Ok, so you say, what is a rebreather and do I need one?

Well, the answer is a bit complex. This is a good place to start, also check the More Stuff and Links.htm page for other authors information, the web is a great learning tool, but cannot substitute for a good instructor who can point to what you are doing right or wrong before catastrophe strikes. Contact me for more info, or if you want to get certified for the Drager Dolphin RB. You must already be a certified diver, with an additional cert in Nitrox or EANx. Enriched air, or Safeair, I can provide this education also.

First, what is a rebreather?

Lets start with what you already know, Open circuit diving, normal everyday first and second stage regulators. The first stage drops, or regulates, the high pressure 3000 psi or 200 bar tank air down to about 150 psi or 10 bar above the ambient pressure (the pressure of the air or water around you). You take a breath, inhaling 21% oxygen and 79% nitrogen (if using air), you exhale a breath that contains 16% oxygen, nitrogen and carbon dioxide (the body’s waste product from combustion), metabolizing only about 5% of the gas you inhaled, the rest is wasted, sent up as bubbles to break, noisily, on the surface, scaring the fish and marking your position. You have also given away some of your body’s heat and hydration, which exist in finite amounts, more on that later.

OK, you know what you are giving away, how do we fix that? Simple, by recirculating that exhaled breath back to you, removing the CO2 (carbon dioxide), adding back the O2 (oxygen) you used, now you can breathe that air again. That’s it in a nutshell, however the mechanics of it are what the science of RB’s is all about.

Lets look at the loop, what we call the parts of the RB that contain the air we breathe. First the mouth piece, it is fitted with hoses on both sides, that lead back to the RB unit. (I’m going to assume Drager flow patterns here, other RBs can and do plumb differently) Following the air flow as the diver exhales gas low in oxygen, high in CO2 the gas travels out the mouth piece through a one way valve and over the divers RIGHT shoulder into the RB housing, there it goes into the EXHALE counter lung, which is just a black bag with some fittings on it. This bag is designed to trap moisture, should any get in from loose lips, or excess water vapor that has condensed from your breath. It also has an over pressure relief valve on it, that is the black valve you see in the center, on top of a drager dolphin or Atlantis RB, this valve is designed to vent excess pressure that has built up in the loop and works just like a dry suit exhaust valve, spring loaded and adjustable.

The gas we exhaled then travels into the scrubber canister, which has another water trap, the reason for the water traps is inside the scrubber canister. We have packed our scrubber canister with soda lime (divesorb, sofnolime etc) which doesn’t like water in large quantities, see the paragraph on caustic cocktails. The soda lime is a chemical that absorbs CO2 and in the process produces heat and a bit of water vapor and slowly changes itself into chalk, not something you can take to school, but chemically it is the same stuff.

So now we have scrubbed out the waste product and are ready to add fresh gas. In a Semi-Closed RB (like the Dolphin/Atlantis, Azimuth, Halcyon etc) fresh gas is added at a mostly continuous rate, in the D/A or Azimuth, gas is added via a constant mass flow orifice, in the Halcyon gas is added keyed to the divers breathing. In any of these SCR’s (semi closed RB’s) the gas added is usually a blend of nitrox, higher in oxygen than air and at a flow rate designed to always have a high enough percentage of oxygen in the loop to keep the diver happy (read alive). So what’s flowed in might be 50% oxygen, but because it is being diluted by our breathing the oxygen out of it the actual percentage of oxygen in the loop might be 10-20% lower than that. There is a formula to figure this out, based on your work load, flow rate and supply gas, that will be in your class book and your instructor will explain how it works and how to apply it to a particular dive plan. The gas is flowed in faster than we can use it, so some the loop volume will flow out of the exhaust OPV (remember the OPV that is in the Exhale CL?). This makes the SCR’s more efficient that OC but they still waste gas, allowing a bit of every breath to escape from the OPV. On the other hand they are much simpler to learn and quite a bit more fool proof than fully closed RB’s (CCR).

In the drager dolphin, the gas coming out of the scrubber then flows into the INHALE CL. Fitted to the inhale CL is the dosage device, it has two functions, one, it feeds gas in through an orifice designed to flow a precise amount per minute. The dolphin has several orifices that you can use, depending on the nitrox blend you have selected, the orifices are designated by the gas that Drager recommends you use. There are 60, 50, 40 and 32% orifices that are available to use. The higher percentage orifices flow smaller amounts of gas per minute. The other function of the dosage device is to keep the breathing loop full of gas as you descend, it acts like a second stage regulator feeding gas to the inhale CL should you try to inhale against an empty loop. What this means in practice is as you descend, the volume of the breathing loop is compressed by the increasing water pressure, just like your BC does and during your descent the volume compresses to the point that you cannot inhale a full breath, at that point, the dosage device is activated by your inhalation, just like the second stage gives you a breath when you inhale, filling the loop and your lungs, keeping the diver happy. A happy diver is one who has a full breath, right?

So now, our gas flow has had its CO2 scrubbed out, vented the excess, had fresh oxygen rich gas added to it and is now ready to get sent back to the diver through the inhale hose and another one way valve into the mouth piece. See how this works? Pretty simple really.

Some options (personally I wouldn’t dive any RB without some form of O2 monitoring) for the Drager include an oxygen monitoring device called an Oxygauge, it is simply a fuel cell placed into the inhale CL that produces electricity in small amounts in the presence of oxygen, the more molecules of oxygen present, the more voltage the cell produces. The display is simply a readout of voltage, mathematically corrected to read in partial pressure, remember your nitrox class? We can survive partial pressures of oxygen between .16 and 1.6 with the best results around 1.0-1.4. Another neat option is the Uwatec Oxy2 and Air Z O2 dive computer system. The Oxy2 fits in the loop between the inhale CL and the hose returning to the diver over the divers left shoulder. It monitors the partial pressure of O2 and transmits it to the AirZO2 which the diver wears on his/her wrist which calculates your dive information based on what you are breathing. Pretty nifty system and well worth the extra cash involved.

Now about that caustic cocktail. Soda lime, when exposed to copious amounts of water, especially sea water, tends to become slightly corrosive and has a metallic taste that is not at all pleasant to have sloshing around in your mouth. It is important to keep your breathing loop as dry as possible, small amounts of water that get in are trapped by the design of the system and are kept on the exhale side of the scrubber, water vapor that is part of your breath make it through the scrubber with no problem and the soda lime’s reaction to the CO2 produce small amounts of water too, this is no problem. In fact in over 100 hours of bottom time on my RB, I have had only one very mild CC, I closed my loop, at the mouth piece, tilted the inhale side up to allow the mixture to travel from the inhale side to the exhale side and then rolled to dump the mixture into the exhale CL trap, reopened the loop and continued the dive, no problem. If you get a major flood, its time to switch to your open circuit bailout and end the dive.

In a fully closed RB,(CCR) you have two gas feed systems, one is only to provide volume to the loop, we call the gas the diluent pronounced dil’-you-lent, and it should have enough oxygen to breathe it directly at the maximum depth you plan to be at, it is either added via pushbutton valve, or added automatically via an ADV (automatic diluent valve, go figure) just like the dolphin does when you bottom out the loop. The other gas system is an oxygen injection system and there are several ways to get O2 into your loop. Some RB’s, like the KISS and my dolphin conversion, add O2 continually via a small orifice, plumbed between a tank full of pure O2 and the breathing loop. The flow rate is controlled by the intermediate pressure of the regulator on the O2 tank combined with the size of the orifice. The flow rate is set by the diver to match his/her metabolic oxygen use rate. Other more sophisticated CCR’s use a system of oxygen analyzers, voting logic, and a solenoid to inject the oxygen into the loop, these systems can also be controlled manually if the electronics fail. RB’s of this type are the Megaladon, Inspiration, CCR 2000 and the Cis-Lunar, just to name a few. All types of CCR’s must have some form of Oxygen Partial Pressure monitoring, if for example, the O2 flow was to fail, the diver must be made aware of it to be able to either fix the problem or end the dive. Hypoxia can sneak up on a diver with absolutely no warning.

OK, so now you may have a clue as to what RB’s are all about, so why would you want one, and is it right for you?

One of the main advantages of RB’s is Efficiency, a SCR is at least 5 times more efficient than OC, and a CCR is at least 10 times, and even more efficient as you dive deeper because your need for oxygen doesn’t change as you go deeper, the CCR only provides the O2 that you metabolize. I can dive for an hour, at any depth, and only use 2-3 cubic feet of oxygen, compare that to your aluminum 80.

You also stay warmer diving a RB. OC sucks heat out of you with every breath, you breathe in cold air, warm it with you lungs and then blow it out into the ocean, with a RB your warm breath comes back around again and again, and the chemical reactions in the scrubber canister are providing more heat too.

OC divers come back from a dive with dry mouths, along with the heat you exhale goes moisture, not so with RB, warm moist breaths are the rule here.

NOISE, NOISE and more noise. You can never appreciate just how nice a quiet dive is until all you hear are the light pops of the one way valves in your mouth piece working until you try RB diving. I can hear OC divers from hundreds of feet away, imagine what the fish hear. There is nothing like having a school of fish completely surround you, not even noticing your presence. I dive occasionally in a place called Clear Lake in central Oregon. It is a mountain lake with crystal clear water that is very cold. I never knew there were fish in that lake until I dove it on a rebreather, the first time with the RB, I saw a huge school of trout milling around the entry, that is until my buddy on OC got into the water, gone, like a shot, the whole school disappeared. Was it his lack of deodorant? I don’t think so. Seals blow air bubbles as a sign of aggression, do you think they will hang around a group of aggressive bubble blowing divers, usually not. I have played with more seals since getting my RB than ever before on OC. If you are into underwater photography or video, consider a RB, you will get shots you never thought possible.

Now for the down side. Rebreathers are EXPENSIVE. The entry level is around $3500usd for a basic SCR with no Oxygen monitoring, your not going to dive a RB without O2 monitoring are you??? A Fully closed KISS RB will run you $4300 plus tanks, BC, Regs. You can have a Megaladon Electronically controlled CCR for about $6500 complete and an Inspiration is around $7500. Did we mention training? This is not the place to go bargain hunting, you are playing with your life here and you need to do your homework. Find the best instructor you can, ask around, get some references from some students he has taught, contact your instructors sponsoring agency for any complaints. Does your instructor regularly dive the unit you want to be trained on? Get your course syllabus in writing, how many dives, how much classroom time, is scrubber included, can you get dates scheduled ahead of time, not just at the instructors “convenience”? What if you need more dive time than the average person? Are books included? Don’t be afraid to ask questions, a good instructor will have answers for you or will get them. Expect to pay 300-500 for training on a SCR and 750-1000 for a CCR. Training should include, theory, math, physics, operation, assembly, cleaning, maintenance, safety, bailout techniques, and actual diving in open water and maybe pool time.

More down side. Rebreathers are not for every diver. If you are the kind of diver who runs out of air frequently, or doesn’t pay close attention to your diving, forgets equipment, gets distracted easily, doesn’t maintain your gear after every dive or weekend, leaves things lying about to get stepped on, makes every purchase with only price in mind and is just generally absent minded or careless, DON’T EVEN THINK ABOUT IT!!!!! You can kill yourself easier on a rebreather than by putting a plastic bag on your head, probably faster too. This is not a hobby that you can dabble at, it is not for the twice a year diver. If you are a serious diver, that wants to extend your bottom time, play with the fishies closer and more intimately than ever before, and don’t mind the extra attention, training, cost and maintenance issues that come with diving a rebreather, then come on in, the water is quiet.