REBREATHER DIVING

A rebreather recycles the gas you breathe instead of venting it to the sea. That single difference changes nearly everything else about a dive: how long you can stay, how much gas you carry, how you decompress, and how unforgiving the dive becomes if you stop paying attention. This page explains how closed circuit works and what it takes to dive one.

What a rebreather does

On an ordinary scuba set you inhale from a cylinder and exhale into the water. Most of that exhaled breath is wasted. Your body consumes only a small fraction of the oxygen in it; the rest leaves as bubbles, along with all the inert gas that came with it.

A rebreather keeps that gas in a sealed loop. Your exhaled breath passes through a canister of absorbent material that chemically removes the carbon dioxide, the oxygen your body has actually consumed is replaced, and the same gas comes back round to be breathed again.

Open circuit and closed circuit

The reason this matters is pressure. On open circuit, the deeper you go the denser the gas you breathe, so consumption climbs with depth. At 60 m you are working through roughly seven times the gas you would use at the surface for the same breathing rate. That is what turns a deep open-circuit dive into a logistics exercise: multiple large cylinders, decompression gases, and a plan for carrying them all.

On a closed circuit rebreather (CCR), oxygen consumption is set by your metabolism rather than by depth. You use much the same oxygen at 60 m as you do at 6 m. A modest oxygen supply and a scrubber can therefore support a dive that would need a small mountain of cylinders on open circuit.

Semi-closed rebreathers (SCR) sit between the two. They continuously vent a proportion of the loop and top it up from a premixed cylinder. Mechanically simpler, but less efficient than closed circuit and with far less control over the gas you are breathing.

Inside a closed circuit rebreather

• Mouthpiece: a dive/surface valve, or a bail-out valve that lets you switch to open circuit without taking anything out of your mouth.
• Counterlungs and hoses: flexible bags that give the loop somewhere to go as you inhale and exhale within a sealed system.
• Scrubber: a canister of absorbent that strips carbon dioxide out of the loop. Its capacity is finite, and it is consumed by time and workload.
• Oxygen and diluent cylinders: oxygen replaces what you metabolise; diluent, air or trimix, keeps the loop at volume on descent and dilutes the oxygen so it stays safe to breathe at depth.
• Oxygen cells: usually three sensors reading the partial pressure of oxygen in the loop. They are consumables with a service life, and validating them is part of the discipline.
• Controller and handset: reads the cells and injects oxygen to hold a chosen setpoint, with the diver monitoring and able to intervene manually.
• Bailout: an independent open-circuit supply, sized for getting you out from the worst point of the dive.

Why divers choose closed circuit

• Duration. Long bottom times at depth stop being a gas-supply problem.
• Better decompression. Holding an optimal oxygen partial pressure throughout, rather than accepting whatever a fixed mix gives you at that depth, means less decompression for the same dive.
• Warm, humid gas. The scrubber reaction returns gas warm and moist, which matters a great deal in British water on a long dive.
• Almost no bubbles. Quiet, and with a very low visual and acoustic signature. Useful for marine life and photography, useful for close survey work on a wreck, and the reason closed circuit is the military standard.

The last two are why our expeditions run on closed circuit. Deep wreck survey around the British coast means long, cold, working dives where the difference between open and closed circuit is the difference between a look and a proper job.

What closed circuit asks of you

A rebreather does not make deep diving easier. It removes one constraint and adds a set of new ones, and it deserves to be approached on those terms.

The three ways a loop can hurt you are hypoxia (too little oxygen), hyperoxia (too much, leading to oxygen toxicity) and hypercapnia (carbon dioxide breaking through an exhausted or badly packed scrubber). What they have in common is that the gas can feel entirely normal while any of them is developing. On open circuit, a gas problem usually announces itself. On closed circuit it often does not, which is why the answer is procedural rather than instinctive: a written checklist every time, a proper pre-breathe, cells you have validated rather than assumed, maintenance done to schedule, and a bailout plan you have actually thought through.

How to start

Most divers come to closed circuit after building solid open-circuit technical experience first, because the unit is easier to learn when buoyancy, trim and gas planning are already second nature. Our training pathway runs Intro To Tech, then Twinset Diver, then Decompression Procedures, before CCR. We run CCR conversion training, and trial dives on the SHARK can be arranged, so ask us.

The SHARK CCR is the unit we specified for our own diving: hybrid oxygen delivery, fully redundant electronics, an aerospace-grade aluminium frame, rated to 100 m, a CE rated scrubber duration of 200 to 250 minutes and up to 12 hours in military use. It is built to our specification in Belgium, certified to CE EN14143, serviced and supported here in the UK, and starts at £10,950.

Common questions

Is rebreather diving dangerous?

It is less forgiving than open circuit rather than inherently reckless. A rebreather can deliver a gas that feels perfectly breathable and is not, so the failures that matter are the ones you cannot taste or feel. That risk is managed the way it is managed in aviation: formal training, a written checklist before every dive, a pre-breathe, validated cells, disciplined maintenance, and a bailout supply you have genuinely planned to use.

How long can you stay down on a rebreather?

Loop duration is set by three things: the scrubber's remaining ability to absorb carbon dioxide, the oxygen supply, and your own decompression obligation. It is not governed by a single cylinder the way open circuit is, which is why gas planning on closed circuit looks nothing like gas planning on open circuit. The SHARK CCR has a CE rated scrubber duration of 200 to 250 minutes, and is worked up to 12 hours in military use.

What is the difference between CCR and SCR?

A closed circuit rebreather keeps the whole loop sealed and injects oxygen to hold a chosen partial pressure, so almost nothing is vented. A semi-closed rebreather continuously vents a proportion of the loop and replaces it from a premixed cylinder. SCR is mechanically simpler; CCR is far more gas-efficient and gives much better control of decompression.

Can a new diver learn on a rebreather?

Requirements vary by agency and by unit. Some recreational rebreather courses accept relatively inexperienced divers on shallow-depth units. The fully closed, mixed-gas units used for deep wreck work are a different proposition, and normally follow solid open-circuit technical experience.

How much does a rebreather cost?

The unit is only part of it. The SHARK CCR starts at £10,950, and beyond the unit you should budget for training, oxygen cells, scrubber absorbent, cylinders, servicing and a bailout setup.