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Rebreathers

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Will your next dive be 3 hours long?

With PADI endorsing the launch of recreational rebreathers with the introduction of their own rebreather courses, there may be a revolution coming.

How do they work?

At shallow depths, a diver using open-circuit breathing apparatus typically only uses about a quarter of the oxygen in the air that is breathed in, which is about 4 to 5% of the breathed volume.

The remaining oxygen is exhaled along with nitrogen and carbon dioxide - about 95% of the volume. As the diver goes deeper, much the same mass of oxygen is used, which represents an increasingly smaller fraction of the inhaled gas.

Since only a small part of the oxygen, and virtually none of the inert gas is consumed, every exhaled breath from an open-circuit scuba set represents at least 95% wasted potentially useful gas volume, which has to be replaced from the breathing gas supply.

A rebreather recirculates the exhaled gas for re-use and does not discharge it immediately to the surroundings. The inert gas and unused oxygen is kept for reuse, and the rebreather adds gas to replace the oxygen that was consumed and removes the carbon dioxide.

Thus, the gas in the rebreather’s circuit remains breathable and supports life and the diver needs only carry a fraction of the gas that would be needed for an open-circuit system. The saving is proportional to the ambient pressure, so is greater for deeper dives, and is particularly significant when expensive mixtures containing helium are used as the inert gas diluent. The rebreather also adds gas to compensate for compression when depth increases, and vents gas to prevent overexpansion when depth decreases.

Rebreather history

  • Around 1620: In England, Cornelius Drebbel made an early oar-powered submarine. To re-oxygenate the air inside it, he likely generated oxygen by heating saltpetre (potassium nitrate) in a metal pan to emit oxygen. Heating turns the saltpetre into potassium oxide or hydroxide, which absorbs carbon dioxide from the air. That may explain why 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 more than two centuries before Saint Simon Sicard’s patent. (!)

  • 1808: The oldest known rebreather based on carbon dioxide absorption was patented in France by Sieur Touboulic from Brest, a mechanic in Napoleon’s Imperial Navy. This early rebreather design worked with an oxygen reservoir, the oxygen being delivered progressively by the diver and circulating in a closed circuit through a sponge soaked in limewater.
    Touboulic called his invention Ichtioandre (Greek for ‘fish-man’). There is no evidence of a prototype having been manufactured.

  • 1849: A patent for the oldest known rebreather for which a prototype was built, also using an oxygen reservoir, was granted to Pierre Aimable De Saint Simon Sicard.

  • 1853: Professor T. Schwann designed a rebreather in Belgium; he exhibited it in Paris in 1878. It had a large back mounted oxygen tank with working pressure of about 13.3 bar, and two scrubbers containing sponges soaked in caustic soda.

  • 1878: Henry Fleuss invented a rebreather using stored oxygen and absorption of carbon dioxide by rope yarn soaked in caustic potash solution, to rescue mine workers who were trapped by water.

  • About 1900: The Davis Submerged Escape Apparatus was designed in Britain for escape from sunken submarines. It was the first rebreather which was practical for use and produced in quantity. Various industrial oxygen rebreathers such as the Siebe Gorman Salvus and the Siebe Gorman Proto, both invented in the early 1900s, were derived from it.

  • 1903 to 1907: Professor Georges Jaubert invented Oxylithe, which is a form of sodium peroxide (Na2O2) or sodium dioxide (NaO2). As it absorbs carbon dioxide in a rebreather’s scubber it emits oxygen.

  • 1907: Oxylithe was used in the first filming of Twenty Thousand Leagues Under the Sea.

  • 1907: Dräger rebreather used for mines rescue.

  • 1909: Captain S.S. Hall, R.N., and Dr. O. Rees, R.N., developed a submarine escape apparatus using Oxylithe; the Royal Navy accepted it. It was used for shallow water diving but never in a submarine escape;

  • 1912: The first recorded mass production of rebreathers started with the Dräger rebreathers, invented some years earlier by Hermann Stelzner, an engineer at the Dräger company. The Dräger rebreathers, especially the DM20 and DM40 model series, were those used by the German helmet divers and German frogmen during World War II.

  • 1930’s: Italian sport spearfishers used rebreathers systematically. This practice came to the attention of the Italian Navy, which developed its frogman unit Decima Flottiglia MAS, which was used effectively in World War II.

  • World War II: Captured Italian frogmen’s rebreathers influenced design of British rebreathers. Many British frogmen’s breathing sets’ used aircrew breathing oxygen cylinders salvaged from shot-down German Luftwaffe aircraft. The earliest of these breathing sets may have been modified Davis Submerged Escape Apparatus; their full-face masks were the type intended for the Siebe Gorman Salvus, but in later operations different designs were used, leading to a full-face mask with one big face window, at first oval and later rectangular (mostly flat, but the sides curved back to allow better vision sideways).

    Early British frogman’s rebreathers had rectangular counterlungs on the chest like Italian frogman’s rebreathers, but later British frogman’s rebreathers had a square recess in the top of the counterlung so it could extend further up toward the shoulders. In front they had a rubber collar that was clamped around the absorbent canister.[9] Some British armed forces divers used bulky thick diving suits called Sladen suits; one version of it had a flip-up single faceplate for both eyes to let the user get binoculars to his eyes when on the surface.

  • Early 1940s: US Navy rebreathers were developed by Dr. Christian J. Lambertsen for underwater warfare and he is considered by the US Navy as "the father of the frogmen".[12][13] Lambertsen held the first closed-circuit oxygen rebreather course in the United States for the Office of Strategic Services maritime unit at the Naval Academy on 17 May 1943.

  • c.1960 to c.1990: In this period in Britain there was very little rebreather use by civilians, and no easy way for the general public to obtain rebreathers, and the BSAC prohibited rebreather use by its members. The Italian firms Pirelli and Cressi-Sub at first each sold a model of sport diving rebreather, but after a while discontinued those models. Some home made rebreathers were used by cave divers to penetrate cave sumps.

  • 1989: The Communist Bloc collapsed and the Cold War ended, and with it the perceived risk of attack by Communist Bloc forces, including by their combat divers. After that, the world’s armed forces had less reason to requisition civilian rebreather patents, and automatic and semi-automatic recreational diving rebreathers started to appear.

  • Long dive times

    Perhaps the most significant advantage is greatly increased gas efficiency. Usually, a diver only uses a small fraction of the oxygen of each inhaled breath and most of the oxygen leaves the lungs unused when the diver exhales. This means that oxygen and other gases in the exhaled gas is wasted when the diver exhales - and as you know, this gets worse with greater depths because of the increased pressure.

    Rebreathers retain most or all of the exhaled breath, processes it, and returns it to the diver as breathable air. Because there are almost no exhaled bubbles at all, there is no change in gas usage efficiency at greater depths. This makes rebreathers more beneficial as the depth increases.

    For example, a standard scuba cylinder contains enough gas to sustain an average resting person for about an hour and a half at the surface. The same cylinder will last only 45 minutes at 10 meters and less than 10 minutes at a depth of 90 meters - but if that same cylinder were filled with oxygen and used to supply a closed-circuit rebreather, the diver could theoretically stay underwater for two days - whatever the depth!

    Decompression Efficiency

    This advantage only applies to closed-circuit rebreathers, not oxygen or semi-closed rebreathers. Oxygen rebreathers are limited to depths where decompression is not an issue.

    The reason it applies only to closed-circuit rebreathers and not semi-closed rebreathers has to do with differences in the breathing gas dynamics of these two types of rebreathers. Semi-closed rebreathers maintain a more-or-less constant fraction of oxygen in the breathing gas, whereas closed-circuit rebreathers maintain a constant partial pressure of oxygen in the breathing gas.

    his means that the non-oxygen portion of the breathing gas which the part that determines decompression obligations is kept at a minimum. This allows the diver to stay longer at depth without requiring decompression and also to speed up the decompression process whenever there is a decomression obligation

    Breathe warm, moist gas

    The breathed gas is warm rather than cold, as in open circuit because of the pressure reduction of substantial amounts of gas just prior to inhalation. So, less gas used in rebreathers means less cool gas is introduced.

    Our lungs contain a dense vascular system and much heat can be lost through exhalation. The rebreather allows the retention of body heat because temperature is maintained, in varying degrees, within the loop. In addition, rebreather gases are also warmed by the chemical reaction during the carbon dioxide scrubbing process.

    The Sound of Silence

    Conventional scuba releases a large burst of noisy bubbles at each breath. This affects the behavior of marine life and fish are often frightened and will not allow close proximity.

    Semi-closed rebreathers reduce the volume of exhaled bubbles, and closed-circuit rebreathers essentially eliminate bubbles entirely. This allows divers are able to approach marine life much more closely without disturbing behavioral patterns - great news for specimen collection and marine photographers.

    Rebreather disadvantages

    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 dircuit 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

    4 Golden Rules

    The last decade of statistics show that equipment brand has not been a factor in accidents. Ergo, the most important decision you can make is not which make of rebreather you will buy, but rather the commitment to use your equipment safely.

    Four factors will prevent 90 percent of the accidents:

  • Use a checklist every time you dive.

  • Do a proper, relaxed and complete pre-breathe for five minutes with your nose blocked.

  • Don’t jump in the water unless all systems are working perfectly.

  • Replace your oxygen cells proactively every year.

  • Factors when buying a rebreather

    Beyond that, several factors may influence a purchase decision:

  • 3rd Party tests. Check for third-party tests for oxygen tracking, canister duration and other factors., as well as for details on their CE or equivalent testing.

  • Weight and size. Is it portable enough for your needs - will it conform to airline baggage weight constraints.

  • Training. Check the availability of instruction from a trusted, experienced and current instructor

  • Support. Make sure there is local support and other divers using the same kit near you. Also check the of support and service, turnaround times, availability of oxygen cells and other parts. Check online for comments and complaints from customers.

  • Manufacturing company. Check the history and stability of the manufacturing company.

  • Alarm systems. Make sure the unit provides visual, and or audio and tactile alarm systems.

  • Other features . Check for features such as "auto on" electronics, redundancy. battery type and sharing or replacement.

  • Cost. You should not buy a cheap parachute, so buy the best rebreathing equipment that you can afford.

  • Self service. Check what you can reliably do yourself regarding needs

  • Consumables. Check the availability and cost of consumables like the scrubber and batteries.

  • Be safe

    Make a pledge to yourself and your family that you will abide by the four Golden Rules.

    The mouthpiece

    Unlike normal gear, you just drop the mouthpiece into the water - because water would fill the breathing loop and the scrubber canister. You will have to close the valve on the mouthpiece before you take it out of your mouth.

    Adjusting boyancy

    You can’t adjust buoyancy by inhaling or exhaling, because the same amount of air just goes back and forth between your lungs and the rebreather. and never changes volume or buoyancy.

    Pressure

    As depth increases, the increasing pressure will collapse the counterlungs just as it collapses BCs and dry suits. Some rebreathers automatically add more gas to the breathing loop while other models require you to add gas manually.

    Using the gas

    You will need to be frugal when inflating your BC or clearing your mask. This is because gas used for those actions is depleting a much smaller total supply. Also, you need to watch your gauges closely and buddies need to check each other‘s kit for air leaks.

    Heavy breathing

    If you are exerting yourself - like pulling something or swimming against a current - your body will take oxygen out of the breathing loop faster than normal.

    Closed-circuit systems and passive semiclosed-circuit systems will sense this and add extra oxygen.

    However, active semiclosed systems will not. This means that you will have to remember to purge the breathing loop. This is done by exhaling the oxygen-poor gas through your nose so the rebreather can replace it with richer gas.

    Ascent

    When commencing an ascent on a semi-closed-circuit rebreather the loop must be purged to enrich your breathing mixture, or the drop in pressure may cause the partial pressure of oxygen may become too low. Closed- circuit systems add oxygen automatically.

    You will also notice that during the ascent the rebreather will vent gas as the counterlungs expand. This is the only time the rebreather dumps a lot of gas, the reason being that "sawtooth" profiles are especially wasteful.

    You may find that you are becoming increasingly buoyant during and ascent, caused by the rebreather not venting gas fast enough. You may have to manually dump from your BC, your dry suit or from the rebreather to maintaind acceptable buoyancy.

    The bottom line

  • It is not only about the equipment - it is especially about you. Don’t even consider a rebreather is you are a fly-by-the-seat-of-your-pants person. You may die without even realising it. With a rebreather, you will have to take full responsibility for your own safety.

  • Rebreathers are more complex than open-circuit gear - and you will have to be able to assemble, clean, maintain and repair the kit. The chances of your local African dive shop having a specialist are slim. The simplest rebreather has all the parts of your open-circuit setup plus a lot more - and all those parts, the connections between them and the 50+ O-rings need regular maintenance.

  • Like a pilot, you will need to be disciplined and religiously follow procedures and checklists. Filling scrubber canister and assembling the breathing loop involve steps that must be followed precisely - and tests that can’t be skipped. During the dive you have to watch gauges more closely than on open circuit.

  • All said, the rebreather frontier offers great advantages - and great dangers.

    Be honest with yourself and make your decision with your head - not with your heart.

     


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