebreather/gallery.htm) (more history)