What is a Rebreather?
By Jeffrey Bozanic

Immersed, Spring 2004, (8:1), p. 38.

"Normal" scuba is an open circuit system. Combining a high pressure cylinder and a demand regulator, you inhale gas at ambient pressure, use a little of the oxygen in the gas, and exhale. When you exhale the gas, it bubbles to the surface, carrying as much as 98% of the original oxygen it contained. The "open circuit" comes from the fact that the exhaled gas is released on every breath.

A rebreather recirculates the gas a diver is breathing, allowing the diver to breath the same gas over and over again. The system removes the carbon dioxide generated by human metabolism, and adds oxygen and other gases to make up what is consumed. Because the gas is breathed over and over again, instead of being "thrown away" with every breath, a diver can remain underwater far longer on much less gas. In fact, for some dives, rebreathers can be as much as fifty times more efficient on gas consumption than standard scuba.

There are three major types of rebreathers. Each has different advantages and disadvantages, and each requires a different type and degree of training. The three classes of rebreathers are: (1) oxygen, (2) semi-closed circuit, and (3) closed circuit.

OXYGEN REBREATHERS are the simplest and least expensive rebreathers.
Diagram of an Oxygen Rebreather

As the name implies, the breathing gas is 100% oxygen. Because of this, the diver incurs no decompression obligation, as there is no inert gas. However, the maximum depth to which the units may be used is 20 feet of sea water (fsw), because of oxygen toxicity. Oxygen rebreathers are completely bubble free, so long as the diver maintains a given depth, or ascends slowly enough to metabolize oxygen by volume to counter gas expansion due to the ascent.

To allow divers to travel deeper an inert gas must be added to the breathing mix. SEMI-CLOSED REBREATHERS (SCR) typically utilize some type of enriched air nitrox (EANx) as the base gas, which is an oxygen-nitrogen mixture containing more oxygen than air. The gas may recirculate through the breathing circuit several times, but a key feature of SCRs is that a portion of the gas is either continually dumped at a constant rate (constant mass flow), or a portion of every breath is exhausted (respiratory minute volume keyed).

Diagram of a Semi-Closed Circuit Rebreather

These systems allow divers to dive to depths of generally about 130 fsw, and are more efficient than open circuit scuba. However, they typically lack sensors that tell the divers exactly what they are breathing, and always have some bubbling occurring.

Finally, CLOSED CIRCUIT REBREATHERS (CCR) close the loop, so to speak, by recirculating all of the gas the diver is breathing. Oxygen from an oxygen cylinder is added to replace that which is metabolically utilized by the diver. As the diver descends, a diluent gas, typically air or trimix, is added to maintain the proper breathing volume.

Diagram of a Closed Circuit Rebreather

Oxygen sensors monitor the oxygen partial pressure, allowing the diver to stay within safe limits. Some CCRs are computer controlled, automatically adding oxygen as necessary, and may also incorporate a dive computer to calculate inert gas status of the diver.

With rebreathers, "bigger" does not necessarily mean "better." As with any piece of equipment, the user must determine what the desired objective is, and select the gear accordingly. This is especially true with rebreathers, where the difference between units may involve a difference in 40-50 hours of training, and a price differential of over $20,000. In fact, for many objectives, traditional open circuit scuba is still the system of choice. However, if your objective necessitates it, nothing can beat the use of the appropriate rebreather.