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Their stealth and extended bottom times have made rebreathers popular with military and technical divers for many years. Recently, lower prices and "user-friendly" designs have made rebreathers more attractive to recreational divers like you and me. Indeed, several models are aimed specifically at the recreational market.

Is this the future of diving? Are conventional open-circuit rigs bound for the oblivion of duck fins and two-hose regulators? Are you ready for rebreather diving? Are rebreathers ready for you?

Maybe, no, maybe and maybe. At least that's my guess after spending most of a month studying and diving rebreathers. It turns out they have some very real, valuable advantages over open-circuit, tank-and-regulator systems. But they have some equally real and serious disadvantages too. If you have some unusual needs and are willing to make some sacrifices of time and money, a rebreather can be a godsend. But most divers, for most purposes, will continue to prefer open-circuit scuba for a long time to come.

Why You Might Want a Rebreather

Long dive times. The biggest advantage of a rebreather is gas efficiency. A single fill of a small gas cylinder or cylinders and CO2 scrubber can last for anywhere from one to six hours, depending on which rebreather it is. Unlike open-circuit scuba, your gas duration on a rebreather is nearly independent of depth, so you could, in theory, spend all that time on the bottom.

Of course, a rebreather does not make you immune to DCS and nitrogen narcosis. Those risks remain, though the more sophisticated closed-circuit rebreathers can adjust your gas mix to reduce the DCS risk. The advantage of the rebreather's long duration for most of us is that you can make several dives on one fill of scrubber and cylinders.

Silence. Rebreathers exhaust few or no bubbles. You don't hear that roar of exhaust bubbles, and neither do the fish. That allows you to get closer to marine life, which is why rebreathers are popular with professional photographers and some researchers. You won't be rendered invisible, but you seem to be less alarming to most fish.

Warm, moist breathing gas. The chemical reaction in the CO2 scrubber actually warms and humidifies your breathing gas. Diving with a rebreather does not give you that cotton-mouth feeling and doesn't chill you as much.

Optimum gas mixture. The more sophisticated rebreathers constantly monitor the partial pressure of oxygen in your breathing mix. They can keep your PPO2 constant regardless of depth or exertion, or alter it on the fly for needs like decompression. The benefit can be less nitrogen uptake and faster offgassing--in other words, more bottom time with less DCS risk. Rebreathers are not created equal, however, and the less-expensive designs do not have this ability.

Why You Might Want To Think Twice

A rebreather failure can go unnoticed. When open-circuit regulators fail, it's immediately obvious. Either you get no air when you suck on the mouthpiece or (more likely) you get too much and a sudden rush of bubbles in your face.

When a rebreather fails, the signs, if any, are more subtle. You're still able to inhale and exhale as before because you are just passing the same gas back and forth between your lungs and the breathing loop. The CO2 content in that gas may be rising and the O2 content may be falling, but this won't be immediately apparent without gauges, monitors and alarms. Rebreather diving is like flying on instruments, not by the "seat of your pants."

On the other hand, if you watch your instruments and detect the problem promptly, you'll probably have more time to deal with it on a rebreather than you would on open circuit. You still have gas to breathe, and its oxygen and CO2 content do not change instantaneously.

A rebreather failure can be deadly. A rebreather is constantly mixing the gas in your breathing loop, removing carbon dioxide and adding oxygen. Either component in the wrong proportion is poisonous. Much of the design effort and much of the complexity of rebreathers goes into making that mixing function as accurate and reliable as possible. But it's never going to reach the certainty of open circuit, where what you breathe is simply what went into the cylinder. An open-circuit "bailout" bottle and regulator is a good idea when diving with conventional gear, but it's a must with a rebreather.

Cost, weight, bulk, convenience, etc. These minor factors all weigh against rebreathers. Though the cost to buy one is hardly minor, you're often told that you'll save money on each dive since you don't have to refill tanks as often. But you do have to buy scrubber chemicals, and maintenance will cost more. And will that $60 two-tank dive boat give you a rebate if you don't need to use its tanks? Probably not.

Rebreathers, including the bailout bottle, are generally bulkier and heavier than a single tank and regulator, and they don't fit well into the tank-rack-and-bungee-cord gear station typical on dive boats. Air travel with cylinders can be a challenge and fills for oxygen and even nitrox can be harder to find, especially in remote locations.

The First Steps

Let's say you've considered the pros and cons and decided you want to dive with a rebreather. What's the drill?

First is making the decision of which rebreather to buy. Rebreathers differ considerably in not only price but capabilities, the great divide being whether they are closed-circuit or semiclosed-circuit in design.

Closed-circuit rebreathers have the lowest gas consumption, the best mixture control and, generally, the most capability, but are more complex and expensive. Semiclosed-circuit rebreathers are simple, robust and less expensive, with gas consumption rates somewhere between closed-circuit rebreathers and open-circuit scuba.

You might also consider such things as time and depth limits, backups to control devices and fail-safe mechanisms, warranties, how many units have already been in use and for how long, and more.

Then there's training. A prerequisite will be nitrox certification since rebreathers either use nitrox or, in effect, mix it on the fly. Following that, virtually every rebreather manufacturer will require you to take a rebreather training course lasting four or five days. The cost of this will be extra, normally at least $500. Part of the curriculum covers in-water skills like how to interpret gauges and monitors and how to switch to backup systems. Another part is training in assembly and disassembly, servicing and maintenance of your particular unit.

Because you will have to be trained by an instructor certified in your particular make and model, you may have to travel to another city and stay there for five days or so for your training course.

Meanwhile, you will start to assemble a special tool and spare parts kit. You'll probably want one or more spare oxygen monitors and various solenoids and sensors for the more complex units. Also gas analyzers and flow-rate test devices, depending on the unit. Add to that mouthpieces, batteries, O-rings, tie-wraps, silicone grease, etc.

A Day on the Water

Even more than open-circuit, a rebreather dive begins before you get wet and ends after you're dry. Predive and postdive care for the rebreather are essential every time and can't be skipped. Expect to spend an extra half-hour on each end of the dive.

Here's generally what to expect, keeping in mind that each rebreather is different and requires its own procedure.

Predive

• Fill cylinder(s). Most semiclosed-circuit rebreathers use a single cylinder of nitrox. You'll have to decide in advance which nitrox mix you'll use so the bypass valve orifice can be matched to it. Most closed-circuit rebreathers use two cylinders, of oxygen and a diluent (usually air, though other gases may be options). In either case, but especially when using nitrox, you will analyze the gas yourself to make certain what is in the cylinder. If your bailout system uses a separate cylinder, you may need to refill that also.

• Fill scrubber canister. Different rebreathers use slightly different types of CO2 absorbent and different granule sizes, but all of it looks a lot like cat litter. You buy it in large plastic jugs or buckets, which you must keep tightly sealed because exposure to air causes the absorbent to react and become used up.

You'll pour the absorbent into the canister, tapping the canister occasionally to make sure the absorbent settles and completely fills the canister. Because the absorbent dust is caustic, you should wear gloves and a breathing mask. Then you'll close and seal the canister and the absorbent bottle or bucket. Many divers do this job at home before the dive trip to minimize the mess.

• Assemble the rebreather. Here, semiclosed- and closed-circuit rebreathers differ considerably. In general, though, you will attach the counterlung (or lungs) to the absorbent canister and install them in the frame of the rebreather. You will test the one-way valves in the breathing hoses and attach them to the counterlungs. You will install the cylinder(s) and check their valves. All this can involve a dozen hose connections.

If you have an oxygen monitor and other electronics, you will test them. If you have a constant-flow semiclosed rebreather, you will need to check the flow rate of the orifice.

You will then test the entire unit for air leaks and water leaks. Leaks are potentially serious. You have so little gas on board that you can't afford to lose any. If water leaks cause the unit to flood, it will become extremely negative. And water in the CO2 scrubber causes a reaction with the absorbent known as a "caustic cocktail"--a nasty mouthful that can chemically burn your lips, mouth and throat.

As should be obvious, this assembly process is both complicated and critical. You need plenty of time and plenty of space to work, and you should use a checklist to prompt your memory. Don't expect to assemble your rebreather between the dive briefing and the "pool's open!" call.

Into the water. Waddling across the deck in a rebreather has been justly compared to carrying double tanks, but once you're in the water most of the difficulty ceases. Several differences to open-circuit diving will strike you immediately, though.

One is that you can't simply drop the mouthpiece into the water, because water would fill the breathing loop and the scrubber canister. There is a valve on the mouthpiece that you have to remember to close before you take it out of your mouth.

Another is that you can't affect your buoyancy by inhaling or exhaling, because the same amount of air just passes back and forth between your lungs and the rebreather and never changes volume or buoyancy. If you're used to exhaling to get below the surface, this won't work.

As you descend, increasing pressure will collapse the counterlungs just as it collapses BCs and dry suits. Some rebreathers automatically add more gas to the breathing loop, others require you to add it manually. When you suck on the mouthpiece and get no air, you push on a dry suit-type valve to reinflate the breathing loop.

You need to be stingy when inflating your BC or clearing your mask because gas used for that is gas lost from a much smaller total supply. For the same reason, you need to watch your gauges closely and you and your buddy need to be vigilant for air leaks.

If you work unusually hard--if you have to swim against a current, for example--your body will take oxygen out of the breathing loop faster than normal. Closed-circuit systems and passive semiclosed-circuit systems will sense this and add extra oxygen. Active semiclosed systems will not, however. In that case you must remember to "purge the breathing loop" by exhaling this oxygen-poor gas through your nose so the rebreather replaces it with richer gas.

At the beginning of an ascent on a semi-closed-circuit rebreather, you must also purge the loop to enrich your breathing mixture. Otherwise, as pressure drops, the partial pressure of oxygen may become too low. Closed- circuit systems add oxygen automatically.

Also, as you ascend and the counterlungs expand, the rebreather will vent gas. This is the only time the rebreather purposely dumps a significant amount of gas, and the reason that "sawtooth" profiles are especially wasteful. You will probably find that the rebreather does not vent gas fast enough and you become increasingly buoyant, so you'll need to manually dump from your BC, your dry suit or from the rebreather.

Postdive
If you are planning another dive that day and have enough gas and scrubber time left, all you need to do is turn the rebreather off during your surface interval. It's a good idea, though, to check the breathing loop for water inside.

If this is your last dive for a few days, you will need to disassemble and clean the rebreather thoroughly. The warm, moist environment inside the breathing loop is perfect for growing bacteria, so it must be disinfected with whatever solution the manufacturer recommends, then rinsed well and dried. Drying the inside of the breathing loop, with its baffles and corrugated hoses, can be very difficult.

If you are diving again tomorrow, however, you need only disinfect the mouthpiece and corrugated hoses.

The used CO2 absorbent must also be dumped and the scrubber canister must be thoroughly cleaned and dried. Electronics and oxygen sensors have their own care requirements. Plan on spending an hour on postdive maintenance at first, although you will get faster with experience.

Long-Term Maintenance

Each cylinder has a first and a second stage regulator which requires annual service. These are normally just ordinary open-circuit regulators that can be serviced by your local dive shop. Cylinders need to be hydro tested and visually inspected like any others.

Oxygen sensors have a life span and need to be replaced, usually every 12 to 24 months, depending on how much they are used. (They deplete themselves in air about half as fast as when diving.) After diving, some divers remove them from the rebreather and seal them to extend their life.

Computer controls have batteries that must be replaced occasionally.

Some manufacturers recommend that the whole unit receive a thorough inspection and overhaul every year.

Rebreathers: How They Work

All rebreathers are built around the principle of a one-way breathing loop. One hose takes your exhaled breath to the CO2 scrubber, and another brings it back (without the CO2) to your mouth.

On each side of the scrubber there is a counterlung, just a flexible bag that expands and contracts to accommodate the on/off nature of your breathing. The counterlung on the exhalation side usually has a relief valve to vent excess gas from the system. The counterlung on the inhalation side has an input valve where more oxygen or nitrox is added.

Add a mouthpiece with a valve to prevent flooding, a one-way valve in each breathing hose so your breath circulates the right way, and some other bits and pieces and you've got a basic rebreather.

How many gases are injected into the inhalation counterlung, and how the injection is controlled, determine whether it's semiclosed-circuit or closed-circuit.

Semiclosed-circuit rebreathers have the simplest gas control mechanism. Basically, it is just a fixed orifice, an opening that permits a constant flow rate into the breathing loop. Any excess above what your body consumes is vented to the water in a stream of small bubbles, which is why the system is called "semiclosed."

The simplest semiclosed-circuit rebreathers constantly add nitrox from a single cylinder. The Drager Dolphin and Ray are popular examples. They are called "mass flow" or "active" semiclosed-circuit rebreathers--active because the unit is always injecting fresh gas. The orifice, which controls the flow rate, must be selected before the dive to match the nitrox mix chosen. This type of rebreather is all on or all off: Whenever the cylinder valve is turned on, gas flows into the breathing loop at the rate determined by the orifice. Manual addition valves and some other plumbing may complicate the picture, but that's the essence.

"Passive" semiclosed-circuit rebreathers inject gas only on demand. Various mechanisms to trigger the gas injection may be used, but they are mechanical. For example, a system of ratchets and levers measures the volume of a counterlung, and when it gets below a certain size (because your body has removed that much oxygen from the breathing loop), it triggers a valve to inject more gas. Passive systems use less gas than active ones, but the actual content of the gas mix in the breathing loop may be more variable.

Somewhat more complicated self-mixing semiclosed-circuit rebreathers add oxygen and a diluent separately through fixed orifices or (in the case of the diluent) a demand valve. They also may use less gas, but may be subject to larger variations in the oxygen content of the gas mix.

Fully closed-circuit rebreathers aim to control exactly the oxygen content in your breathing gas. They add only the gas you need, when you need it, and don't waste any. Thus, no bubbles most of the time and a longer gas duration. This fine control of gas addition comes from some electronic wizardry. Normally, sensors analyze the oxygen content of the breathing loop and inform a computer, which adds oxygen or diluent as needed to maintain a preselected "set point" for the oxygen partial pressure. Redundancy (often three oxygen sensors and two computers) makes the wiring and plumbing diagrams confusing, but again the concept is fairly simple.

Are You Ready for a Rebreather?

You need to look not only within the rebreather but within yourself. Some personality types are more suited than others to the demands of using and caring for a rebreather. And some people probably shouldn't even consider it.

Are you comfortable with "nuts and bolts"? Rebreathers are more complex than open-circuit setups, and you will have to be self-sufficient for assembly, cleaning, maintenance and a lot of the repair, since the chances of your local dive shop having a specialist are slim. Even the simplest rebreather typically has all the parts of your open-circuit setup, plus a lot more. All those parts, and the connections between them (there are 50 or more O-rings in a typical rebreather), must have regular maintenance.

Are you self-disciplined? Predive, during the dive and postdive you have to make up your mind to follow procedures and checklists exactly. Filling the scrubber canister and assembling the breathing loop before the dive involve steps that must be followed precisely and tests that can't be skipped. The same care must be taken when disassembling and cleaning the rebreather after the dive. And during the dive you have to watch gauges more closely than on open circuit. Are you meticulous about maintenance of your open-circuit gear, for example, or do you "hose it and go"?

Do you accept responsibility for your safety? You have to take the attitude that the correct operation of your rebreather depends on you alone. The idea that the manufacturer, the instructor or someone else is responsible may be gratifying to your heirs but will not save your life in the event of a failure. Are you comfortable letting the boat crew or your dive buddy set up your open-circuit gear for you, or do you insist on doing it yourself?

Can you resist temptation? Rebreathers promote what might be called "mission creep." Many of them are capable of dives far beyond the training of most recreational divers. Units that can change the gas mix during the dive are especially suited for decompression diving. It's human nature to "see what this baby will do," but rebreather training is not technical training. You will learn how to operate the unit, but not the specific disciplines of tech diving like deep diving and cave penetration, for example. "Mission creep" can take the apparently innocuous form of adding other nontechnical but demanding equipment too soon. Using a complex camera rig can distract you from monitoring your rebreather carefully, for example.

Scuba diver image courtesy of Shutterstock.