This guide is designed to explain how to choose a rebreather for scuba diving. It helps you to understand the basic features you might need depending on the type of diving your do.
However you have to be trained by a qualified rebreather diving Instructor in order to use or even buy a Closed Circuit (CCR) or a Semi-closed Circuit Rebreather (SCR).

1st question: do you really need a rebreather?

It might sound like a strange question but it’s an important one. A closer look at the Internet forums and mailing lists and you’ll discover a lot of people selling their expensive and brand new rebreathers, the main reason being that most of the time, they just don’t use it! Yes you read it properly: after months of research and comparisons, after a financial investment the size of a small car, divers discover that they don’t need it, it’s too much hassle, they don’t have time or they don’t like it.

So better to think beforehand about cost versus benefits for you and the dives you do. It depends on:

-    The type of diving you do. A rebreather can definitely make sense for a diver who spends his/her time doing deep wreck dives or long video dives. But does it make sense for short and shallow reef dives in a tropical environment? It’s a question that only you can answer, but think about all the hassle to travel with a rebreather (even a small one!), filling Nitrox or Oxygen tanks, buying Sodalime, preparing the unit before the dive, cleaning it afterwards. All that work for something that can easily be done with a single tank you rent anywhere!

-    The number of dives you do. Consider the initial investment for a rebreather, plus the maintenance (batteries, O2 sensors, yearly regulator maintenance). Then divide this number by the number of dives you do in three years (that’s the average time people own a specific rebreather). Then add the additional expenses for each rebreather dive (sodalime, tank fill, etc). If you end up with a cost per dive very close to your monthly salary, please consider diving open circuit.

-    Your complacency level. It’s a proven fact: rebreathers need to be taken care of. And they are jealous. After some dives on open circuit, don’t expect to come back to your rebreather and find it working flawlessly. Chances are that something will not work properly. You need to dedicate a lot of time to your rebreather, for maintenance, pre-dive and post-dive checks and routine skills underwater. But if you spend more time to brush your teeth in the morning than to prepare, check and properly pack your diving equipment, you’ll have to change your habits if you have a rebreather. And if post-diving maintenance simply means having a nap after the dive, then rebreather diving is not for you.

 

2nd question: What type of rebreather do you need?

There are as many rebreathers on the market as there are three-syllable words in a Rambo movie.
And all these rebreathers are either Closed or Semi-closed circuit rebreathers. Since a few years now, the public seems to prefer closed-circuit rebreathers (CCR) to Semi-closed ones (SCR), mainly because their respective prices tend to be closer than in the past, unlike their respective performance. Purely mechanical, SCRs loose some gas during every exhalation and rely on a constant percentage of oxygen, something that is not as ideal as CCR on a decompression standpoint. Therefore you find less and less SCRs still being built.
So the choice is more on the CCR side. Apart from some very specific applications, pure Oxygen CCRs are not really used in sort diving because of their very shallow depth limit. So the two main types of CCRs left are the mCCRs (Manual) and the eCCRs (Electronics), the main difference being the way oxygen is injected in the breathing loop:

-    mCCR. Most of mCCRs rely on a very small constant O2 leak that keeps the loop content safe to breathe. The user has to frequently check the loop content (through a handset showing the pO2 reading of generally three Oxygen sensors) and manually injects more oxygen if necessary. These rebreathers are simple and reliable but the diver has to carefully monitor his/her oxygen level.

-    eCCR. eCCRs are more complex units using a computer (or multiple computers) to constantly monitor the Oxygen level in the loop in order to electronically maintain a pre-determined pO2 (the “setpoint”).
The choice for you depends on your bank account (eCCRs are more expensive than mCCRs), the type of dives you do (eCCRs are usually less depth-limited than mCCRs) and your personal level of trust in electronics underwater.

3rd question: What features should you look for in a CCR?

That’s where we start to become more technical. So if you don’t want to spend your nights reading technical brochures, graphics, curves and test results, let’s keep it simple.
 
-    Counterlungs: The bags where you’re breathing from determine to a certain extend, the way you’ll breathe underwater, in different positions. Some rebreathers have these bags back-mounted giving a nice chest-free configuration. Other have them “over-the-shoulders” for a better work of breathing but a more cumbered chest. Some manufacturers give you the option to choose the size of the counterlungs: too small and you’ll fight to get some gas, too large and you’ll have a lot of unnecessary drag.

-    Scrubber: One of the most important of the unit, the scrubber holds some Sodalime absorbing the CO2 and cleaning up the gas you’ll breathe. You can choose between different types (granules or solid state cartridges), different architectures (axial or radial for a longer duration) and different sizes. Remember that in this case, size matters, as it gives you more time underwater, or more reserve for the same duration.

-    Electronics: That’s where the changes were the most impressive in the last few years. SCRs and mCCRs usually display simple pO2 readings. eCCRs can have much more advanced displays with redundant pO2 and setpoint reading, built-in decompression softwares with OC bailout capabilities, battery level, O2 sensors voltage and even scrubber monitoring. Just make sure you can easily read the most important information!

-    Gas Supply: all CCRs have a small tank of Oxygen and a small tank of air (or other diluent). Some manufacturers give the possibility to use different tank sizes, depending of what you find locally. Other manufacturers chose the “hard-case” way with just enough room in it for a specific size and shape of cylinders. Again here, size matters, even for rebreather tanks, as these tanks are also been used for BCD (and eventually dry-suit) inflation purpose. And even if rebreather divers always carry an off-board bailout tank, the on-board tank might also be used in bailout emergency. In this case, the more gas, the better!

4th question: Is there any other additional features you might want?

-    HUD: Most of the rebreathers on the market come some forms of safety devices like visual and audible alarms. However not all come with a Head-up Display (HUD), a very nice way to monitor your loop content without using your hands, something very handy when you take pictures, hold on a shotline in a strong current, or play cards during a long decompression stop.

-    BOV: A Bail-Out Valve is a rebreather mouthpiece combined with an OC 2nd stage. Even if you carry a bailout tank with an extra 2nd stage, this regulator might not be available when you need it (dangling, entangled or … used by your buddy). The BOV gives you the ability to immediately switch to Open Circuit if something goes wrong with your rebreather and your breathing loop is full of un-breathable mix … or water. A highly desirable features, as some divers have reported that some symptoms of intoxication were their inability to think properly and to quickly locate their alternate air source.

-    Off-board plug-in: When it comes to gas supply depletion, the more options you have the better. Some rebreathers allow you to plug in the loop any gas tank you might find, as long as it has a standard LP hose fitted. This gives you some flexibility when dealing with what shouldn’t be an emergency with a rebreather.

5th question: Is there anything else to consider buying?

Air, Oxygen and Sodalime are the most obvious things to consider for every rebreather dive. Sodalime comes in different brands, sizes and flavours. Air and oxygen don’t.
But you’ll also need a bailout tank with a regulator fitted. No need for a very big tank, unless you dive deep. One of the most popular bailout tank for recreational rebreather divers is the Aluminium 40cuft (5.5L) but it all depends on your OC gas consumption. You need enough gas to safely ascend from your maximum depth and complete all your decompression, and still have some extra gas left in the tank (at least one third).
You’ll have to change the batteries from time to time, as solar panels don’t work fine underwater. The same applies to the oxygen sensors that need to be changed at least every 18 months.

Also consider some additional accessories like a neck strap or a Full-Face Mask that help to prevent you to drown if you become unconscious underwater. It’s not something you plan to do every day but you don’t plan for a car crash but you still buckle up your safety belt every time you take your car, don’t you?
A rebreather-friendly dive computer can also give you the opportunity to take full advantage of the optimal mix in your breathing loop. Several dive computers on the market have a constant pO2 capability. Some even have the option to plug in an additional oxygen sensor for real-time pO2 calculation.
And maybe the most important purchase that goes hand in hand with your rebreather: a proper training course. There is no safe rebreather diving without implementing good habits first. Rebreather diving is like horse riding. It can be a lot of fun if you do it properly but it can also be dangerous. A rebreather is like a horse, having a life on its own. You have to stay in charge at all time, to avoid to be exposed to potential risks that you need to be aware of.

This guide was written by Cedric Verdier, a PADI Course Director and a Trimix Instructor Trainer for IANTD, PSAI, ANDI, DSAT and TDI.  Cedric spends most of his time teaching Cave and Mixed-Gas Rebreather courses at the diver and the instructor level.  He has written 9 books and more than 150 articles about diving. He is the current chairman of DIRrebreather, an organization implementing safe standards and procedures into rebreather diving.