Most closed-circuit rebreather (CCR) accidents do not begin with catastrophic equipment failures, but quietly, with small, manageable deviations that go uncorrected. Over time, these deviations align into an accident chain that becomes increasingly difficult—and eventually impossible—to break.
This is why CCR incidents are best understood not as single events, but as process failures.
The Accident Pit: From Minor Problem to Unsurvivable Situation
Accidents typically follow a predictable escalation pattern:
• Minor problem occurs
(Easily resolvable — normal operational region)
Basic capacity and attention are sufficient to correct the issue.
• Serious problem occurs
(Increased stress — higher cognitive load)
The diver must actively think, analyze, and prioritize.
• Emergency
(Rapid, correct response required)
The risk of panic increases. Automatic responses and ingrained habits are critical.
• Unresolvable problem
(Survivability is lost)
The diver no longer has the physical or cognitive capacity to recover.
Never allow a minor problem to migrate downward through this funnel.
1. The Setup Phase — Errors Before the Dive
The accident chain often begins before the diver enters the water. Common contributors are:
• Rushed or incomplete pre-dive checks
• Poor scrubber packing (causing channeling) or extending scrubber duration beyond conservative limits
• Oxygen sensors nearing the end of their service life or not reading correctly
• Small leaks dismissed as “minor”
At this stage, nothing feels wrong. The unit powers on. PO₂ responds. The diver believes the system is reliable when it is already compromised.
The danger lies in false confidence.
When a CCR diver enters the water with a compromised system—without knowing it—they have already moved closer to the edge of the operational envelope.
2. Normalization of Deviation
During the dive, the diver begins to notice that something is off:
• PO₂ fluctuates more than expected
• Solenoid behavior feels unusual
• Work of breathing is slightly elevated
• Oxygen sensors show disagreement
Instead of stopping and investigating, the diver adapts, thinking:
“It’s probably just depth change.”
“It’s within limits.”
“This has happened before.”
Each time a deviation is accepted without correction, it becomes normalized. This step is subtle—and extremely common—but also one of the most dangerous in the accident chain because it feels reasonable.
The dive continues. Stress remains low. But the diver has unknowingly shifted from problem-solving to problem accommodation.
3. Task Loading and Distraction
Most CCR accidents escalate when the diver becomes distracted or task-loaded. Common sources include:
• Filming or photography (camera work can consume a significant amount of awareness)
• Navigation or survey work (workload creates perceptual narrowing)
• Scooter use (speed, drag, enjoyment, propeller noise, changes in depth, and one-hand occupation all contribute to distraction and dismissal of subtle warnings)
• Environmental stress (current, restrictions, low visibility, or even a leaking drysuit during a cold-water dive can consume mental capacity)
• Team separation (a major stress trigger, especially in complex or demanding environments)
PO₂ checks may now become less frequent. Breathing patterns change. Small warnings go unnoticed.
At this point, the diver is no longer leading the dive—they are falling behind it.
4. Physiological Impairment — The Turning Point
This is where the accident chain becomes extremely difficult to stop. Depending on the failure mode, the diver may experience:
• Rising CO₂ (CO₂ hit / hypercapnia) → Can be caused by an overused or channeled scrubber, improper gas selection, excessive exertion, or restricted breathing. Symptoms include anxiety, air hunger, confusion, poor motor control, rapid breathing, perceptual narrowing, headache, panic, and unconsciousness.
• Dropping PO₂ (hypoxia) → Occurs when the brain is deprived of oxygen (PO₂ too low, or breathing hypoxic gas at an inappropriate depth on open circuit). Symptoms often go unrecognized, which is why hypoxia is so dangerous. They include narrowing attention, delayed reactions, loss of judgment, confusion, disorientation, and convulsions.
• Rising PO₂ (hyperoxia) → The central nervous system (CNS) is affected by elevated oxygen partial pressure, even during short exposures. Symptoms include dizziness, twitching, visual disturbances, irritability, nausea, ear ringing, and convulsions.
The critical danger here is that the diver may not recognize the impairment. Symptoms often feel psychological rather than mechanical. Decision-making degrades before the diver realizes something is wrong. The ability to analyze, troubleshoot, or even choose to bail out diminishes rapidly.
5. Delayed or Inappropriate Response
Because CCR problems are often silent, the diver may be unable to react—especially during the most dangerous situations such as hypoxia or hyperoxia—or may hesitate to bail out before conditions worsen.
Even during less critical problems, divers often fail to abort the dive, thinking:
“It’s probably fine.”
“The dive is nearly over anyway.”
By now, the problem has moved well beyond the “minor” category in the accident pyramid. The chain is tight. Options are shrinking.
6. Loss of Control
As impairment worsens, situational awareness collapses, buoyancy control deteriorates, team communication fails, and motor skills degrade. The diver may lose consciousness, and the situation becomes unresolvable.
Because there are no bubbles and little noise, teammates may not notice immediately. This is why many CCR accidents are not witnessed in real time—and why rescue windows are often tragically short.
Why This Chain Is So Dangerous
Each individual step is common and seemingly manageable. But combined, they create a self-reinforcing spiral that reduces awareness and decision-making, delays response, and incapacitates the diver.
The CCR does exactly what it was designed to do: quietly keep the diver breathing—until it shouldn’t.
Breaking the Chain
The most effective prevention strategies are:
• Building ruthless pre-dive discipline
• Zero tolerance for unexplained deviations
• Frequent, deliberate PO₂ cross-checks
• Early bailout decisions (appropriate to your training and diving level)
• Training failure recognition—not just mechanical drills
• Maintaining low task loading during critical phases
Most importantly:
Experience does not always protect you.
Discipline does.
