Rebreather Diving: Silent and Deep

What a Rebreather Does

A standard scuba regulator (open circuit) works simply: compressed gas from the cylinder flows through the regulator on demand; the diver breathes it; exhaled gas exits through the exhaust valve as bubbles. Roughly 75% of the gas in each breath is unused nitrogen; a large fraction of the oxygen is also exhaled. The total gas consumed is the sum of all breaths taken — typically 12-20 litres per minute at recreational depths.

A rebreather recirculates the exhaled gas. The loop:

1. Diver exhales to gas passes through a CO2 scrubber (a canister packed with soda lime, which chemically absorbs carbon dioxide)
2. Scrubbed gas passes through a counterlung (flexible bag that expands on exhalation and contracts on inhalation)
3. A solenoid valve controlled by oxygen sensors adds small amounts of oxygen to maintain the correct partial pressure of oxygen (PPO2)
4. Diver inhales the recycled, CO2-free, oxygen-topped gas

The result: gas consumption drops to the rate of oxygen metabolism only — approximately 0.5-1 litre per minute versus 12-20 litres on open circuit. A diver can spend 3-4 hours at depth on the gas supply that would support 45 minutes on a standard scuba rig.

Types of Rebreather

Closed-circuit rebreather (CCR): The full system described above. Manages both CO2 removal and oxygen partial pressure electronically. Capable of deep, long dives. Requires intensive training and maintenance.

Semi-closed circuit rebreather (SCR): Simpler system; adds a fixed small flow of fresh gas continuously. Less efficient than CCR but simpler and less expensive.

Recreational CCRs: Designed for recreational depth limits (40m). Examples: Poseidon MKVI, Hollis Explorer, Mares Horizon.

Technical CCRs: APD Inspiration/Evolution, JJ-CCR, rEvo, Megalodon. Capable of extreme depth and duration in qualified hands.

The Advantages

No bubbles: Rebreather divers can approach marine life more closely and observe natural behaviour more accurately. The difference is most noticed with shy animals — seahorses continue feeding undisturbed by rebreather divers; parrotfish schools don't part as you approach.

Extended bottom time: 3-4 hours at depth fundamentally changes what is possible — cave diving, mixed-gas technical diving, and marine research all benefit enormously.

Optimal gas mixture: A CCR automatically maintains the correct oxygen partial pressure for the diver's current depth — essentially diving on optimal nitrox at every depth.

The Risks

Rebreathers have a higher per-dive fatality rate than open-circuit scuba. The causes are well-documented:

• CO2 breakthrough: If the scrubber canister is exhausted, improperly packed, or wet, CO2 is not removed. CO2 poisoning produces rapid unconsciousness with little warning.
• Hypoxia: If the oxygen sensor fails and the system under-delivers oxygen, the diver can lose consciousness without experiencing the urge to breathe.
• Hyperoxia (oxygen toxicity): If sensors fail high and over-deliver oxygen at depth, seizure risk increases.

Rebreather training requires significantly more time and commitment than open-circuit; minimum courses are typically 5-7 days of intensive instruction. Pre-dive checks are non-negotiable.