IT’S CLOSE TO THREE YEARS since training agency colossus PADI launched its ground-breaking rebreather-training programmes for recreational divers.
At the time I reviewed its course for DIVER (PADI Goes Bubble-Free, April 2012) there was only one unit that met the strict specifications laid out by PADI for type R (Recreational) rebreathers, and this was the Poseidon MkV1.
A year or so later, Cornwall-based rebreather specialist Ambient Pressure Diving launched its own version, the Evolution (R), a reformed version of its existing unit with software and hardware modifications enabling it to gain approval by PADI for use in its recreational programmes. Both these units are fully closed-circuit rebreathers (CCRs).
A third unit has recently been launched, this time from across the Atlantic, by Hollis Gear of San Leandro, California. This one is a semi-closed-circuit rebreather (SCCR) designed and developed in the UK specifically to meet the requirements for recreational rebreather diving. It’s called the Explorer.

The Explorer is the brainchild of UK engineer, underwater adventurer and technical diving instructor Kevin Gurr. He’s the computer science whizz who developed the popular Pro Planner software, changing the way mainstream mixed-gas divers went about their business.
Gurr launched his Dorset-based VR Technologies company and in 1997 incorporated the Pro Planner software in the VR3, the world’s first mixed-gas dive computer and the first to support CCR diving.
His passion for rebreather diving and its associated technology dates back to 1987 and his first foray under water using a CCR unit. He went on to dive with his mentor and rebreather pioneer Dr Bill Stone through the 1990s, gaining the knowledge and desire to build his own rebreather units.
Gurr launched his first CCR, the Ouroboros, under the VR Technologies banner in 2005.
Three years later he followed this with the Sentinel, a closed-circuit machine that was the first to incorporate a gaseous carbon dioxide (CO2) sensor capable of detecting scrubber failure as part of a comprehensive safety-monitoring system.
Gurr’s latest creation, the Explorer SCCR, features his sophisticated resource-management system, envisioned more than 20 years ago and bought to fruition after the sale of VRT’s Explorer technology to Hollis.

Philosophy & Design
When PADI launched its recreational rebreather specifications, the response of most manufacturers was to ask themselves: “How do we make our rebreathers safer for recreational divers”
Their answer was to let clever electronics and computer software make decisions for the diver, resulting in units that, to a certain extent, automate out the potential for human error and make them consequently easier to use. They applied this philosophy to existing closed-circuit technology.
Gurr took a different approach. He asked: “How do we make rebreathers safe for recreational divers” The difference saw him thinking out of the box as he assessed what was actually needed when factoring in the average experience and attention levels of recreational divers.
With fully closed-circuit systems he saw the use of 100% oxygen as a problem; it can be a dangerous gas to handle, especially at high pressures, not only for the end user but also for the filling technicians.
There are also supply issues, with very few dive centres having the logistics needed to provide pure oxygen.
Nitrox, on the other hand, is readily available almost everywhere. Gurr started to think along the lines of semi-closed systems, but these have their own inherent problems in that they waste gas.
Existing semi-closed units work by constantly feeding gas (nitrox) into the breathing loop at a rate calculated to match the user’s consumption. They then periodically dump excess loop gas in large quantities, reducing duration.
Gurr’s answer was to combine closed-circuit electronics (with three oxygen sensors, a PO2 controller and a solenoid valve) within a semi-closed system.
The result is the first hybrid SCCR designed specifically for diving within what is commonly termed the “recreational envelope”, offering most of the benefits of closed-circuit units with the simplicity of a gas system using single nitrox mixes between 32% and 40%.

The Hardware
At first glance, the Explorer looks very futuristic. It features a white high-impact plastic back cover with strategically placed vents, and its shape and gloss finish is reminiscent of a Star Wars Stormtrooper’s armour.
The units we were using had been fitted with aluminium backplates to make them less of a luggage burden (stainless steel is also available) and Hollis C45LX wings finished in red and black (a traditional-style BC is also available).
A dumpy 5-litre cylinder sits sideways across the base of the unit. Above this is a large-bore tube holding the CO2 scrubber canister.
There’s an electronic control head at one end and a blank cap at the other with a green indicator that becomes visible only if the CO2 scrubber canister is fitted. Without the scrubber installed the diver will very quickly succumb to the potentially fatal effects of CO2 poisoning, so this little green button could prove to be a life-saver.
Internally and underneath the scrubber tube sit a pair of thick-walled polythene counterlungs and the Loop Control Valve (LCV).
The breathing loop consists of a pair of corrugated hoses with a switchable mouthpiece integrated with an open-circuit bail-out regulator.
The electronic brain for the unit is housed along with integrated, rechargeable lithium-polymer batteries in a clear plastics case. The batteries are UL-listed (flight-safe) and are double-sealed to reduce the chance of leakage.
The VRX-style wrist-mounted computer with two push-buttons and OLED display is connected by wire to the electronics case, as is the LED head-up display (HUD).
The clear case is there for a reason. Inside are more LEDs that mimic those on the HUD so that your buddy can also see your safety status in real time while under water. The electronic components are sealed inside the head, keeping them safe from water ingress should a catastrophic flood occur.
A battery-charging connection is included in the electronics head. This charges the main batteries as well as the handset batteries from a mains electricity supply.
Three oxygen sensors are fitted to a sensor module, which also has the facility to take an optional gaseous CO2 sensor. We didn’t have one and, although it is a nice extra, I didn’t feel that we were missing anything.
A temperature probe forms part of the sensor module. This sits inside the central recess of the scrubber canister, measuring the temperature increase as the absorbent crystals do their work when chemically scrubbing out CO2 produced during the breathing cycle.
This gives a real-time indication of the duration of the scrubber material during use.
The sensor module gains power and communicates to the electronics head via induction plates, so there are no pins or fiddly connections with which to struggle, and it helps to keep the head watertight.
Lastly there’s a regulator-type first stage that connects directly to the onboard cylinder with hose connections to the regulator-type bail-out valve, wing-inflator and an octopus second stage. A high-pressure wireless transmitter communicates with the onboard computer system.
All the Explorer’s main components are built from high-grade polymers or impact-resistant plastics with precision-fit mechanisms.

The Software
The Explorer’s Life Support System (LSS) is designed around the breathing loop, high-pressure gas source and electronics control system. These all integrate to give an intelligent but simple display of status.
The wrist-mounted computer provides a simple “Check and Dive” functionality, keeping the interface between machine and diver as easy to use as possible. The software uses intelligent monitoring to determine the appropriate tests and checks that need to be performed to get the Explorer ready to dive.
Once under water any failures or potential problems are communicated to the diver using a visual and vibrating alarm system via the HUD’s three-light display, and mimicked via the Buddy Universal Display (BUV) as well as shown graphically or as instructions on the handset.
The HUD and BUV have three main warning levels: flashing red plus a vibration alarm are activated when the dive should be aborted on open circuit; flashing green or blue indicates a manageable error situation; and solid green indicates that the system has no detected problems and is functioning correctly. Cleverly, the HUD lights colour combination has been configured for those who may be colour-blind.
The Explorer software won’t allow a dive to be carried out without conducting the required sequence of pre-dive checks. The handset gives guidance in performing these checks, which are displayed in sequence on the wrist display screen.
Some of these checks rely completely on the diver to perform them correctly. Other tests can be positively checked by the electronics control system but the user needs to confirm that the settings are OK for diving.
The handset displays are also colour co-ordinated, with green graphics indicating that the system is functioning without any problems and yellow indicating a low-level alarm such as low gas-cylinder pressure.
Magenta indicates that the reading is unreliable, as with a failed sensor. Finally, as in the HUD, red indicates a major hitch prompting immediate action. There is the facility to scroll through the unit’s recourses to check on such aspects as remaining scrubber life, gas usage and no-decompression limits.
As required by the PADI specification, the unit has a fail-safe system, which reduces the chance of diver error and includes a breathing detection feature that automatically turns the unit on to warn of the potential for accidental injury or worse.

Learning to Explore
I booked a course in the Red Sea to learn how to dive using the Hollis Explorer with Kent-based Blue Ocean Diving’s owner, PADI Course Director and technical instructor-trainer Ryby Stonehouse.
Ryby had kindly offered to supply the units plus his time and expertise to teach the PADI Rebreather and Advanced Rebreather Diver courses in conjunction with the Hollis Explorer’s specific skills.
The prerequisites for the initial course are that you have to be at least 18 years old, an Open Water and Enriched Air Diver or equivalent, and have a minimum of 25 logged dives.
The venue was Roots Luxury Camp, situated a stones throw from the beach in Egypt’s El Quseir. We had at our disposal a training pool, an air-conditioned classroom, workspace and tools for unit maintenance, and unlimited shore access to the sea.
A sandy-bottomed seabed at depths suitable for the progression of virtually any course makes this an ideal venue for instructors and trainees alike.
The combined modules were conducted over a four-day period (three days plus one for the Advanced qualification) with theory and practical in-water sessions that started off in the pool and progressed to the sea.
The PADI theory portion required the completion of knowledge reviews at the end of each module. Two PADI and
one Hollis final exam needed to be successfully completed before certification could be awarded.
Of note is that earlier this year PADI introduced a self-packing module for any unit with a manufacturer-approved, user-packable scrubber. We covered the technique for this safety-critical operation in depth.
Our open-water training dives were carried out initially above the 18m-depth limit imposed in PADI training standards. They were completed without the need for external bail-out tanks.
Various skills were taught and practised both mid-water and on the seabed, the most important being bailing out onto the onboard open-circuit system.
As the course progressed, this vital safety skill became second nature.
Past 18m we were into the Advanced section of the course and needed to include the use of single side-mounted stage tanks for off-board bail-out. Again, switching to open circuit and ascending safely quickly became automatic.
Other core skills included responding to a simulated rebreather emergency, mask-clearing, in-water leak checks, dive-control-parameter adjustments, SMB deployment, HUD and status monitoring and clearing water from the breathing loop.

Taking the Explorer under water, I had to get myself into the mindset for rebreather diving. Buoyancy isn’t controlled in the same way as open circuit, where an inhalation or exhalation would be enough to fine-tune your trim.
The nature of rebreathers is that they don’t waste exhaled gas in the form of bubbles, they just move that gas
between your own biological lungs and the unit’s counterlungs without affecting buoyancy.
So it makes sense to remain (as much as possible) at a constant depth and consider going around an obstacle rather than over or under it, controlling trim via the wing or BC as you go.
Also I had to get into the habit of constantly monitoring the HUD and wrist display, scrolling through my resources so that, at any given time, I knew and understood the unit’s and my own dive status.
As a hybrid SCCR that uses a single gas (nitrox) in its operation, the machine dumps small amounts of excess loop gas via the rear cover vents through a unique hydrostatically balanced loop control valve (LCV).
This happens about every 20 seconds, depending on the diver’s breathing rate.
The gas is vented gradually at the top of a breath, giving out a small cloud of bubbles that’s behind the diver. However, this is a silent and pretty inconspicuous event, so the Explorer behaves and feels very much like a closed-circuit unit.
The advantage of silent diving became graphically clear on one of the training dives, when a juvenile eagle ray swooped down to the seabed and lay in an empty clamshell nibbling the edge to feed on algae. Our group of four divers joined it, lying in a semi-circle just a foot from the seemingly unaware fish, and this is the closest I’ve ever been to what has always been my favourite species.
During the dives the HUD gave a bright but unobtrusive indication of the Explorer’s status. Any change in colour is instantly noticeable and an immediate check of the handset will let you know what’s occurring.
In the unlikely event that you miss this indication, the secondary BUV display can alert your buddy that there’s something amiss; two pairs of eyes are certainly better than one when it comes to safety.
The Explorer’s automated user interface was simple to use, right from the pre-dive checks through to the dives. The cleverly thought-out resource management system gives the diver useful options during the dives.
The Explorer is designed to give underwater durations of around two hours, although as in any diving the no-decompression limits and gas consumption rates determine your time under water.
In our case the available nitrox, as is common in Egypt, was 32%, which enabled us to dive to 40m but conversely shortened in-water times due to reduced NDLs.
We did however enjoy 90 minutes under water on most of our dives.

There are benefits to diving with rebreathers, including longer in-water times with extended decompression limits, less disturbance and noise from bubbles, warmer breathing gas and lower gas consumption.
Unfortunately they have their downsides too, the foremost being that they’re a huge investment, requiring time-consuming and meticulous preparation before every dive, and they also require constant attention and expensive servicing.
The question has to be asked: who’s going to invest their hard-earned wonga on this type of unit
The two existing CCR units that meet the PADI specs are upgradeable – indeed, a percentage of their users will use the recreational courses as a stepping-stone to progress with further training and experience to full-on mixed-gas technical diving.
The Explorer can’t be upgraded, leaving those who want to progress with the only option of further investment. So where does the Explorer sit in the marketplace
I contacted PADI’s Director of Rebreather Technologies, Vikki Batten, for her opinion. Vikki sees the Explorer as a unit that through clever design is unique in its approach and by default has its own niche in the market.
“Initially the ‘early adopters’, who are by nature a bit more adventurous, are the most likely to want to progress to tec,” she told me. “However, the vast majority of divers don’t even own their own kit and prefer to rent it when they’re on holiday (possibly the only time they go diving).
“The rental market for the Explorer, with its simple-to-use controls and safety features, is therefore huge. It’s a place where recreational divers can discover and enjoy the enhancement and new dimension that this genre of diving brings.”
Vikki went on to say: “Silent diving and closer marine encounters along with more time under water is most divers’ dream scenario. All recreational rebreathers offer this, but we’re seeing a lot of interest from divers in a unit that’s small, white and non-tekkie.
“In time I see the Explorer becoming a popular choice for the vast majority who don’t see themselves as technical divers, instead wanting just to enjoy its benefits.”
For me, the Hollis Explorer offers real-life advantages over the competition – most notably the gas.
Nitrox is safer to handle and use than pure O2 and it’s readily available almost anywhere, and although you are likely to have to take the 5-litre cylinder with you when travelling, at least you need to take only one.
The whole unit weighs in at less than 20kg, so for around £25 each way it can be pre-booked as extra baggage for overseas trips.
Thanks to Ryby, and his excellent approach to teaching, the courses were a doddle to complete. They should have been for me, as an existing rebreather diver, but although highly experienced the other two divers on our course had always dived open circuit, yet agreed that the training presented no problems.
The Explorer is a complex machine on the inside, but its user interface is simplicity itself.
Kevin Gurr and Hollis Gear may have hit the jackpot with this cleverly engineered bit of technology – indeed, some industry insiders believe this could be the tipping-point for the recreational rebreather market.
It’s still early days, but it’s looking promising.

PRICES: Hollis Explorer Sport Rebreather, £4295. PADI RRB Advanced Combo course, £850.

TRANSPORT: Nigel Wade flew from the UK to Hurghada with Monarch Airlines, on which standard baggage allowances of 20kg are enhanced by 5kg on production of valid diving certification. The Explorers were transported as excess baggage for a pre-booked price of £50 each for the return journey.

TRAINING PACKAGE: The Scuba Place, Ryby Stonehouse (Blue Ocean Diving) and Roots Luxury Dive Camp have teamed up to provide the PADI Rebreather and Advanced Rebreather Diver certification in 2015. Dates are available throughout the year and prices, including flights, transfers, seven nights’ soft all-inclusive accommodation, unlimited house-reef diving and the full course, certification and manuals, start from £1699pp.

CONTACTS: Hollis Gear,
Blue Ocean Diving,
Roots Camp,
The Scuba Place,