1At a glance
- Primary Function
- Recycle exhaled gas for extended dive times
- Breathing Gas Efficiency
- Highly efficient; conserves oxygen and inert gases
- Bubble Production
- None (closed-circuit) or minimal (semi-closed)
- Dive Duration Potential
- Significantly longer than open-circuit
- Oxygen Partial Pressure
- Actively managed by electronics and sensors
- Training Requirement
- Specialised and extensive
- Complexity
- High; requires diligent maintenance and pre-dive checks
2How a rebreather works
A closed-circuit rebreather (CCR) operates on a continuous loop principle. The diver inhales from a counterlung, a flexible bag that holds the breathing gas. As the diver exhales, the breath passes through a carbon dioxide absorbent canister, typically containing a chemical called 'sorb' (e.g., Sofnolime), which chemically removes CO2 from the gas. After CO2 removal, the now-cleaned gas returns to the counterlung.
During this process, the diver's metabolism consumes oxygen. Electronic sensors continuously monitor the partial pressure of oxygen (PO2) within the breathing loop. When the PO2 drops below a pre-set level, the rebreather's electronics signal a solenoid valve to inject a precise amount of pure oxygen from a small cylinder into the loop, restoring the desired PO2. A separate 'diluent' gas (often air or trimix) from another cylinder can also be added manually or automatically to increase the volume of gas in the loop, especially during descent to compensate for compression, or in an emergency.
This system ensures that the diver always breathes an optimal gas mix for their depth, leading to reduced decompression stress and extended no-decompression limits or significantly longer decompression dives. The absence of bubbles means no gas is wasted, and the diver remains stealthy in the water, making it ideal for marine observation or photography. However, the complexity of the system requires rigorous pre-dive checks, extensive training, and a thorough understanding of potential failure modes and emergency procedures.
3Rebreather vs. Open-Circuit Scuba vs. Semi-Closed Rebreather
| Feature | Closed-Circuit Rebreather (CCR) | Open-Circuit Scuba | Semi-Closed Rebreather (SCR) |
|---|---|---|---|
| Gas Usage | Recycles exhaled gas; highly efficient | Exhales all gas to ambient water; inefficient | Recycles some gas, vents a portion; moderately efficient |
| Bubbles | None (silent) | Abundant (noisy) | Minimal (some noise) |
| Dive Duration | Very long (limited by sorb/O2) | Short-to-moderate (limited by tank volume) | Longer than open-circuit (limited by sorb/gas) |
| Gas Mix | Maintains optimal PO2 throughout dive | Fixed mix (e.g., air, nitrox, trimix) | Fixed mix, dilutes with fresh gas |
| Complexity/Training | High; extensive specialised training | Low; basic Open Water Diver training | Medium; additional training required |
4What a rebreather means for your liveaboard trip
Bringing a rebreather on a liveaboard trip significantly enhances dive capabilities but also introduces specific logistical considerations. The primary benefit is the ability to conduct longer, deeper, and more frequent dives, often with reduced decompression obligations compared to open-circuit diving. This can translate into more time exploring specific sites, observing elusive marine life, or accessing deeper areas beyond typical recreational limits.
However, liveaboard operators need to be informed well in advance if you plan to dive with a rebreather. Requirements typically include storage space for the unit, spare parts, and consumables like sorb. Many liveaboards offer dedicated rebreather support, including oxygen and diluent fills (air, nitrox, or trimix if available), sorb (carbon dioxide absorbent) provision or secure storage, and charging stations for batteries. Some even provide specific rebreather-friendly dive briefings and guidance on gas planning.
It is crucial to verify the liveaboard's rebreather policy and support capabilities before booking. Divers must be self-sufficient and highly trained, as rebreather diving carries higher risks if not executed meticulously. The silence of rebreather diving can dramatically improve wildlife encounters, making it particularly appealing for photography and videography enthusiasts who seek close, undisturbed interactions with marine fauna.
5Rebreather diving logistics for liveaboards
| Logistical Factor | Importance | Liveaboard Provisioning | Diver Responsibility |
|---|---|---|---|
| Sorb Availability | Critical | Often provided/sold; storage for used sorb | Ensure fresh sorb supply or confirm purchase options |
| Gas Fills (O2/Diluent) | Critical | Oxygen typically available; diluent (air/nitrox/trimix) depends on vessel capability | Pre-order specific gas mixes (e.g., Trimix) if required |
| Charging Stations | High | Dedicated 230V/110V outlets for batteries | Bring correct chargers and adapters |
| Storage Space | High | Designated area for unit assembly/disassembly, spares | Keep gear organised; ensure unit is secured |
| Technical Dive Support | Medium | Experienced dive guides for deep/decompression dives | Plan dives within training and confirm guide's expertise |
6Common misconceptions
Myth: Rebreathers are purely for technical divers. Fact: While rebreathers are widely used in technical diving due to their extended duration and gas efficiency, there are also recreational rebreather courses and units designed for use within no-decompression limits. These units offer silence and extended bottom times for non-technical recreational divers.
Myth: Rebreathers are inherently more dangerous than open-circuit scuba. Fact: Rebreathers are more complex and require a higher level of training, diligence, and meticulous pre-dive checks. If not properly maintained or operated, the risks are higher. However, when operated by a well-trained diver following proper procedures, they can be a safe and efficient way to dive, offering advantages like optimal decompression. The key is proper training and adherence to protocols.
Myth: Rebreathers are completely silent and produce no bubbles at all. Fact: Closed-circuit rebreathers (CCR) produce no exhaust bubbles. However, semi-closed rebreathers (SCR) vent a small amount of gas periodically, producing some bubbles, though far fewer than open-circuit systems. Even CCRs can occasionally 'burp' small bubbles if the diver ascends rapidly or if there's a slight overpressure in the loop.
Myth: Rebreathers eliminate the risk of decompression sickness (DCS). Fact: While rebreathers allow for precise control of oxygen partial pressure (PO2), which can optimise inert gas off-gassing and potentially reduce decompression stress, they do not eliminate the risk of DCS. Divers still absorb inert gas (from the diluent) and must follow appropriate decompression schedules and ascent rates. Inadequate planning or execution can still lead to DCS.
FAQ
What is the main advantage of diving with a rebreather?
The main advantage of a rebreather is the ability to recycle exhaled gas, conserving expensive breathing gases like helium and oxygen. This allows for significantly longer dive times compared to open-circuit scuba and eliminates bubbles, providing a silent approach to marine life.
Do I need special training to use a rebreather?
Yes, rebreather diving requires specialised and extensive training beyond conventional open-circuit scuba certifications. This training covers the complex operation, maintenance, emergency procedures, and specific physics and physiology associated with rebreather diving.
Are rebreathers suitable for all types of diving?
Rebreathers are particularly well-suited for extended bottom times, deep dives, repetitive dives, and activities requiring stealth, such as marine photography or scientific research. While recreational rebreathers exist, they are generally not used for very shallow or short duration dives where open-circuit scuba is simpler and sufficient.
What is 'sorb' and why is it important for rebreather diving?
Sorb (carbon dioxide absorbent) is a chemical compound, typically in granular form, used in rebreathers to remove carbon dioxide from the diver's exhaled breath. It is crucial for preventing CO2 build-up, which can be toxic. The sorb must be fresh and correctly packed before each dive or series of dives, as its effectiveness diminishes with use.
Can all liveaboards accommodate rebreather divers?
Not all liveaboards are equipped to accommodate rebreather divers. It is essential to confirm with the liveaboard operator in advance regarding their rebreather policy, availability of oxygen, diluent gas fills (air, nitrox, trimix), sorb, charging facilities, and guide experience with rebreather diving. Some liveaboards specialise in technical or rebreather-friendly itineraries.
What are the primary safety considerations for rebreather diving?
Primary safety considerations for rebreather diving include meticulous pre-dive checks of the unit, thorough gas planning, vigilant monitoring of oxygen partial pressure, and an absolute adherence to training protocols. Redundancy in oxygen sensors, diluent gas, and emergency bail-out procedures are critical due to the system's complexity.