A rebreather is a SCUBA breathing system that recirculates exhaled gas. Unlike open-circuit scuba, which releases each exhaled breath into the water, a rebreather removes carbon dioxide and replenishes oxygen so the gas can be breathed again.
How Does It Work?
The diver breathes through a breathing loop. Flexible bags called counterlungs receive exhaled gas and supply gas for the next breath. A canister containing carbon dioxide absorbent, called the scrubber, removes the carbon dioxide produced by the diver.
A mixed-gas closed-circuit rebreather (CCR) carries oxygen and a second gas called diluent, which may be air or an appropriate helium mixture. Oxygen replaces what the diver consumes. Diluent provides additional breathing-loop volume when needed, including during descent. It is a gas mixture, not necessarily an inert gas.
Oxygen sensors measure the partial pressure of oxygen (PO2) in the loop. The target PO2 is called the setpoint. This is different from maintaining a fixed oxygen percentage: the percentage changes with depth for a given PO2.
Manual and Electronic Control
On a manual CCR (mCCR), the diver controls oxygen addition, often assisted by a mechanical oxygen feed. An electronic CCR (eCCR) uses sensor readings and a controller to operate an oxygen-addition valve. Designs vary, and many electronic units also provide manual controls. Both require the diver to monitor the breathing system; electronic control does not remove that responsibility.
A semi-closed rebreather (SCR) also recycles gas, but deliberately releases part of it and replaces it with fresh breathing gas. Its operation and gas-management requirements differ from those of a CCR.
Why Do Divers Use Rebreathers?
- Less noise and fewer bubbles. A CCR normally releases little gas, which can improve opportunities to observe or photograph marine life.
- Efficient use of breathing gas. Recycling gas can make longer dives and some technical dives more practical. It does not provide unlimited dive time: scrubber capacity, gas reserves, bailout, exposure, and decompression remain limits.
- Control of oxygen exposure. Managing PO2 can offer decompression advantages compared with some open-circuit gas choices. The benefit depends on the dive profile and gases; decompression planning is still required.
What Are the Tradeoffs?
Rebreathers require additional preparation, checks, cleaning, maintenance, and consumables. Ownership cost includes training, servicing, oxygen sensors, absorbent, and suitable bailout equipment as well as the unit itself. Costs vary substantially by design and the diving you intend to do.
Too little oxygen, too much oxygen, or a failure to remove carbon dioxide can make the breathing gas dangerous. These problems may not provide a reliable warning through how the gas feels to breathe. Rebreather diving requires training on the specific unit, consistent preparation and monitoring, and a planned alternative breathing supply.
A rebreather can be a useful tool when its benefits match your diving, but it is not automatically the best next step. Read our Rebreather Training Overview and Tony Land’s article Choosing Your First Rebreather to explore the decision.