The Prelimaryof any modern Rebreather is just a Bag that is operated as Counterlung. This system was already in use more than 2000 years ago. The trick is quite simple: breathing is stimulated by stress and CO2, fainting by lack of Oxygen. So itīs easier to breath into and from a bag than to stop breathing the same time although it does not really prolong the Oxygen. Breathing from the Live-Jacket, rebreathing and changing the gas any 4th breath was (by principle) the next step. This could prolong 100barl to last up to 10minutes (depends on depth, training etc.).
Classical Helmet-Diving works just the same way: the pump on the surface brings down enough bar*l air to give the diver enough Oxygen to preserve fainting, the divesuit is also in function as counterlung, and sometimes a wet flannel towel with tartar (Potassiumhydrogentartrat = Kaliumhydrogentartrat) already acts as CO2 Absorber ("scrubber").
are the next Step. They just realize the fact that the body needs O2 and dumps CO2. It depends to the intended purpose if the O2 is added manual, if required or by constant flow. The IDA57 is a tiny oxygen-rebreather. In most Oxygen Rebreathers for SCUBA usage the O2 is added if required by an under-pressure triggered demand valve. Manual Add was used on submarine escape units, constant flow on fire escape units and by oldtimers (because they called gases "Oxygen" what we would call "Nitrox" or "Hydrox"). The CO2 is absorbed, in most cases by lime. It is vital essential to keep the loop clean of other Gases than O2 to prevent fainting. This means getting rid of Nitrogen that was in the Lungs before the dive, the Nitrogen that was in the body of the diver and "dirt" that was in the Oxygen tank. High Oxygen partial pressures are toxic, this causes the limits in using time and depth.
Chemical-Mixedgas-Rebreatherextend the range, because they allow to use mixed gases in a simple circuit. A chemical produces O2 while consuming C02. The most popular of those Rebreathers is the Navy CCCR from CCCP the IDA71. How deep you can use those Rebreathers depends on the diluent you use, but also donīt forget to keep the loop clean.
Semiclosed-Rebreather(SCR) are in use since about 1726, long before Cousteau and Gagnan developed the Aqualung. The CO2 is scrubbed out by lime like in the O2-Rebreather, the difference is how Gas is added:
Electronic-Closed-Rebreatherare controlled by an electronic unit, they make no bubbles. (Sure, when you ascend and dump expanding gas this makes some bubbles.) The electronic sensor measures the oxygen partial pressure (ppO2) and the electronic adds O2 to the breathing loop by triggering an electromagnetic valve (the so called "solenoid") when nessesary, the CO2 is scrubbed out by lime like in the O2-Rebreather. ECCRīs have two bottles with gas: 1) the oxygen, to be added by the electronic and 2) the diluent gas, to be added manually or by volume control. The "Buddy Inspriation" is a CCR (fully Closed mixed gas Circuit Rebreather) with manual diluent add, it allows to dive the twilight-zone when using trimix as diluent gas.
Liquid-Rebreatherdo not use gas to ventilate the divers lungs but special liquids, this allows unlimited deep diving, even without decompression stops. It is unknown if that is still science fiction like in the movie "the abyss" or if it is/was in military use. Liquid ventilation already is in hospital use, the chemical named "Liquivent" is manufactured by Höchst. I heared about diving experiments in 1994 that were abandonated due to problems with CO2 and with eye sight (how to see without an gas-bubble for the eyes / how to decompress the eyes in the case of having used a gas bubble). It is known that Liquid-Rebreathing can not solve the HPNS-Problem.
http://Rebreather.de/rebreather/principles.htm © Karl Kramer, 09.10.98