Rebreather

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File:Inspiration front.JPg
Inspiration closed circuit diving rebreather

A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycles exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather a lightweight and compact breathing set for long durations in environments where humans cannot safely breathe from the atmosphere.

Basics

Rebreather technology is used in many environments:

  • Underwater - where it is sometimes known as "closed circuit scuba" as opposed to Aqua-Lung-type equipment, which is known as "open circuit scuba".
  • Mine rescue and in industry - where poisonous gases may be present or oxygen may be absent.
  • Space suits - the outer space is a vacuum where there is no oxygen to support life.
  • Hospital anaesthesia 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. Here the rebreather is big and is connected to the air in the habitat.

As a person breathes, the body consumes oxygen and makes carbon dioxide. A person with an open-circuit breathing set typically only uses about a quarter of the oxygen in the air that is breathed in. The rest is breathed out along with the nitrogen.

With a rebreather, the exhaled gas is not discharged to waste. The rebreather recovers the exhaled oxygen for re-use. It absorbs the carbon dioxide, which otherwise would accumulate and become poisonous. It adds oxygen to replace what was consumed. Thus, the gas in the rebreather's circuit remains breathable and supports life processes. Nearly always, the oxygen comes from a cylinder, and the carbon dioxide is absorbed in a canister full of some absorbent chemical such as soda lime.

History of rebreathers

Also see Timeline of underwater technology.

Around 1620 in England, Cornelius Drebbel made an early oar-powered submarine. Records show that, to re-oxygenate the air inside it, he likely generated oxygen by heating saltpetre (sodium or potassium nitrate) in a metal pan to make it emit oxygen. That would turn the saltpetre 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 accidentally made a crude rebreather nearly three centuries before Fluess and Davis: see this link.

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 (e.g. the Siebe Gorman Salvus and the Siebe Gorman Proto) 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.

US Navy rebreathers were developed by Dr. Christian J. Lambertsen in the early 1940s for underwater warfare. Dr. Lambertsen, who currently works at the University of Pennsylvania, is considered by the US Navy as "the father of the Frogmen."

Advantages of rebreather diving

The main advantage of the rebreather over other breathing equipment is the rebreather's economical use of gas. With the "open circuit" Aqua-Lung, which is another 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 need 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

File:Rebr rn parts labelled.jpg
A simple naval-type diving oxygen rebreather with the parts labelled
File:Inspiration back.JPG
Back of an Inspiration Diving Rebreather, with its casing opened

There are several design variations of diving rebreather. All types have a gas-tight loop that the diver inhales from and exhales into. The loop consists of components sealed together. The diver breathes through a mouthpiece or a fullface mask (or with industrial breathing sets, sometimes a mouth-and-nose mask) connected to one or more tubes bringing inhaled gas to or exhaled gas from the diver, a counterlung or breathing bag to hold gas when it is not in the diver's lungs, and a scrubber containing carbon dioxide absorbent to remove the carbon dioxide from the loop. Attached to the loop there will be at least one valve letting gases, such as oxygen and perhaps a diluting gas, be injected into the loop. There may be valves letting gas be vented manually or automatically from the loop.

Most modern rebreathers have a 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 gas passes through the same tube in opposite directions. The mouthpiece often has a valve letting the diver take the mouthpiece from the mouth while underwater or floating on the surface without water being allowed to enter the loop. Many rebreathers have "water traps" in the counter lungs, which prevent large volumes of water entering the loop if the diver removes the mouthpiece underwater without closing the valve, or if the diver's lips get slack letting water leak in.

The active ingredient of the scrubber is often soda lime. All gas moving through the loop must pass through the absorbent so its carbon dioxide component 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 causing carbon dioxide poisoning. The diver must monitor the exposure of the scrubber and replace it when necessary. Carbon dioxide gas sensors 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 rebreather absorbent called "Protosorb" supplied by Siebe Gorman had a red dye, which was said to go white when the absorbent was exhausted. 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 mixed gas 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.

A 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.

A basic need with a rebreather is to keep the amount of oxygen in the mix, or more technically known as the partial pressure of oxygen or ppO2, from getting too low (causing anoxia or hypoxia) or too high (causing oxygen toxicity).

In some early rebreathers the diver had to manually open and close the valve to the oxygen cylinder 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 breathing bag controls the oxygen flow like the demand valve in open-circuit scuba. Most modern closed-circuit rebreathers have electro-galvanic fuel cell sensors and onboard electronics, which monitor the ppO2, injecting more oxygen if necessary or issuing an audible warning to the diver if the ppO2 reaches dangerously high or low levels.

With humans, the urge to breathe is caused by a build-up of carbon dioxide rather than lack of oxygen. When using a rebreather, carbon dioxide is removed from the breathing gas by the scrubber, suppressing the body's natural warning. The result of serious hypoxia is sudden blackout with little or no warning. This makes hypoxia a deadly problem for rebreather divers.

In many rebreathers the diver can control the gas mix and volume 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 breathing bag, on the chest, over the shoulders, or on the back, has an effect on the ease of breathing. The design of the rebreather also affects the swimming diver's streamlining and thus ease of swimming.

Some rebreather sets include a bailout, a second (usually open-circuit) supply of air or other breathing gas to help the diver to reach safety if his main breathing set fails.

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 that it supplies is oxygen. As pure oxygen is toxic when inhaled at pressure, oxygen rebreathers are limited to a depth of 6 meters (20 feet); some say 9 meters (30 feet). Oxygen rebreathers 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.

In some rebreathers, e.g. the Siebe Gorman Salvus, the oxygen cylinder has two first stages in parallel. One is constant flow; the other is a plain on-off valve called a bypass; both feed into the same exit pipe which feeds the breathing bag. In the Salvus there is no second stage and the gas is turned on and off at the cylinder. Some simple oxygen rebreathers had no constant-flow valve, but only the bypass, and the diver had to operate the valve at intervals to refill the breathing bag as he used the oxygen.

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 oxygen injection rate must be carefully chosen and controlled to prevent either oxygen toxicity or 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 automatic closed-circuit systems, a mechanism 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.

Rebreathers whose absorbent 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.

Cryogenic rebreather

There have been plans 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 and is replaced from the oxygen tank.

Such a rebreather called the S-1000 was built around or soon after 1960 by Sub-Marine Systems Corporation. It had a duration of 6 hours and a maximum dive depth of 200 meters of salt water. Its ppO2 could be set to anything from 0.2 bar to 2 bar 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 liquid nitrogen or helium depending on the depth of the dive. The set could freeze out 230 grams of carbon dioxide per hour from the loop, corresponding to an oxygen consumption of 2 liters per minute. If oxygen was consumed faster (high workload), a regular scrubber was needed. See Fischel H., Closed circuit cryogenic SCUBA, "Equipment for the working diver" 1970 symposium, Washington, DC, USA. Marine Technology Society 1970:229-244.

See also Cushman, L., Cryogenic Rebreather, Skin Diver magazine, June 1969, and reprinted in Aqua Corps magazine, N7, 28, 79.

There are articles on the web about a cryogenic rebreather called Titanic II. These articles are a hoax; some of them include unrealistic technology.

Other designs

In the Siebe Gorman Proto (see above) the absorbent was loose in the bottom of the breathing bag and not in a canister.

Risks and precautions with rebreather diving

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 too high nor too low. 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 flushes out the old mix and replaces it with a known proportion of oxygen from the diluent.

Divers using oxygen rebreathers are advised to flush the system when they start the dive, to get surplus nitrogen out of the system.

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. A particular problem when using a closed circuit rebreather is the drop in ambient pressure caused by the ascent phase of the dive, which can reduce the partial pressure of oxygen to hypoxic levels leading to what is sometimes called shallow-water blackout.
  • Seizures due to oxygen toxicity caused by too high a partial pressure of oxygen in the loop.
  • Disorientation, panic, headache, and hyperventilation due to excess of carbon dioxide caused by failure or inefficiency of the scrubber. This can happen if the diver is producing carbon dioxide faster than the absorbent can handle(e.g. during hard work or fast swimming). The solution is to slow down and let the absorbent catch up. It can also be caused by depth where the increased concentration of other gas molecules, due to pressure, prevents all the carbon dioxide molecules coming into contact with the active ingredient of the scrubber.
  • The rebreather diver must keep breathing in and out all the time, to keep the gas flowing over the absorbent, so the absorbent can work all the time. Divers need to lose any air conservation habits that may have been developed while diving with open-circuit scuba. In closed circuit rebreathers, this also has the advantage of mixing the gases preventing oxygen-rich and oxygen-lean spaces within the loop, which may give inaccurate readings to the oxygen control system.
  • "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 bail 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

External links

Diving rebreather manufacturers

  • Ambient Pressure Diving - maker of the Inspiration and Evolution rebreathers
  • Drager - maker of various semi-closed circuit rebreathers
  • Halcyon - maker of the semi-closed circuit rebreather
  • Jetsam - maker of the KISS rebreather
  • Megalodon & Mini Meg - The Megalodon Expedition class rebreather
  • Steam Machines - Prism rebreathers
  • Siebe Gorman, see also Siebe Gorman. Important in diving history, but now closed down.
  • Rebreather Electronic Upgrades - The Hammerhead Electronic System
  • Cis-Lunar. The firm has closed down, but there is information on this link. They made computer-controlled automatic rebreathers. Their production model was in a streamlined casing that had a characteristic "frog's back" humped shape. Their name is Latin for "on this side of the moon": the firm's first plan was to develop spacesuit kit.

Other information sources

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