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Keep It Simple Sidewinder

This month, we continue to explore innovative rebreathers on today’s market with a dive into KISS’s split-canister designed mCCR, the Sidewinder, launched in 2016. Designed to address some of the specific challenges of sidemount rebreathers, the Sidewinder has gained a passionate following among cave divers and explorers, as tech instructors Jake Bulman and Skanda Coffield explain.



By Jake Bulman and Skanda Coffield. Header image: Skanda decompressing in “Cenote The Void”, a deep cave currently under exploration by the team. Photos by Alvaro Herrero at: @mekanphotography unless noted. Full disclosure: Protec Dive Centers owner Patrick Widmann is the KISS Training Director.

Since 1998, KISS Rebreathers has been designing and producing rebreathers known for their simplicity, reliability, and field serviceability. The KISS Classic, designed by the original owner Gordon Smith, has been used by explorers around the world for more than two decades. The Orca Spirit is a lighter back-mounted option from the KISS Classic, and features scrubber design that would later be carried over to the Sidewinder. As the need for a sidemount CCR grew and grew, KISS released the Sidekick. Surrounded by a hard metal shell, this was specifically made for harsh environments. Due to being an independant, self contained unit it could easily be donned/doffed at the surface or underwater, pushed through restrictions, or worn as a bailout rebreather (BOB).

One of our more well-loved units around Protec Dive Center

The Sidewinder is unique in that instead of being designed to meet the needs of a specific environment or dive type, fulfilling a specific person’s needs was the goal. Josh Hotaling, a Marine Corporal who lost both his legs in combat, wanted to start using rebreathers for his dives, but the available sidemount rebreathers had changes of balance throughout the breathing cycle due to the counterlung being on one side of the diver. Mike Young, with input from Edd Sorenson, worked on developing a unit that would be balanced while still being light, small and reliable. In 2016, the Sidewinder was born.

Left to right: Jake, Skanda, and Danny (student), during the final day of his Air Diluent class.

Our first real exposure to the Sidewinder came after Phillip Lehman of the Dominican Republic Speleological Society bought one and trained with Edd Sorenson in Florida. Between the two of us, we had previously owned, dove or been instructors on several units, including JJs, Sidekicks and Pelagians. While all of them have some nice features, they also have their downsides. Either they were very heavy, making buoyancy significantly more difficult (particularly in the shallow caves of Mexico), too large for the size of the caves we aimed to explore, or suffered from other issues. Small restrictions, cenote entrances that are difficult or impossible in backmount, and logistics of reaching some of the cenotes in the first place, meant we needed a sidemount rebreather.

Sidemounted CCRs in a Pre-Sidewinder World

At the time, sidemount rebreathers had two principal drawbacks: While each unit has taken a different approach, and sometimes solved one, a solution to both that doesn’t introduce new problems has proven difficult.

First of all, these rebreathers replace one of the sidemount tanks, giving divers the vertical profile of a standard sidemount configuration while they carry one bailout tank (which serves as diluent for most of them as well). This not only causes redundancy problems in the case of equipment failure on that tank, but also bail out range problems. Many people move to a rebreather to do bigger dives than they can on open circuit, and one bailout tank does not get you very far. Additional bailout tanks can be worn as stages, but this creates a larger vertical profile, significantly diminishing one of the main benefits of sidemount rebreathers. Stages can of course be removed, pushed through restrictions and put on again but it adds significant time and effort to passing through each restriction.

The second major drawback of sidemounted rebreathers has to do with the movement of gas during the breathing cycle. When divers exhale, the gas moves into the counterlung which is inside the rebreather on either the left or right side of the diver. This creates a shift in lateral trim that, while manageable, undoubtedly adds some level of task loading, even if it is subconscious.

Skanda swimming through one of many lower sections of the area

On the Sidewinder, unlike other sidemounted units, the scrubber is split into two canisters that allows them to sit alongside divers’ bodies, on top of the two standard sidemount tanks. Problem one, bailout placement and redundancy, solved. In this position, the canisters  slightly add to a diver’s profile, but it is hardly noticeable. In fact, in many photos it can even be hard to see that a diver is wearing a rebreather at all, with only the loop giving it away. 

Having two scrubber canisters also provides a level of redundancy regarding CO2 management. Scrubbers, designed to “scrub” the CO2 from the gas, can allow CO2 to pass through them for various reasons including packing errors, water, or overuse. Having two helps to reduce the likelihood of most causes of excess CO2.

The counterlung, which sits on the divers back and connects the two absorbent canisters, is the next critical piece to the puzzle. By being so close to the position of the diver’s lungs, the change in trim throughout the breathing cycle is negligible. This difference cannot be overstated enough. Regardless of how comfortable one becomes with the shifting of gas on other side-mounted rebreathers, it will always use some level of attention and energy to compensate for that shift. This is most clearly noticed in the performance of entry-level rebreather divers. Problem two, lateral trim, solved.

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The List Goes On

The Sidewinder is one of the lightest rebreathers on the market. [Ed. The Sidewinder is 12.3kg/27.1 lbs., which is lighter than the Liberty SM 22kg/48.5 lbs., but heavier than the RBK 8.62 kg/19 lbs., which is the lightest on the market] This can be a real consideration for divers with back or shoulder injuries, or who need to transport the unit long distances, across difficult terrain or through dry caves. Like open circuit (OC) sidemount, the tanks can also be carried to the water separately. The rebreather can be carried in a single bag, whether for jungle hiking or taking your carry-on luggage during air travel making it very nice to travel internationally with! 

Once in the water, the other advantage of a lightweight rebreather becomes apparent. The heavier the unit, the more gas is required to offset that weight. Sometimes this can be done with thicker undergarments or lighter tanks, but this is not always possible. When changing depths, managing buoyancy requires constant adjustments due to the compression or expansion of gas. With lots of gas between the three gas spaces (wing, dry suit, and CCR) the change in buoyancy happens very quickly. 

The Sidewinder is incredibly comfortable and easy to dive because of the drastically reduced gas needed to offset the weight. This gives the diver more time to react before the ascent/descent speed starts to pick up. With buoyancy not being a constant struggle to maintain (especially in shallow and constantly changing cave profiles),  divers are able to devote more of their focus to monitoring themselves, their team and their surroundings, allowing for greater awareness.

The Sidewinder is a mechanical rebreather, meaning that the oxygen is added via a constant mass flow orifice rather than an electronically controlled solenoid. Constant mass flow works via a combination of a small orifice and a fixed IP (internal pressure) first stage. Unlike a normal first stage that compensates for depth by increasing the IP during descent, the fixed IP first stage remains constant regardless of depth.

 If a diver descends to the depth where ambient pressure is equal to that of the first stage, the flow stops, and the manual add button will not add anything. This creates a depth limit, usually 80-90 m/260-300 ft. The flow of oxygen at shallower depths is matched to the amount metabolized by the diver, which maintains the PO2 in the loop, and makes buoyancy much easier. This shifts more control of the rebreather to the diver and removes potential failure points when compared with electronic rebreathers (eCCR) which use a solenoid to automatically add oxygen, when needed to maintain PO2.

Solenoids can be frustrating due to unfortunately timed injections changing a diver’s buoyancy; hence many technical divers will opt to run the rebreather manually to avoid this. Manually running a rebreather requires practice, and increases task loading. That being said, electronic rebreathers do have a place, and several reliable, proven machines exist on the market. This is a personal choice.

The Sidewinder handles water quite well. While water cannot be removed from the unit while diving, it continues working even with substantial water inside. Not only does the diver have two canisters, but the counterlung lies between them and acts as a water trap. It is not uncommon to find water in the bottom of a counterlung if they had water ingress, but the inhalation canister will be dry except in the worst of floods.

If absorbent is fully flooded, there will not be enough contact with the gas to react with it and remove the CO2. However if the sorb gets wet, then water is allowed to move out of the canister, and the sorb will continue working to a certain extent.

Skanda surveying a newly explored section of cave

Who is Using the Sidewinder and Why?

Historically, rebreathers were used by people who needed them for a given complex dive or lengthy project, in spite of the extra cost, serviceability, hassle, bulkiness, difficulty, and risks involved. If rebreathers were not needed, they were not used. The Sidewinder was not much different in this regard, with almost all of the early users being cave divers or explorers. However, as many of these barriers have been either removed or reduced, many divers have begun using them, not because they need to, but because they want to.

Cost: Sidemounted rebreathers are generally smaller, and in the case of KISS units, do not feature expensive electronics. This has brought cost to nearly half of what some of the mainstream back-mounted units cost, making it feasible for a much larger audience.

Serviceability: Just like open circuit regulators, rebreathers do require maintenance and servicing. In general, more complex and complicated equipment is more difficult to service yourself , should you even want to. Many eCCRs require that the electronics be sent to the factory or authorised service centre if repairs are needed. The Sidewinder is incredibly simple, it has few parts, and it does not require specialised tools to service (except the orifice tool). The servicing of the unit involves replacing o-rings and not much else! This is great for divers who want to service their own gear or who live far away from someone who services technical dive equipment. Also, during remote expeditions, the rebreather can be serviced without time delays.

Hassle: Setup and breakdown of the Sidewinder is quick and easy, requiring as little as 10 minutes each way for experienced users. It does not require altering your sidemount setup, grabbing different tanks, or accessing a bench in order to put one on. For people who do not need it, this goes a long way towards making it something they want to do.

Bulkiness: Larger rebreathers are often wider than some of the smaller framed divers, and nearly as heavy. Nobody likes carrying heavy equipment around, so the lighter it is, the better. We often find ourselves talking with other divers in the parking lot while in our suits and rebreathers which would definitely not happen with heavier equipment.

Difficulty: From setup to disassembly, and all the diving in between, the sidewinder is extremely comfortable and easy to use. This only gets more pronounced as you put more time into it. The days of first time rebreather tryouts being a difficult, frustrating experience are behind us, and it will only get easier.

Risks: Rebreathers will always come with risks, and that does not change between units. In the past, people accepted the risks because risk was required for  their particular dives; however, accepting the risks because one wants to dive is perfectly valid as well. As long as the risks are fully understood—which can only be managed through high quality training—each diver can make their own decision.  The vast majority of risks taken in life are for things we want to do, not that we need to do. 

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Due to these differences, divers of all backgrounds are dipping their toes into rebreather diving, with many deciding it’s something they want to do after they try it. For anyone who previously thought CCR diving took too much time and cost too much money—for instance, people who struggled to carry heavier units, divers wanting to have close encounters with marine life, photographers needing the time to shoot their perfect image without bubbles, instructors without a lot of time to set up their units, or divers on vacation wanting to spend it relaxing—rebreathers are suddenly not only feasible, but desirable.

That being said, there is still a community of hardcore cave explorers utiliizing the Sidewinder. People have experimented with using one in a dual-ccr setup; although, to our knowledge, this has not been put into practice yet in any significant way. A Sidewinder does fit the bill for that purpose however, and can be easily fit with a back mounted unit or a “tube style” sidemount rebreather. We imagine in the coming years we will see this on some of the bigger projects around the world.

How Our Diving has Changed

Our team has been using rebreathers for exploration for quite some time now. But we would never have gone through the effort of using one unless necessary. That was mainly because it took more time to set up and breakdown, was less comfortable, required more equipment, and limited where we could access compared to sidemount OC. The idea of the rebreather being the rule instead of the exception was unimaginable, especially considering a majority of the diving here in Mexico is shallow.

Yet, you will almost always find us diving the rebreather regardless of what we are doing. It no longer takes extra time, adds difficulty to the dive or runs the risk of expensive repair costs. It feels natural. And makes dives more fun!

No longer are we tourists in an alien environment relying on equipment that shifts our balance each time we take a breath. Nor do we need to sacrifice comfort and freedom of movement to extend our limits. With the Sidewinder, we feel like locals, completely adapted to the environment around us and free to move about as we like. The limiting factor is now the diver, not the equipment.

Jake floating in the shallows after practicing skills with newly certified sidewinder divers

The Sidewinder has Changed Our Game

The Sidewinder is incredibly easy to use. It not only creates an enjoyable experience, but contributes to safety. The level of awareness, understanding of the unit, control and stability that can be seen in entry level divers is shocking. After a few minutes of swimming around in a tryout, first time CCR divers could often be mistaken for highly experienced rebreather divers based on their buoyancy and control in the water. The simplicity of the unit regarding design and “diveability” has allowed us to spend more time training people to be safe, competent divers who truly understand their rebreathers, as well as the risks that come with them and how to manage those risks..

Training aside, it has also changed the game when it comes to exploration in caves around the world, which has led to some truly epic discoveries, but that is a story for another day!

Dive Deeper

Sidemount Pros: KISS Sidewinder 100 Hours In

Website: KISS Rebreathers

InDEPTH: My Journey Into Sidemount Rebreathers by Becky Kagan Schott

Originally from Canada, Jake Bulman is a full-time cave diving and CCR instructor at Protec Dive Centers in Mexico. The last several years of teaching have been almost exclusively sidewinder focused, from try dives to CCR Cave classes, 4C to 24C, and in several countries around the world. Outside of work, he can be found on exploration projects in local caves of a wide range of depths, distances, and sizes.

Since 2016 Skanda Coffield has been living in Mexico, working at ProTec Dive Centers. Originally from Australia, he made the move to be close to the longest underwater cave systems in the world. When not teaching cave diving or training new sidewinder divers, he spends his time diving his Sidewinder and exploring new and old cave systems.


Hydrogen, At Last?

In February, Dr. Richard Harris aka Dr. Harry and the Wet Mules conducted the world’s first hydrogen rebreather dive to a test depth of 230 m at Pearse Resurgence, New Zealand. The purpose of the 13.5 hour dive was to determine the practicality and efficacy of using hydrogen to improve safety and performance of über-deep scuba dives. Harris will present the work up and details of the dive at Rebreather Forum 4 in Valletta, Malta on 22 April. In preparation, we thought it useful to review some of the history and research regarding hydrogen diving. Here’s what you need to know.




By Michael Menduno. Header image: Dr. Richard Harris and Craig Challen at the entrance of Pearse Resurgence . Photo courtesy of Dr. Simon Mitchell.

🎶 Pre-dive clicklist: Hydrogen Song by Peter Weatherall, the singing scientist 🎶

Harris’s recent dive is the latest of an estimated 54 experimental hydrogen dives that have been conducted over the last 80 years by military, commercial and, yes, technical divers. It was the first reported hydrogen dive made on a rebreather, in this case, dual Megalodons connected by the bailout valve (BOV)—one charged with trimix diluent, the other with hydreliox (O2, H2, He). It was also the first hydrogen dive conducted in a cave.

The idea of using hydrogen to improve diver safety and performance on deep dives has been a long time in the making. The Swedish Navy began a program led by 26-year-old diving inventor and engineer Arne Zetterström (1917-1945)—think a young Lamar Hires, Bill Stone, or perhaps Jona Silverstein—who was the first to experiment with hydrogen as a possible deep diving gas during World War II. At the time, the United States had the only known helium reserves. And, because of its use for military “barrage balloons” filled with non-flammable lighter-than-air gas, the US had instituted the “Helium Control Act of 1927,” which forbade the export of helium. Zetterström’s mission was to determine if hydrogen was a possible alternative.

Zetterström, whose father was a Swedish naval architect, developed a method for producing hydrogen from ammonia. He had worked out the flammability issues (hydrogen gas is explosive when mixed with more than 4% oxygen), and calculated a rudimentary decompression plan for hydrox. Ultimately, he conducted six surface-supplied dives on hydrox 4/96 (4% O2, 96% H2) to: 40m/131 ft, 70m/230 ft, 110m/361 ft and 160m/525 ft, with one case of decompression sickness (DCS). Bottom times ranged from 10-25 min with 140 min of air and oxygen decompression. Tragically, Zetterström was killed on the 160 m dive in 1945, when tenders mistakenly pulled him up from depth without decompression.

It was nearly 40 years before interest in hydrogen as a diving gas was rekindled. At the time, commercial diving was getting deeper as North Sea oil production boomed, and commercial diving contractors were pushing the limits of heliox (an oxygen helium mix) diving as depths approached and exceeded 306 m/1000 ft. They believed that hydrogen might offer a solution. Hydrogen is half the molecular weight of helium, reducing breathing gas density, and research by Duke University’s Dr. Peter Bennett on trimix suggested that the narcotic properties of hydrogen might ameliorate the effects of High Pressure Nervous Syndrome (HPNS), which is a major limiting factor on heliox dives. 

Commercial Divers Do It Deeper

Pioneering French commercial diving contractor Comex (Compagnie maritime d’expertises) launched its hydrogen program dubbed Hydra in 1982. Beginning with animal experiments, over the next decade, Comex conducted a series of manned wet and chamber complex dives, Hydra II-Hydra X, to determine the efficacy of hydrogen diving. 

In February 1987, the Undersea Hyperbaric Medical Society held its 33rd workshop in Wilmington, North Carolina, titled “Hydrogen As A Diving Gas,” which included presentations by Comex. The following year, during Hydra VIII, four Comex divers and two French Navy divers successfully performed six days of work at 530 m/1738 ft on an offshore platform near Marseille with dramatically improved diver performance when compared to heliox diving. 

Comex saturation divers working on an offshore platform at 530m depth breathing hydreliox.

In 1991, Comex conducted a final record chamber dive to 701 m/2300 ft at its Marseille headquarters. Serendipitously, as then publisher and editor-in-chief of aquaCORPS Journal, I was invited to witness the dive, which was in progress in the Comex chamber complex. Heady stuff indeed! Special thanks to decompression engineer, JP Imbert, who worked at Comex at the time, and diving safety officer George Arnoux who made that possible.

Following the UHMS Hydrogen Meeting, the US Navy’s Naval Medical Research Institute (NMRI), in Bethesda, Maryland, began a program that lasted more than a decade (1990-2001) researching hydrogen under the direction of research physiologist Dr. Susan Kayar. Kayar and her team showed that breathing hyperbaric hydrogen had no ill biochemical effects on mammals that had been previously overlooked.

Next they demonstrated the feasibility of “biochemical decompression,” aka biodec, which reduced the incidence of DCS in in animal test subjects following hydrogen dives by roughly half. With a human biodec protocol, divers would take a capsule of special mammal-friendly, H2-eating gut bugs—think probiotics—which would consume the H2 gas and convert it to methane that would then be released to the atmosphere through the path of least resistance. Small wonder that Dr. Kayar, who measured flatulence from pigs and rats decompressing from hydrogen, became known as the “Queen of Farts.”

Susan Kayar aka the “Queen of Farts”. Photo courtesy of S. Kayar

Comex and the research by the US Navy demonstrated that hydrogen could significantly improve the safety and performance of working divers, and in doing so extend their range, much as the introduction of helium breathing mixes has been able to greatly improve diver safety and performance over deep air diving. Unfortunately, hydrogen was very expensive to mix and handle in a saturation diving environment, and it arguably arrived on the scene too late.

By the late 1980s, the commercial diving industry was already transitioning to robotics, especially for deep diving, due to the economics—it was far less expensive and safer to put a remotely operated vehicle (ROV) at depth for many tasks, instead of a diver. As a result, hydrogen was never “operationalized” as a diving gas, and has remained an exotic vestige of hyperbaric research in search of an application.

Bring on The Tekkies

Hydrogen was on the minds of tech pioneers during the emergence of the “technical diving revolution” i.e., the adoption of mixed gas technology, in the early 1990s. Legendary cave explorer Sheck Exley cited hydrogen in an interview I did with him for aquaCORPS,“Exley on Mix,” in the fall of 1991. Exley had already made several sub-260 m/852 ft dives using trimix at a time when the majority of the sport diving industry would have been hard pressed to spell N-I-T-R-O-X, let alone know what it was used for. Ironically, that was the year that Skin Diver magazine editor Bill Gleason ingloriously dubbed nitrox the “Voodoo Gas,” a moniker that could arguably better be applied to hydrogen, whether hydrox or hydreliox.

Sheck Exley at Zacatón in the early 1990s. Photo from the aquaCORPS archives

As Exley explained, “From what I’ve been learning, there seems to be some real potential for hydreliox, though no one has looked at its use for deep bounce dives. With the kind of compression rates that I was dealing with at Mante, you need the heavy nitrogen to avoid HPNS, but I’d rather have the hydrogen.” 

You may be surprised to learn that Harris and the Wet Mules are not the first technical divers to conduct an experimental dive with hydrogen. That distinction goes to diving engineer Åke Larsson, and six tech diving colleagues. Fortuitously, Larsson worked at the Hydrox lab Swedish Defense & Research Institute under Dr. Hans Örnhagen. 

In 2011, in collaboration with the Swedish Diving History Society (SDHF) and Royal Institute of Technology Diving Club, they formed the Hydrox Project led by fellow techie Ola Lindh, with the goal of revisiting the work of Zetterström. Specifically, they focused on developing a mixing station, procedures and measuring system, and investigated flammability suppression, in the hopes of recreating Zetterström’s 40m dive.

Åke Larsson and colleague prepare for a 42 m hydrox dive. The yellow cylinder contains hydrox 4/96. Photo courtesy of Åke Larsson

In July 2012, the seven Hydrox Project divers each completed a single hydrox open circuit dive to 42 m/138 ft in an isolated quarry. The divers descended on air back gas, switched to the hydrox 4/96 stage cylinder at depth and breathed for five minutes, switched back, and then began a padded deco schedule on air, no oxygen for operational simplicity, using an extrapolated ZH-L16 algorithm. According to Larsson, “Breathing hydrox was a surprisingly pleasant experience! We planned the dive using air, so I was slightly narked but feeling well at 42 m. When we switched to hydrox gas, it felt cold but almost slippery and very, very easy to breathe compared to air. My brain cleared up in less than a minute and the nitrogen narcosis was gone.” 

Where Art Thou H2?

Though the Hydrox Project dives were technically not tech dives, and not very deep, there is reason to believe that hydrogen could improve diving safety and performance on über-deep untethered dives. First, and most importantly, hydrogen can improve the work of breathing (WOB), which is a major risk factor on deep dives.

The gas density of trimix 4/90 (4% O2, 90% He, balance N2) at 250 m/816 ft is 7.3 grams/liter, considerably above the 6.0 g/l threshold where the risk of a negative outcome dramatically increasesNote: it is believed that a high WOB killed Dave Shaw who suffered respiratory insufficiency during a 270 m/880 ft dive at Bushmansgat. With half the molecular weight of helium, hydrogen could reduce gas density to safe levels resulting in lowered work of breathing. For example, the density of hydreliox 4/30 (4%O2, 30% H2, balance He) at 250 m is 4.56 g/l, equivalent to breathing normoxic trimix 21/35 at 40 m.

Hydrogen, which is narcotic at deep depths (PH2≥ 15-20 bar), has also been shown to ameliorate HPNS, which is another major limiting factor at depth. And with market interest in hydrogen as a fuel source on the rise, the gas is plentiful, and cheap relative to helium, though mixing costs could be pricey.

However, there remains a HUGE caveat. Though technical divers were successful in adapting helium-based mixed gas technology to scuba diving, the situation with hydrogen is completely different. Helium diving was well established in the military and commercial diving communities by the late 1980s/early 1990s, and the technical community was able to draw on that experience. That’s simply not the case with hydrogen diving, which has not been operationalized, and most of the research has dealt with saturation diving.

In fact, there are significant challenges that must be addressed if self-contained hydrogen diving is to be possible, let alone successful. These include the real risk of fire and explosion, both above and below the surface, respiratory heat loss—hydrogen has about 3x the specific heat of helium and could lead to acute respiratory heat loss (RHL), followed by dyspnea, cough, and hyper-secretion; its high specific heat also impacts scrubber efficacy, particularly in cold water. Then there’s H2 to He isobaric counter diffusion, hydrogen narcosis. Oh, and the issue of decompression, which according to one expert may be the least of one’s worries {Ed. note: Because the other factors would likely kill you first!], and then there’s the matter of a suitable breathing platform. It’s a formidable list.

The Wet Mules at Pearse Resurgence for their February 2023 expedition. Photo courtesy of Simon Mitchell

Dive Like A Mule

The Wet Mules have been unrelenting innovators in their quest to explore Pearse Resurgence, a project that Harris first became involved with in 2007. Ironically, in 2012, a few months before Larsson and his team made their historic 42 m hydrox dive, Dr. Harris was explaining to the delegates at Rebreather Forum 3.0, in Orlando, Florida, why he and team mate Craig Challen were giving up on open circuit bailout at Pearse—it required some 28 cylinders each to bailout from a 220 m/718 ft dive with 30 minute bottom time. Instead the pair of push divers planned to dive dual back-mounted Megalodon rebreathers connected at the BOV—a configuration they still use today. They rejected sidemount bailout rebreathers due to poorer work of breathing (WOB). They have also worked with O’Three to adapt suits and electric heat to stay warm in the chilly 6ºC/43ºF Pearse waters, and created a system of decompression habitats in the cave to make the decompression safer, and more manageable.

One could surmise, it was only a matter of time before ‘Dr. Harry,’ who is a  deep diving anesthesiologist after all, would feel compelled to hit the H2. Feeling lucky today Punk?

Craig Challen helps Dr. Harry gear up. Note the dual Megalodon. Photo courtesy of Simon Mitchell

Harris will be presenting his report on his historical 230 m hydrogen rebreather dive at Pearse at the closing banquet for Rebreather Forum 4, in Valletta, Malta 22 April. His talk will be videotaped. He and Dr. Simon Mitchell, who was present with the Mules for the dive, is also preparing a paper for the Journal of Diving and Hyperbaric Medicine, and Harris will prepare a paper for The RF4 Proceedings, which will be issued in the fall.

Hydrogen Dreamin’

It is uncertain whether we’ll see a team of NIXIE tech divers—you know who you are— exploring a newly discovered 460 m/1500 ft deep shipwreck, supported by diving bells and an ROV in the next 10-20 years—think Britannic expedition on steroids! However, as tech dives continue to get deeperthe ten deepest cave dives today average 284 msw/926 fsw (adjusting for altitude and freshwater) or 75m/246 ft deeper than those of the 1990s thanks to rebreathers, while the deepest shipwreck dives today average 176 m/576 ft or 55m/180 ft deeper than those of the past it is possible that we will see limited use of hydrogen for special, well-funded tech projects.

It is perhaps more likely that one day soon, clandestine military divers will be locking out of a subsea platform wearing dual or triple H2-enabled rebreathers to carry out some über-deep mission—though we will surely never know.

Then again, hydrogen may well just remain another geeky diving dream that proved to be a dead end, albeit one that inspired us to continue think big and boldly about underwater exploration, and helped increase our understanding and knowledge of what is required for humans to safely breathe underwater, and dive deeper, and stay longer. Isn’t that what diving is all about?

Below please find a selection of curated hydrogen diving articles and resources. We hope you find them useful. And do consider coming to Malta to hear Dr. Harry’s talk. It will be historic.—M2

Rebreather Forum 4

Playing with Fire: Hydrogen as a Diving Gas

As every tekkie knows, helium is essential for deep diving due to the fact it’s non-narcotic and offers low breathing gas density. But it’s conceivable that hydrogen may one day become a part of the tech tool kit for dives beyond 200 m/653 ft, by virtue of the fact that it’s light, a little narcotic and offers the possibility of biochemical decompression. Diver Alert Network’s Reilly Fogarty has the deets.

High Pressure Problems on Über-Deep Dives: Dealing with HPNS

If you’re diving beyond 150 m/490 ft you’re likely to experience the effects of High Pressure Nervous Syndrome (HPNS). Here InDepth’s science geek Reilly Fogarty discusses the physiology of deep helium diving, explains the mechanisms believed to be behind HPNS, and explores its real world implications with über-deep cave explorers Dr. Richard “Harry” Harris and Nuno Gomes. Included is a list of sub-250 m tech diving fatalities.

The Case for Biochemical Decompression

How much do you fart during decompression? How about your teammates? It turns out that those may be critical questions if you’re decompressing from a hydrogen dive, or more specifically hydreliox, a mixture of oxygen, helium, and hydrogen suitable for ultra-deep dives (Wet Mules, are you listening?). Here the former chief physiologist for the US Navy’s experimental hydrogen diving program, Susan Kayar, aka the ”Queen of Farts” gives us the low down on biochemical decompression and what it may someday mean for tech diving.

Operation Second Starfish: A Deep Diving Novel of Submarine Rescue, Science, and Friendship

You read about the science of hydrogen diving last month in a “A Case for Biochemical Decompression,” by former U.S. Navy researcher, Susan Kayar. Now read her science fiction: where experimental diving may one day take us! Retired scientific director of the Navy’s Experimental Diving Unit (NEDU), John Clarke reviews Kayar’s new novel, “Operation Second Starfish.“


NDRI: Arne Zetterström and the First Hydrox Dives by Anders Lindén and Anders Muren. Swedish National Defence Research Institute. 1985

H2HUBB:  The History of Hydrogen

InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties by Michael Menduno and Nuno Gomes.

InDEPTH: Maintaining Your Respiratory Reserve by John Clarke

John Clarke Online: Hydrogen Diving: The Good, The Bad, the Ugly (2021)

Hydrogen Diving – A Very Good Year for Fiction (2018)

Diving with Hydrogen – It’s a Gas (2011)

Undersea Hyperbaric Medical Society: Hydrogen as a Diving Gas: Proceedings of the 33rd UHMS Workshop Wilmington, North Carolina USA (February 1987)

Michael Menduno/M2 is InDepth’s editor-in-chief and an award-winning journalist and technologist who has written about diving and diving technology for more than 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, and he produced the first tek.Conferences and Rebreather Forums 1.0 & 2.0. In addition to InDepth, Menduno serves as an editor/reporter for DAN Europe’s Alert Diver magazine, a contributing editor for X-Ray mag, and writes for He is on the board of the Historical Diving Society (USA), and a member of the Rebreather Training Council. 

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