Rebreathers in Caves: How They Differ, the Bailout Concept and What Training Involves

A closed-circuit rebreather (CCR) recycles the diver's exhaled gas: it removes carbon dioxide with a chemical absorbent and adds oxygen to replace what the body has used, so very little gas leaves the system as bubbles ^[2]. Rebreathers have become common in technical and cave diving because they make long, deep dives possible on small cylinders. They also change the nature of the risk, and cave training for rebreather divers is built around that change. This article explains the concepts; it is not a description of rebreather procedures, which must be learned on a specific unit from a qualified instructor.

How a CCR differs from open circuit

With open-circuit scuba, every breath is drawn from a cylinder and exhaled into the water. Gas supply is the limiting factor, and the gas in the cylinder has a fixed, known composition. With a CCR, the gas the diver breathes is a mixture constantly adjusted by the machine and the diver. Its oxygen level is measured by sensors and controlled by electronics, a manual valve or both; its carbon dioxide is removed by a scrubber whose capacity is finite.

The benefits for cave diving are clear. Potts, Buzzacott and Denoble note that rebreathers let divers stay underwater longer without carrying additional tanks ^[2], and the 2023 Rebreather Forum 4 consensus describes CCRs as widely adopted by technical divers to reduce gas consumption and extend depth and duration ^[3]. Exley noted in 1979 that exhaled bubbles can dislodge silt from a cave ceiling ^[6]; a rebreather that recycles its gas releases far fewer bubbles, although we found no study quantifying the effect in caves.

The trade-off is complexity. The Rebreather Forum 4 statement describes rebreathers as technologically complex with many failure points, and their use as apparently associated with a higher accident rate than open-circuit scuba ^[3]. The most serious failures do not announce themselves. Too little oxygen (hypoxia), too much (hyperoxia) and a build-up of carbon dioxide (hypercapnia) can each impair or incapacitate a diver, and these three appear among the stated causes in an analysis of 181 recreational rebreather deaths between 1998 and 2010 ^[1].

What the fatality research says

Andrew Fock's 2013 analysis in Diving and Hyperbaric Medicine estimated that recreational rebreather deaths occurred at several times the rate of open-circuit recreational diving, with figures ranging from about four to ten times depending on assumptions ^[1]. His fault-tree comparison found that a CCR on its own carries roughly 25 times the risk of a critical component failure of a manifolded twin-cylinder open-circuit system, but that this risk can be offset by carrying redundant open-circuit bailout ^[1]. Two-thirds of the deaths were associated with a high-risk dive or high-risk behaviour, including starting or continuing dives with alarms sounding or known faults, and Fock identified many points in the human-machine interface where errors could lead to a fatality ^[1]. No particular brand or type of unit was over-represented ^[1].

Cave-specific data are thinner. In the review of trained cave divers who died in the United States between 1985 and 2015, the number using rebreathers rose from one in the first 15 years to six or seven in the second (the paper's text and table differ slightly), an increase that was not statistically significant given the small numbers ^[2]. Individual cases illustrate the failure modes: one diver had not opened the valve of his rebreather's oxygen cylinder, and another is thought to have used a unit with out-of-date oxygen sensors and a malfunctioning solenoid ^[2]. The authors noted that the classic cave diving rules were written before rebreathers and do not specifically address them ^[2].

The first recreational production CCR is dated to 1997 by Potts and colleagues and to 1998 by Fock ^[1][2]; Potts and colleagues add that rebreathers soon became popular with technical divers, including cave divers ^[2].

The bailout concept

Because a rebreather can fail in ways the diver cannot fix underwater, rebreather divers carry an independent way to breathe, called bailout. In most cave diving this means open-circuit cylinders carried alongside the rebreather, enough to get the diver out. Fock's analysis showed why this matters: with redundant open-circuit bailout included, the modelled risk of a mission-critical equipment failure becomes similar to that of an open-circuit diver who depends on several cylinders to complete a decompression dive ^[1].

In a cave, the exit can be long, and bailout must be planned for the worst place to have a problem, the farthest point of the dive. Training standards state this directly. GUE's CCR Cave standards require divers always to have enough bailout gas to exit the cave from the maximum penetration ^[4]. The NSS Cave Diving Section's CCR Cave Diver course requires open-circuit bailout with a margin over what is needed to exit from maximum penetration, carried as two or more cylinders or a single cylinder with a dual-outlet (H) valve ^[5]. The NSS-CDS notes that many students first complete its open-circuit Apprentice Cave course, partly to learn how much open-circuit gas an exit really requires ^[5]. How bailout is calculated is taught in those courses and is not reproduced here.

The Rebreather Forum 4 consensus statements add related points: a bailout valve (a mouthpiece that can switch from loop to open circuit) can ease the transition in some emergencies but needs a high-performance open-circuit supply, mouthpiece-retaining straps help prevent drowning if a diver loses consciousness, and practices around bailout rebreathers (a second rebreather carried as bailout) should be developed and documented ^[3].

Training implications

Rebreather cave training sits on top of two separate foundations: rebreather competence and cave competence. The NSS-CDS CCR Cave Diver course requires rebreather certification, at least 50 logged dives and at least 25 dives and 35 hours on the same rebreather that will be used ^[5]. GUE requires its CCR technical certification on the same configuration, 50 non-training dives on that configuration and 25 non-training dives at its second cave level ^[4]. GUE's course topics include risks specific to rebreather diving in an overhead environment, equipment configuration, and exits in zero visibility while monitoring the unit by its head-up display ^[4].

Rebreather cave divers still carry the full cave kit. Both agencies require one primary and two backup lights, a primary reel per team, safety spools or reels and line markers on their rebreather cave courses ^[4][5].

Broader rebreather research points to human factors as much as hardware. Fock recommended checklists and designs that reduce opportunities for error, and suggested that initial training should build core skills before more complex options are introduced ^[1]. Rebreather Forum 4 endorsed brief pre-entry checklists (for example confirming the unit is switched on and the oxygen cylinder is open), head-up displays for safety-critical information, refresher training because skills fade, and reporting of incidents and near-misses to DAN; it also stated that solo diving may increase the likelihood of a fatality during a rebreather incident ^[3].

Summary

Rebreathers give cave divers range and duration, but they replace a simple, robust breathing system with a complex one whose failures can be silent. The cave diving response is layered: an independent bailout sized for the exit, the same lights and line equipment as any cave diver, rigorous checks, and training that first builds rebreather experience and then applies it to the overhead environment.