Choosing a Closed Circuit Rebreather
by Capt Tom McCarthy
StoneRust.com | EastCoastWreckDiving.com
This article is intended to be a primer for those who are considering utilizing rebreathers in their diving. It takes a broad and pragmatic approach to selection and looks at all options objectively. It is not intended to be your only source of information when making a decision, but rather is a jump off point for you to begin your research.
What is a rebreather? At its core it removes CO2 and adds oxygen to replace what your body burns up in the normal course of operating. It has a flexible container that you inhale and exhale from. Some kind of mouthpiece to breath through. It must have a way of keeping gas moving in a constant direction. It also has some way to add more gas to the system (the loop in rebreather jargon) as we go deeper and/or metabolize O2.
What I've just described is the simplest rebreather. It is the Oxygen Rebreather. Mainly used for military operations, the oxygen rebreather is created for simplicity, portability, and stealth.
It does have one major limit however, it can't be used deeper than 20' because the only gas available is pure Oxygen. If you're a human this is bad. This makes the Oxygen rebreather all but useless for most divers.
This unit essentially adds pure oxygen whenever the volume of gas in the loop becomes smaller than the one your lungs hold. It’s like sucking from a balloon that automatically adds more air when you remove all the gas from it.
Why does this volume decrease though? Well remember, our body will burn this Oxygen to function, all the while our loop volume is decreasing. Most oxygen rebreathers have some method of automatic and mechanical Oxygen addition that activates whenever this loop volume hits a certain level. By having this function, the Oxygen rebreather, in normal function, will always add enough gas to the system for you. This is all great but for us sport/technical divers we need a way to go deeper.
Semi Closed Circuit Rebreathers
Knowing that the Oxygen depth limit is our controlling factor for an oxygen rebreather, why don't we just replace the oxygen bottle with something that can be breathed deeper? Ok, let's just pop a bottle of nitrox on our oxygen rebreather.
“First breath, doing great. Few more breaths. The adv fired. That should be good. Few more, it fired again. few mer et foyerd agin... Few………thud."
So what just happened? I blacked out from lack of oxygen. There's one crucial difference between our oxygen rebreather and this new nitrox rebreather, the inert gas nitrogen. As our body used up the oxygen the loop volume fell, this in turn activated the addition of more nitrox (which has more nitrogen in it). This keeps happening as our body burns the oxygen and exhales the unused nitrogen back into the system. This will keep taking place until the point where the loop's oxygen level is too low to sustain consciousness. So how can we fix this problem? We need to remove some of the nitrogen and replace it with oxygen. The only way to remove an inert gas using a nitrox, however, is to forcefully purge it from the loop. This is why we call this a Semi-Closed Circuit Rebreather (SCR).
There are two different ways to accomplish this purging of gas. The first is to continuously flow fresh nitrox through some sort of orifice like a running garden hose in a bucket. This causes a small amount of inert gas to continuously overflow from the system. The downside of this method is that it does not adjust to your body's activity level so we may be wasting gas needlessly.
The second method is a bit more complex mechanically but helps fix this problem, this is the Passive SCR (PSCR). The Halcyon RB80 is an example of this system. The loop, through a system of small bellows (think accordion), vents a small portion of every exhaled breath and adds back the lost loop volume using an automaticity fired mechanical system. Thus removing the need for a constant flow of gas. This system is very simple in operation. We have created a wonderful gas extender tool, generally around 8 times more efficient than the same volume of gas used with normal Open Circuit SCUBA. Additionally, we only have gasses which are breathable at our current depth, plumbed into the system. This gives us the benefit of always knowing what our loop contents are and eliminates the need for electronic gas monitoring systems.
There are, however, a few drawbacks to Semi-Closed. Logistically speaking, the semi-closed unit rebreather can present more complex planning with regards to diving in remote areas. Because it only extends a predetermined gas mix, we must bring, and use, multiple gasses on deeper/longer dives. This can lead to the need to bring additional filling equipment/ and gas in order to complete the mission at hand. This can be especially difficult when dives are conducted on small vessels at sea or very remote regions with little to no access to diving gasses or filling equipment. Enter the CCR.
Closed Circuit Rebreathers
The Closed Circuit Rebreather (CCR), unlike its semi-closed cousin, has a distinct advantage in that it is essentially a portable mixing station on your back. In normal dive operations we may only need to fill two small rebreather cylinders. One cylinder is filled with oxygen, the other with a bottom mix commonly called a diluent (such as air/trimix/heliox). This can dramatically simplify logistics with regards to how much gas is needed on expeditions. The only time we would need to fill large cylinders would be in the event of a rebreather failure where open circuit bailout gas was used. CCR also provides convenience for divers who may conduct dives of various depths regularly. A CCR diver may have multiple standard bailout gasses filled and at the ready but may rarely need to refill them. It’s not unusual for a rebreather diver to use the same bailout cylinder for an entire dive season without ever needing to fill it.
So why is it that we need such little gas with the closed circuit rebreather? This can be attributed to one thing, the introduction of pure Oxygen. While the semi closed circuit rebreather must vent the inert portion of the loop gas in order to raise oxygen levels, the CCR utilizes a designated cylinder filled with 100% Oxygen to make up for this decline. In normal operation and constant depth no gas is ever vented from the system (hence the name Closed Circuit). Every CCR operates under this same principle, however, the way in which the Oxygen is delivered into the system can vary greatly.
As opposed to Semi Closed Rebreathers, a common factor between all types of CCR’s is the potential concern for the loop’s oxygen to fluctuate to dangerously high or low levels. This introduces the concept of electronic PPO2 monitoring systems. These systems almost all function in the same way, utilizing consumable oxygen sensors similar to any nitrox analyzer. Due to the constantly changing electronics market I won’t get into specific models or makes of PPO2 monitors but in general a PPO2 monitor is merely a volt meter that displays linear sensor voltage output translated into PPO2’s. When considering any electronics it’s important to select an established and reliable system. This can only really be done by polling the community at large for their own experiences with using a specific piece of hardware. This is the most fluid aspect of rebreather selection and future innovations from quality CO2 monitoring to significantly improved Oxygen Sensors could very well be just around the corner. We’ve already seen so me really fantastic innovation as of late by way of real time scrubber prediction systems and the earliest CO2 sensors. Who knows what the future will hold for rebreathers?
Now then, let’s take a closer look into the three main functional distinctions of CCR’s commonly found in todays market.
Manual Closed Circuit Rebreather (MCCR)
The first and simplest type of CCR is the Manual Closed Circuit Rebreather (MCCR). The MCCR, like all CCR’s, has a diluent cylinder and an oxygen cylinder. The oxygen needed to maintain PPO2 is added by the user by pressing a manual add valve (similar to a BC Inflator) as well as through a constant trickle of gas via what is known as a Constant Mass Flow (CMF) orifice. Constant Mass is different from constant volume in that the same number of molecules of Oxygen are allowed to flow into the system rather than the same volume (the physics behind this are actually quite interesting and it’s worth reading about them). The way in which this is achieved is by adding a non-flexible metal disc to the first stage regulator’s environmental seal. This prevents the 1st stage from compensating for depth by raising the intermediate pressure output. In a normal regulator intermediate pressure is kept at a constant pressure above ambient pressure, not so with this “1 atmosphere” add on. If we used a normal regulator system the raise in intermediate pressure output would result in a greater and greater mass of oxygen being added into the loop as we go deeper. In our capped system this trickle of Oxygen is set to a flow rate just slightly lower than the divers metabolic rate while at rest. This prevents the PPO2 from creeping up due to over addition of Oxygen. It can be manually adjusted by the diver between dives via changing the intermediate pressure of the regulator. The benefit of this method of operation is that it allows the diver complete control over maintaining PPO2 via mechanical only means. Meanwhile, the orifice reduces frequency in which the diver needs to manually inject oxygen throughout the dive.
The down side to this system, however, is that with a set intermediate pressure we’ve now created a depth limit on the rebreather. If the intermediate pressure of the first stage is set at 10BAR than the flow of oxygen will decrease as we approach the equivalent ambient water pressure. That’s not to say that these units cannot be dived deeper. Some have been dived well in excess of 600’ (185m). Modification in equipment and technique is generally necessary to achieve this however.
There is also a rarely utilized type of MCCR which is worth mentioning. This MCCR uses a normal depth compensating 1st stage oxygen regulator and an adjustable needle valve in order to control the flow of oxygen into the loop. This requires the user to constantly monitor the rate at which their PPO2 is falling or rising and adjust the valve accordingly. This system, however, removes the depth limitations set by a 1 atmosphere regulator system.
Electronic Closed Circuit Rebreather (ECCR)
Next we have the electronic closed circuit rebreather (ECCR). The ECCR’s method of Oxygen injection is a bit more complex than that of the MCCR. It relies on computer based Oxygen addition. The rebreather’s electronics constantly monitor the PPO2 levels of the loop and add oxygen via a gas solenoid. Sort of like an oxygen dam. The ECCR essentially does the same thing an MCCR does, however it does it handsfree. Think of this as cruise control. Like cruise control, however, we cannot just lean back and take a nap while relying on the system to handle everything. An ECCR must be monitored just as closely as any other rebreather. The potential for high or low PPO2 is ever present and the diver must keep diligent about it. The addition of a solenoid also adds the potential for one more system to fail, either adding too much or too little Oxygen into the loop.
Hybrid Closed Circuit Rebreather (HCCR)
The final type of rebreather is referred to as the Hybrid Closed Circuit Rebreather (HCCR). Hybrids are just that, a hybrid between MCCR and ECCR. They utilize both the CMF Orifice and the Oxygen Solenoid to maintain PPO2 levels. Many divers operate these units manually while using the ECCR solenoid as a backup (or parachute) should the diver fail to maintain a sufficiently high PPO2.
Backmounted vs Front Mounted
The next choice when deciding between CCR’s is the questions of Front or Backmounted counterlungs.
The positioning of the counterlung can be highly polarizing for CCR divers. I’ll offer the proclaimed advantages and disadvantages of both from both sides.
Front Mounted Counterlungs
The front mounted counterlung system has been around for a long long time. The system has several advantages. The first is flood tolerance. Front mounted counterlungs, when designed properly, will allow a significant amount of water to be trapped within them before it’s able to make its way to any of the critical systems of the rebreather (such as the scrubber canister or the oxygen sensors). Most units will have a dump valve located on the counterlungs that allow the user to over-pressurize the loop and force water out of the system. This can be a great option in the event that water enters the DSV. However, some will argue that this is human error and any diver who accidentally lets water in through the DSV should call the dive (whether it happens before it begins or during) and take the unit apart to ensure that water hasn’t entered critical areas of the loop. I agree with this stance. The other argument that is heard regarding flood tolerance is that any rebreather diver who continues to stay on the loop while having an unknown flooding issue is asking for trouble. A failed o-ring or other sealing system doesn’t generally fix itself and the idea of staying on a sinking ship that is taking on water while you have another option is not advisable. I wont pick a side on this but some will argue that over reliance on flood tolerance can cause a user to stay on a failed rebreather longer than they should. The other issue commonly heard is that having counterlungs on the front of the body results in a cluttered chest that in turn reduced maneuverability and ability to access chest d-rings. As someone who is trained on and has dived a front mounted counterlung unit for many years I have to agree with this statement. It can, at times, be more difficult.
Lets’s focus on the positive now. One of the major advantages of front mounted counterlungs is the work of breathing. At one point this was certainly the case. It is much easier to design a low work of breathing unit with front mounted counterlungs than it is to obtain the same work of breathing for a back mounted unit. Why? It’s a matter of hydrostatic pressure. Having the lungs on the chest means that the diver has an easier time inhaling from the loop but a more difficult time exhaling. Human beings are generally better at exhaling than inhaling, this is why musical instruments produce sound through exhalation. While this was generally the case for quite some time it is more important to look at a manufacturers testing data with regards to work of breathing than to just look at counterlung positioning. Some back mounted counterlungs units breath better than some front mounted counterlung units and vice versa.
Back mounted counterlungs
Back mounted counterlung rebreathers are exactly what they sound like. These units move the counterlungs to the users back. This provides the advantage of an uncluttered chest area which allows for better maneuverability and streamlining. In the past backmounted counterlung units have had a higher work of breathing when compared to most front mounted units. However, with improved engineering abilities and ever improving designs we cannot unequivocally state this to be true. One of the arguments that some front mount users will make is that having the counterlung on the back reduces flood tolerance. Some units will make up for this with dump valves located strategically on the counterlungs, others have small water traps that are intended to collect water before it enters critical areas. Again, while flood tolerance may allow a user to stay on a failed rebreather as a last ditch effort, it is more important to focus on proper bailout gas planning which allows users of both systems to safely surface from a dive in the event of any doubts produced from flooding.
Top of Shoulder Counterlungs
Recently some manufacturers have begun to produce top of shoulder counterlungs as a middle ground between the two. They provide an uncluttered chest while keeping some of the flood tolerance and work of breathing properties of front mounted lungs. I’ve heard mixed results about these, some people love them, others claim that they greatly restrict head movement. Either way, it’s worth mentioning as one more option.
With the ever growing trend of sidemount diving, the use of sidemount rebreathers should be mentioned. Traditionally sidemount rebreathers have been a fringe section of the rebreather community, mostly being in the form of quasi homebuilt units. More recently however, models such as the SF2, Flex and Sidekick have taken sidemount rebreathers into mainstream production. While sidemount rebreathers can be an excellent tool to accomplish some truly remarkable dives, they are hardly for everyone. They can be notoriously difficult to dive and present a slough of new configuration and technique concerns that, in this authors opinion, should only really be attempted by those who have learned to dive closed circuit on a backmount version first. Things, such as changes in trim while passing through small vertical restrictions, can have greater influences on breathing resistance by way of hydrostatic loading. A concern that may cause an unexperienced rebreather diver to panic or handle the situation poorly.
Regardless of which unit you chose, I feel that one of the biggest considerations when choosing a new rebreather is that of your adjacent community. The benefits of having access to team members and mentors who dive the same rebreather cannot be overstated. They will be there for you when you need help, they will be your spare parts store, and they will be there to teach you the little tricks and tips that can only be gained through experience. Rebreather training is merely a stepping stone towards becoming a truly proficient diver. It is these people who will walk with you on that path. It is also important to note that while most CCR’s essentially function in the same way, there is very little standardization in the community with regards to gas connections. Anytime you wish do dive with new team members this should be discussed before hand.
It is also important, when considering a new rebreather, that you go with an established and reliable rebreather company. While making a rebreather can be a very capital and intellectually intensive task, there have been several examples of fly by night companies that would eventually stop offering support for the limited number of rebreathers that they produced. It should also be mentioned that CE testing is considered a great barometer, by some, while examining a specific rebreather. The large time and capital expense that is put into completing these tests could prove that a company is in it for the long haul. Additionally, CE testing ensures that a slough of quality and performance specifications are met. Some argue that these tests may go too far and actually hinder innovation by requiring expensive retesting for every change made to a specific unit. That is a discussion for another time though.
In all, it can be quite difficult to chose a new rebreather. This is amplified when choosing your first rebreather. You don’t know what you don’t know yet so often times divers will not be sure of what they want in a unit until they’ve developed some experience on one. The best thing you can do is to talk to everyone you can. Not just instructors, who are financially invested in a specific unit, but those everyday end users as well. Every unit will have it’s strengths and weaknesses but most function perfectly well for the majority of diving one will do. Once you have a clear idea of what it is that you’d like to use a rebreather for, it also makes the process of choosing one a bit easier. In the end, no matter which unit you choose, remember to always respect the tool at hand and never allow yourself to become complacent. Safe diving!