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Rise of the Recreational Rebreather

An explorer begets the Explorer, the new Hollis Gear semi-closed unit that just may change and reinvigorate sport diving 

Text by Michael Menduno

Photo: Hollis


Kevin Gurr has been passionate about computing and rebreathers since his first rebreather dive in 1987. The prolific 54-year old British explorer, tech instructor and engineer got his start rebuilding the electronics for the Biomarine CCR155—one of the first mixed gas rebreathers to be adopted by sport divers—and went on to build the first nitrox dive computer, the ACE Profile, in 1991 before most divers could even spell n-i-t-r-o-x. He sold several hundred units and planned to add a data logger, gas sensor, onboard analyzer and ‘look-ahead’ capability, but the project got shut down for lack of funding. Two years later he released Pro Planner desktop decompression software to address the needs of the emerging technical diving market.

In 1997, Gurr rolled out the world’s first mixed gas diving computer, the VR3, which he manufactured and distributed through his newly formed company VR Technology Ltd. based in Dorset, UK. The VR3, which was capable of handling multiple nitrox, trimix and heliox mixes was also the first computer that supported closed circuit rebreather (CCR) diving, and later, the first to run the popular Varying Permeability Model (VPM)—the decompression model behind ‘deep stops’. As a result, the VR3 quickly became the computer of choice among technical divers and remained so for more than a decade.

But Gurr, who mentored with rebreather pioneer Dr. Bill Stone in the 1990s, wanted to build his own rebreather. In 2005, VR launched the Ouroboros closed circuit rebreather, aka ‘Boris’. Three years later they rolled out the Sentinel, the first CCR to incorporate a gaseous carbon dioxide (CO2) sensor capable of detecting a scrubber failure as part of a comprehensive CO2 monitoring package. That year, Gurr was awarded Eurotek’s Lifetime Achievement Award for his contributions to technical diving.

Now the soft-spoken tech pioneer has turned his engineering prowess on the challenges of recreational diving.

Recreational Rebreathing

His latest brainchild, the Explorer, a ‘recreational rebreather’ that features a sophisticated resource management system that Gurr first envisioned more than 20 years ago, is being rolled now to select dive centres around the world. The unit is manufactured and distributed Hollis Gear of San Leandro, California, which purchased the technology from VR Technology, which will continue to work with Hollis to provide R&D and support.

Though the Explorer is the latest of three ‘Type R’ rebreathers (the R stands for Recreational vs. T for Technical) that have been certified by PADI, along with the Poseidon MKVI and the Ambient Pressure Diving (AP Diving) Evolution (recreational version)—call it Evolution(R)—closed circuit rebreathers, it’s fair to say that Gurr has taken a different approach.

While other manufacturers asked the question, “How do we make a closed circuit rebreather safe for recreational divers” in designing their unit, Gurr took a more fundamental approach asking, “How do you make rebreathers safe for recreational divers?” The difference is profound. As he explained to me over the phone, “When you look at what recreational divers really need and factor in their experience and attention levels, you realize that you have to be way more design specific with regards to depth and duration limitations. Then you can really streamline the product.”

The result of Gurr’s ruminations is a unique, 32-pound (14.5kg), electronically controlled semi-closed rebreather that costs under $5,000 and may well represent a tipping point in making rebreathers accessible to a wider base of users. Industry insiders who are familiar with the unit call it a potential game-changer.

Automating Out Human Error

It’s been two years this November since PADI—the world’s largest dive training agency—launched its rebreather training program for recreational divers. The program is based on a new class of machines—so-called Type R units—that automate many of the functions of rebreathers, and in some cases make decisions for the diver, making them easier to use and therefore reducing the potential for user errors which can lead to accidents.

Rebreathers offer many benefits over open circuit SCUBA, including extended duration, near optimal decompression (read: longer no-decompression limits), improved thermal profile, reduced weight and the ability to get closer to wildlife without those noisy bubbles. And don’t forget the ‘cool’ factor. But their complexity and maintenance requirements have resulted in a poor safety record that has besmirched the technical diving community—one of the few groups outside of the military to use mixed gas rebreathers—over the last decade and a half.

A recent study examined 180 recorded rebreather related deaths from 1998-2010 and found that fatality rates for rebreather diving were 8 to10 times higher than for open circuit SCUBA, or about 4 deaths/100,000 dives compared to about 0.5 deaths for SCUBA. In almost all of these incidents the primary trigger was the human-machine interface, that is ‘user error’. For example, divers forgetting to turn on their rebreather, not analyzing their gas, assembling the unit incorrectly, not packing their scrubber properly, or misinterpreting sensor data and making poor decisions. Divers also engaged in risky behaviors such as exceeding limits, carrying insufficient bailout and solo diving.

Photo: Rosemary E. Lunn / The Underwater Marketing Company

Military users, who benefit from a high degree of training and discipline, manage these risks through standards-based diving including mandatory pre- and post-dive checklists, adherence to the buddy system, the use of full-face masks and or mouthpiece retaining straps to prevent drowning in the event that a diver loses consciousness, and the reliance on diving supervisors, along with an extensive support infrastructure. However, these factors are largely absent in the sport diving community.

Instead the recreational industry has turned to automation, increased engineering and simplified diving protocols to overcome what is essentially our human fallibility and lack of consistency.

As PADI’s vice president of rebreather technologies Mark Caney explained to me, “The only way to reduce errors with recreational divers is to have the rebreather making decisions and shielding the diver from complexity that they don’t need to know.” He compares it to a car with an automatic transmission, anti-locking brakes, power steering, auto-lighting and active cruise control.

“I know that’s alarming to some people,” he acknowledged. “But we routinely rely on automation to make complex machines easier to use in the rest of our lives. Recreational diving is no different.” Caney was responsible for developing the Type R and Type T specifications for recreational and technical rebreathers covered under PADI training courses.

Of course, some people believe that recreational divers shouldn’t be diving rebreathers at all. The Hollis Explorer may change their views.

Thinking Outside the (Yellow) Box

While Poseidon and AP Diving have focused on adapting their closed circuit technology for recreational use, Gurr decided to tackle the problem of building a recreational rebreather from the ground up. His ‘Ah-Hah’ moment?

“The big thing, my epiphany if you like, came when I realized that oxygen is the problem for the average recreational diver, and the dive store and the boat operator that caters to recreational diving. It forces them to handle a potentially dangerous high pressure gas and they really aren’t prepared or educated for that,” Gurr told me. “Oxygen is fine for technical divers but for the masses, the risks go up exponentially. So we realized we had to take it out of the equation.”

That left him with another problem: How to eliminate the oxygen and still meet his design goals of providing a two-hour dive time and sufficient onboard bailout, without forcing the diver to carry large and or multiple cylinders? In response, Gurr turned to semi-closed technology.

An electronic closed circuit rebreather mixes oxygen and a diluent gas, such as nitrox or trimix, into the breathing loop in order to maintain a specified PO2 level, or set point, as measured by its oxygen sensors. It then adds oxygen to replace the quantity consumed by the diver. In contrast, an active addition semi-closed rebreather, which is purely mechanical, feeds a constant stream of a single gas such as nitrox into the breathing loop at a rate calculated to match the diver’s consumption. The excess gas in the loop is periodically dumped into the environment, hence the term ‘semi-closed’. There are no sensors, electronics or multiple gases to deal with, making the units much simpler.

The Semi-Closed and the Explorer

It’s not surprising then that the first rebreathers designed specifically for recreational diving, the Dräger Atlantis and Grand Bleu Inc.’s Fieno-S, were semi-closed rebreathers. Both released in 1995, they preceded by two years the launch of AP Diving’s ‘Inspiration’, which was the first production sport diving CCR. At the time, many industry experts believed that semi-closed systems, which had no electronics, were likely more suitable for recreational use because of their relative simplicity and lower cost.

However, active semi-closed units have their own problems. First, they waste gas. This is one of the reasons that the $3,500 Atlantis likely never took off; it offered scant improvement in duration over an aluminum 80 cylinder. Second, the PO2 in the loop, and therefore the diver’s decompression obligation, is dependent on the diver’s actual workload. The higher the workload, the more oxygen is consumed, resulting in shorter no-decompression limits and the risk of hypoxia (low oxygen levels) in shallow water. Finally, semi-closed systems like the Atlantis dump the excess loop gas in large burps, which are noisy and interfere with the diver’s buoyancy.

Photo: Rosemary E. Lunn / The Underwater Marketing Company

Gurr’s solution was to marry closed circuit electronics, that is, three oxygen sensors, a PO2 controller and a solenoid valve, with a semi-closed system. So, just like a closed circuit rebreather, when the diver works harder the Explorer adds more nitrox to maintain PO2 levels. An ordinary semi-closed system can’t do that. But Gurr went further to automate the PO2 controller in order to maximize the time that the diver can stay underwater.

The resulting hybrid offers most of the benefits of a closed circuit system with the simplicity of a single gas, semi-closed system.  PADI’s Caney describes it as, “A bit of genius. It has the advantages of a CCR, without the penalties,” he said.

A Sophisticated Life Support System

The futuristic white Explorer looks like Star Wars storm trooper battle gear. The rebreather uses a single 40 cubic foot plus cylinder of nitrox (32-40 percent) and is designed for no-decompression dives to depths of about 80 feet (24m) to 110 feet (33m) with dive times up to two hours. There is a built in open circuit bailout valve (BOV), but the diver needs to carry an additional small bailout cylinder for dives in excess of 60 feet (18m).

The Explorer is able to offer much of the performance of a true CCR because its PO2 controller automatically adjusts the set point to maximize the diver’s dive time based on the their no-decompression limit (NDL) time and remaining gas supply. It also maintains oxygen levels within safe limits to minimize the risk of hypoxia.

Presumably, that’s what recreational divers really care about. As the diver descends, the controller adds more nitrox to the loop, increasing oxygen levels and therefore maximizing the diver’s NDL, which is typically a limiting factor at depth. If the diver is making a deep dive and begins to run low on gas and get close to their ‘bailout reserve’, the system will tell the diver to ascend and simultaneously reduce the set point to save gas since the gas remaining and not the NDL is now the limiting factor. Similarly, if the diver ascends during the course of a dive, increasing the NDL so it’s no longer a limiter, the system will automatically reduce the set point to save gas and increase dive duration. The diver can also adjust the Dive Control Parameter, or DCP, manually to maximize NDL or gas.

The Explorer is also smart in how it handles the ‘excess’ gas in the loop. Unlike the Atlantis, where the diver felt a slight increase in buoyancy, followed by a massive gas dump and drop in buoyancy, the Explorer vents excess gas gradually in tiny bubbles at the top of each breath by means of a unique hydrostatically-balanced Loop Control Valve (LCV), that maintains a constant loop volume. As such the Explorer feels like a closed circuit rebreather. The loop controller also makes buoyancy control easier. The gas dump is connected to the exhale counter-lung, so that body fluids (like snot), which can accumulate in the loop, get evacuated on every breath. So does some of the exhalation CO2, which enables the petite 3.3-pound axial scrubber, which Gurr calls a ‘filter’, to provide two hours of dive time.

One of the most striking features of the Explorer is the user-interface, which is a model of simplicity. In dive mode, the handset displays three numbers: depth, elapsed time and the remaining dive time along with the current ‘limiting’ factor or resource, which is arguably all that a recreational diver really needs to know. It also includes a graphic ascent rate indicator. That’s it.

The computer, which contains more than 73,000 lines of code and is housed in the case (not the handset), monitors the diver’s gas supply, NDL, expected filter duration, battery power remaining, the diver’s CNS clock and the overall system, along with PO2 and PCO2, and dynamically calculates the diver’s remaining dive time based on which resource is limiting the dive, whether it’s gas, NDL, the filter, etc. The diver can cycle through the status of each of these resources if he/she is so inclined, which are displayed in minutes remaining – example, ‘Filter: 119 minutes (99 percent)’. As well the diver can view his/her current PO2 and PCO2. The computer does the work to make it simple for the user. The diver just pushes the button and goes.

The heads up display (HUD) follows suit, with three lights; Green for ‘Have a Nice Dive’, Flashing Green/Blue for ‘Look at your display, I have something to tell you’, and Flashing Red accompanied with a vibrating alarm for ‘Danger, danger, bailout now’. There is also a secondary HUD visible to the buddy. The system relies on commands to communicate with the diver. It tells you to Ascend Now, Slow Down, Bailout Now, instead of informing the diver of a problem, such as ‘High PO2’, and leaving the decision to them. The unit will support limited (unplanned) decompression in the event the diver exceeds his or her NDL limits.

The Explorer continually monitors its internal workings and performs error checks. In the event of a serious problem, such as water egress or sensor failure, the unit automatically goes into ‘failsafe mode’, injects gas every three seconds and tells the diver to bailout. The unit has three batteries for redundancy.

Like the VR Sentinel, the Explorer offers an add-on ($500) CO2 monitoring package to the base price of $4,450, which includes a gaseous sensor designed to detect CO2 in the breathing loop. Included as standard are a thermal stick, which measures canister duration, a CO2 calculator (based on O2 consumption) and a simple canister timer.

Mechanically, the unit is designed with interlocking parts that can only be assembled in one way, and goes together in far less time than a closed circuit rebreather. If the user leaves something out, the unit will fail its pre-dive check. If the filter is missing, the user is unable to breathe on the loop and a large green knob protrudes from the case. Disassembly is just as easy.

Per the Type R spec, the Explorer walks the user through the pre-dive checklist. Early on Gurr told me if you can open a cylinder valve, open and close the mouthpiece, and breathe for a minute, the system will take care of everything else (including analyzing the gas). He was right. “The average person has no idea how his or her smart phone works,” he said. “They tap an icon and it works. That’s what we have done with the Explorer. It takes the human out of the chain, because we know that’s the most likely failure point.”

In Search of A Recreational Market

Gurr and the Hollis team are one of three groups that have been driving rebreather automation.

The Poseidon MKVI, which licensed its technology from Dr. Bill Stone, was the first Type R rebreather to be certified by PADI (in 2011) and represented a paradigm-shift in rebreathers with a slick automated user interface, automated pre-dive checklist and auto-calibration. Stone also pioneered their patented dual-sensor ‘active validation’ system that yields a single PO2 value for measuring oxygen in the loop; all other CCRs display three varying values, one for each sensor, that the user must interpret. It’s the first major innovation in oxygen sensing in more than 50 years.

Similarly, AP Diving, which launched the first production sport rebreather, the Inspiration, in 1997, and is now the world’s largest rebreather manufacturer, has been a pioneer in machine automation with the patented dual oxygen controller, auto-set point change, auto-calibration, auto-device detection, as well as being the first to offer a canister duration monitor in 2005. As a result, all of their rebreathers produced since September 2012 are compatible with the Type R spec by simply turning on a software key, and modifying some plug-n-play hardware, such as removing the manual oxygen add valve—the equivalent to adding a governor to a car. An AP Diving Evolution(R) runs about $9-10,000.

Photo: AP Diving

Both the MKVI and the Evolution(R) are upgradeable to tech level rebreathers, part of their strategy, and this is a major difference from the Explorer. Poseidon’s Executive Vice President of Sales, James Roberton, told me that about one third of MKVI users to date have purchased tech upgrades, which can add $1,000 or more to the base price of $8-9000. He said that the MKVI’s upgradeability is a key to their future success.

AP Diving offers instructors the ability to upgrade their student’s Type R machine using the software key at no cost. Managing Director, Martin Parker—the Henry Ford of rebreathers—doesn’t view sport diving through the Tec vs. Rec prism. He told me that AP’s target market is the ‘enthusiast diver’ (like Parker himself) who wants to dive longer, or deeper or probably both. He considers his Type R offering as a stepping-stone for ‘holiday divers’ to discover the advantages of CCR diving and master the basic skills, as they advertise: “From shallow reef to deep wreck, one rebreather does it all.” All AP rebreathers now offer four software-switchable levels of capability.

Rather than rely on a single unit to bridge both markets, Hollis’s strategy is to market its Prism 2 closed circuit rebreather for technical divers, and address the ‘pure’ recreationalists with the Explorer. And therein lays the challenge. In addition to pioneering a radical new technology, Hollis will be pioneering a new market segment that hasn’t been tapped before.

Vikki Batten, Director of Rebreather Technologies, PADI, called it a “brave move” when I reached her at her office in Bristol, UK. She believes the market that Poseidon and AP Diving are targeting—users that will eventually move to a technical CCR— is likely an easier sell though nowhere as big as the potential market of recreational divers that currently haven’t even considering buying rebreathers.

“Hollis may be able to open up the market the way they did with sidemount diving, but it will require lots of education,” she said, adding, “We’re seeing lots of interest from women in a unit that’s small, white and non-tekkie.”

Currently the sport diving rebreather market is very much in its infancy. Industry insiders estimate there are at most 10,000-15,000 active rebreather divers worldwide. Most of these are technical divers. Sizing the ‘recreational’ market is tricky. Agency sources estimate there were about 4,000-4,500 rebreather course certifications issued worldwide in 2012; by comparison, PADI’s website indicates the agency issued over 900,000 entry level and continuing education certifications across its worldwide system during the same period. Roughly half of the 4,000 plus rebreather certifications were classed as ‘basic’ and likely to be attached to a machine sale, but only a percentage of those were Type R  (versus technical). That could mean as few as 1,000 to 1,500 Type R machines were sold in 2012.

Photo: Karl Shreeves

Ambitious Goals

Sources I spoke with said that the main inhibitors to sales were price and market awareness, which largely depends on the number of dive centers that sell rebreathers, the number of instructors who teach them and the number of units in the field.

Hollis Chief Executive Officer, Nick Hollis, has ambitious goals for the Explorer. He told DIVER that his company hopes to sell 1,000 Explorers in 2013, which would nearly double the market from last year. They’ve already presold 300 units, purchased mostly by instructors. Currently about 200 instructors have been trained but only about 30 to 40 are active, having completed the required hours on the unit. In addition, Hollis hopes to set up 100 Hollis CCR Centers in the U.S., which would support both the Prism and Explorer, from their base of nearly 300 U.S. dealers. They currently have upwards of 25.

Clearly, Hollis has the price advantage with the Explorer at 50 to 60 percent of the price of the Poseidon or Evolution. “Our customer base is interested in rebreathers but they are not committed to drop $10k,“ said Michael Thornton, an Explorer instructor and principal of Dive Addicts in Salt Lake City, Utah, which is a Hollis CCR center. “That’s probably the biggest reason they’ll consider the Explorer outside of the cool factor, and its simplicity. It costs only slightly more than high-end [open circuit] dive gear.” Thornton has already sold several Explorers to photographers.

But building an instructor base and distribution will likely take time. Poseidon’s Roberton said that it’s taken his company two and half years to build their instructor base and supporting dive centers. “Getting 50 hours on the machine is not insubstantial for a busy instructor,” he explained. Poseidon reportedly has 58 rebreathers centers in the U.S. out of 153 worldwide.

Certainly the Explorer’s ‘wow’ factor and price point will help.

Explorer and filmmaker Jill Heinerth, who was the first person after Gurr to dive an Explorer prototype believes that the potential is enormous. “It could be a game changer,” she said. “We need something that’s cool, hip and sexy if we’re going to reignite the public’s interest in diving.”

If that happens, Rec could become the new Tec.



7 Comments Leave A Reply

7 Responses to “Rise of the Recreational Rebreather”

  1. Stephen Bell

    This article, “Rise of the recreational Rebreather” was a wonderful education for me. I am considering a rebreather and this article helped me understand who’s who in the business and their take on rebreather technology for Tec and Recreational diving. I feel like I can make a better decision on a rebreather now. But this article was obviously written some time ago since it references 2012 sales of rebreathers. I would like to know where things stand now with this technology. Did the Explorer take off as Hollis suspected it would? Where can I learn about rebreathers today to make a better decision?

    • Andy wallace

      The explorer is a sweet unit. I just got certified (February 22, 2015), and am renting one to take on a 3tank dive this weekend. Also am considering buying one. So far I have 3 or 4 friends, none of whom sell the things, that are sold on them as well. 😎

  2. terje olsen

    Is it true that it doesn’t frkghten fish life? As an underwater videographer quitt g CCR due to risk, this is a major selling point to me. Really appreciate a tip o this… Terje

    • Laura T

      It doesnt scare the fish life at all. Two of us took our Explorers to the Maldives last year and had sharks coming right up to us to investigate what we were. Of course there were no OC divers around us as they had all run out of NDL. It’s amazing how much more you will see. Unfortunately on that occasion I had a macro lens in my camera. I was kicking myself on the surface!

  3. Badluck Bob

    This something I would seriously consider. 5 g’s is a lot of dough, but doable with a little (or a lot) of fiscal planning. What other cost factors, such as maintenance, cost of gas and filters per dive verses good old regular air fills, and the # of places you can get the refills etc….?

  4. Александр

    Rebreather diving for recreational purposes is generally classed as technical diving, and the training is provided by the technical diver certification agencies. Training of scientific divers on rebreathers is usually done by these same technical diver training agencies as the use of rebreathers by the scientific diving community is usually insufficient to justify separate in-house training.

  5. Mark McLeod

    It is currently August 24/2020. What is the latest info on rev rebreathers ?


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