Rebreather Safety: Sensors, Scrubbers, and Bailout

Safety is the question that comes up more than any other when experienced open-circuit divers consider making the switch to a closed-circuit rebreather. It is a legitimate concern. A CCR is a more complex system than a regulator and a tank, and the consequences of a malfunction are different in nature and potentially more serious if not managed correctly.

The honest answer is that a well-maintained rebreather in the hands of a properly trained diver is an exceptionally safe piece of equipment. But that safety comes from understanding the specific risks, knowing how the engineering addresses them, and maintaining the discipline to follow procedures every dive. Here is what you need to know about the three primary hazards in CCR diving and how modern design, specifically the AP Diving Inspiration, mitigates each one.

The Three Core Risks in CCR Diving

Every CCR hazard ultimately comes down to one of three physiological threats: too little oxygen, too much oxygen, or too much carbon dioxide. Understanding each one is fundamental to safe rebreather diving.

Hypoxia: Too Little Oxygen

Hypoxia occurs when the partial pressure of oxygen (ppO2) in the breathing loop drops below safe levels. On open circuit, this is virtually impossible because every breath delivers fresh gas directly from the cylinder. On a CCR, the loop must be actively replenished with oxygen as your body metabolizes it. If the oxygen addition system fails or falls behind, ppO2 can gradually decline.

Hypoxia is particularly insidious because the early symptoms, a feeling of well-being, mild euphoria, and reduced critical judgment, are easy to miss underwater. By the time a diver recognizes something is wrong, they may already be significantly impaired. This is why relying on how you feel is never an acceptable monitoring strategy on a rebreather.

How the Inspiration addresses this: The dual oxygen controllers independently monitor ppO2 and add oxygen to maintain your setpoint. If C1 fails to maintain ppO2 and levels drop to 80% of setpoint, C2 automatically assumes control and fires the solenoid to correct the situation. Both controllers provide visual warnings on the 2020 Vision display, audible alarms, and the fiber optic HUD shifts from green to red. The diver receives warnings through three independent channels before the situation becomes critical.

Hyperoxia: Too Much Oxygen

Hyperoxia occurs when ppO2 rises above safe levels, which in recreational and technical diving means sustained exposure above approximately 1.6 bar. At elevated ppO2, oxygen becomes toxic to the central nervous system and can cause seizures underwater, which is typically fatal in a diving context.

On a rebreather, hyperoxia can occur if the solenoid sticks open and continues adding oxygen beyond the setpoint, if the diver descends rapidly without the controller compensating, or if pure oxygen enters the loop unintentionally. The risk is manageable but must be monitored.

How the Inspiration addresses this: Both C1 and C2 controllers continuously compare the readings from three independent oxygen cells. If ppO2 exceeds safe limits, the system generates immediate warnings through the display, audible alarms, and the HUD. The system does not rely on a single cell reading. Three cells provide voting logic so that a single faulty cell reading (which could be erroneously high or low) is identified by comparison against the other two. The dual controller architecture means that even if one controller’s solenoid malfunctions, the second controller detects the rising ppO2 and alerts the diver.

Hypercapnia: Too Much Carbon Dioxide

Hypercapnia results from CO2 accumulation in the breathing loop when the scrubber fails to adequately remove carbon dioxide from exhaled gas. This can happen if the Sofnolime is exhausted, improperly packed (allowing gas to channel around the absorbent), or if the scrubber canister is compromised.

Symptoms include headache, increased breathing rate, anxiety, and confusion. Unlike oxygen-related issues, there is no electronic sensor in most CCR units that directly measures CO2 in the loop. This makes prevention through proper scrubber management absolutely critical.

How the Inspiration addresses this: While the system does not directly measure CO2 concentration, the Temp-Stick scrubber monitoring system tracks the thermal wave through the scrubber bed in real time. The exothermic reaction between CO2 and Sofnolime generates heat, and the array of temperature sensors along the scrubber center monitors how far through the bed this reaction has progressed. The 2020 Vision display shows a graphical representation of remaining scrubber capacity. When the thermal front approaches the exit end of the canister, the diver has a clear visual warning that scrubber capacity is diminishing and it is time to end the dive. This system works in both cold and warm water and provides reliable tracking even with partially used scrubber fills from previous dives.

Oxygen Cells: Your Eyes Inside the Loop

Oxygen cells are electrochemical sensors that measure ppO2 in the breathing loop. They are consumable components with a finite lifespan, and their accuracy degrades over time. The AP Diving Inspiration uses three cells positioned in the scrubber lid, providing triple redundancy for the most critical measurement in the system.

The cells feature co-axial connectors with gold-plated push-on, pull-off connections for secure, reliable electrical contact and easy replacement. The four-pronged base provides stable anchoring, and the Teflon front face disperses moisture while allowing rapid gas transfer for real-time ppO2 readings. Both oxygen controllers read all three cells independently, and the 2020 Vision display shows each cell’s reading individually so you can spot a degrading cell before it causes a false reading.

Cell maintenance is one of the most important aspects of rebreather ownership. Replace cells within their recommended lifespan, verify accuracy before every dive, and never ignore a cell that is reading differently from the other two.

Bailout Planning: Your Safety Net

Bailout planning is the CCR equivalent of carrying a backup. Every rebreather diver should carry sufficient open-circuit bailout gas to safely ascend from maximum planned depth, including any required decompression stops, in the event of a complete CCR failure that cannot be resolved underwater.

The specifics of bailout planning vary by dive profile:

• Recreational depths (under 40m): A single bailout cylinder with an appropriate nitrox mix is typically sufficient.
• Moderate technical depths (40-60m): Multiple bailout cylinders with different mixes (bottom gas, travel gas, decompression gas) may be required.
• Deep technical dives (60m+): Comprehensive bailout gas planning with trimix bottom bailout, multiple decompression gases, and consideration for extended decompression under emergency conditions.

AP Diving’s Projection Dive Planner software supports both closed-circuit and open-circuit bailout scenario planning, including overhead environments. Good bailout planning assumes the worst case and works backward from there.

The Safety Culture That Matters Most

Engineering and design can only do so much. The AP Diving Inspiration has more redundancy built in than any other recreational CCR, but ultimately the most important safety system is the diver’s discipline. That means completing the full pre-dive checklist every time, monitoring the HUD and display throughout every dive, maintaining the unit according to the service schedule, replacing consumables on time, and never diving beyond your training or the unit’s certified limits.

Rebreather diving is not inherently more dangerous than open-circuit diving. It is differently dangerous, and the mitigation strategies are different. With proper training, disciplined maintenance, and respect for the engineering, CCR diving offers an extraordinary combination of capability and safety.

Questions about rebreather safety? Silent Diving has been supporting AP Diving rebreather owners across the Americas for over 20 years. We are happy to discuss any aspect of CCR safety, from unit selection to training to maintenance.

Learn about the Inspiration safety systems | Talk to our team