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by Michael Menduno
Divesoft was born when Czech Information Technology (IT) entrepreneurs Ales Prochaska and Lucie Šmejkalová, who ran a successful online banking software business for 20 years, decided to forgo the corporate life and apply their considerable expertise to their shared passion for diving. Beginning with an innovative helium-oxygen analyzer, the “He/O2 analyzer,” which uses a patented (2012) acoustic helium sensor that Prochaska created and built for himself and his friends, the pair of brainiacs went on to create the Liberty Rebreather, which is arguably one of the most fault-tolerant systems on the market, along with their line of Freedom dive computers.
After launching the company in the spring of 2013, Prochaska, Šmejkalová, and their team exhibited at the Diving Equipment & Marketing Association (DEMA) in 2014. I was there and remember the crowd of tech divers huddled around their booth, peppering the Divesoft staff with questions about the unique fault-tolerant equipment on display. I was one of them.
Since that time, Divesoft has grown to become a serious contender in the global rebreather market, offering both back mount and sidemount versions of the Liberty. The company has attracted numerous high-profile users like Canadian filmmaker and educator Jill Heinerth; Polish deep cave diving record setter Krzysztof Starnawski, award-winning U.S. cinematographer Becky Kagan Schott, Italian explorer Edoardo Pavia, UK’s man in the Yucatan Steve Bogaerts, and many more. This summer, the innovative equipment makers introduced their “Analyser Solo,” a new lightweight, easy-to-use helium-oxygen analyzer. We caught up with Prochaska over the summer. Here is what the banking-software-guru-turned-tech-diving-inventor had to say.
InDepth: You describe your He/O2 analyzer as the “foundation stone” for the company. What motivated you to create this analyzer? Of all the diving equipment you could create, why a He/O2 sensor?
Ales Prochaska: When I started diving helium blends, there wasn’t an easy-to-use analyzer that could show me the complete blend composition in a single step. So, I created the analyzer mainly for my own use. When I commissioned the electronics board, I had ten of them made, in case any fellow divers were also in need of an analyzer. Turns out they were, and the whole stock of analyzers sold within two days.
I understand you use a standard electrochemical oxygen sensor in the analyzer. How did you come up with the idea for an acoustic helium sensor versus a chemical sensor? It seems like a brilliant solution and, in retrospect, an obvious approach in some ways. Divers are, of course, very aware of helium’s acoustical properties from listening to our Donald Duck voices!
I researched all the physical properties of helium I could think of. There were many pages to go through. I studied them and tried to imagine a sensor based on each. And the acoustic principle won, being easy to implement and very accurate.
How does it work?
The helium content is determined based on measuring the speed of sound in the analyzed mix. The speed of sound depends on the content of helium and oxygen, and the temperature of the mix. The dependence of the speed of sound on pressure is small and can be disregarded under normal atmospheric pressure.
Did your educational background help you create the analyzer?
Yes, in the case of the analyzer, I found my previous studies in engineering, electrical engineering, and software engineering to be very useful. Skills from all three fields were needed to design and manufacture the analyzer.
What was your and Lucie’s diving background? Are you both tech divers?
From my first diving course, I realized that technical diving was the direction I wanted to follow and explore intensively. Lucie, on the other hand, is not as attracted to technical diving or caves. She prefers a dive in the warm tropical seas among the coral fish!
How long did it take you to build that first analyzer?
The first analyzer was sold in 2004, but the development began a little earlier.
And you have the patent. Brilliant! What benefits does the acoustic sensor bestow on users compared to other chemical-based He analyzers?
Yes, the sensor is patented. The acoustic principle used allows fast and efficient measurements, even on flowing gas. It’s also as easy to use as a conventional nitrox analyzer, and I think that was the biggest benefit of our analyzer.
Divesoft just introduced the “Analyzer Solo.” How does it differ from its original He/O2 Analyzer?
The Solo Analyzer is a lighter and more simplified version of the He/O2 Analyzer. However, the original is still produced because it allows the attachment of additional equipment such as a pressure sensor and others alike. The Solo has no plug-in connector.
What came next; the Liberty rebreather or the Freedom dive computers or both?
They were supposed to launch simultaneously because the computer and its software were developed at the same time as the Liberty control unit (with which it has a common base), but we managed to release the dive computer a little earlier.
What year was that?
We launched the company in 2013!
How did you go from a He/O2 analyzer to a rebreather, the flagship of your company? What was your motivation to build a rebreather, or was that always the goal?
The motivation to develop Liberty was similar to that of the analyzer. We wanted to dive with a rebreather, but none of the devices on the market at that time had all of the features we found important. We had a clear idea of the qualities a rebreather should have and knew we could design it. This was in and of itself a great motivation to try.
Necessity is the mother of invention! What made you think you could build a rebreather in terms of expertise and experience? How did your earlier work in IT inform your design ideas and implementation?
At first glance it doesn’t appear as such, but the fault-tolerant rebreather is an extensive software masterpiece above all. We had a lot of experience with fault-tolerant systems through our IT and even our underwater backgrounds. At one point, we had developed a software that was supposed to be perfectly resistant to failure. When Prague was hit by the huge flood in 2002, it destroyed the main data center. The system we designed, however, transferred the activity to a backup center and continued working as if nothing had ever happened. Since then, we knew that we wanted to pursue a system that could be sunk underwater deliberately, not just during a flood.
Wow. Very cool. Clearly “fault-tolerance” is part of the DNA of your rebreather. Was that your starting point, then, for your design?
Yes, that was a clear goal from the beginning. We knew that other rebreathers were not completely fault-tolerant. Their standard solution was to go into restricted safe mode or report an error and wait for the user to deal with it. Our intention was to develop a rebreather that, in the case of a control electronics failure, anything from discharging the battery to interrupting the solenoid coil, would continue to operate without any restriction. In technical diving, there may be situations where several problems gradually accumulate and prohibit divers from emerging in less than a few hours. In those cases, a rebreather that remains working despite having a defect may be necessary.
Were dive computers just a logical add-on?
Yes, once we had mastered the rebreather control software, including the decompression model and the waterproof rebreather handset, it would have been a shame not to use it to design an independent diving computer. All we needed was to cut the rebreather cable, refill the battery and the computer was (almost) finished. (Ales smiles.)
Talk to me a little about having four O2 sensors in the unit? How does that work?
The number of sensors is closely related to the fault-tolerant properties. The standard number of sensors is three, sometimes the ‘3 + 1’ or ‘3 + 2’ arrangement is used, where the added sensors serve as an additional check on the functioning of the three main sensors. But Liberty has two complete, full-featured control units, each with its own sensors. That’s why we used four sensors, because this number is easily divided by two [Ales smiles]. Of course, this does not mean that each control unit works with only two sensors. The units keep communications with one another, and both are aware of the measurements of all four sensors.
I remember from Rebreather Forum 3.0, Nigel Jones, who worked with Poseidon, said that three sensors in a “voting logic” algorithm do NOT offer true redundancy, and in some cases offers much less redundancy than divers imagine. Does your algorithm offer something stronger? Please explain.
The degree of redundancy depends on the evaluation algorithm i.e. “voting logic.” If properly designed, more sensors will always be more secure. At Liberty, we monitor not only the instantaneous values of the sensors (plus the elimination of the obviously defective, for example, flooded sensors), but also their course over time. We know when the system adds oxygen and how much and the depth and volume of the breathing circuit; from that we can calculate how much the sensor should measure. From the reaction of the other sensors, we can also manually add oxygen or diluent. We also know how and how fast the sensors should respond to depth changes. From all of this, we can identify the faulty sensor and exclude it from the measurements. And, of course, the system always informs the diver of such an event, as they have the final say in how to proceed.
Fascinating! It sounds like the Liberty uses some of the same principles that Bill Stone used in designing his “Active Validation” approach in Poseidon rebreather, that is comparing instantaneous sensor values with what you expect them to be.
But what Nigel Jones said, of course, still applies. The diver should never assume the control unit will solve everything. The control unit solves only what it is programmed for, so, for example, the quality of the sensors and their replacement must be supervised by the divers themselves. There was a case where a diver had two faulty sensors and one good sensor in a rebreather, and the control unit ignored the values of the correct sensor. The unit decided based on two faulty sensors, which ‘outvoted’ the right sensor.
Why two helium sensors? The FHe is not going to change during the dive, is it?
Helium concentration changes during the dive because the oxygen concentration changes. The more oxygen, the less diluent and thus less helium. This logic could also be reversed and the oxygen concentration could be calculated by measuring the helium concentration (with a known diluent composition). Thus, the helium sensor not only serves for the correct calculation of decompression based on the actual He content, but could be used in emergencies to measure the oxygen concentration under certain conditions. There are two He sensors, with each control unit having its own.
Ah of course. In the loop! Clever! Dual computers as well, right?
Of course, a duplicate computer is necessary for fault-tolerant devices. Each computer has its own battery and is connected to the other only by a data bus. The data bus is, of course, waterproof and short-circuit-proof, so no conceivable failure of one computer could affect the other.
I believe your back mount CCR came first, yes? What motivated you to create a sidemount unit as well?
Back mount CCR came first because it was and still is the main type of rebreather arrangement. Sidemount was the logical successor of the back mount. It uses the same body and a variety of other parts as the back mount but is horizontally mounted on a special rack. The need for sidemount originally came from cave divers, so we wanted to provide them with the opportunity to use Liberty in this arrangement as well.
Is a sidemount CCR a specialized niche product, or do you think it will replace or at least equally compete with back mount CCR?
Liberty sidemount is a complete rebreather, including bottles with diluent and oxygen. It can be used not only for cavers, but also as a backup rebreather, handed to another diver without having to switch hoses from off-board gases.
The number of divers who use sidemount as the main unit during diving is increasing. The sidemount’s arrangement has some advantages over the back mount and there are now known and proven procedures as well as technical equipment for diving with sidemount. Its growth is inevitable. While I do not expect the back mount to be replaced altogether, it seems the usage of sidemounts is increasing and many divers will choose it as their main unit.
What would you say are the critical issues right now in CCR diving?
The biggest problem is divers pursuing rebreather diving without adequate training. They see the usage of rebreathers increasing and think that diving with a rebreather is a common thing. But the fact is that diving with a rebreather is still more demanding in terms of skill and self-discipline than an open circuit, and divers who dive with a rebreather without undergoing any type of training are putting themselves at risk.
Would you say CCR diving is getting safer?
It is a little safer, especially since divers, instructors, and rebreather manufacturers are taking lessons from past accidents to try and avoid them. (Similar to the efforts of those in the auto and aviation industries.).
I see you have just added Synchrony Bank financing option for purchases of Divesoft gear. How did that come about? Has financing been a problem for dive consumers?
Synchrony Bank has been funding our products since the beginning of August solely for the US market where rentals are popular and widespread. We wanted to accommodate customers purchasing our goods by credit card.
Where is Divesoft headed? Do you have a collective vision for the future? What’s on the horizon?
We want to be innovators in the field of equipment for technical divers with a significant overlap in the field of recreational diving, of course. In addition to analyzers, our main domains are still rebreathers and diving computers. In these areas, we are already in development and planning stages for major improvements and innovations. We are currently preparing several innovations to be launched in November 2019 at DEMA, followed by BOOT 2020 in Dusseldorf. Stay tuned for those. [Ales smiles]
Divesoft has been sponsoring the exploration of Hranice Abyss. Are there other projects that Divesoft is sponsoring as well?
We sponsor the Greek Seahorse Rescue Station Hippocampus Marine Institute, as well as Czech police divers during mine clearance operations in the waters of the Sava, Una, and Drina rivers in Bosnia and Herzegovina. Last but not least, the Hranice Abyss.
Thank you so much. You have taken a unique approach, and it’s evident that it is really paying off for you. Congratulations!
Aleš Prochoaska’s patent: Device for measuring oxygen concentration in gas mixtures containing helium and/or hydrogen
Michael Menduno is InDepth’s executive editor and, an award-winning reporter and technologist who has written about diving and diving technology for 30 years. He coined the term “technical diving.” His magazine “aquaCORPS: The Journal for Technical Diving”(1990-1996), helped usher tech diving into mainstream sports diving. He also produced the first Tek, EUROTek, and ASIATek conferences, and organized Rebreather Forums 1.0 and 2.0. Michael received the OZTEKMedia Excellence Award in 2011, the EUROTek Lifetime Achievement Award in 2012, and the TEKDive USA Media Award in 2018.
Forbes Magazine article (In Czech):
The RB80 Semi-closed Rebreather: A Successful Exploration Tool
What rebreather has arguably logged the most exploration kilometers since its market introduction in 1998—an estimated 160 km plus (100 miles plus for you Imperialists)—and continues to rack up the klicks? It’s Halcyon’s RB80 passive-addition, semi-closed rebreather. Here WKPP greybeard and RB80 instructor trainer David Rhea reports on the RB80’s history, design & workings, training, and he offers the lowdown on its new sidemount progeny, the RBK. Looking for an electronics-free, sidemount bailout rebreather? Halcyon may just have your number.
By David Rhea
Header image by David Rhea
Full Disclosure: Halcyon Dive Systems is a sponsor of InDepth.
In the early 1990s, the cave exploration conducted by the Woodville Karst Plains Project (WKPP) in the Woodville Karst Plains of Florida, especially Wakulla Springs, was becoming quite complicated. With dives averaging depths of 89 m/290 ft, with penetration being measured in miles, and decompression taking hours, it was becoming obvious that rebreathers would be necessary to move forward. In 1996/97, the WKPP began using a semi-closed circuit rebreather known as a Passive Variable Ratio-biased Addition Semiclosed Rebreather (PVR-BASR), nicknamed “The Fridge,” to extend their exploration and decompression obligations. This piece of equipment was a very large, bulky, and complex unit, and while it was uncomfortable above and below the water, the PVR managed to do what was intended and allowed for further exploration.
In 1996, a team of European explorers called the European Karst Plains Project (EKPP), who utilized the “Doing It Right” (DIR) techniques and philosophy of the WKPP, began using a semi-closed rebreather called the RB-2000. The unit was developed by the EKPP founder and director Dr. Reinhard Buchaly, who was inspired by the great success of French cave explorer Olivier Isler had at Doux de Coly and other cave systems using a custom made, triple redundant, semi-closed rebreather, the RI 2000 designed with the help of French engineer Alain Ronjat.
The RB-2000 unit was much smaller than the PVR-BASR, and utilized a very clever, intuitive, and reliable design. This design complemented the DIR philosophy used by both teams, and would become the choice for both groups moving forward. By 1999, WKPP explorer Jarrod Jablonski and Robert Carmichael, the owners of Halcyon Dive Systems, worked out a deal with Buchaly to have Halcyon manufacture, sell, and service an American version known as the RB80.
Designing a Semi-closed Rebreather
Being a cylindrical design 185 mm/7.28 in. in diameter, and 660 mm/25.98 in. tall —virtually identical to the size of an aluminum 80 cylinder—helped the RB80 get its name. The RB80 was designed to fit between the cylinders of a double tank configuration utilizing a specially designed frame, manifold, and switch block system. The economy of parts allows for maximum efficiency with only about 130 parts total.
This design complemented the DIR philosophy of maintaining all of the diver’s back gas for emergencies, utilizing stage tanks for exploration and decompression. The gas switches utilized the same procedures taught by Global Underwater Explorers (GUE) but instead of swapping regulators from the mouth, the diver plugs a special QC6 swagelok fitted hose from the stage bottle into a switch block that feeds the gas into the rebreather.
The stage bottle regulator is a standard open circuit (OC) configuration with the addition of one extra hose with a QC6 connector. The switchback also allows for a hose from the back gas to be plugged into the block in case of a stage failure or other emergency.
The vertical design of the RB80 has a very clever water removal tube that runs directly through the center of the scrubber bed and vents water along with a small volume of discharge gas. The unit is a passive addition semi-closed design, with no depth compensation and is tied to the diver’s respiratory rate. Roughly 1/10 of the respired volume of breathing gas is discharged into the water with each breathing cycle via the inner bellows and a familiar over pressure relief valve (OPV), commonly used on most dry suits. See Figure 2 and 3 below.
The RB80 utilizes a dual bellows counterlung system (versus traditional counter lungs), which reduces the loop volume each breathing cycle. When the loop volume is sufficiently reduced, it triggers the injectors made of components of an open circuit regulator that function quite similarly. Once the injectors fire, the loop volume is replenished with fresh gas. The unit has dual injectors for redundancy, which can be isolated at the switchblock if necessary.
The scrubber bed lies above the bellows in this vertical design and is manually filled by the diver before each dive. The scrubber is a 3.2 kg/7.05 lb design and will last approximately ten hours, based on more than twenty years of operational experience. Note that semi-closed rebreathers generally get longer duration on a scrubber given that a percentage of the breathed gas is expelled and replaced with fresh gas.
The mouthpiece design incorporates a bail out valve (BOV) allowing the diver to switch from the rebreather to OC at the turn of a lever conveniently located in the center of the mouthpiece block. A hose routed from the right post regulator of the back gas is always live and gives gas immediately once the lever is turned.
Assessing the Work of Breathing
New rebreather divers often state they feel the RB80 has a lot of breathing resistance. This is generally due to the fact that they are accustomed to modern OC second stages which deliver almost effortless on-demand breathing. Typically, modern second stages use VIVA (Ventura-Initiated Vacuum Assist) technology. This technology, along with the geometry of the second stage and the fact that the second stage is balanced, make for incredible light cracking effort. Upon inhalation, the initial cracking effort lowers the gas volume in the case, which pulls down the flexible diaphragm, activating the lever that opens the valve and allowing gas to flow to the diver. The VIVA then keeps the gas flowing at the same rate without the need for the diver to continue to draw on the regulator.
By comparison with the RB80, the diver is simply pulling the gas through a one-way check valve, drawing the available gas in through the right inhalation hose, beyond the valve, into the mouthpiece block, and into the divers mouth. This entire system of gas flow does not have any boost effect like its OC sister and therefore feels as if you are working hard when, in fact, it is quite effortless. As a “virtually” closed loop, one simply draws the available gas through the inhalation hose into the diver’s mouth, and then exhales out a one-way valve at the mouthpiece, back through the left exhaust hose into the breather where the gas is scrubbed of CO2, water is removed, and the gas is replenished.
Similar to OC, the rebreather does vary in breathing performance based on the diver’s position. If you have ever stood on your head diving OC, you feel a change in performance, as the second stage is much deeper than the lungs. The RB80 historically being a back mounted rebreather keeps the unit at a slightly shallower depth than the diver’s lungs, making inhalation slightly harder and exhalation slightly easier. This difference is virtually indistinguishable; however, extreme head down or head up positions can seriously affect rebreather breathing efforts. When worn in a side mount or stage position, the unit is in equal position with the lungs, making for very easy breathing. Fortunately, when a diver is in near perfect trim, the RB80 performs best, as this is the ideal position for ease of breathing.
Extending a Diver’s Breathing Gas
The RB80 is a serious gas extension device, providing 8-10 times the gas mileage of OC. By rebreathing one’s gas and only losing 1/10 of each breathing cycle into the environment, the RB80 can take a single aluminum 80 cf/11 ltr stage bottle and turn it into roughly 640 cf/18m3, or the equivalent of eight AL 80s.
In cave exploration, we always start a project by setting up the cave with “safeties.” These are caches consisting of two bottles each equipped with a stage regulator complete with an OC second stage as well as a QC6 equipped drive hose to plug into the RB80, and a submersible pressure gauge (SPG). These bottles are placed roughly every 3,000-5,000 ft/900-1,500 m in the cave, and will remain there throughout the exploration. The safeties are checked by support divers prior to every push to ensure function and adequate gas volume. The bottles are properly filled and marked with the proper Maximum Operating Depth (MOD) gas for the dive, and they are labeled “SAFETY.”
With rebreather diving, it is paramount that adequate bailout gas be available in case of a single point failure on the rebreather. Rebreathers, while quite robust, have many single failure points, i.e., the breathing hoses, one way valves, OPV, the bellows in the case of the RB80, and even the diver’s mouthpiece. As mentioned, the injectors have redundancy and can be isolated in case of issues, and a spare mouthpiece is always carried by the diver in case of a serious tear or damage.
Any other single point failure could render the rebreather inoperable, forcing the diver to return and complete all decompression on OC, demanding eight times the amount of gas that had been used at this point in the dive. So, in addition to 100% of the back gas being maintained for bailout, cave exploration demands the discipline of staging the cave with safety bottles, safety scooters, as well caches of decompression gas, and proper support personnel.
RB80 vs. an Electronic-controlled Closed Circuit Rebreather (eCCR)
A variety of eCCRs are available by manufacturers. These units are extremely efficient, as no gas is lost from the breathing loop. The eCCR can be 25-50 times more efficient than OC. However, in addition to the single point failures listed above, which are common on all types of RBs, the eCCR, has additional concerns that prevent it from being a consideration for many cave exploration groups like the WKPP and GUE-affiliated El Centro Investigador del Sistema Acuífero de Quintana Roo (CINDAQ) foundation, which hosts the Mexico Cave Exploration Project (MCEP) in the Yucatan.
Most eCCRs have three oxygen sensor cells that must be meticulously maintained and work together with a solenoid and an electronic controller, using a concept called voting logic. Together with an oxygen bottle and a diluent bottle, the eCCR mixes the diver’s gas during the dive within a (PO2) set point range that is predetermined by the divers. By having three oxygen cells, the controller will side with the two that have the most similar reading if one were to start to read differently from the other two.
Unfortunately, voting logic is inferior to the gold standard—triple redundancy: main unit, back up, back up for the back up—and has been known to be incorrect i.e., in the case of a double cell failure. Discipline, and pre- and post-dive maintenance, are the key to maintaining good sensor reliability.
When diving an eCCR, it is necessary for the diver to constantly monitor the gas mix in their loop in order to ensure that they safely avoid hypoxia or hyperoxia. For an easy-to-see reminder that the unit is working within the safe limits set by the diver, most eCCRs rely on a heads-up display (HUD)— generally mounted to the inhalation hose—that shows a small series of lights indicating green for good, yellow for caution, and red for danger, in case the PO2 in the breathing loop is getting out of range. This is of course driven by a controller that gives real time PO2 that can be viewed on the diver’s handset. Most eCCRs provide at least one handset as well as the HUD to ensure proper redundancy.
One of the reasons many cave exploration groups like the WKPP strictly use the RB80 is its simplistic mechanical, reliable design. With the RB80, the gas is premixed into the stage bottles, and the back gas is always mixed for the MOD of the max depth expected to be reached during the dive. With the RB80, there is no gas mixing during the dive; the gas is plugged into the switch block similar to doing an open circuit gas switch. The gas is filled, properly analyzed, and the content label is attached to the neck of the bottle prior to leaving the dive center.
The bottles all have properly placed MOD stickers on two sides of the bottle for easy identification by both the diver and his team mates, plus a MOD sticker placed on the bottom of the cylinder that can be identified by teammates when being viewed from behind. In the water, the proper stage bottle is selected for the MOD, and the gas is safely plugged in at the proper switch depth, but only after the bottle has been properly identified, verified by the buddy, and the drive hose confirmed with the bottle that has been chosen, similar to open circuit gas switches.
The most dangerous thing about the RB80 (and semi-closed units in general), is the oxygen drop, especially in shallow water [See the Loop Gas calculations section of the RB80 page in Wikipedia]. Due to the fact that oxygen is being consumed during respiration, and gas is discharged from the inner bellows with each exhalation, the oxygen drops slightly with each breathing cycle until fresh gas is replenished from the injectors, typically every two to four breaths. For this reason, one must be cautious when using the RB80 at shallow depths (when the ambient pressure is low) or when using mixes with a lower oxygen fraction as a travel gas.
The drop in oxygen levels also means there is a slight increase in inert gas that remains in the loop and that needs to be taken into consideration for decompression. Both of these nuances of the RB80 are easy to calculate and adjust for prior to the dive.
During RB80 training, both the oxygen drop and the increase in inert gas load are addressed and easily able to be factored in. The theory is discussed in an RB80 class, and software is available to easily do quick calculations. All of this can then be programmed into GUE’s Buhlmann-based desktop decompression program, DecoPlanner, for proper dive planning. Like most rebreathers, the RB80 has additional complexities requiring proper pre-dive assembly, testing, maintenance, and post-dive discipline.
Training on the RB80
The WKPP was established in 1995, and from the beginning, adopted a standardized approach to gear configuration and procedures. Initially, this approach was called “Hogarthian,” after early WKPP pioneer Bill Hogarth Main. Later, project director George Irvine added to the standardization and coined the phrase “Doing It Right,” or DIR, to represent this standardized approach. In 1998, Jarrod Jablonski founded GUE, which offered exploration-based training utilizing WKPP’s standardized approach and gear configuration. Once Halcyon started building the RB80, GUE began offering formal training. Currently they are the only training agency to do so.
From the beginning, GUE’s RB80 training has been exploration-based, with a heavy emphasis on failure-based training i.e. dealing with equipment failures as a team, similar to other GUE courses. Exploration-level cave diving has complex exposures that require divers to return from deep inside the cave, and then make a vertical ascent to return to the surface. With the addition of the RB80, divers are able to extend their penetrations exponentially, adding as much as 12-14 hours of decompression on some dives alone. Conventional rebreather training does not properly prepare someone for these types of exposures.
GUE divers have historically been required to take Fundamentals, Tech 1, and Tech 2 with a minimum of 25 dives at each level between classes prior to beginning their RB80 training. This is in addition to the Cave 1 and 2 level training and experience required to begin cave exploration. The investment of time, energy, and resources necessary to become a GUE/WKPP exploration cave diver makes for a very serious explorer who has the skills and experience necessary to conduct dives with this level of exposure. The failure-based training also builds the diver’s confidence, repetitive learning, and instincts necessary to safely explore.
One of the many reasons for the long term success of the RB80 has been this extremely regimented training by GUE’s four active RB80 instructors. In addition to the most intense and demanding rebreather training available, GUE RB80 students must purchase the unit prior to taking the training. This alone narrows the attendance to only the most serious explorer, as no rental option is considered.
Until fairly recently, GUE divers were the only ones using the RB80. Even then, only those willing to take the robust training who had an exploration mindset learned to dive the unit. Currently there are 150-200 GUE divers certified to dive the RB80. The discipline and attitude of these explorers has ensured that the RB80 has been responsible for more kilometers/miles of cave exploration than any other rebreather in the world. I estimate that more than 161.6 km/100 mi of cave passage has been explored using the RB80.
The discipline and attitude of these explorers has ensured that the RB80 has been responsible for more kilometers/miles of cave exploration than any other rebreather in the world. I estimate that more than 161.6 km/100 mi of cave passage has been explored using the RB80.
A Specialized Exploration Tool
For 30 years the WKPP has been mapping the underwater labyrinth of the Woodville Karst Plain, having mapped over 56,609 m/185,000 ft of cave passage with more than 35,189 m/115,000 ft below 58 m/190 ft. The RB80 has been one of the most vital keys to this success, including the world record dives in Wakulla and the following traverse. It is the only rebreather used for exploration on Woodville Karst Plain projects. Presently, virtually all exploration being conducted by the WKPP below 61 m/200 ft is exclusively done on the RB80.
Over the years, and especially during the Wakulla exploration heydays, one of the growing concerns was running out of scrubber material during the dive. On the biggest dives, the entire RB80 double tank configuration would be swapped at the deep portions of the decompression for a fresh ‘breather with smaller double five-liter bottles and fresh scrubber material. Note: an advantage of the RB80 over an eCCR is that the valves can be closed and the unit reliably stored underwater like a stage bottle for bailout. It can then be quickly turned on and dived.
In 2008, CINDAQ’s MCEP project also adopted the RB80 and has done countless hours of exploration in the caves of the Yucatán. Between January 2018 and December 2020, for example, MCEP exploration divers mapped in excess of 180,000 m/594,000 ft of new cave passage in Ox Bel Ha alone using RB80 technology.
As CINDAQ board member and co-owner of Zero Gravity Dive Center, Christophe Le Maillot, explained, “It is such a sturdy and intuitive unit. In all the years we used it, we have never had to terminate or cancel a dive because of a malfunction. It’s a real work horse!” Like their sister WKPP team, the MCEP exclusively uses GUE-trained RB80 divers for their exploration dives.
GUE divers have also utilized the RB80 for cave exploration projects in China, the Nullarbor caves in Australia, caves in the south of France, cave and wrecks of Italy including the Pantelleria project, the Alviela cave project in Spain, and other karst areas around the world. In addition to cave exploration, the RB80 has been utilized by GUE divers on the west coast for the ghost net removal, and by GUE wreck divers in Canada and around the world.
Introducing the RBK, a Sidemount RB80
In response to explorers wanting a stageable version of the RB80 as both a travel and/or bailout rebreather, Halcyon began working to develop a modified sidemount version of the RB80, called the RBK. The first version was called the RBK1 and after several years of modifications Halcyon produced two more revisions, the RBK 2 and the RBK 3, referred to simply as the RBK.
The overall diameter of the RBK is the same as the RB80, but by reducing the height of each section, the overall length of the unit has been reduced to 50 cm. Though the scrubber was reduced in volume to 2.4 kg/5.29 lb, the scrubber duration is rated for approximately eight hours based on user experience [See InDepth’s Rebreather Holiday Shopping Guide for add’l spec details]. Because of the smaller form factor, the RBK offers a 6-8:1 gas extension versus 8-10:1 on the full RB80.
The sidemount RBK has been used as a sidemount, travel, and bailout rebreather by both the WKPP and the MCEP, which has been testing and helping to refine various RBK prototypes since 2015. On recent long-range explorations through small passages, the RBK has proven to be an outstanding tool for shallower, long distance cave exploration. MCEP instructors are now working with the GUE board of directors and other RB80 instructors to develop a RBK sidemount training course, which should be available in the not-too-distant future.
New Non-GUE Users
Over the last few years, Halcyon has made the RBK available to select non-GUE divers. They have sold custom versions of the RBK to militaries around the world. In addition, they have provided RBK units to exploration divers from Karst Underwater Research (KUR), who have been using the RB80 as a side mounted bailout breather for their recent long range exploration dives at Weeki Wachee Springs and other systems. The divers received their RBK training directly from Halcyon.
As KUR project director Andy Pitkin put it, “It is undeniably true that ‘simplicity is the ultimate sophistication,’ as Leonardo Da Vinci once noted. The RBK has proved itself to be close to a perfect tool for our particular application, far exceeding my initial reserved expectations.“
From its conception, it was quickly obvious that the RB80 would be around for a very long time. The simplicity, safety, and the robust mechanical nature of the unit, combined with rigorous training, and highly experienced users, arguably make RB80 and RBK the ultimate exploration tools.
InDepth’s Rebreather Holiday Shopping Guide (2020)
Halcyon: Using The RB80 As A Sidemounted Bailout Rebreather by Andy Pitkin, Karst Underwater Research (2018)
GUE: DOUX DE COLY: GUE Expedition with RB80 (2004)
Introducing the RB80 by Michael Waldbrenner and Dr. Reinhard Buchaly
Deep Tech: Victory At Last (1998): Olivier Isler is setting penetration records with a triple-redundant semi-closed rebreather
David Rhea is an active GUE instructor and instructor evaluator, having been with GUE since the earliest days. An avid explorer with the WKPP since 1998, David has explored caves in China, Florida, Australia, Mexico, and France. A passion for diving started at age six, leading David to make his first dives at age nine. He became a scuba instructor at age 18. David has worked full time in the scuba industry for over 40 years, and has worked for Scubapro since 1995. David is as passionate today about exploration, teaching, and underwater photography and managing his Florida Scubapro territory as he has ever been.
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