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What to think about before buying a rebreather

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What to think about before buying a rebreather

Thu, 13/10/2011 - 23:21
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One morning, sooner or later, you will wake up with a strange sensation—as if some minor and weird changes happened in your body overnight. You are not turning into another Dr Jekyll and Mr Hyde, but in the recent weeks you have co me to find your dives to be very noisy with all the bubbles escaping from your regulator starting to get on your nerves. All your dive gear has also started to feel very heavy, with all these tanks and regulators to carry everywhere.

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And then you have this fancy and expensive dive computer full of features you cannot use because they are designed for divers with little in common with you, using state-of-the-art and highly complex equipment on their back and breathing esoteric mixes coming from remote planets.

So, after reading magazines, walking your way along many booths in the dive shows, surfing the net looking for rebreather-related websites, you have finally arrived at an important threshold in your life. You are ready to buy a rebreather. You have made up your mind, checked that the mortgage for your nuclear bunker is in place and you won’t have to replace your two-year-old pink Jacuzzi anytime soon. What do you do then? Asking yourself the following questions is a good place to start:

Do I really need a rebreather?

It might sound like a strange question, but it is an important one nonetheless.

Take a peek at the internet forums and mailing lists and you’ll discover a lot of people selling their expensive and brand new rebreathers. Why? The reason stated most of the time is that the present owner just doesn’t use it! Yes, you read it correctly. After months of research and comparisons of products, prices and offers, and after making a financial investment equal to the one needed for buying a small car, a lot of divers discover that they don’t really need it, or it’s too much of a hassle—they don’t have time, or they don’t like it. So, it is better to think twice beforehand, weighing cost against benefits, taking into consideration what types and frequency of the dives you usually do or plan to do. Points to consider should include:

The type of diving you do.

Using a rebreather can make a lot of sense for a diver who spends a lot of time doing deep wreck dives or long video dives. But does it also make sense for short and shallow reef dives in a tropical environment? Only you can answer that question. But consider the hassle of travelling with a rebreather (even a small one!), filling nitrox or oxygen tanks, buying scrubber, preparing the unit before the dive, cleaning it afterwards and what not. All that work for something that can possibly be done just as easily with a single tank you can 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 the span of three years, which is the average time people own a specific rebreather. Then add the additional expenses for each rebreather dive (scrubber, tank fills, etc). If you end up with a cost per dive very close to your monthly salary, it might be wise to consider sticking with your current open diving open circuit.

Your complacency level

There is no way around it—rebreathers need proper care. And they can be temperamental pieces of equipment. 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 be working properly as it was before. It is necessary to dedicate some time to your rebreather— for maintenance, pre-dive and post-dive checks and routine skills underwater. So, if you spend more time brushing your teeth in the morning than preparing, checking and properly packing your diving equipment, you’ll have to change your habits if you want to dive with a rebreather. If post-diving maintenance means having a nap after the dive to you, then rebreather diving is not for you.

What type of rebreather do I need?

There are about as many rebreather models on the market now as three-syllable words in a Rambo movie. These rebreathers are either semi-closed rebreathers (SCR) or fully closed circuit (CCR) rebreathers. In the recent years, the favor of the public has gravitated towards the fully closed-circuit rebreathers. Firstly, the once significant price gap between the SCRs and CCRs full bred units have narrowed over the years. Secondly, the performance and accessibility of the CCRs have kept improving.
 Purely mechanical SCRs constantly loose gas by every exhalation, and they work by providing a constant percentage of oxygen—something that is not as ideal as CCR from a decompression standpoint. Therefore, the current trend is still that fewer SCRs are being built, with CCRs clearly being the preferred choice overall.
 Aside from some very specific applications, pure oxygen CCRs are not used in sport diving because their use is limited to a maximum depth of only six meters. (Any deeper than that, the partial pressure of the pure O2 reaches toxic levels). That leaves two main types of CCRs to share the scene: mCCRs (manually operated CCR) and eCCRs (electronically controlled CCR). The main difference lies in the manner by which oxygen is injected in the breathing loop:

mCCR

Most of mCCRs rely on a tiny constant feed of O2 into the loop making the content safe to breathe. The user has to frequently check the loop content on a display showing the pO2 readings from a set of (most often three) oxygen sensors and manually inject more oxygen if necessary to keep oxygen content at the correct levels. These rebreathers are simple and reliable but require the diver to constantly keep a close eye on the oxygen monitors.

eCCR

eCCRs are more complex units using a computer (sometimes several) to constantly monitor the oxygen level in the breathing loop and electronically maintain a pre-determined constant pO2. This level is known as a “setpoint”.
What type you choose may in part depend on the contents of your piggy bank, as eCCRs are more expensive than mCCRs. Also, the type of dives you do means something, as eCCRs are usually less depth-limited than mCCRs. It may also be a matter of which degree you are comfortable with relying on electronics under water.

This is where we get technical. So, to spare you long nights of ploughing through heaps of technical brochures, graphics, curves and test results, let’s get right to point.

Counterlungs

These are the ‘bags’ on the breathing loop where your exhalation goes. The shape and position of these determine some of the breathing characteristics and how easily you breathe in different positions underwater. Some rebreathers have the counterlungs back-mounted, giving a nice chest-free configuration. This configuration aids exhalation but makes inhalation harder. The opposite is true for chest-mounted counter-lungs.
 Others have them configured “over-the-shoulders” for a better work of breathing, but a more encumbered chest. In any case, the counterlungs should be positioned as close to your real lungs as possible to minimize work of breathing (See info box). Some manufacturers also provide the option of different sizes of counterlungs. Size is another another trade-off, which has to be balanced. Too small and you may find yourself in a struggle to get enough gas, too large and they create a lot of unnecessary drag.

Scrubber

One of the most important components of the unit, the scrubber canister holds the absorbent (i.e. Sodalime) which absorbs the exhaled CO2 cleaning up the gas you’ll breathe. You can choose between granular absorbent that you pour into the canister yourself or solid state cartridges, which you just slide in as a whole package.

There are also different scrubber architectures—axial or radial. An axial scrubber is essentially a wide cylindrical pipe through which gas flows from one to the other in a ‘vertical direction’. In a radial scrubber, gas flows in a radial direction from a central pipe at the hub and towards the side of the canister.

Radial has a longer duration and is generally considered to offer a lower breathing resistance. Scrubbers, too, come in different sizes. A bigger scrubber gives you more time underwater, or more reserve for the same duration.

Duration?

Probably one of your least concerns. Chances are you will be starving long before your scrubber expires. Unless you are a technical diver planning very long dives, most scrubbers will outlast any dive you can endure before you crave the next meal or need to go to the bathroom. It is mostly a matter of convenience and how often you need to change. A typical scrubber will last you 4-6 hours depending on the size of the canister and the temperature of the water.

Granular absorbent or prepacked cartridges?

To some extent, this is a matter of choice of economy versus convenience. Pouring granules and packing the scrubber is not outright messy but a bit of manual work. It can also be contended that prepacked cartridges are packed uniformly and correctly by a manufacturer ensuring that channelling won’t happen. (see box next page). But there is not always an option. There aren’t cartridges available for each and any size of canister or rebreather, while some rebreathers, i.e. the Poseidon CCR, only accepts proprietary cartridges.

Gas supply

All CCRs have a small tank of oxygen and a small tank for air or other diluent. Some units are able to use different tank sizes, depending on what you find locally. Other manufacturers make their units with a “hard-case”, which leaves just enough room for a specific size and shape of cylinder. Once again, size matters—even for rebreather tanks, as these tanks are also being used for inflating BCD and possibly a drysuit, too. And even if the rebreather diver carries an off-board bailout tank for emergencies, the on-board tank might also be used as a backup in a bailout emergency. In any case, the more gas, the better!

Electronics

This is the area in which the changes and advances have been the most impressive in the last few years. SCRs and mCCRs usually display simple pO2 readings. eCCRs, on the other hand, tend to have more advanced displays with redundant pO2 and setpoint reading, built-in decompression softwares with Open Circuit bailout capabilities, battery level indication, O2 sensors voltage and even scrubber monitoring. Just make sure you can easily read the most important information!

Are there any other additional features I may want?

HUD

Most of the rebreathers on the market come with some forms of safety devices like visual and audible alarms. However, not all come with a Head-Up Display (HUD), which is 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 open circuit regulator second stage. Even if you carry a bailout tank and regulator, this regulator might not be immediately available when you need it—you could be dangling from something, entangled or the bailout is already being used by your buddy. The BOV gives you the ability to immediately switch to Open Circuit by flipping a switch if something goes wrong with your rebreather and you find your breathing loop filled with an unbreathable gas mix or even water. It is a highly desirable extra feature, as some divers have reported that some symptoms of intoxication showed up as the inability to think properly and to quickly locate their alternate air source.

Off-board plug-in

When it comes to gas supply and avoiding 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 become an emergency with a rebreather.

4th sensor

Semi-closed Circuit rebreathers used to have no form of electronics whatsoever. Then divers discovered that an oxygen sensor might help them to better monitor their loop content. Some mCCRs have two cells but problems arise when on of these cells doesn’t agree with the other one. Most eCCRs come with three oxygen sensors fitted, using what is called the “Voting Logic” to double check their reading between each other (a faulty cell is voted out by the two others). Now, the idea might be to have four cells fitted to eliminate any potential situations where two cells display wrong information but are considered to be right by the computer.

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