Rebreather Configurations

Joseph Jonathan Bosquez

Rebreather configurations are a critical element of the diving system just like conventional open circuit diving, such as backmount and sidemount. Configurations are a balance between the diving environment and the user’s comfort. Normally, dictated by the underwater environment, rebreathers must take a step further by being an ambient pressure system, operating underwater, be robust, perform well, be comfortable to wear, and must even travel well!

Divesoft's vision was to reimagine the closed circuit rebreather design and begin again from the ground up. The Liberty Classic configuration was created unconventionally by designing the unit around work of breathing and fault tolerance. The only conventional aspect of the classic was that it is worn on the divers back and employed front mount counterlungs. This initial design has grown and evolved into the line up that we see today of the Liberty Classic, Heavy, and the popular Liberty Sidemount rebreather.

It’s easy to go beyond the scope of this post by diving into the history of rebreather design and how they were worn and configured. We are going to focus on popular configurations, Chest mounted rebreathers, backmounted (BM) rebreathers, and sidemount (SM) rebreathers.

The configuration and design of the rebreather is critical because it affects the hydrostatic pressure between the diver and rebreather (Closed Circuit Rebreather or CCR). The hydrostatic balance of the counterlung location, the divers trim, and the unit’s design all affect the overall work of breathing (WOB). WOB is a critical factor because the divers lungs move the gas through the system, if there are any disruptions or imbalances that create more work, this can cause fatigue, CO2 retention, hypercapnia and even asphyxia.

Historically rebreather design and configurations being mostly shaped by WOB which meant placement and orientation of the counterlung system lead to design focus. Logically the best hydrostatic balance was what was close to the user's chest, this led to the obvious chest mounted rebreathers. Simple and effective units that began life as primarily oxygen systems and later evolved into mixed gas systems are now popular again as they travel extremely well and often have good WOB.

Moving forward in rebreather evolution and even skipping ahead of semi-closed systems (robust and simple designs that were a branch away from fully closed rebreathers to work around pesky electronics) leads us to mixed gas electronic CCRs such as the CCR Liberty Classic. The first backmount configuration started life with front mounted counterlungs. This was the go-to configuration for high performance rebreathers because they utilized the reliable WOB close to the users chest but able to easily add gasses and integrate a simple and effective automatic diluent valve. Many users prefer this configuration as it is a very effective and proven configuration for deep rebreather diving. Other than front mounted counterlungs we also had encased counterlungs housed within the rebreather assembly on the divers back. We continue to see this around with a few manufacturers.

In the last 12 or so years there has been a revival of rebreathers and what is now available on the market has given the consumer a huge variety in rebreathers and how they are configured. Backmounted counterlungs (CL) that are free and unencumbered by a case have become more popular than frontmounted counterlungs. Gas addition manual add valves have gone from a handheld block to the Divesoft Liberty’s popular mouthpiece manual add valves (MAVs). The phrase, “If you can find your mouth, you can find your MAVs”, has proven this configuration to be very user friendly. Back-mounted counterlungs have a variety of advantages, such as, a clear chest, good work of breathing, lung placement allows for the diver to exhibit good trim, effective breathing in all orientations, dewatering capabilities, and effective protection of water intrusion to the electronics through water traps and “T” pieces.

Sidemount rebreathers have also become another very popular configuration with exploration because they allow us to utilize rebreather technology through restrictions safer than before. A true sidemount rebreather is completely self contained and is worn similar to a single sidemount cylinder. The Liberty sidemount configuration specifically is a complete system with both diluent and oxygen gas on board, easily removable, and has redundant computers, and redundant displays.

Sidemount rebreathers are an incredible tool and great way to configure a rebreather for expedition diving because they are able to travel well, can easily offboard gas into the unit, and should be removable in a manner similar to a SM cylinder. Limitations of Sidemount rebreathers are that they are not as flood tolerant as a backmounted rebreather because any water trap or “T” piece would hamper the work of breathing, thus, any water intrusion will migrate directly to the rebreather's electronics and scrubber. Thus, it is critical for a sidemount rebreather to employ a robust water resistant scrubber such as a radial scrubber and resistant electronics. Hydrostatic loading is also very dependent on the unit’s orientation within the water. The diver’s trim has a significant impact on the work of breathing. Combined with the water vulnerability and easy changes in WOB from the divers trim, they often defend the notion that sidemount rebreathers are more advanced and having CCR experience with a BM unit will help the divers experience with SM rebreathers.

A rebreather diver can combine the configurations of a double BM rebreather or combine a BM rebreather with a sidemount rebreather to employ a primary rebreather with a backup rebreather or “bailout” rebreather. This configuration is relatively new to the community and has been defined between a “Dual Rebreather or Dual CCR” or a “Bailout Rebreather”. This different nomenclature defines two different functionalities, one that both rebreathers are being used in conjunction during the dive and the diver is swapping between the two breathing loops in a certain time interval not only for functional verification of the second unit.

Two, the bailout rebreather is defined as the purely backup rebreather for the diver. This means that the diver is on one primary system and only changes between loops to verify breathing loop integrity during depth changes or over time. Loop integrity and function check are critical to be constantly monitored. Discussion on the management of this goes well beyond the scope of rebreather configurations. A double BM rebreather would employ a set of backmounted CL and frontmounted CL. A primary BM unit and a SM unit would ideally use BM CL to maintain a clear chest for the SM rebreathers loop to remain at the ready for the diver.

With all of the development in rebreathers in the last 24 years one begs to ask the question. What does the future of rebreather configurations look like? Will it look like dual systems such as Closed Circuit Rebreather combined with an Open Circuit system backup? Or will we move to a trifecta of a Primary Closed Circuit Rebreather, Bailout Closed Circuit Rebreather, and an Open Circuit system? Or will we remove the Open Circuit system completely and just have a primary CCR and bailout CCR?

The OC scuba system will forever be utilized SCUBA systems because we will always need a diluent and inflation gas systems to drive the rebreathers. It will likely just move from being a complete breathing gas to primarily drive gas system for the rebreathers that the diver may also breathe from through a bailout valve (BOV) during a transition between both systems.

Whichever way the community goes, it will likely be a combination of things as every diver has a different experience and dive sites have different requirements. What will be critical is that we will need manufactures that are willing to innovate and be receptive to the divers requirements in order to continue the sport of underwater exploration. We are truly living in an exciting time as there has never been more resources and support for underwater exploration than there are today.

Author: Joseph Jonathan Bosquez