When Rebreathers Go Wrong: The Failures Every Diver Needs to Understand

Rebreathers represent one of the most significant technological advances in modern diving. By recycling breathing gas, removing carbon dioxide, and carefully replenishing oxygen, they allow divers to stay underwater far longer, move quietly, and operate more efficiently at depth. These advantages have made rebreathers popular among technical, scientific, military, and exploration divers.

However, the same complexity that makes rebreathers so capable also makes their failures more dangerous. Unlike open-circuit scuba, where problems are often noisy, obvious, and quickly detected, rebreather failures can be subtle, silent, and fast-acting. When a rebreather goes wrong, the diver may have only seconds to recognise the problem and respond correctly.

Understanding how rebreathers fail, and why those failures are so unforgiving, is essential for anyone diving closed- or semi-closed-circuit systems.

How rebreathers change the risk landscape

In an open-circuit system, the diver exhales waste gas directly into the water. Gas composition is fixed by the cylinder mix, and carbon dioxide is removed naturally with every breath. A rebreather fundamentally changes this process.

A closed-circuit rebreather recirculates exhaled gas through a breathing loop, removes carbon dioxide using a chemical scrubber, and injects oxygen to maintain a target partial pressure. This means the diver is now dependent on sensors, electronics, valves, scrubber material, and procedural discipline to stay alive.

According to safety guidance published by Divers Alert Network, rebreather diving introduces failure modes that do not exist in open-circuit scuba, most notably oxygen control failures and carbon dioxide retention. These risks are manageable, but only when divers understand them and operate within strict procedures.

The three primary causes of fatal rebreather accidents

Hypoxia: the most common killer

Hypoxia occurs when the oxygen partial pressure in the breathing loop drops below the level required to sustain consciousness. In rebreather diving this can result from incorrect sensor calibration, sensor failure, solenoid malfunction, depleted oxygen supply, or simple human error during setup.

Investigations compiled in Rebreather Accident Investigations show that hypoxia is the single most frequent cause of fatal rebreather accidents. What makes hypoxia especially dangerous is the lack of warning. Divers often feel little distress before losing consciousness, which explains why many victims are found with no attempt to bail out.

Hypercapnia: carbon dioxide poisoning

Hypercapnia occurs when carbon dioxide is not adequately removed from the breathing loop. This can happen if the scrubber material is exhausted, incorrectly packed, contaminated with water, or overwhelmed by high work rates.

Carbon dioxide toxicity produces symptoms such as air hunger, headache, confusion, panic, and rapid breathing, but these symptoms can escalate quickly and impair decision-making. Research discussed in NOAA’s Rebreathers and Scientific Diving Proceedings highlights that increased work of breathing at depth can accelerate CO₂ retention even when scrubber duration limits have not technically been exceeded.

Hyperoxia: oxygen toxicity

Hyperoxia occurs when oxygen partial pressure becomes too high, increasing the risk of central nervous system oxygen toxicity and underwater seizures. This can result from controller errors, incorrect setpoint selection, sensor drift, or excessive oxygen injection at depth.

While less common than hypoxia or hypercapnia, hyperoxic seizures are often immediately fatal underwater unless the diver can be stabilised and switched to open-circuit gas instantly.

Human factors remain central to most incidents

Although equipment failures do occur, accident analysis consistently shows that human factors play a decisive role. Common contributors include incomplete pre-dive checks, skipped checklists, inadequate maintenance, overconfidence, and pushing beyond training limits.

Training agencies such as PADI emphasise that rebreather divers must maintain strict procedural discipline, carry sufficient open-circuit bailout gas, and remain within conservative depth and exposure limits, particularly outside controlled training environments.

A recurring theme in fatal accidents is normalisation of deviance, where small shortcuts gradually become accepted practice until a single failure cascades into a fatal outcome.

Mechanical and electronic failure modes

Rebreathers rely on multiple layers of hardware redundancy, but that redundancy only works if the diver understands it and monitors it correctly.

Common mechanical and electronic failures identified in accident investigations include oxygen sensor drift or current limitation, solenoid sticking or failure, battery depletion, flooding of the breathing loop, damaged mouthpiece valves, and improperly assembled components following maintenance.

Unlike open-circuit failures, which usually result in rapid gas loss and obvious symptoms, rebreather failures may continue unnoticed until physiological limits are exceeded. This is why constant monitoring and cross-checking of sensors is critical throughout the dive.

Why rebreather accidents are difficult to investigate

One of the challenges in improving rebreather safety is that many fatal failure modes leave little physical evidence. Hypoxia and hypercapnia do not leave clear post-mortem markers, and electronic data is often lost or damaged during recovery.

This has led investigators involved in the Rebreather Accident Investigations project to stress the importance of conservative operation, robust training standards, and voluntary incident reporting to improve collective knowledge and prevent repeat scenarios.

Practical steps to reduce risk

Once authority has been established, the remaining guidance draws on synthesis rather than additional sourcing.

Rebreather safety depends on layered risk reduction. Divers should maintain absolute discipline with checklists, pack scrubbers exactly to manufacturer specifications, replace sensors proactively rather than reactively, and treat any abnormal breathing sensation as a reason to abort the dive.

Carrying adequate, immediately accessible open-circuit bailout gas is non-negotiable. Bailout planning should assume the worst-case scenario at maximum depth, including decompression obligations.

Currency also matters. Skills degrade quickly without practice, and infrequent rebreather use significantly increases risk.

A technology that demands respect

Rebreathers are not inherently unsafe, but they are unforgiving. They offer extraordinary capability, yet demand a level of procedural compliance more comparable to aviation than recreational scuba.

When rebreathers go wrong, they usually do so quietly, quickly, and without second chances. The divers who use them safely over long careers are those who respect the machine, respect the process, and never assume that yesterday’s successful dive guarantees today’s safety.