By Ron Akeson

Several of the current models on the market today originally started off as home-built units then eventually progressed into commercially viable versions available to the public, such as the KISS Rebreather (Vancouver, BC in Canada). Others were specifically designed for the production line right from the beginning.

Why?
So why would you as a diver want to dive with a rebreather? Perhaps you are an underwater photographer or videographer and would like to get closer to your subjects, or just get rid of those nagging bubbles in your wide-angle shots. Maybe you would like to extend your bottom time for work or pleasure. Personally I got into rebreathers so I could spend more time underwater. It always seemed like a waste to literally take a whole day, with travel included, to do two dives totaling an hour and a half or so. Now I can do multiple two-hour dives and really see most of what a site has to offer.

Alan Studley, a KISS rebreather instructor comments; “Most of the people I teach how to use KISS Closed Circuit Rebreathers are interested in either exploration, or photography. They want to go places and see things they couldn't on open circuit. Plus they have the financial means and time to invest. Personally, I switched from open circuit so I could get closer to wildlife and stay there longer.”

Dave Hancock, owner of Ideations and creator of the Dive Alert signaling device, has similar feelings, “I switched to closed circuit rebreathers because I can take a week’s worth of diving gas on my small boat without having to find a fill station. My Megalodon rebreather allows me to go to the same dive sites that I’ve been going to for years and experience a totally new dive; but on the deeper side. Using a rebreather has revitalized my love for diving and I’ve been diving since 1969. Double tanks just do not have the appeal to me.”

Lower your gas usage
If you are a technical diver, a rebreather will significantly lower your gas usage and expense, and has the potential to extend your dives deeper or longer than tanks will currently allow. If something unexpected happens on the bottom, such as getting lost, you do not have the added stress of a limited gas supply to figure out the problem and start your ascent. Granted in that situation you better have contingency tables or computers that can handle your dive modification and a means to stay warm for the time needed, but these are all parts of good dive planning.

Rob Wilson, who regularly dives many of the deep shipwrecks in the Pacific Northwest adds, “I think rebreathers are a safer way to do exploration on the deep wrecks we like to dive here. Quite simply I feel my closed circuit rebreather gives me the flexibility and extended range open circuit units can't provide. I just like using the right tool for the job. It makes the logistics of a multiple day remote expedition much simpler too. You can dive a whole week with a trans-fill whip and a tank of oxygen and diluent.”

Silence
I have heard numerous scuba divers comment on how much they enjoy the silence of being underwater. After all, was it not Jacques Cousteau who thrilled us with the film Silent World? What most divers do not realize is that scuba is anything but silent. When I dive with my rebreather I can usually find my way back to an anchor line just by following the directional sound of the open circuit divers and their bubble noises! If I can hear it, you can bet the marine life can too. It’s no wonder most rebreather divers say they can approach marine life easier, such as hammerhead sharks, manta rays and wolf eels. Eliminate the bubbles and you have true silence.

On the first open water dive I did when learning to dive the Draeger Dolphin rebreather back in 1995, my instructor and I came upon a wolf eel lying out on the bottom not moving. As we approached I thought it might be dead so I gently touched its tail. I don’t know who jumped more, me or the wolf eel who jerked its head around only to see two divers staring at it. As it swam quickly away I felt bad upon the realization that it had just been sleeping and that we must have scared the crap out of it!

If you get cold easily while diving, a rebreather actually produces moisture and heat as a byproduct of the CO2 (carbon dioxide) scrubbing capability. This equates into breathing warm, moist gas which is great on a long and/or cold dive. On open circuit you lose a lot of your heat underwater by warming each breath up to body temperature, only to exhale it out into the water. You also become significantly more dehydrated throughout a dive when using open circuit because each breath is humidified in the lungs before it is exhaled. With a rebreather you no longer have that cottonmouth feeling and arguably have less chance of getting decompression illness since dehydration is a major predisposing factor.

So how do rebreathers work?
In simple terms they are very similar to a two hose regulator. They consist of the breathing loop, with the addition of an inhalation and exhalation counter lung, one or more tanks and a scrubber for removing the carbon dioxide out of the re-circulated air.

As a diver exhales into the rebreather mouthpiece the gas travels through a corrugated hose into the exhalation counter lung, mounted on the back of or over the divers shoulders. From there the gas travels through the scrubber canister, where by chemical reaction the carbon dioxide is removed and moisture and heat are released. Next the gas travels from the scrubber to the inhalation counter lung where it can be re-breathed by the diver through another corrugated hose attached to the other side of the mouthpiece, thus completing the loop.

To monitor the oxygen percentage of the gas the diver is breathing there are usually one to three oxygen sensors, but not all rebreathers have them.

Construction
Before we look at the types of rebreathers, let’s go into a little more detail on the counter lungs and scrubber. The counter lungs are similar in construction to a buoyancy vest or BCD, only smaller. Their position on the diver makes a big difference in how easy a unit breathes. But as in life, many things are a trade off.

Mount the counterlungs in the back and you can reduce a lot of bulk on the front of the diver, but breathing resistance will be increased in positions other than horizontal, which is the position most divers strive to achieve. In a vertical position it is much easier to inhale than exhale. This is because the air is less buoyant than the water and wants to flow up, while on the exhalation you have to push the gas down to the counterlung against the water pressure.

The other common type of counterlung is the ‘over-the-shoulder’, and although they breathe easier in all positions they are significantly bulkier hanging down over the diver’s chest. To illustrate ease of breathing with different counterlung styles, stand in front of a mirror and notice the distance between where the counterlung sits and where your mouth is. The closer the vertical distance is the less the breathing resistance will be. Now picture yourself in a horizontal position, then in a head down position; the same physics apply.

Scrubber canister
Another thing that has an affect on breathing resistance is the design of the scrubber canister. The two main types are axial and radial. In axial scrubbers the gas flows lengthwise through the media (material that produces the chemical reaction to remove the CO2). This produces a higher work of breathing but is less likely to have a breakthrough (when the material is used up and quits removing the CO2). The packing of an axial scrubber is also a little more forgiving than a radial. This is because if some settling occurs you still have a significant distance the gas still has to travel through, unlike radials.

In radial scrubbers the gas passes widthwise (picture a doughnut) from the middle outward. This increases the duration of the scrubbing material because each particle of scrubber is more completely used, unlike in an axial, and decreases the breathing resistance. The big downside of radial scrubbers is that if they are pushed beyond their rated time limit, or not packed properly, they can have a catastrophic breakthrough, unlike an axial which more gradually breaks through. As mentioned earlier packing is also more critical due to the fact the gas has less material that it comes into contact with so anything that affects the thickness of the material also significantly affects the duration.

One thing to keep in mind for any diver with aspirations of deep diving beyond 300 feet (90 meters), radial scrubbers are the recommended route to go. Most rebreather manufacturers do not rate their axial scrubbers beyond that depth due to the increased work of breathing from the increase in gas density. It is sort of like trying to drink a milkshake with a straw.

Types
Now that we have a basic understanding of the components, there are the four main types of rebreathers; oxygen, semi-closed, passive semi-closed, and closed circuit. The vast majority of rebreathers currently on the market are of the latter category.

Oxygen rebreathers
Oxygen rebreathers historically have been for military use in clandestine operations. They are very simple consisting of a counterlung, the breathing loop, a small oxygen tank, and a CO2 scrubber canister. While usually compact and chest mounted, the oxygen rebreather is not practical in recreational diving due to its 20-foot (6m) depth limitation, which is the depth pure oxygen becomes potentially toxic. There are currently no oxygen rebreathers commercially available.

Semi-closed circuit rebreather
The active Semi-closed circuit rebreather (SCR) consists of the breathing loop, inhalation and exhalation counterlungs, the scrubber canister, and gas supply. This type of rebreather then uses a preplanned Nitrox mixture which it bleeds into the breathing loop. The Draeger Dolphin is a good example. It has three stock orifices for setting the Enriched Air Nitrox; 40, 50, and 60 percent. Roughly 20% of each breath is vented to the surrounding environment. For decompression calculations divers can calculate their inspired oxygen percentage based on their estimated personal oxygen consumption, backed up with an oxygen gauge and sensor along with the flow rate of the gas. This number could then be plugged into any Nitrox computer. While the Draeger Dolphin is no longer in production (you can still find them used on eBay.

The KISS GEM rebreather is entering the market however, ‘GEM’ standing for Gas Extending Mechanism. The GEM can be used in conjunction with any scuba tank the diver currently has. The GEM mouthpiece has a special valve that vents roughly a third of each breath, allowing any tank to last three times as long as it would on open circuit. The scrubber canister attaches onto the tank and the counterlungs are chest mounted. KISS also has future plans for a mouthpiece that will exhaust 20% of the gas (tank will last five times longer) allowing divers to go deeper and/or stay longer.

Passive addition
A subset of the semi-closed rebreather is the passive addition (PASCR). Rather than gas flowing continuously into the loop as in active systems, passive rebreathers are keyed into a divers breathing rate thus minimizing gas wastage. A percentage of each exhalation is exhausted into the environment and gas is only added when the diver bottoms out the counterlung on inhalation.

The Halcyon RB80 unit is a good example of a passive addition semi-closed rebreather (80 because it is roughly the same size as an 80 cubic foot tank). Halcyon quotes a gas savings of eight times more than open circuit, which is significant for longer or deeper dives. This type of rebreather is a favorite among Hogarthian divers (those who believe that you make things as simple as possible, eliminate unnecessary items, and thereby minimize as many failure points as you can) because there are no electronics to fail and it has similarities to open circuit diving. Decompression calculations also differ very little from open circuit.

Semi-closed rebreathers, whether SCR or PASCR, all have in common that they utilize axial scrubbers and if they have electronics it is only for confirmation of the oxygen partial pressure (PPO2) for decompression calculations

CCR
Closed circuit rebreathers (CCR) as the name implies, have a fully closed loop. A single cylinder is replaced by two, the diluent or bottom gas, and pure oxygen. The diluent is the main breathing gas while electronics monitor the partial pressure of oxygen in the loop and replace any that is metabolized by the diver. The only gas that is vented from the loop is during ascents or if a diver’s technique is sloppy.

They also differ significantly from active SCR’s because they do not have a continuous flow of gas, only that which is needed to maintain the loop volume or oxygen partial pressure. Along with the electronics there are three oxygen sensors for redundancy, and a solenoid triggered by the electronics to add the oxygen. This is what the majority of models available on the market today are and what I personally dive.

While I started out diving an SCR, I upgraded to a CCR in 2001 when the opportunity arose. Back then if you wanted a new CCR, not a rebuilt military one, you either ordered the Inspiration and had it shipped from England (which I did) or you mortgaged your house and bought a Cis-Lunar for around $20,000 US plus training.

No regrets
I am currently on my second CCR, a Megalodon, and have never regretted my decision. As a technical instructor trainer, I do enough deep trimix diving that my CCR pays for itself in gas savings about every three years. As my diving progresses into the 400-foot (121 m) range, I don’t have to get larger tanks or change my gear, just dress up warmer for longer dives.

To illustrate the point, an aluminum 80 cubic foot scuba tank would last the average diver just minutes at 330 feet (100m), while the same tank filled with oxygen and connected to the CCR would last closer to 24 hours! Of course the scrubber would not last that long and you would get hungry or need to pee, but it just goes to show how efficient closed circuit rebreathers are.

I choose the Megalodon because I now have the option of using an axial or radial scrubber, depending on how deep I want to go and it is also made locally so service is quick.

Slow flow
A deviation on the CCR theme is to replace the solenoid with an orifice that allows a slow flow of oxygen into the breathing loop. As long as the amount flowing into the loop is less than the diver is metabolizing, only a small amount needs to be manually added periodically to make up the difference. Both KISS and the ISC Copis (Constant Oxygen Pressure Injection System) rebreathers are good examples of this.

Fail safe
The belief here; as in PASCR units is if you eliminate the solenoid, you have one less failure point. Since the solenoid is what allows the addition of a specific amount of oxygen determined by the electronics, if it fails it can have catastrophic consequences. While the solenoid can fail open or closed, if it fails closed oxygen will no longer be automatically added to the loop and the diver can potentially become hypoxic. If the solenoid fails open, the uncontrolled addition of oxygen in the loop can cause hyperoxia. This is one of the many reasons a rebreather diver needs to be alert and monitor their unit at all times.

issues
One issue with a constant oxygen injection CCR is the fact that the oxygen first stage is unbalanced. This means that unlike normal scuba first stages which increase flow with increased depth, the unbalanced first stage supplies the same flow of oxygen no matter the depth (a good thing). If you go deep enough, usually in the 240 foot (72 m) range, when the ambient pressure equals the intermediate pressure of the first stage, oxygen stops flowing and the rebreather becomes a manual CCR. The other possible consideration is that you need to continually monitor your PPO2 and have a hand free for manual oxygen addition, not usually a concern unless you are managing a scooter in heavy current for example.

Efficiency
From a decompression standpoint, the CCR is the most efficient rebreather because of the constant partial pressure of oxygen, followed by the PASCR, and lastly the SCR system. In simple terms this equates to less decompression time for a CCR than SCR on a given dive.

So what is the difference between the individual units and which one should you choose? That is a very good question and one that does not have an easy answer. Hopefully by now you have a good handle on the basic features that are available. The rebreather you choose should match the type of diving you do and it should offer the features that you feel are important.

Local instructor
Another thing to consider is whether there is a local instructor that can train you on the unit you select. An additional cost may incur if you need to travel. Spend some time on each of the manufacturer’s websites and see what they have to offer. Hopefully you can talk to other rebreather divers and get their opinions as well. Keep in mind however, if you asked ten different people what car you should buy you might very well get ten different answers; rebreathers and dive gear are no different.

Major expense
I am in no way encouraging divers to run out and buy a rebreather. The buying process should be handled as with any other major expense, with a lot of research and questions. They are expensive, require extensive training, and create another level of complexity underwater. The rebreather that is best for you is the one that fits your type of diving and your pocketbook, similar to buying a new car. But if you find yourself wanting to investigate more about any of the above points, a rebreather might be in your future.

For more information and details about rebreathers and their individual applications, I have provided a list of current rebreather manufacturers, many of which have manuals online.

http://www.customrebreathers.com (Megalodon, COPIS, Pathfinder, and Predator)
http://www.diverite.com (Optima)
http://www.divenautilus.com (Nautilus)
http://www.halcyon.net (RB80)
http://www.hollisgear.com (Prism)
http://www.kissrebreathers.com (Kiss and GEM)
http://www.ouroboros.technologyindepth.com (Ouroboros)
http://mkvi.poseidon.com (Discovery MK VI)
http://www.rebreather.us (Hammerhead)
http://rebreathers.com.au (Abyss and Stingray)
http://www.revo-rebreathers.com (Revo)
http://www.silentdiving.com (Inspiration and EVO)
http://www.technologyindepth.com (Sentinel)
http://www.titandivegear.com (Titan)

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Peter Symes, Red Sea approx anno 2000 -