A Closed Circuit Rebreater (CCR) is already an incredibly efficient life-support system. It allows us to recycle breathing gas, maintain a constant partial pressurre of oxygen (PPO₂) and significantly reduce the amount of gas required during deep or long dives.

So if one CCR is good, two must be better. Right?

There is also the obvious advantage that dual CCR currently happens to be one of the coolest things in technical diving. So there is that.

Before going any further, a disclaimer:

I am (currently) not a trained dual-CCR diver. I do, however, dive and teach different rebreather configurations. My experience spans over backmount and sidemount units and I am a JJ-CCR, as well as a Side Winder (2) instructor, and I am currently working towards additional CCR instructor qualifications. And yes, I will dive Dual CCR when there is a good reason for it. Watch this space.

Disclaimer 2: 
This is not an argument for or against a specific unit or configuration.

It is a conversation about redundancy, bailout philosophy and one question that I think deserves considerably more attention: What happens when your backup becomes your only life-support system?

Why do we bail out in the first place?

To understand the discussion around dual CCR, we first need to consider why we carry bailout. As I mentioned before, a CCR is extremely efficient. We breath from the loop, metabolize a relatively small amount of oxygen and exhale the remaining gas back into the loop. Instead of throwing every breath into the water as bubbles, as we do on open circuit, we recycle most of it. The carbon dioxide produced by our metabolism is removed by the scrubber and oxygen is replaced either mechanically or electronically, (mCCR or eCCR) depending on the design of the rebreather.

As long as the unit can remove carbon dioxide, add oxygen, cycle the gas around, everything is working correctly and there is little reason to leave the loop. Which means, something significant is about to happen, before we decide to bail out. 

The rebreather (or our confidence in it) has been compromised.

At introductory CCR levels, the philosophy is therefore deliberately simple: If in doubt, bail out.

The reason is not that every CCR problem is impossible to solve underwater. Far from it. The reason is that troubleshooting a life-support system while stressed, task loaded or potentially becoming physiologically impaired, can turn a manageable problem into a fatal one.  We have good data to back-up this claim. Open circuit removes most of that uncertainty.

Put a regulator in your mouth and breathe. Get yourself home. Simple.

And simplicity has enormous value when things are going wrong.

But not every CCR failure requires bailout. As our understanding of the rebreathers develops, we can work on more “complex” solutions.

Hypoxia. Perhaps the onboard oxygen cylinder is empty, or the solenoid fails closed, or an orifice becomes blocked. If the breathing loop itself remains functional and we have the appropriate offboard capability, we may be able to supply oxygen externally and manually maintain PPO₂ using a MAV.

Instead of abandoning the rebreather completely, we effectively operate it manually. This is considerably more gas- and decompression efficient than open-circuit bailout.

Hyperoxia may occur because of a stuck-open solenoid, excessive oxygen injection because of a stuck MAV for example. If the problem is clearly identified, the onboard oxygen supply can be isolated. The diver can then supply oxygen from an offboard source and manually add it.

But it comes at a cost: task loading.

Now the diver must actively monitor PPO₂ and manually add oxygen while simultaneously maintaining buoyancy, navigation, team awareness, decompression and everything else associated with the dive.

For an experienced diver who has trained specifically for this, it can be a viable strategy. But the reality is that even very experienced CCR divers struggle to maintain PPO₂ with increased task loading during ascents. 

So for a novice CCR diver this is probably not a good idea.

There are however other failures where our options become considerably more limited. For example when you no longer (can) trust what the rebreather is telling you.  One of the fundamental requirements of CCR diving is knowing the PPO₂ inside the loop.

• You cannot smell PPO₂
• You cannot taste it

Guessing is a bad idea, you certainly should not attempt to estimate it based on how you feel.

Some CCRs provide independent or redundant monitoring systems. For example, a separate HUD or secondary monitor, can provide another method of interpreting oxygen-cell information if the primary controller (or monitor) fails. staying on the on the loop therefor may be possible.

But what happens when you cannot reliably determine PPO₂?

Perhaps the controller / monitor fails due to cable damage, or a battery is drained, or the handset is flooded? Perhaps the cells provide contradictory or unstable information due to condensation buildup and provide incorrect or misleading information. At some point, you must ask yourself a very simple question:Do I actually know what I am breathing?

If the answer is no, bailout becomes the wise option you (hopefully) trained for, and besides attractive: It’s your only safe option!

Because an incorrectly displayed PPO₂ can go in both directions.

The actual PPO₂ may be much higher than indicated, exposing the diver to the possibility of an oxygen-toxicity event or it may be considerably lower, resulting in hypoxia and potentially unconsciousness or a decompression obligation you are not aware of and therefor risk DCI.

Neither situation gives you much time for philosophical debate. I am not saying trying that a diluent flush is unable to give insight or provide a better understanding what cells are not working, you have a bit of time, but it definitely adds to task loading.

Flooding the CCR changes the situation however.  A damaged loop due to an entanglement or mistake with the DSV (certainly on dual CCR), might result in a flooded unit. Water reaching the scrubber increases the work of breathing and there is a significant risk of a caustic cocktail. When this happens, the machine simply stops being a viable breathing system and there may be no meaningful troubleshooting solution for this underwater. You have to bail out.

Which brings us to perhaps the most discussed CCR emergency of all: Hypercapnia

Hypercapnia deserves particular respect because it can rapidly destroy a diver's ability to solve the very problem causing it. When Carbon dioxide rises  your breathing rate increases. Work of breathing increases which means you works harder, meaning more CO₂ is produced and the situation can spiral very quickly. CO₂ is dense and extremely narcotic. Panic may develop.

Trying to remain on a breathing loop while experiencing hypercapnia may be an extremely poor decision. Open circuit gives us something extraordinarily valuable in that situation. Every exhaled breath disappears into the surrounding water. The carbon dioxide is gone. The open circuit regulator does not need to scrub it. The rebreather does not need to chemically process it. You breathe in fresh gas and dump the exhaled gas directly into the water.

With hypercapnia, gas consumption can initially be enormous. Personally I have seen a diver breath with a SAC rate of over 65 liter per minute. (at 45 meter this means 65 * 5.5 = 357.5 / minute! This empties a S80 in 5 minutes!)

And that creates one of the central problems with deep CCR bailout planning.

Bailout can get very big, very quickly.

Imagine a deep CCR dive. As long as the CCR operates correctly, the amount of gas consumed is relatively small. But with bailout planning we cannot assume that everything continues to work "normal" and that we remain our normal “low" SAC rates. That is literally why bailout strategies exists. ISO standards dictate that for the first 5 minutes we should calculate 50L/min.

Now imagine you have to bailout at significant depth while breathing extremely hard. The volume of open-circuit gas required can become substantial. Add on top of this extended decompression since PPO₂  won't be constant all the way to 6 meter. Add a conservative SAC rate. The numbers get big.

Suddenly we are carrying multiple cylinders largely because of something we hope never to use.  Three, four cylinders or five cylinders? Perhaps more depending on the dive. 

More cylinders mean more available gas. But they also mean additional drag, regulators, gas switches, cylinder management and therefor complexity and how often do we practice this?

There is no free redundancy. 

Every solution introduces another system that must itself be managed.

And this is where dual CCR becomes extremely attractive.

Enter dual CCR

Instead of carrying a huge amount of open-circuit bailout, why not carry another rebreather? If the primary CCR floods, close the loop. Switch to the second CCR. Establish the bailout rebreather is set correctly. Go home. From a gas-efficiency perspective, the idea is brilliant.

Instead of carrying enough open-circuit gas to breathe all the way out from maximum penetration or depth including decompression, just carry another extremely efficient breathing system. That second CCR might provide hours of breathing capability using comparatively tiny cylinders and will provide constant PPO₂. On paper, the redundancy looks extraordinary. Primary life-support system fails. Move to secondary life-support system. Problem solved!

Except: What happens if CCR two has a problem?

This is where I struggle with some interpretations of dual-CCR philosophy.

Imagine your primary CCR has suffered a genuine, unrecoverable failure.

• Flooded
• Cells readings are inconsistant
• Hypercapnia

Whatever the reason, you have made the decision that the rebreather can no longer safely be operated. You switch to the second CCR.

At that moment, the second CCR is no longer your backup. It is your only functional rebreather and gas source.

Now imagine you are 100 metres deep. Or several hundred metres into an overhead environment. And something happens to that second CCR. What are you going to breathe?

Two three-litre diluent cylinders are not suddenly going to transform themselves into a meaningful open-circuit bailout supply for a long ascent, decompression or cave exit.

And that introduces a very different psychological situation. Before the first failure, you had redundancy. After the first failure, you may no longer have redundancy at all. You have simply moved onto another complex life-support system with the knowledge that if you break it you are done.

The mental side matters and this discussion that cannot easily be represented on a spreadsheet.

Because now I know, that another serious failure on my CCR may leaves me with very limited options. That awareness should change my decision-making.

If you conduct an OC bailout and you have a regulator that is not working properly. Doing a gas switch eliminates this problem. Sure you might run low on BO gas, but you can still make gas switches and if required move faulty regs onto working cylinders. Yes complicated but doable. 

That is the part of dual CCR that I think deserves more discussion.

Redundancy is not simply "two of everything"

True redundancy is not necessarily having two complicated systems that depend on similar technologies, procedures or human actions.

Redundancy is having a credible solution after failure.

Sometimes that solution is another CCR. Sometimes it is open circuit. Sometimes it is team bailout. 

Different divers may justify different answers. But the correct question is not: How many rebreathers am I carrying?

The better question is: After my first major failure, what credible options remain?

And then: What happens after the next failure?

For example, if you experience hypercapnia (for whatever reason), will you be able to make the switch to your bailout rebreather and can slow down breathing before that second unit becomes compromised? It’s not the carbon dioxide in the rebreather that might have caused the problem. Your body is loaded with carbon dioxide that needs to be removed by a significant enough CO2 pressure gradient (that open circuit gives you), allowing it to be washed out, until you can be absolutely certainly that if and you can switch to the bailout rebreather, you don't break it with loading it with carbon dioxide it can't coupe with. Because if you do break your backup rebreather, you are done. 

So sanity breaths are a good idea. Always.

But dual CCR is not a bad idea either.

I want to be very clear about that. Dual CCR can be an extremely powerful configuration. For deep diving, long decompression and major overhead penetration. it's gas-efficiency advantages are obvious and perhaps superior. It may reduce the enormous open-circuit bailout volumes that would otherwise be required and maybe impossible to carry with you. And with appropriate design, procedures, training and experience, switching between two rebreathers can absolutely become a practiced skill.

But simply adding another CCR does not magically make the dive safer.

Where open circuit brings gas-volume limitations, dual CCR can give procedural complexity, managing multiple buoyancy sources and computer. But most importantly it increases task loading and might suppresses diver performance.

So, should you dive dual CCR?

Maybe. That is probably the most irritating answer possible! But I think it is the correct one.

Dual CCR should not be considered automatically safer simply because there are two rebreathers. Nor should it be dismissed simply because it introduces more complexity. The configuration needs to be examined against the actual dive.

• Depth.
• Penetration.
• Decompression.
• Available (team) BO gas.
• Environmental conditions.
• Workload.
• Training

And, most importantly, what remains available after the first system is genuinely lost.

Because redundancy only has value when it still exists after you need it. And that leaves me with the question I started with:

If your primary CCR is gone and your backup CCR becomes your only viable life-support system, is it going to bring you back up home?

Cherry on top of the cake and final thought: As per RF4.0 observation; rebreathers themselves appear to have become safer, but the estimated fatality rate per dive has not improved nearly as much as we might expect. Meaning it’s likely the training or operator contributing to the problem is a second CCR really a good idea?

Paul Emous

Technical Extended Range Instructor Trainer | Expedition leader

Paul is a Technical Extended Range Instructor Trainer and expedition leader in advanced and remote diving. He teaches Just Culture and decision-making under pressure, where risk is real and accountability is absolute.

Contact: paul@mousemedia.nl