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Omne Trium Perfectum

Good things come in threes. At least that’s what explorer and former commercial diver Axel Schoeller believed when he needed a specialized rebreather platform for a deep open water project. The result of several years’ effort is a triple closed-circuit rebreather configuration—one back mount with two sidemount CCRs— that Schoeller dubbed his “CCR Trilogy.” The author explains his DIY motivations, how he went about constructing the rig, and a little about his dives.



Text and Images by Axel Schoeller unless noted.

🎶 Pre-dive clicklist: Il barbiere di Siviglia by Gioachino Rossini🎶

1 JUNE, 2023—We are greatly saddened to report that the 60-year old author, Axel Schoeller died on a 100-meter solo rebreather dive on his CCR Trilogy rig near Protaras, Cyprus, less than a day after we published his story. We are awaiting details. RIP Axel. According to DAN, which verified Axel’s death, he is the third rebreather diver to die in the last seven days.

The phrase, “Omne Trium Perfectum” (Three is perfection), conceived of by Confucius in 500 B.C.E. and later translated to Latin, definitely applies to the concept of a triple closed-circuit rebreather (CCR) or “CCR Trilogy.” If you google “CCR Trilogy” you may end up with a baby carriage, which besides being used for NASA applications, isn’t helpful for the purpose of this article. Although of course, astronauts use rebreathers during their extra vehicular activities (EVAs).

Credits: NASA/JSC.

OpenAI’s language model GTP-4 does a bit better and suggests that groups of three are inherently satisfying and harmonious. I can live with a rebreather configuration like that.

Planting a Seed

Once upon a time, a rather challenging open ocean scientific dive project appeared on my to-do list and demanded a custom-built dive system to get to the job and back, with a sufficient bailout arrangement and efficient parameters for working dives. The task was to collect unique bottom samples and record data at depths beyond the air diving range at specified locations in Cyprus, and slowly increase the range in small increments and steps if feasible. This is in cooperation with Cyprus government, university and commercial academia in marine biology and geology.  As a safety factor, I wanted to eliminate time constraints. In order not to add this layer of stress. During a recent visit to Cyprus, cave explorer Jim Bowden, a team mate of the late Sheck Exley) explained to me that working under time restrictions creates dangerous risks. The “no time limit” also gave me ample opportunity to gradually field test the CCR Trilogy.  

Photo by T. Buchan

As always, it starts with a maybe crazy idea which has somewhere an atom of possibility attached to it. Latter is the driving force, for I was able to persuade a small team of mad divers to follow my cult. “Think big” was the necessary agenda and, in order to maximize dive range, we started to build a bailout system around a CCR. Scuba Tech Diving Center in Cyprus, is the number one goto place for CCR diving on the island and my primary source for technical advice and implemetation during the building phase of the CCR Trilogy. 

Searching through our workshops of what we had flying about, we took a Megalodon CCR and wedged it between two 20-litre tanks and started to play the configuration. Then we added another two 20-litre tanks as wing tanks plus two five litres for independent inflation. Our limit was defined by what the diver could carry—no extra staging and self-reliant solo diving. Although my CCR Trilogy concept is built for such diving I do on occasion have support divers for in-water checks and for monitoring and stand-by during the return in the upper deco levels.

Editor’s note: Global Underwater Explorers does not sanction solo diving. Furthermore, solo CCR diving was recently addressed by Rebreather Forum 4 in a consensus statement which reads, “The forum recognizes that solo diving may increase the likelihood of a fatality in the event of a rebreather diving incident.”

Photo by S. Patient

The next challenge was to incorporate a Draeger Nova full face mask (FFM), so I didn’t have to fumble too much with swapping regs and to minimize clutter. The first solution we came up with was to connect the rebreather and two regs to the mask with custom machined adapters made by TecMe in Germany which delivered a precision job. The gas was ultimately supplied via a switch block on each side mounted at the chest (two gases per block per side). In case of need, four OC gas mixes could then be delivered via two regulators including the supply to the rebreather.

Photo by S. Patient

All was great, and it worked—in the water, that was. The nightmare was at the surface—to set up, get dressed, into the water, and then, of course, the reverse with about 150kg+ of equipment strapped to you. It was no fun in a sun-burning climate with average summer temperatures of 35C/95f. Drysuits were needed, of course, for the water temperatures of 15-17C/60-63f. 

At times, life got easy, as I was able to convince my commercial dive employer to provide us with a small diver support vessel (DSV) with a crane. We custom built a Launch and Recovery System (LARS) platform out of waterproof pallets and got deployed and hauled in and out of the sea. The fun was short lived as the vessel was commissioned for a proper commercial surface supply dive job, and we had to resort to a RIB. Just to move around in the boat (with the kit on) to enter the water was a project in itself. But we pushed on and made it happen.

Photo by L. Evans
Photo by L. Evans
Photo by S. Patient

Evolution and Metamorphosis

The SARS-CoV-2 bug interfered, and all dive ops came to a stand-still. The coming periods of quarantine and seclusion gave us ample time to think the monster system over. It became clear that open-circuit was not cutting it: The open circuit system was too bulky, heavy and a logistic nightmare to move around out of the water and to get dressed in and out. Obviously open circuit is also wasteful of gas. So I went back to the drawing board. On top of that,  the dive industry started producing sidemount rebreathers. 

As in all sectors of diving there is a constant technological upgrade and development process due to demand. Efficiency is king. Rebreathers rule!

The majority of dive apparatus in military applications are rebreathers. Commercial saturation (SAT) diving uses surface supplied reclaim systems and rebreathers as bailouts. As early as the 1980s, Draeger advertised its bailout CCR 450 rated to depths of 600m/1969 ft.

In scientific diving, rebreathers become more of a choice because of their efficiency and economics for increasingly challenging underwater research work. In the world of open circuit (OC) SCUBA, the transition from simple cylinders to twinsets or triplets and then added wing tanks was a natural evolution. Sport CCR will likely go the same way. Dual CCRs are more often seen nowadays. And there are many sidemount units around. Just put them all together and—voila—it’s a CCR Trilogy. If you can carry two OC breathing devices, one on each side, and one on your back—why not CCRs? Of course there are some little disadvantages, but nothing we couldn’t overcome.

OC has its place and won’t go away, but a rebreather gives you much more carry-able breathing time, bailout range, and many other advantages, especially when multiplied by three. In a bailout scenario, I do not want to be overly concerned about limited breathing gas supplies or different gas mixes.

  • Fourth Element
  • Halcyon Sidemount

Standardizing on Divesoft

The first thing we did was to ditch the two 20-L cylinders and swap them for sidemount bailout rebreathers. We concluded that a mix of CCR models was messy and not an ideal thing to do. After some good research and deliberation, we chose to go the whole nine yards. Our choice fell on Divesoft in the Czech Republic. As a commercial diver, I was able to pile up some stash and, together with some leftover emeralds from the Atocha, Darwinism took its path. I emptied my Swiss bank account, salvaged some Nazi gold from an U-Boat, plus I donated an arm and a leg for two Liberty Sidemounts, a Liberty heavy backmount, and all the other bits and pieces, plus extras. 

Besides the Liberty being an excellent piece of engineering with a lot of redundancy and flood resistance built in, another reason for choosing Divesoft was that it is built in Europe. The latter was an economic advantage, and an added bonus was no red tape when dealing with foreign technology. Quick and economic access to service and parts paired with outstanding customer support. Plus you always get a rubber chicken with your kit. Although I am sure some at Divesoft had to consume an increase in calming medications when dealing with me. 

Another nice feature of the Liberty CCRs are the two built-in checklists on the individual handset controllers. The inner components of the heads have a dual redundancy with 2 separate batteries, 4 x O2 sensors, 4 depth sensors, 2 solenoids and as an option 2 HE sensors. If one component fails, it automatically is taken over by the back-up. Both systems compare each other continuously. If the handset or Optical Display Adapter (ODA), a HUD, fails the remaining display will still run on and provide the necessary info to maintain the CCR unit. 

Multiple gas mixes and set-points can be pre-programmed for automatic switching at predefined depths including a built-in stand-by bailout-CCR option, which halts the firing of the solenoids when the unit is not in use. Although I try to maintain the same gas mix throughout the CCR Trilogy it sometimes happens that there are differences between the unit gases. No problem for computing or switching CCRs. 

Divesoft incorporates the individual bail-out gases for the CCRs in stand-by mode within the software electronics. When switching to another CCR, the different gas mix will automatically be taken over and calculated to maintain the correct dive profile algorithm on all the CCRs. 

During controller setup, attention to detail is of the essence. On average, it takes me at least a long day to set up the CCR Trilogy. Checklists rule!

Not Off The Shelf

You don’t buy a CCR Trilogy off the shelf. A CCR Trilogy is formed for a specific task as is evident from the few divers using such a configuration. In this case, a standardised setup as advocated by some dive organizations won’t happen any time soon. A CCR Trilogy is a customized life support system, which needed modifications as well as associated in-water testing. My system was specifically designed for open water environments and NOT cave diving or dives in restrictive spaces.

The SMs took a good deal of time to rig and trim to my liking. The Eastern Mediterranean, our main operation theater, is highly salty, and we had to add weights to the SMs for trim. Also, we chose extended scrubbers which, under the right conditions, can provide 6-8 hours of dive time each according to Divesoft. We cut it at max 6 hours or even after one dive when underwater work demands. I never swap scrubbers or CCR body parts between the units. Regardless of remaining time, the scrubbers stay with the specific unit. On each dive I regularly switch between the three CCRs for performance checks and CCR switch drills and, of course, pay close attention to tracking the three remaining scrubber times.

The backmount (BM) system has the same scrubber and head as the sidemounts (SM). As a remembrance to the original outset, we mounted 2 x 12 litre 300 bar carbon tanks to the back mounted unit with a LOLA valve connection system, similar to Global Underwater Explorers (GUE) configuration. The twin 12s function as diluent for the BM plus as OC switch gas when swapping between the CCRs.

For peace of my mind, I decided to equip both SMs with Bail Out Valves (BOVs) fed by the BM diluent via quick disconnects. That gives me OC switch gas at any time no matter which CCR I am on (during writing this article, IQSub came out with a new specialized BOV for SM rebreathers).

Then the naughty 12 L twins decided to have babies. One 3 L inflation tank and one 3 L O2 joined the tank family. Both little ones hang on with a Metalsub quick-release system at the back of the BM CCR. The configuration outline was to be able to get dressed in and out of the CCR Trilogy alone and in and out of the water from a boat without breaking a sweat when solo diving. Things have to be organized. If kitted on on the boat, and the last piece of a small item to clip on is out of reach, you will invent numerous loud R-Rated expressions.

I have color coded most of the CCR Trilogy elements for ease of identification and labeled so that no accidental mixup occurs during MOB, DEMOB and service. Especially the scrubbers. Also, I use a left-to-right approach in the setup of the units. 

The BM unit is more comfortable and easier to dive than the SMs. For this reason I dedicated it as the last Bailout CCR for a worst case scenario. The two 12 L / 300 bar carbons may also provide enough OC gas through any of the three CCR units to reach shallow depths if all three CCRs fail. My dive planning was based on the use of one CCR with the remaining two dedicated as bailout only, with the additional limited possibility of open circuit use.

With the CCR Trilogy to fix a unit-specific problem underwater is nearly obsolete. I regard my rebreathers as body parts and listen and feel them while monitoring them as required. Anything dodgy going on, and the unit in question is ditched and replaced by the next CCR. I have named the CCR Trilogy individuals after the Greek mythological Moirai: Clotho, Lachesis, and Atropos. It’s personal. Like in any good relationship.

Photo by S. Patient

At the beginning, while messing about with building up the CCR Trilogy configuration and the endless trials and tests after each change, one problem soon became very clear. First trials are always in the pool with another observer to help with adjustments. In one of the pool trials we originally used SM diver surface valves (DSV) connected to the FFM. But the mask started drawing water. Quite pissed off, we searched for the leak source and, soon enough, I found my dumb mistake. We had a custom made mouthpiece built into the mask to access individually any of the connected rebreathers. That didn’t work out because of the DSV blow-out function. The solution was simply to get rid of the mask. Unfortunate, but I had a sexy mask replacement, and the upgrade to BOVs came along.

Photo by S. Patient

Putting The Trilogy To The Test

Sea trials commenced and the CCR trilogy concept proved its worth: During a dive test, one of the loop hoses came off a unit I was on. Total flooding and acid taste. Rebreather swapped and all was in the green. Besides the four sensors, there was no issue with the electronics of the CCR head after the flooding. Disassembling, freshwater rinse, and drying. The inner works of the Divesoft CCRs are engineered with abusers like me in mind.

While modifying the CCRs, I had to shorten the loops of the SMs which I cut to a comfortable length. When reconnecting, I didn’t tighten one of the hose connections well enough, which then got loose underwater. It’s good if you can bail out to another rebreather or two. Like on an airplane where three engines may increase your survival by a lot. The mistake was not so much that I didn’t tighten the loop connection enough, which passed the pre-dive neg/pos pressure check, but rather my failure of not paying enough “attention to detail.”

To minimize clutter, I did away with all analog pressure gauges and replaced them with eight transmitters on short hoses. Besides the pressure contents displayed on the individual handsets and the three ODAs, I also use two Shearwaters. With their ability to display four pressure contents, I programmed the respective tanks of the CCR Trilogy in sequence from left to right on screen. Put your hands together—and all eight tank pressures are visible at once. Shearwater lets you also assign a two letter/number code to the individual transmitter. That helps to identify a specific tank. If a transmitter acts up or dies, go to the next CCR.

When not in use, I park the loops for the SM CCRs to the waistband at the front of the harness with forwarded D-rings next to the buckle. The BM loop stays around the neck under the chin. It is no obstruction and has no floating disturbance. All BOVs have a chin rest and are quite comfortable.

  • Halcyon Sidemount
  • Fourth Element

So far so good, and after lengthy trials and test dives, the CCR Trilogy is up and running. It took us about five years to reach this point. The complete CCR Trilogy system for an innerspace activity costs about 60,000 Euros. This  excludes all necessary certifications to operate the units, gas, time, and other logistic costs.

Some words to a CCR Trilogy ops:

  1. Attitude, attention to detail, checklists, tests, back-ups, drills, and training.
  2. Visualization. Preparation and planning will take most of the time of a dive ops. Over the years I developed a habit of visualizing not only a dive but also its MOB and the CCR assembly. This visualization routine I keep up several days or more ahead. A side effect is to be considered unsocial.
  3. Buoyancy management in a blue water environment. To control vertical buoyancy during an ascent while handling the five inflating gas compartments can be a bit of a challenge. Needless to say, it is of absolute necessity to be able to easily and comfortably reach, use, and manage all the dump systems. Blind and fast. Forward thinking, preparedness (if a dump fails), and plenty of training helps not to become a rocket with five boosters. Minimizing clutter, and maintaining a clean golden triangle is of advantage. 

During ascents, I found the ODA head up displays a vital help. It is similar to an airplane piloting when applying visual flight rules (VFR) vs. instrument flight rules (IFR), which are two different required licences.

At the moment, there are only a few divers who use a CCR trilogy but, with the rise of dual rebreathers, I have a feeling the CCR Trilogy club will soon expand.

Photo by G. Flaherty
Photo by T. Buchan


InDEPTH: When Easy Doesn’t Do It: Dual Rebreathers in Extended-Range Cave Diving by Tim Blomëke.

Born in Vienna, Austria, in 1963, Axel Schoeller descended from the surrounding alps to the high seas at age 15 to work in the German merchant marine as a mess-boy, and ended his sailing profession as Chief Officer at Sea Shepherd. He was first to dive the lakes and the Baltic areas in the new unified East Germany. In the USA, he joined the team of Mel Fisher and the divers of the Atocha with further similar exploits on the 1715 fleets of East Florida and up to North Carolina where he participated on the research team for DAN chamber dives, studying the effects of flying after diving. In the late 90s, he began his technical diving career in Key West, Florida, with Billy Deans, and further education with Kevin Gurr and Dave Thompson in the UK. Certification and training on several different CCR units in Europe and Asia ensued. Once back in Germany, he switched from submarine treasure hunting to marine archaeology, participating in projects in Asia, the UK, Spain, Greece, and ultimately in Cyprus followed by a career in commercial diving which allowed him to travel worldwide for construction, salvage projects and last as a commercial inspection diver in the offshore oil and gas industry. Now settled in Cyprus, he wastes his money on CCR diving in his free time, and works to advance local scientific diving.

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Configure Me This: The Annotated Sidemounter

We make a deep dive into the world of sidemount diving, and examine seven leading sidemount systems with guest editors Steve Davis and Stratis Kas.




This feature was created by Steve Davis, Stratis Kas and Michael Menduno. Introduction by Michael Menduno. Special thanks to Michael Thomas and brand representatives who helped us. Cover collage by SJ Alice Bennett with photos by Jason Brown of Bardo and Stratis Kas. Images by Stratis Kas unless noted.

🎶 Pre-dive Clicklist: Dave Brubeck – Take Five 🎶

At last, I am pleased to present InDEPTH’s long awaited sidemount edition, which has been in the works for over a year. The purpose of the issue is to celebrate the art and practice, as well as the equipment and culture of sidemount diving. Call it, “The Joys of Sidemount.” But whether you already follow the Bogarthian way, or simply regard it all as “widemount,” I promise you there is something for you in this issue.

The seed of an idea for a sidemount issue grew out of two things: First was the popularity and interest of our NOV 2021 feature that celebrated our innate gear headedness: “Annotated Tekkie,” which examined the question of what and how much kit is required to safely explore our underwater world. Diving is a technology-dependent activity to be sure.

Second, was, that we’ve been seeing tremendous growth and interest in sidemount diving we’ve been seeing. Like technical diving itself, what started in the cave community with small groups of experienced divers experimenting with DIY sidemount configs some three plus decades ago, has now blossomed into a mainstream, commercial tech diving activity in both cave and open water. Sidemount diving has now even spilled over into recreational diving—the configurational equivalent of recreational nitrox in the era of tech mixed gas diving.

One of the important inflection points was bringing sidemount diving out of the caves where the configuration was born, into open water. Sidemount pioneer Jeff Loflin was one of the individuals instrumental in bringing sidemount to recreational divers and the open water tech community. He explained it this way to me in our interview, “We were taking sidemount from the dark, and bringing it into the light.”

“We were taking sidemount from the dark, and bringing it into the light.”—Jeff Loflin

Interestingly, over the last decade, sidemount rebreathers, used as both a primary or a bailout, or both, have also gained significant traction driven by innovative vendors like KISS/XDEEP, SF2, Divesoft, and others. In fact, seven vendors are currently producing sidemount rebreathers. Note that we have also seen an increase in the number of chest-mounted units that can be integrated with sidemount systems. However, we decided to focus on open circuit sidemount for the purpose of this issue, andl address sidemount rebreathers at a later date.

Today, virtually all of the tech diving agencies offer open sidemount courses, with the possible exception of NAUITEC. For example, these range from “Intro to Tech” level sidemount courses, at RAID, which are aimed at getting divers into sidemount at the beginning of their tech journey. At the other end of the spectrum, Global Underwater Explorers (GUE) only offers a cave sidemount course to those who have completed GUE’s advanced cave training (Cave 2) and have at least 50 post-class cave dives.

Similarly, the majority of the major recreational agencies offer a recreational sidemount class for open water divers, including CMAS, NAUI, PADI, RAID, SDI, and SSI. Collectively, these agencies are issuing thousands of certifications a year.

So how big is sidemount diving? Good question, right?!?

How big is tech diving?!? We don’t really know. As an industry we suffer from a severe case of data insufficiency syndrome (DIS). [We can combat this—see our survey below!] However, in speaking with insiders involved in the sidemount business, some rough comparisons emerged.

Sidemount diving is likely bigger than rebreather diving, which according to estimates presented at Rebreather Forum 4 in April, likely represents about 15-20,000 divers globally. This seems like a plausible estimate given the significant cost difference, familiarity with the equipment, and perhaps greater accessibility to training and equipment with open circuit sidemount compared to rebreathers.

Sidemount rebreathers have gained significant traction. Here is Divesoft’s Joseph Bosquez diving a Liberty Sidemount in Ginnie Springs, Florida. Photo by Marissa Eckert.

Another comparison: sidemount diving is likely bigger than cave diving! “Sidemount is not just used for cave diving anymore, but I would say that at least 80% of cave divers have at least tried, or have sidemount in their tool kit. Fifty percent or more dive it exclusively,” Dive Rite general manager Jared Hires explained. Add in tech sidemount diving in open water and wrecks, and the growing number recreational sidemount divers, and we are likely to arrive at what sidemount instructor Steve Davis of Sidemount Pros estimates as “Cave divers plus some.” 

On the recreational side, a PADI exec told InDEPTH that PADI’s Sidemount Standard Specialty is now as popular as its Dry Suit Specialty, which is PADI’s third most popular Specialty Diver course, behind Enriched Air (Nitrox) Diver and Deep Diver, and ahead of Peak Performance Buoyancy. No drills on the knees, puhleez! I kid our friends at PADI.

Despite its growth and relative new-found prominence, sidemount still remains a kind of a best kept secret—the elephant in the restriction?—by which I mean many tekkies (including myself until recently) lack awareness and working knowledge of sidemount diving and may not have even tried it.

Not surprising divers being what they are, even among sidemount instructors and users, there’s a range of opinions on its use and application. On one hand, some view sidemount as a specific tool to be used exclusively for diminutive karst cave, on the other, many consider sidemount more of a platform choice. In fact, Davis, who you’ll hear more about in a minute, offered this question and assertion in our interview, Speaking Sidemount, and then went on and made his case:

 “Is backmount the best tool for cave diving? No, it’s not. Sidemount is far and away the best tool for cave diving!”—Steve Davis

Certainly, many including GUE, would challenge this assertion. But on the other hand, who died and left legacy twinsets in charge? Jacques Cousteau? Is backmount intrinsically safer? Does it offer better performance? If so, please share the data with everyone. There is room for respectful debate. Of course, every platform has its strengths and weaknesses, and environmental conditions, and individual physical and preference differences play an important role in choosing the appropriate platform. However, because of the lack of awareness, knowledge, and direct experience in the overall diving community, the limitations of sidemount get exaggerated, and its application, perhaps, under appreciated.

“There’s absolutely nothing I can’t do in sidemount that you can do in doubles, and there are a shit ton of things that I can do in sidemount, that you can’t do in doubles.”—Edd Sorenson

As instructor trainer and cave rescuer Edd Sorenson explains in his Who’s Who interview, “People used to tell me all the time all the things I couldn’t do in sidemount, and still do. ‘You can’t dive off a boat, you can’t do this, you can’t do that, you can’t scooter, you can’t double stage, you can’t quad.’ I would tell everybody the same thing. There’s absolutely nothing I can’t do in sidemount that you can do in doubles, and there are a shit ton of things that I can do in sidemount, that you can’t do in doubles.” Note in his interview Sorenson recounts taking GUE founder and president Jarrod Jablonski for his first sidemount dive. A trust me dive with Edd? Trust me, you don’t want to miss it.

Not surprising, it seems that many, or most of the dedicated sidemount divers we spoke to for this issue (See: The Who’s Who of Sidemount Diving) offered some version of the following, and or, would likely agree with this meme:

Once You Go Side, You’ll Always Dive Wide.

Ha! I kid our backmount sisters and brothers. None the less, it’s clear that sidemount is here to stay!

Now I’m not a sidemount diver, yet (it’s on my list), and only one member of InDEPTH’s team dives sidemount, in caves. So, to help us with the issue, we enlisted the help of two sidemount experts to serve as guest editors. We reached out to New Zealand-based sidemount instructor, ambassador, and host of the “Speaking Sidemount” podcast Steve Davis, principal of Sidemount Pros. We also enlisted the help of Greek photographer/filmmaker, author, long time InDEPTH contributor, and cave and sidemount instructor, the inimitable Mr. Stratis Kas. Both of them helped with the selection people and sidemount systems we planned to highlight, and authored content for the issue.

Fourth Element

Beginning with this lead story, “Configure Me This,” we explore seven recognized and storied sidemount systems, all of them a little different. Who knew there were that many? In contrast, it’s fair to say, backmount configs have generally become fairly standardized in the global tech community. Please note, that the majority of models are shown wearing their personalized branded gear not necessarily stock products.

Next, we offer the perspectives and stories of 34 leaders in the field, in a piece titled, “The Who’s Who of Sidemount Diving.” We also dive into the philosophy, culture, and practice of sidemount diving in “Speaking Sidemount” and “The What, Which and Why of Sidemount Diving’‘ with our guest editors.

In addition, we offer A Brief History Sidemount Diving with Lamar Hires, Bill Renaker and Patrick Widmann, and specifically the evolution of cold water vs warm water sidemount titled, “The Evolution of Sidemount System Design: Two Distinct Paths Shaped by Florida and Mexico,” by sidemount instructor and author Andy Davis. Finally, the issue wouldn’t be complete with a bit of DIY sidemount heresy from long time scuba engineer and troublemaker, Dave Mclean, call it Sidemount Heresy! Trust me, you’ll be better for the exposure!

Here then is InDEPTH’s celebration of sidemount diving and culture. We want to thank our forward thinking sponsors: DAN EuropeDive Rite, DivesoftFourth ElementHalcyon, Shearwater and XDEEP for making this issue possible. We also want to thank Nicole Alarid, Orie Braun, Jared Hires, Nick Hollis, Michael Thomas, and Patrick Widmann, who helped us sort out sidemount configurations and Elena Vivaldo for researching recreational sidemount. And of course, we want to thank our models: Robert Thomas (CDG), Ricardo Castillo (Dive Rite), Emöke Wagner (Halcyon), Melodie Trevino (Hollis), Marcelin Nebenhaus (Razor), Mélissa Bezaz (Toddy) and Tamara Adame (XDEEP). Looking good divers! We do plan to create a free downloadable poster from this issue. Watch this space!

Please note, we realize this issue primarily represents the views of advocates who are making a case for sidemount. We haven’t focused on stories of divers who have had bad experiences with sidemount or spent much ink delving into its downsides, to the extent that these exist. Consequently, I invite readers who feel so moved to submit their views and or experience, call it, “The Trouble With Sidemount ” or “Never Do This is Sidemount..” . I’m joking about the titles but serious otherwise. We will happily run thoughtful stories. Let’s get a conversation going.

Finally, we would like to acknowledge that as human diving journalists and artists on deadline, we have likely made errors and omissions, and/or failed to identify important items that our geeky readers will no doubt discover. Our apologies in advance. If you do find any errors, omissions, or needed tweaks, please let us know, and we will endeavor make corrections. Thank you.—Michael Menduno/M2

Please take a minute and complete our new: Sidemount Diving Survey. We will report the results in a coming issue.

Use the following navigation links to dive into your favorite configurations 

Cave Diving Group

Dive Rite Nomad Ray

Halcyon ZERO GRAVITY™ Sidemount System

Hollis Katana 2


Toddy Style

XDEEP Stealth 2.0 Tec

  • Fourth Element
  • Halcyon Sidemount

Cave Diving Group (CDG) Sidemount System 

The CDG sidemount system has evolved many times since the 1960s but it’s still a lightweight system for diving sump’s generally found after significant dry caving. The important thing to remember is that it is not just one system. Each diver will have a slightly differently made harness but basically doing the same thing. 

Photos courtesy of the CDG.
Weight System
  • Traditionally the weights go around the waist on a CDG harness, a more modern version of the CDG harness such as the RMTD harness (in the photo) weights can be fitted down the spine. 
  • The harness itself is a simple design with minimal adjustment but designed to walk or cave while wearing wetsuits and small cylinders. It can also be used with larger cylinders. Steel cylinders are the norm for uk cave diving. The harness has fixed D rings and no sliding D rings for use with Aluminium cylinders. 
  • Some CDG harnesses are adapted to use with vertical access equipment (SRT) so the caver/diver can descend the vertical dry cave then use the same harness to dive. 
  • The CDG harness has no wing, because generally buoyancy is not required when using wetsuits and small cylinders in sumps. It was common for divers in British Sumps when using drysuits to use the suit for buoyancy and use no wing. In more recent years a small wrap around wing such as the X deep classic or home made equivalent can be easily added if the diver is using bigger cylinders or carrying equipment through the sump to further dry cave exploration and requires buoyancy.

The CDG harness is designed to take weight loading on the divers’ shoulders and hips. Karabiners are generally used for attaching cylinders to the harness. A bolt snap is not a weight loading piece of equipment and should not be used to lower equipment down and then up vertical sections of dry cave, karabiners also work when covered in mud or grit! It’s not a harness that will get cylinders in that perfect internet trim but it is a harness designed to explore flooded cave sections found a long way underground. All modern harnesses took the sidemount system from the original Cave Diving Group harness design. 

Dive Rite Nomad Ray

The Nomad Ray is the latest sidemount system offering from Dive Rite. It includes a number of notable upgrades to the Nomad range and was designed to be easily adjustable, suitable for a range of body types and diving applications, including the use of both steel and aluminum cylinders.

Weight System
  • The Nomad Ray has three padded, internal pockets each of which can hold up to 5 lbs/2.3 kg of either hard or soft weight.
  • The weight pockets are located on the back of the wing and accessed via a zipper, making for easy weight adjustments while fitted.
Photos courtesy of Dive Rite.
  • The harness consists of independent shoulder, waist/hip, and crotch straps each of which are adjustable to fit the system to a variety of diver body types.
  • The harness incorporates custom hardware including two fixed angled D-rings on the shoulders, two fixed angled D-rings on the waist/hip strap plus, two sliding waist D-rings, two chest bungee-retainer slides, two rear drop D-rings for primary tank attachment, and one “dog-bone” crotch strap double D-ring for accessories, plus a DPV D-ring.
  • The harness is in Y configuration and attaches to the BCD/wing via woven D-rings at the top and through woven loops and screwed connection at the bottom.
  • For the fashion conscious, the wing comes in six colors.
  • The BCD wing is shaped to provide lift at the lower lumbar region. It comprises a Denier outer shell with an inner bladder and has 42 lbs/19 kg of lift.
  • The upper backside panel is abrasion-resistant for restrictions.
  • It incorporates a standard inflator that can be routed from the left (standard) or right side.
  • The BCD includes two pull dumps which are also OPVs. One is located at the inside top center of the wing with a left shoulder pull dump activated via a sheathed cord. The second is located at the bottom left of the wing. The locations of the dumps may require an alteration of diver position to dump all of the gas from the wing e.g. roll to the right to dump from the left side lower dump. Note this would also align with dumping gas from a drysuit. 
  • “Belly-band” bungees locate and wrap the outer wing extremities to the diver’s body. The bungee includes a bungee hook attachment and two wing grommets for easy and quick adjustments by simply moving an overhand knot on the bungee.

The Nomad Ray is a significant upgrade for Dive Rite and evolving to meet the needs of modern sidemount divers. In particular, the easy-access weight system, shoulder dump, easy-to-adjust “belly-bungee,” plus the method of attaching the harness and the independent crotch strap to the wing, are notable improvements.

Halcyon ZERO GRAVITY™ Sidemount System

Halcyon’s Zero Gravity sidemount system was a collaborative creation with our Mexico distributor, Zero Gravity. The Zero Gravity system puts the buoyancy over the hips and along the sides of the rib cage where buoyancy is desired. Extremely streamlined, the air chamber positioning gives it a flat profile on the back of the diver. The Zero Gravity is suited for double aluminum 80’s and a wetsuit or small steel cylinders used with dry suits and heavier undergarments. The Zero Gravity features an adjustable harness, and an optional weight pouch that will hold up to 15 lbs./6.8 kg, and optional inflator placements on the right or left side.

Weight System
  • The added weight attachment is designed and built to lay on the diver’s back in line with the spine. Like our traditional weight options, this allows the diver to keep perfect trim while adding weight. With the three-pocket design, you can add up to 5 lbs/2.3 kg in each pocket as needed and disperse them at different placements for that balanced distribution. The velcro additions allow for secure storage and keep even soft lead pouches from moving around while diving. 
Top left and right photo courtesy of Halcyon Dive Systmes.
  • A harness system with multiple point adjustment is styled after the standard backplate harness, with the lower portion of the strap going behind the diver, allowing for greater freedom of movement and comfort, especially for the women in the diving. This harness system is easily changeable when needed over time, allowing the diver to modify the colors of the webbing if desired for personal customization.
  • The newly redesigned door handles provide divers with several options when choosing the right cylinders and placement. With the added grommets, the diver can adjust the door handles into three different locations on the back, as is, higher, or in towards the diver’s back. This placement is essential when setting up the cylinders. The added feature of using curved door handles over traditional square door handles is that when the cylinders are clipped to the system, the bolt snaps do not snag or get hung up in a corner. This keeps the cylinders and the bolt snaps in a free space for easy reaching, clipping, and removing as needed throughout the dive. 
  • The adjustable bungees make it easy to use with different regulator configurations, whether the regulator is pointed up or down. The waist strap D-rings on the right and left hip allow for AL80’s to be clipped forward as they become buoyant. We achieve this with a non-fixed D-ring by adding an O-ring to the design. This allows the D-ring to be in an upright position for easy finding and clipping while at the same time allowing the D-ring to move freely if in a tight passage or corridor without becoming a snag point. 
  • The uniquely positioned cylinder bungee runs across the back of the system and diver giving you more stretch when wrapping your valves while keeping your cylinders high, tight, and secure. This placement of the bungee allows the diver to adjust the bungee length very easily, whether in training or for the more experienced sidemount divers. This bungee is very easy to change out by the user if they prefer a lighter or heavier bungee without compromising the design of the system.
  • The streamlined design with a unique U-shaped bladder provides an almost free and clean back that reduces your profile and drag. The U-shaped bladder allows stable, uniform lifting along each side of the diver where they need it and across the rear of the diver so they have adequate lifting potential without sacrificing profile and trim. This keeps the profile of the diver very low enabling them to access smaller passages without rubbing the bladder. Top to bottom while trim position does not change even as the bladder is fully inflated. This was a key component in the design for low-profile passages and doorways.

“As an avid sidemount diver and explorer in North Florida, I have used a variety of side mount systems on the market. It wasn’t until I dived the Zero Gravity system, that I was able to pass through several low passage tunnels without feeling the top of the system scraping along the way. The low profile of the system, even when fully inflated, allowed me as the diver, to keep a low profile in the cave system. This truly became a game changer when it came to exploring new tunnels.” Orie Braun, Halcyon Sales Manager

Hollis Katana 2

The Katana 2 is the latest sidemount offering from Hollis and brings them into line with the other top contenders in the sidemount space.

Designed with support from cave explorer Edd Sorenson, the Katana 2 incorporates a number of innovative features including a “Quick-Fit” system for easy adjustment, the ability to configure as a H or Y style harness, out of the box support for the KISS Sidewinder, and a BCD top dump valve with shoulder pull.

Photo courtesy of Melodie Trevino.
Photo courtesy of Hollis.
Weight System
  • The Katana 2 weight system features a drop-in Velcro pouch design with 4 x 5 lbs/2.3 kg pockets along the spine. The spine positioning of the weight system can assist with trim adjustment as required.
Photos courtesy of Hollis.
  • The Katana 2 sidemount harness features an innovative “QFS” Quick Fit System which allows users to easily tailor the one-size-fits-all harness to their specific size.
  • The design also allows for either an “H” or “Y” style harness configuration. H-style is most commonly used when donning the system in a drysuit and can be less restrictive, while the Y-style is most similar to a backplate configuration and provides a snug but comfortable fit when properly sized. Both are suitable for various types of diving and user preferences.
  • The Katana 2 is the first harness with attachment loops built-in to support the popular KISS Sidewinder CCR and similar units now in the marketplace.
  • The wing lift capacity is 40 lbs/18 kg and features a tapered design to keep the diver profile as low as possible, without the “turtle shell effect” which causes drag. 
  • The top of the wing is two- dimensional, versus the bottom which has a three-dimensional design for targeted lift placement to promote better horizontal trim. D-ring attachment points for accessories are placed on both sides of the wing, which are easier to reach than the traditional crotch strap attachment. The wing is also available in either a single or dual bladder option.The Katana 2 wing has a 1000D Nylon Cordura outer shell for abrasion and puncture resistance, and a 420D nylon inner bladder. The wing includes a top dump with shoulder pull that allows venting of gas while in trim.

“The coolest thing about the Katana 2 is the way it ships out of the box. You can customize it to your preference thanks to the innovative harness and adaptable wing design. Technical divers love to customize their gear … and they can make all of the adjustments themselves without permanent modifications on this rig.” Hollis Brand Manager, Nick Hollis


The Razor Harness embodies simplicity and elegance with just two continuous pieces of webbing and one closure point. Its design is minimalistic, yet strong, rugged, and reliable. The harness offers a comfortable and custom fit for divers of all sizes, thanks to its quick and easy setup and adjustable, standardized hardware.

Weight System
  • The Razor Harness allows precise placement of weights for optimizing trim. Additional weights can be easily added to the Razor Pocket Weight, the Waist Strap, or both if more than 13 lbs/6 kg are needed.
  • All attachment points, such as D Rings, on the Razor Harness can be swiftly and easily adjusted for personalized equipment placement. Each Shoulder Strap/Waist Strap can be adjusted at the Mini Back Plate.
  • The length of the Lumbar/Crotch Strap can be adjusted at the Delta Shoulder Plate. The height of the Waist/Hip strap can be adjusted at the Mini Back Plate. Extra attachment points can be added if necessary.
  • The BCD/wing is simple to use and attach or detach from the Razor Harness, secured by just two button head bolts. The mounting position can be easily adjusted to accommodate different-sized divers.
  • The wing is exceptionally durable, constructed with three layers: two outer layers of abrasion-resistant 1000 denier ballistic nylon with a layer of heavy gauge polyurethane in between. The wing is ultrasonically welded, and all edges are finished with edging tape. All attachment points feature reinforced grommets for added strength.
  • The primary wing provides 45 lbs/20 kg of lift and is equipped with a low-profile, heavy-duty manual dump/OPV valve from DSS, along with a standard power inflator. This allows inflation of the primary wing either using the power inflator or orally.
  • The fittings for the dump valve and corrugated hose/power inflator are interchangeable, enabling divers to use them on either the left or right side of the wing according to their preference.
  • The Redundant wing also provides 45 lbs/20 kg of lift and is fitted with a very low-profile “coin” dump/OPV valve and oral inflator.

The Razor Harness and BCD/Wing system offers divers a sleek and minimalist design without compromising on functionality or durability. Its simple yet robust construction provides a comfortable and secure fit for divers of all sizes. The weight system allows for precise weight distribution to optimize trim, while the adjustable attachment points offer personalized equipment placement. The BCD/wing is easy to attach and remove, and its rugged three-layer construction ensures long-lasting performance. With interchangeable fittings and low-profile valves, the system offers versatility and convenience. Overall, the Razor system combines simplicity, reliability, and versatility, making it an excellent choice for divers seeking a streamlined and efficient diving experience.

Toddy Style

The Toddy Style Sidemount System offers divers a unique and innovative approach to sidemount diving. Its “sandwich” style weight system, adjustable harness, and thoughtful design elements enhance comfort, balance, and ease of use. With the ability to customize weight distribution, easily adjust harness straps, and utilize specialized clips, this system provides divers with a streamlined and efficient sidemount experience. The Toddy Style Sidemount System is a reliable choice for divers seeking enhanced maneuverability and minimal drag during their underwater explorations.

Weight System
  • The “sandwich” style system consists of a thin backplate that holds the wing together while protecting it from accidental restriction impact. A backmount-style backplate finishes the sandwich-style system. These backplates also act as a weight system. If the user desires, additional backplates can be added, fitting perfectly on top of each other and distributing the weight evenly over the diver’s back, rather than concentrating it on the spine. This allows for better balance and reduces the “rotation” effect.
  • For divers who want to travel with the system, there is a more traditional sidemount weight system. It consists of a weight holder that can be added to the backplate, again placed away from the spine, to enhance balance and minimize the “rotation” effect.
  • Finally, there is a flexible butt extension with a special mount for precise addition of weight to achieve optimal trim.
  • The system’s shoulder straps are easily adjustable using a Velcro system that secures it in place with wide elastic wraps.
  • The shoulder straps are connected by a removable bungee that keeps them in place when divers turn on their side. It also acts as a temporary clipping place for items such as pigtail marker clips, lights, etc.
  • The inflator extends from behind, near the neck area, resembling the position in a backmount system. There is no valve, but instead, a direct, extremely durable, and resistant connector. This eliminates a fragile point present in all other systems.
  • The wing’s bladder is easily accessible and can be changed even in the field.
Regulator accessories
  • The system also utilizes custom-made clips for the regulator’s second stages. These clips allow divers to secure the regulators to their chest D-rings, resulting in a more streamlined profile and easy access in case of an emergency.

The Toddy Style Sidemount System stands out as a top choice among cold water backmount divers looking to transition smoothly to sidemount diving. It offers a convenient alternative because of its compatibility with both existing regulators and hose lengths, as well as not requiring dedicated cylinders. The presence of a backplate, reminiscent of traditional backmount setups, provides a sense of familiarity and ease of adaptation. Whether for cold water diving or any other diving environment, the Toddy Style Sidemount System offers divers a reliable and comfortable sidemount configuration to enhance their underwater experiences.


The STEALTH 2.0 TEC was designed for deep decompression diving and extended cave penetrations. It provides 42 kbs/19 kg of lift to support multiple cylinders required for advanced diving. It effectively manages gas movement and position to ensure stability, balance, and trim at any inflation level.

Photos courtesy of Mekan Photography.
Weight System
  • The Xdeep Stealth 2.0 features a customizable central weight system located on the spine. It can be adjusted to accommodate the maximum weight preferred by the diver.
  • Additionally, there are droppable weight pockets available in different sizes: S-size (2 x 4.4 lbs/2 kg), M-size (2 x 6.6lbs/3 kg), and L-size (2 x 13.2 lbs/6 kg).
  • For fine-tuning trim purposes, there are also trim pockets in M-size (2 x 4.4 lbs/2 kg) or L-size (2 x 6.6lbs/3 kg) that can be placed anywhere on the harness.
  • The harness is one universal size with a wide adjustment range. It consists of independent shoulder, waist/hip, and crotch straps, each made of different thicknesses and fully adjustable with tri-gliders to fit various diver body types.
  • The harness utilizes custom hardware, including two fixed large D-rings on the shoulders, two fixed D-rings on the waist/hip strap, and two rubber sliding waist D-rings (metal version available as an upgrade).
  • It also has two rear drop square attachments for primary tank attachment, a crotch strap D-ring for accessories below the dump valve, and a DPV D-ring.
  • The Y-configured harness attaches to the BCD/wing at the top.
  • The BCD wing is designed to provide lift at the lower lumbar region. It is constructed with a Cordura 1100 dTEX outer shell fabric and a Nylon 440 dTEX inner bladder with a 0.2 mm TPU coating. It offers 42 lbs/19 kg of lift.
  • The upper backside panel is abrasion-resistant for durability.
  • The BCD includes an inflator available in three sizes, with a standard length of 16″/41 cm (14″/36 cm and 19″/48 cm available upon request).The inflator can be routed from the left (standard) or right side.
  • One central low pull dump also functions as the system’s OPV.
  • Bungees keep the wing attached to the waist harness, ensuring a snug fit around the diver’s body.
  • The wing’s color combinations can be customized on Xdeep’s website:

The XDEEP Stealth 2.0 is a well-designed and versatile diving system that caters to the needs of deep decompression diving and extended cave penetrations. Its robust weight system, adjustable harness, and efficient gas management make it a reliable choice for advanced divers. The BCD/wing combination offers ample lift and durability, while the customizable options allow divers to personalize their gear. Note that the modular system can also be configured from recreational sidemount. Overall, the XDEEP Stealth 2.0 combines functionality, comfort, and performance, making it a strong contender for serious diving adventurers.

Please Take a Minute And Complete our New: Sidemount Diving Survey. We will report the results in a coming issue.

Steve Davis  is the producer and host of the acclaimed podcast, “Speaking Sidemount,” author of the books, “The Canterbury Wreck – A Diver’s Guide” and the eBook, “Sidemount Fundamentals.” He is a specialist sidemount diver/instructor, dives exclusively in sidemount, and is the principal instructor and founder of Sidemount Pros. Steve travels the world diving sidemount in caves, wrecks, and open water. Through Speaking Sidemount Steve’s mission is to share his passion for sidemount diving and provide a medium for the world’s top sidemount divers, instructors, and explorers to share their experiences and thoughts on sidemount diving.

Stratis Kas, a Greek-Italian professional diving instructor, photographer, film director, and author, has spent over a decade as an esteemed Advanced Cave instructor, leading expeditions to extreme locations worldwide. His impressive diving achievements have solidified his expertise in the field. In 2020, Kas published the influential book “Close Calls,” followed by his highly acclaimed second book, “CAVE DIVING: Everything You Always Wanted to Know,” released in 2023. Accessible on, this comprehensive guide has become a go-to resource for cave diving enthusiasts. Kas’s directorial ventures include the documentary “Amphitrite” (2017), shortlisted for the “Short to the Point” Film Festival, and “Infinite Liquid” (2019), which explores Greece’s uncharted cave diving destinations and was selected for presentation at Tekdive USA. Kas’s expertise has led to invitations as a speaker at prestigious conferences, including Eurotek UK, Tekdive Europe and USA, Tec Expo, and Euditek.  For more information about his work and publications, visit

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. Menduno is the organizer of Rebreather Forum 4.

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