Why Divers Switch to Rebreathers (And Why Some Switch Back)

Rebreathers are one of those topics where everyone in the tech diving community has an opinion — usually a strong one. The people who love them are evangelical. The people who went back to open circuit are philosophical about it. Almost nobody is neutral.

Here’s an honest breakdown of what rebreathers actually offer, what they demand, and how to think about whether one belongs in your future.

What a Rebreather Does

On open circuit scuba, every breath you exhale goes into the water as bubbles. You use roughly 25% of the oxygen in each breath; the rest — along with all the nitrogen — gets wasted. A 12-liter cylinder at 200 bar gives you maybe an hour at 20m.

A closed-circuit rebreather (CCR) recirculates your exhaled gas through a scrubber canister that removes CO₂, then injects just enough oxygen to bring the mix back to a usable level. The loop is sealed — almost nothing leaves the system.

The practical effects:

Extended bottom time. A CCR diver at 40m on a 3-liter O₂ cylinder and a 3-liter diluent cylinder can dive for hours on what an OC diver would burn through in 30 minutes.

Optimized decompression. The computer maintains a constant partial pressure of oxygen (PPO₂) — usually set to 1.2–1.3 bar — throughout the dive. This means you’re always breathing the best possible mix for decompression at whatever depth you’re at. Less decompression time overall, especially on complex profiles.

No bubbles. Marine life doesn’t scatter. Photographers get closer. Wreck environments aren’t disturbed. This alone converts some divers permanently.

Thermal efficiency. You’re breathing warm, humidified gas the entire dive. Longer dives in cold water are noticeably more comfortable.

The Cost of Admission

None of that comes free.

It will try to kill you in different ways than OC. The failure modes on a rebreather are not the same as open circuit. The primary ones:

Hypoxia — not enough oxygen in the loop, often insidious and symptom-free until blackout. Happens if the O₂ injection system fails or if you’re not monitoring your PPO₂.
Hyperoxia / oxygen toxicity — too much oxygen, usually from a stuck-open solenoid or incorrect diluent choice at depth. CNS oxygen toxicity can cause convulsions underwater. With a regulator in your mouth on OC, this is survivable. On a CCR, with the mouthpiece in a loop… it’s not.
CO₂ breakthrough — exhausted or channeled scrubber canister letting CO₂ through. Fast onset, feels like sudden breathlessness and anxiety, can be disorienting enough to cause panic.
Incorrect setup — diving a CCR with a known problem is far more dangerous than diving OC with a dodgy tank O-ring. Pre-dive checks are not optional and not abbreviated.

The learning curve is real. CCR courses are longer and more expensive than equivalent OC tech courses for good reason. The bailout procedures, the monitoring habits, the pre-dive checks — these need to be automatic, not recalled under stress.

Most experienced CCR instructors suggest that the first 100 CCR dives are genuinely higher-risk than the same dives on OC, because you haven’t built the automatic habits yet.

Maintenance is a job. Scrubber canisters need to be packed correctly and tracked for duration. O₂ cells have a lifespan (typically 1–2 years) and need replacement before they fail. The loop itself needs regular cleaning and inspection. If you don’t have a systematic maintenance habit, a CCR will eventually catch you.

The cost. A quality CCR runs $6,000–$12,000. Training is $800–$2,000. The ongoing costs of O₂ cells, scrubber media, and servicing add up. This is not a budget option.

The KISS Classic

For divers who want CCR without electronics dependency, the KISS Classic takes a different philosophy: mechanically simple, manually controlled.

Rather than an electronically-controlled solenoid injecting O₂ based on sensor feedback, the KISS Classic uses a constant-flow O₂ injection combined with manual adjustment via a bypass valve. The diver monitors PPO₂ from three O₂ cells and adjusts their setpoint manually.

More work per dive. Fewer electronics to fail. A favorite among divers who understand the trade-off and trust their monitoring habits over a computer’s automatic decisions.

Why Some Divers Go Back to OC

It’s more common than the rebreather community sometimes admits.

Logistics. Traveling with a CCR is a production. Scrubber media isn’t available everywhere. Some dive operations don’t support CCR diving. If you dive in remote locations, the complexity multiplies.

Shallow diving. The efficiency advantage of a CCR shrinks dramatically at shallow depths. For reef dives at 15–20m, the extra equipment, cost, and pre-dive overhead often don’t pencil out.

Stress. Some divers find the cognitive load of CCR diving — constant PPO₂ monitoring, scrubber tracking, bailout awareness — takes away from the experience. Diving should be enjoyable. If the unit makes every dive feel like work, that matters.

Life changes. People dive less. Kids arrive. The 3-hour weekly diving habit that made CCR maintenance sustainable stops being possible. CCR and infrequent diving is a genuinely risky combination.

The Honest Assessment

Rebreathers are exceptional tools for divers doing extended technical dives — particularly anything involving deep decompression, extended range, or underwater photography where bubble suppression matters.

They’re not the next step for every diver who finishes their advanced OW card. The right path to a CCR runs through solid OC technical diving, genuine comfort with deco profiles, and a realistic assessment of your diving frequency and maintenance commitment.

If that describes you, a CCR will change how you dive. If it doesn’t yet, build the foundation first. The rebreather will still be there.

Log your CCR dives in Abyssi — track your setpoint, scrubber hours, and decompression profile alongside your open circuit log.