Dual Rebreather Diving: When a Second CCR Changes What Is Possible
A real-world technical diving case study from Crete
Closed-circuit rebreathers have fundamentally changed what is possible in technical diving.
They allow us to extend range, reduce gas consumption and optimize decompression. But every CCR diver eventually faces the same fundamental question:
What happens if the rebreather fails?
For most CCR dives, the answer is straightforward: carry sufficient open-circuit bailout gas to safely terminate the dive and reach the surface.
As depth, duration and distance increase, however, that simple answer can become considerably more complicated.
At some point, the amount of open-circuit gas required to cover a realistic worst-case scenario may become so large that carrying it creates another problem altogether.
This is where the concept of a bailout rebreather — or dual-rebreather configuration — becomes particularly interesting.
And sometimes, the best way to understand why is through a real dive.
A 5.8 km Problem
The dive described here was not originally designed as an exploration dive.
It was a job.
The objective was to inspect an underwater freshwater pipeline running along the south coast of Crete between Loutro and Glyka Nera (Sweet Water Beach).
The pipeline supplies freshwater to the isolated village of Loutro, where the water is subsequently managed for the local community.
The task was simple to describe:
Follow the pipeline and inspect it for possible damage.
Executing that task underwater was considerably less simple.
[IMAGE 1 – Map showing Loutro → Glyka Nera / straight-line distance 3.3 km]
The straight-line distance between the beginning and end points is approximately 3,3 kilometres.
But the pipeline does not follow a straight line.
Its exact underwater route was not accurately known before the dive.
Neither was its maximum depth.
The information available suggested that the deepest section could be somewhere around 50–60 metres, but there was insufficient information to build the dive around a precisely known depth profile.
That uncertainty fundamentally changed the bailout problem.
Planning for What You Don’t Know
Technical diving normally depends on defining parameters before entering the water.
Maximum depth.
Bottom time.
Gas consumption.
Decompression obligation.
Turn pressure.
Bailout requirements.
But what happens when several of those parameters cannot be accurately established beforehand?
In this particular operation, we knew approximately where the pipeline went.
We did not know precisely how it was lying on the seabed.
We did not know its exact maximum depth.
And because the purpose was inspection, we could not know exactly how long the operation would take.
Consequently, we could not accurately predict the final decompression obligation either.
Trying to calculate conventional open-circuit bailout for one specific profile would therefore have solved the wrong problem.
The challenge was not:
“How much bailout gas do I need for this dive profile?”
It was:
“How do I build a diving system capable of safely accommodating the range of profiles that this operation may produce?”
That distinction was crucial.
Building the Dive Around Capability Rather Than Prediction
My solution was to configure myself for considerably more than the expected dive.
The anticipated maximum depth was approximately 50–60 m.
I prepared the system with operational capability to approximately 90 m, providing a substantial envelope beyond the expected depth.
Likewise, rather than building the dive around one predicted runtime, the breathing systems were configured with sufficient endurance for approximately 4.5–5 hours of diving.
The primary breathing system was a JJ-CCR.
The second system was a Halcyon Symbios chest-mounted CCR.
The result was effectively a dual-rebreather configuration.
The second rebreather wasn’t there simply to increase the duration of the dive.
Its fundamental purpose was redundancy.
If the primary CCR became unusable at the worst practical point of the operation, the solution did not depend exclusively on carrying enough open-circuit gas to breathe all the way through a potentially long ascent and decompression schedule.
Instead, another closed-circuit breathing system was available.
Why Open-Circuit Bailout Becomes Difficult
This is where the physics becomes important.
A CCR’s gas consumption is largely decoupled from depth in a way that open-circuit consumption is not.
With open circuit, breathing-gas consumption increases approximately in proportion to ambient pressure.
At 10 m, the diver is at approximately 2 ATA.
At 50 m, approximately 6 ATA.
At 90 m, approximately 10 ATA.
A diver with a surface respiratory minute volume of 20 L/min would therefore theoretically consume approximately 120 L/min at 50 m and 200 L/min at 90 m before accounting for stress, workload or emergency factors.
That is why bailout planning for deep CCR diving can quickly produce very large gas requirements.
And those cylinders then have to be carried.
They create drag.
They influence propulsion.
They complicate gas switches and equipment management.
And eventually there is a point where adding redundancy can itself increase complexity.
This isn’t merely theoretical. A 2026 peer-reviewed paper examining operational experience from the Wetmules and COBRA Divers describes precisely this problem: for sufficiently extreme dives, open-circuit bailout can become logistically impractical, leading teams independently toward dual-rebreather solutions.
A Second CCR Is Not Simply “More Gas”
This distinction is important.
A bailout rebreather should not be viewed simply as the equivalent of a very large bailout cylinder.
It is another life-support system.
And another life-support system introduces its own failure modes.
The diver must know its status.
The breathing loop must be available.
Oxygen must be controlled.
CO₂ must still be managed.
Gas supplies must be considered.
The diver must be capable of transitioning between systems while simultaneously dealing with the original failure.
And the configuration must remain manageable enough that the redundancy does not create unacceptable task loading.
Therefore:
Two rebreathers do not automatically make a dive twice as safe.
The benefit comes only when equipment configuration, procedures, training and diver experience allow the second system to provide meaningful redundancy rather than additional complexity.
This point is particularly relevant because different experienced exploration teams have independently arrived at different dual-CCR philosophies. The recently published Wetmules/COBRA paper describes contrasting approaches, illustrating that there is no single universal configuration appropriate for every mission.
Why a Chest-Mounted CCR Changes the Equation
The physical configuration of the second rebreather matters considerably.
For this operation, I used the Halcyon Symbios chest-mount CCR alongside the JJ-CCR.
The chest-mounted architecture makes this particularly interesting because the second breathing system can be incorporated without simply placing another conventional back-mounted CCR alongside the first.
Halcyon describes the Symbios as a compact, modular chest-mounted eCCR designed to integrate into different diving configurations. The unit uses 2.35 kg of absorbent and is designed and tested according to EN 14143:2013.
That doesn’t remove the requirement for proper bailout planning — Halcyon’s own manual explicitly requires bailout appropriate to the planned profile and foreseeable emergencies.
But from a configuration perspective, a compact chest-mounted CCR opens some very interesting possibilities for advanced technical diving.
Halcyon Symbios Chest Mount Rebreather
The Other Essential Tool: DPV
Breathing gas was only half of the problem.
5+ kilometres underwater is a propulsion problem as well.
The dive was conducted using a Seacraft Future DPV.
The manufacturer’s published specifications give the Future a maximum quoted range of 15.7 km and maximum operating depth of 150 m, depending on configuration and operating conditions.
For this particular operation, the DPV wasn’t simply a convenient way of swimming faster.
It was part of making the mission possible.
The diver needed to follow the pipeline while maintaining the ability to stop, inspect and photograph areas of interest.
Navigation and route recording also allowed the actual position of the pipeline to be documented.
[IMAGE 2 – Seacraft recorded route / actual pipeline inspection track]
The blue track above shows the actual route followed during the operation.
This is considerably more informative than the straight 5.8 km line between Loutro and Glyka Nera because it shows the real underwater path followed while tracking the pipeline.
Modern Seacraft navigation systems are specifically designed to support waypoint navigation and underwater route recording, including applications for scientific and professional divers.
The Dive Became an Exercise in Managing Uncertainty
This is perhaps the most interesting lesson from the operation.
The difficult part was not simply depth.
It wasn’t simply distance.
And it wasn’t simply decompression.
It was the combination of uncertainty in all three.
If we had known that the pipeline reached exactly 52 m, followed an accurately surveyed route and required a predictable runtime, conventional bailout planning would have been relatively straightforward.
But we didn’t.
The operational question therefore became:
How can we create enough breathing-system autonomy that an unexpected depth or longer-than-expected runtime does not immediately invalidate our bailout plan?
For this particular dive, the answer was:
JJ-CCR + Halcyon Symbios bailout CCR + long-range DPV + conservative operational envelope.
The equipment wasn’t being used to push the diver toward his limits.
It was being used to keep the dive inside those limits despite uncertainty.
That is an important difference.
Bailout Rebreathers Are Not for Every Dive
None of this suggests that every CCR diver should carry another rebreather.
For the overwhelming majority of technical CCR dives, appropriately planned open-circuit bailout remains straightforward, robust and highly effective.
Adding another CCR means adding another system that must be assembled, checked, understood and managed.
There is therefore a crossover point.
Below it, additional open-circuit bailout may remain the simpler solution.
Beyond it — because of depth, duration, distance, penetration or logistical constraints — the amount of OC bailout required can begin to create significant practical problems.
Finding that crossover point is much more interesting than simply declaring one system superior to another.
And it is precisely the type of decision-making that separates equipment configuration from technical dive planning.
Redundancy Should Expand Safety Margins, Not Ego
There is another lesson from this particular operation that I consider even more important.
Advanced equipment can dramatically expand what a diver is capable of doing.
That does not mean it should automatically be used to expand how far the diver pushes the dive.
In this case, the dual-rebreather configuration provided capability considerably beyond the expected 50–60 m maximum depth.
That additional capability was not there because the objective was to reach 90 m.
It was there because we didn’t know exactly what we would find.
Likewise, several hours of breathing-system endurance did not mean the intention was to remain underwater for five hours.
It provided reserve.
This is perhaps one of the most useful principles in advanced technical diving:
Capability and planned exposure are not the same thing.
The greater the uncertainty, the more valuable that distinction becomes.
Where Dual-Rebreather Diving Fits Into Modern Technical Diving
Dual-rebreather diving remains a highly specialized area.
But it is becoming increasingly relevant as technical divers undertake longer cave penetrations, deeper wreck exploration, scientific projects, remote expeditions and complex underwater operations.
The equipment is evolving.
DPVs are becoming increasingly capable.
Compact CCR architectures make previously cumbersome configurations more practical.
Navigation and dive-computer technology provide information that would have been extremely difficult to obtain underwater only a few years ago.
But technology does not replace the diver.
If anything, increased capability makes training, experience, situational awareness and disciplined planning more important.
The question should therefore never be:
“Can two rebreathers allow me to go further?”
A better question is:
“At what point does a second independent breathing system become a more appropriate solution to the bailout problem I am trying to solve?”
That is a much more interesting discussion.
And it is one that technical diving will probably be having increasingly often.
Dual CCR Is a Skill, Not Just a Configuration
Dual-rebreather diving is not something I recently decided to experiment with. Before my current JJ-CCR + Halcyon Symbios configuration, I had already spent years diving a dual SF2 setup, combining a ScubaForce SF2 backmount with an SF2 sidemount.
This matters because carrying two rebreathers is not simply an equipment choice. Managing two life-support systems requires specific procedures, continuous practice and significant in-water experience before the configuration can truly provide redundancy rather than additional complexity.
This is not about going deeper or further simply because the equipment allows it. It is about building the experience, skills and redundancy necessary to safely undertake dives where conventional solutions may no longer be practical.
About Technical & CCR Diving in Crete
At Chania Diving Center, technical diving isn’t simply an extension of recreational diving. CCR, mixed-gas diving, DPV diving and advanced dive planning form an important part of what we do.
Our technical and CCR programs are focused not only on operating equipment, but on understanding why a particular configuration makes sense for a particular dive.
For divers interested in progressing into CCR, Technical Extended Range, Hypoxic Trimix, DPV or advanced rebreather diving, Crete provides an environment where training can be combined with real-world technical diving experience.
EXPAND YOUR DIVING.