Technical 10 minutes
Rebreathers: breathing the same gas over and over
No bubbles, huge endurance, and three ways to kill you that give little or no warning
An open-circuit diver exhales almost all the oxygen they breathe: of every breath, the body consumes only a small fraction and the rest leaves as bubbles. A rebreather recovers that gas, strips out the carbon dioxide, replaces the oxygen consumed, and gives it back to you. It's as elegant as it is unforgiving.
In this article you'll see how a rebreather works, what each of its components does, what real advantages it offers over open circuit, what its three characteristic risks are, and why checklist discipline is inseparable from using it.
The rebreather is the technology that most expands what a diver can do, and also the one that forgives improvisation the least. This article is educational and descriptive material: using one requires model-specific training and continued practice.
The loop and its parts
The whole system is a closed loop that the gas travels around.
Exhaled gas leaves the mouthpiece and enters the loop, a ring of corrugated hoses fitted with one-way valves that force the gas to circulate in a single direction. That detail is essential: without them, the gas would slosh back and forth without passing where it needs to.
The counterlungs are flexible bags that absorb the volume of each exhalation and return it on the next inhalation. They're what gives the loop its elasticity and makes breathing feel natural, and their position has a big effect on breathing effort depending on the diver's posture.
The scrubber is a cartridge filled with chemical material that strips carbon dioxide out of the exhaled gas through a reaction that also generates heat. It's the heart of the system and also its consumable: it has a limited duration, measured in hours of use, and its exhaustion is one of the characteristic risks.
Two small cylinders feed the loop. One carries pure oxygen, injected to replace what the body consumes. The other carries diluent, air or a helium mix, added on descent to maintain the loop's volume and to dilute the oxygen at depth, keeping its partial pressure from spiking.
And the control system: several sensors continuously measure the partial pressure of oxygen inside the loop, and electronics with a display decide how much oxygen to inject and show the diver what's happening. On manual models, that decision is made by the person.
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