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by Jakub Šimánek
Photos courtesy of Divesoft unless otherwise noted. Illustrations by Aleš Procháska. Header photo by Martin Strmiska.
Full Disclosure: Divesoft is a sponsor of InDepth.
With open circuit diving, there is a general consensus that what is simple is safer and more functional, and the community has adopted simple, yet sufficiently redundant configurations. However, as rebreathers have come to the fore, it is time to ask what is necessary and simplest, and what can no longer be simplified.
The simplest way is not always the best way
The simplest, functionally ingenious, and—in principle—trouble-free rebreather is undoubtedly the oxygen rebreather. Simplicity itself. Breathing bag, CO2 absorber, oxygen bottle, oxygen regulator, breathing hose, and directional mouthpiece is all you need. I wish this was the final solution! Unfortunately, as we know, with only oxygen we can not safely dive very deep and it is necessary to complicate the device quite a bit to do so.
Rebreathers have been developed through semi-closed, electronic closed circuit (eCCR), manual closed circuit (mCCR, and passive semi-closed (PSCR). We already know that if we want a safe device that allows us to dive deep, we cannot do without electronics, because all mechanical solutions have their limitations. SCR wastes too much gas, PSCR is much more economical, but decompression is not ideal. Manual CCR is as powerful as electronically controlled closed circuit in terms of gas savings and decompression efficiency, but has depth limitations due to the blocked reference ports in the first stage regulator, which sense ambient pressure. As a result the intermediate and ultimately the low pressure delivered by the regulator does not increase with depth.
While an open circuit regulator valve delivers a fixed pressure of gas above ambient pressure, a mCCR constant flow valve, aka a leaky or trickle valve or fixed orifice, delivers a fixed pressure of gas, in this case oxygen, independent of depth. Accordingly, the valve will deliver oxygen until the ambient pressure is equal to the pressure of gas exiting of the flow valve. So, for example, if the flow valve delivers a pressure of 10 bar, the depth will be limited to 90 m/294 ft or 10 bar of pressure ATA.
This is not the only thing that is problematic on mCCR. mCCRs have been designed with maximum simplicity and with the elimination of “dangerous” electronics—excluding oxygen sensors—in mind. This moves the most critical part, i.e., the control of the partial pressure of oxygen, to the constant flow nozzle with manual addition by the diver. However, the diver is a human who can suffer from bad moods and bad concentration. They can underestimate the situation, have limited ability to concentrate on multiple things at once, especially in ‘critical’ situations and are impacted by limited visibility, cramped space, inert gas effects and great depths etc.
We want to avoid electronics and take control, but we must acknowledge that the weakest link in the whole chain is ourselves. How does our human error rate compare with electronics? How accurate are our oxygen injection calculations vs the machines? Aren’t human factors the hottest topic of recent times in the diving world? The answers are obvious when we consider that one makes three to six mistakes per hour.1
Personally, not counting the testing of pre-production prototypes, I have not had a dive computer fail underwater since 1996. There are no statistics on underwater electronics failure, but I would argue that the ratio of human error rate vs. the error rate of electronic dive computers is quite high. And what is the difference between a dive computer and a rebreather control electronics? Only that the control electronics of the rebreather processes information from multiple sources and has a software algorithm for controlling the solenoid.
Yet some people choose mCCR anyway. Research2 has even shown that people are more willing to trust other people and forgive inevitable human mistakes rather than trusting computers.
Machines can’t think, and we can’t do that for them. We have to think for ourselves, but we can entrust straight forward calculations to computers. They are much better at it than we are.
Important Things Must Be Redundant
Electronic devices have become an integral part of our lives. We wake up with them, we move with them on the ground, in the air, in space, and underwater. We spend the whole day with them, and we fall asleep again with them in the night. Some electronic devices work 24 hours a day, 7 days a week.
Electronics help us in critical situations. When we are driving a car (ABS and other electronic systems), we let it navigate us from point A to point B. We entrust our lives to it when we travel by airliners, which today cannot function without electronics at all. The development of electronics is constantly advancing, but we must admit that electronics, like any other part of the device, can fail. If it is a mobile phone or a television, it is not usually a tragedy, but if it is a hard drive on which you have your family photos, or the results of your many years of work, for example, or a device on which human life depends, the data or device must have a reliable back up.
We all know it from both diving and everyday life, that those who do not back up their data will be sorry when they lose it. Those who do not have a backup plan in diving, a backup source of gas, can lose their lives. We are talking about redundancy.
Redundancy when diving with open circuit means having two first and two second stages, a buddy team, enough gas for the diver and their partner to surface, a buoyancy compensator that is backed up by a dry suit and any measuring device, such as a computer, sufficiently backed up by a second measuring device. So there is partly a kind of team backup. CCR failures cannot neccesarily be solved by team backup; a complete OC bailout ascent should be the last solution when there is no other option left. CCR cannot be backed up by a team member; CCR cannot be shared by two divers. The rebreather must contain its own back up.
What is a Fault Tolerant System?
A fault tolerant system is a feature that allows a system (often a computer system) to continue to work properly even if one of its components fails. Fault tolerance is desirable for systems with high availability or providing vital functions such as a space shuttle or an aircraft, so why not a rebreather? Life depends on it just as much.
As already mentioned, a rebreather is a complex system. What does such a system consist of? The electronic control unit receives information from several sensors, evaluates the data, and calculates the appropriate next action such as firing the injector, adjusting the decompression calculation, etc. The input systems are as follows: a pressure sensor, oxygen sensor, RTC (real time counter), and possibly additional helium or CO2 sensors. We also need a power source, i.e., a battery, and a user interface in the form of a handset or a simple display of PO2 values. Those are the basics.
Therefore, in order to appreciate what a fault tolerant rebreather is, let’s first look at a standard eCCR as shown in Fig. 1. The system is very vulnerable. A single error can cause us to either execute special skills or go straight to bailout. Try to imagine a failure of a battery, a single sensor, a solenoid, or a control unit.
The next diagram illustrates a fault tolerant rebreather schema—namely the Divesoft Liberty CCR (Fig.2)
We start the dive with a fully functional rebreather. The loop is not very different from a conventional rebreather, except that all elements are doubled (a manual add valve (MAV) and automatic diluent valve (ADV), an oxygen MAV and two solenoids). In addition, the electronics are doubled: two control units, two main displays, two secondary displays (HUD and buddy display (BD), solenoids, batteries, and all sensors are doubled. These are actually two self-sufficient systems that are interconnected and can communicate with each other.
Let’s phase out the individual components and observe how robust and resilient the fault tolerant system is. Let’s say I just cut off the handset cable in the wreck. Nothing is happening, I still have a second handset that shows me all the data and I am able to control the whole device. In addition, water does not leak into the device, because the cables are protected by watertight partitions on all inputs and outputs.
Oops, I broke the second handset against a rock. Still, nothing is happening. All the sensors work and I am able to read the ppO2 from the HUD. One of the batteries is exhausted/failed. This means the loss of one CU. But, I still have two oxygen sensors and therefore two O2 sensors, depth sensors, a solenoid. I can still continue on the unit and return safely to the surface without any need to intervene.
How To Make A System Fault Tolerant
The fault tolerant rebreather design consists of three basic parts:
1. A robust software solution
2. Hardware redundancy
3. A fault detection system
Complex software for modern CCRs consists of individual software modules. The software modules (tasks) are: ppO2 measurement, ambient pressure measurement, ppO2 regulation, decompression calculation, and the user interface. Which of these components probably experiences the most errors? Of course, every software engineer sees it right away; it’s the user interface—the most complex part of almost any software. What’s with that?
The solution is simple. We separate the user interface into another hardware-separated unit, i.e., a handset. It communicates with the rest of the system, i.e., with the control unit via the bus, in a fixed, formal, precisely defined manner. In the event of a fault, it only stops working, but the vital core of the system goes on (Fig.4).
Because the control unit (CU) does not include a user interface, it can be programmatically divided into small, simple modules that are easier to verify.
The second principle is hardware redundancy. For hardware redundancy to really work as fault tolerant, it requires a sophisticated system.
Figure 5 shows a primitive rebreather without redundancy. One mistake is enough and it is inoperative (Fig.5).
We improve the system by placing three oxygen sensors. In addition to these, we connect a backup monitoring system, and the battery is doubled (Fig.6).
Figure 7 shows another improvement: a separate oxygen sensor for backup. As in the previous case, the backup is short-circuit-proof on the measuring bus (Fig.7), all of which combine to form a fail-safe system. If any one element fails, it may not necessarily remain functional, but we will immediately learn about the problem and go to the bailout.
Further improvements: everything is completely doubled. In case of any one fault, the system remains functional (Fig.8). In this case, we can already talk about a fault tolerant system.
Third principle: a fault detection system? If I only have two oxygen sensors (Fig. 9) in each part of the system, it is not possible to automatically evaluate which one is defective (but a diver can decide manually because he or she sees all four sensors). How do we solve the problem?
Add three plus three sensors. This will, of course, double the cost of sensors. (Fig.10)
Alternatively, enable each system to see all four sensors. The problem is that it is not at all easy to prevent a short circuit on one system (ex: flooding with salt water) not to short the other system at the same time (Fig.11).
Or, simply connect both systems via a bus, through which they can send the measured values from the sensors to each other. The bus is much easier to disable in the event of a short circuit thanks to analog measurement.
And, other sensors (depth, temperature, etc.) are treated in the same way, effectively doubling them.
Internal complexity does not mean complexity for the user.
If we look at the fault tolerant rebreather scheme, one may think that the system is too complex, but that’s just on the inside. We have already shown that an increased number of components does not lead to a greater risk; on the contrary, in the case of a completely redundant system, it leads to greater safety.
Externally, on the other hand, the control of such a system is very simple and the user does not feel the internal complexity at all. It’s like driving a modern car full of the latest cutting-edge technology, and all we need to do is step on the gas and turn the steering wheel and enjoy the d(r)ive.
- Edkins G., Human Factors, Human Error and the role of bad luck in Incident Investigations, May 23th 2016
- Andrew Prahl and Lyn M. Van Swol; “The Computer Said I Should: How Does Receiving Advice From a Computer Differ From Receiving Advice From a Human,” presented at the 66th Annual International Communication Association Conference, Fukuoka, Japan, 9-13 June 2016.
- Procháska Aleš; Principles of Fault tolerant rebreather“ 2016, powerpoint presentation
aquaCORPS #12 Survivors: Designing a Redundant Life -Support System by William C. Stone (1995)
Jakub Šimánek graduated as a biology and physical education teacher from Charles University in Prague. Thanks to his father, he has been diving since childhood. He transferred his experience from teaching, biology, diving and sports to his instructor activities. Since 2012, he has been working in Divesoft, where he participates in the analysis and development of diving equipment, mainly rebreathers. He has been working as a Factory Instructor Trainer since 2014 and is an author of training procedures for this device. He is currently actively involved in developing and diving with bailout rebreathers.
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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The Life & Times of a West Coast Photogrammetrist: Could it be the Almirante Barroso?
Seattle-based instructor and photogrammetrist Kees Beemster Leverenz recounts the challenges he and his team faced trying to amass sufficient detailed...