by Andy Davis
The origins of sidemount can be traced back to the UK, where it was initially developed as a specific technique for pushing beyond flooded sections during sump exploration. Nevertheless, it was the pioneering innovation of leading cave divers in Florida and Mexico that shaped it into the coherent, diving-focused system that we see today. This article examines how sidemount design was influenced to evolve along two distinct paths by the differing environmental requirements and practices of both Mexican and Floridian cave diving—and how those two developmental strands are now becoming unified as a holistic, singular, design approach.
Why did two distinctive approaches to sidemount system design occur?
The primary factor dictating sidemount design evolution was the water temperature. The Mexican cenotes have consistently tropical water temperatures, whereas the cave systems in Florida are temperate in nature. Cooler water conditions necessitate thicker exposure protection which, in turn, dictates a logical advantage in utilizing higher-capacity steel cylinders. Conversely, in tropical water conditions, aluminum cylinders are ubiquitous, and are preferable for achieving a balanced rig approach when diving with lightweight wetsuits.
For that reason, the development of sidemount design in Mexico reflected the best solution for cave diving with lightweight aluminum cylinders; whereas, in the Florida caves, sidemount design reflected a straightforward solution that evolved for diving with heavier steel tanks.
Sidemount development in the USA
The need to support the high-capacity steel cylinders commonly used in US cave diving demanded a sidemount BCD with substantial lift capacity. The logical solution for that was to adapt existing backmount systems and, consequently, US sidemount system design reflected the modification of existing donut or horseshoe style wings. Without cylinders in place at the rear to restrain the wing it would taco (fold up).
That issue led US sidemount pioneers to sandwich the wing between two conventional backplates. However, because a metal backplate wasn’t needed to support banded doubles, they were soon replaced with lightweight soft backplate systems, along with an external nylon plate to retain the wing. With further design devlopment, dedicated oval or triangular-shaped sidemount wings started to replace conventional backmount wings, but the use of soft, backplate style harnesses persisted. That fundamental design was ubiquitous to American sidemount system manufacturers until the last few years.
Sidemount development in Mexico
Cave divers in Mexico were not constrained by the necessity to support heavy steel cylinders. This allowed for greater functional improvisation in what could be used as a buoyancy wing; the use of 10 liter MSR hydration bladders was a prime example. These smaller-capacity wings could easily be mounted upon bare harness systems; positioned exactly as desired because of their smaller size. To reduce superfluous weight, bulk, and inflexibility, it was possible to minimize the standard one-piece backplate by splitting and reducing it into two small plates at the shoulder and lumbar areas purely for threading the harness webbing. Those plates, being separate, were connected by an adjustable-length spine strap that could be tailored to the diver’s height. Beyond simply minimalizing weight and bulk, this length-adjustable harness system also has specific functional advantages for diving with aluminum cylinders, which I shall describe.
The operational difference between using steel and aluminum cylinders
Beyond merely dictating necessary wing capacity, the buoyancy characteristic differences between high-capacity steel and aluminum cylinders present more nuanced demands for sidemount diving. Those characteristics have a direct effect upon the sidemount diver’s ability to retain trim and stability throughout their dive.
High-capacity steel cylinders are relatively simplistic in terms of their buoyancy characteristics; they remain negatively buoyant even when depleted. Due to that consistency, a sidemount diver using steel cylinders can achieve trim throughout their dives by using the common diving approach of balancing weighting across both sides of their center of gravity.
In contrast, aluminum cylinders transition from negative to positive buoyancy as gas is consumed. That shift in overall buoyancy is mostly apparent in effect toward the base of the cylinder, as the weight of cylinder valves and regulator first-stages retains some negative buoyancy at the top of the tank.
Particular to sidemount diving, attaching aluminum cylinders at the hips has a direct and dynamic effect on the divers’ trim. While aluminum cylinders remain negatively buoyant, as with steel cylinders, the diver will be pulled down where the cylinder attaches at their hips. However, when an aluminum cylinder becomes positive, that force will pull the sidemount diver upwards from their hips. They will shift from being leg-heavy to leg-light as their dive progresses.
That cylinder buoyancy dynamic effect may be minimal and unremarkable for recreational sidemount divers, but it can become unmanageably destabilizing for technical sidemount divers carrying a greater number of cylinders for much longer stops.
This aluminium cylinder buoyancy dynamic prohibits simple trim weighting strategies; as weighting to achieve horizontal trim at the outset of the dive would only exacerbate and exaggerate unbalanced trim in the later stages of a dive.
How Mexican-style sidemount design resolves the issue of inconsistent trim
The fundamental difference between Mexican-style sidemount and Florida-style sidemount is how the respective systems enable horizontal diver trim throughout the dive. Where sidemount systems evolved for diving high-capacity steel cylinders to allow simple trim weighting distribution across the center of gravity, Mexican-style sidemount systems seek to balance variable buoyancy forces predominantly on one side of the fulcrum.
This is achieved by keeping the vast majority of wing buoyancy, and ballast weighting, as far as possible below the divers’ center of gravity. At the outset of a dive, with full aluminum cylinders, the effect of negative buoyancy upon the lower body is countered by lift from the wing. Later in the dive, as cylinders become depleted and gas is removed from the wing, the effect of positive buoyancy upon the lower body is counter-affected by the divers’ weighting. Optimally weighted, and with an ideally configured sidemount system, the diver enjoys consistent trim by balancing out the changing positive and negative buoyancy forces at their hips.
It is this need to keep both wing buoyancy and weighting as far as possible below the divers’ center of gravity that makes the Mexican-style harness design so effective for diving with aluminum cylinders. Specifically, the ability to adjust the length of the spine strap between the lumbar and shoulder plates so that the wing and weighting can be positioned for the most effect directly over the hips, where the cylinders are attached. That ability does not exist, or at least is severely restricted, in the fixed-length soft backplate systems employed within Florida-style sidemount design.
Modern unification of the Florida and Mexico-style sidemount design paths
For several decades, there was ongoing debate between warm and cold water sidemount divers. Mexican-style designs were adept for aluminum cylinders, but the need to retain the majority of wing buoyancy across the lower torso proved problematic to streamlining when a greater volume of gas was needed in the wing. Simultaneously, warm-water sidemount divers using aluminum cylinders struggled to achieve innate trim throughout their dives when using Florida-style systems. At their most desperate, the divers improvised by adding weights to their cylinders to replicate the buoyancy dynamics of steel cylinders.
What I consider to be the latest generation of sidemount system designs overcomes those barriers. They utilize the concept of a length-adjustable harness along with providing the majority of wing buoyancy and weight placement options on the lower torso. At the same time, they utilize a wrap-around wing that provides sufficient lift for the heaviest cylinder options without being detrimental to streamlining. Unlike predecessor generations of sidemount design, these systems, such as the XDeep Stealth and Hollis Katana 2, have gained immediate traction and popular approval among both the cold and warm water sidemount diving communities.
I anticipate that more dive gear manufacturers will incorporate this unification of design paths to embrace the principles and features needed to make sidemount systems universally optimal for any cylinder approach or regional diving practice.
How to Choose A SM Instructor by Andy Davis
Andy is a RAID technical instructor specializing in sidemount and advanced wreck diving. Over 25 years of actively diving, he has also earned instructor ratings with BSAC, PADI TecRec, ANDI and SSI. He is known as a sidemount pioneer in SE Asia and as a writer for various diving magazines and his popular blog; scubatechphilippines.com. Andy has designed technical diving courses for several scuba training agencies. He retired from the Royal Air Force as an officer in 2007 and has been a full-time technical diving professional ever since.
InDEPTH’s Holiday Rebreather Guide 2023
Making a list. Checking it twice. Gonna find out which breathers are naughty or nice. That’s right! It’s time again for InDEPTH’s Holiday Rebreather Guide. This year, we are featuring 32 models of back, sidemount and chest mounted rebreathers, including five new units for your shopping enjoyment. So, get out your Pre-Buy Checklist, and that Gift Card (you do have a gift card don’t you?!?), and buy the breather of your dreams. Ho, ho, hose!
by Michael Menduno, Amanda White and Kenzie Potter. Holiday images by Jason Brown, BARDO CREATIVE.
A Guide to Backmount, Sidemount and Frontmount Rebreathers
6 Dec 2023 – Ho ho ho! InDEPTH’s Holiday Rebreather Guide continues to pick up steam (machines). This season we added Mares Horizon semi closed rebreather and Lombardi Undersea Research’s new RD1 back mounted oxygen rebreather. We also added Lungfish Dive Systems “Lungfish,” And iQSub Technologies’ new FX-CCR front mounted breather along with the Flex2 sidemount CCR. As such we believe the Guide is the most complete one on the market! Pst, pst Mr. Scammahorn, are you still there? Happy shopping divers! Ho ho hose!
Remember you can find all of the Rebreather Forum 4 presentations here on GUE.tv: REBREATHER FORUM 4
1 Dec 2022 – Ho ho ho! Once again, we have updated InDEPTH’s Holiday Rebreather Guide adding two new rebreathers; the new Gemini sidemount, needle valve mCCR from Fathom Systems, and the Generic Breathing Machine (GBM) front mounted, needle valve mCCR, with a dive computer-compatible, solid state oxygen sensor from Scubatron. We also updated the features on the Divesoft Liberty sidemount, and the JJ-CCR. This year, Vobster Marine Systems was acquired by UK-based NAMMU Tech, which plans to rename and re-issue a version of the VMS Redbare. See link below.
Finally, Innerspace Systems’ founder Leon Scamahorn agreed to work on getting us the needed information to add the storied Megalodon to the Guide. Scratch last year’s coal, Xmas cookies for you Mr. Scamahorn! Happy holidays shoppers, here is our updated rebreather guide! Mind those PO2s!
17 Dec 2021 – Ho Ho Ho! We have updated our Holiday Rebreather Guide with new rebreathers and updated features. Despite repeated requests, the only major closed circuit rebreather we are missing is Innerspace Systems’ Megalodon and its siblings. Tsk, tsk Leon Scamahorn, you’ve been a naughty boy! Behold, here is our updated guide. Mind those PO2s!
However, it took the fledgling tech community at least a decade to adapt mixed gas technology for open circuit scuba, including establishing the necessary supporting infrastructure, which was the first and necessary step in the move to rebreathers. A little more than a decade after Stone showcased FRED, British diving entrepreneur Martin Parker, managing director of then AP Valves, launched the “Buddy Inspiration,” the first production closed circuit rebreather designed specifically for sport divers, earning him the moniker, the “Henry Ford of Rebreathers.” [The brand name later became AP Diving] KISS Rebreathers followed a little more than a year later with its mechanical, closed circuit unit, now dubbed the KISS Classic. The rest as they say, is history, our history.
Today, though open-circuit mixed gas diving is still an important platform, rebreathers have become the tool of choice for deep, and long exploration dives. For good reason, with a greatly extended gas supply, near optimal decompression, thermal and weight advantages, bubble-free silence, and let’s not forget the cool factor, rebreathers enable tech divers to greatly extend their underwater envelope beyond the reach of open circuit technology.
As a result, divers now have an abundance of rebreather brands to choose from. Accordingly, we thought it fitting this holiday season to offer up this geeky guide for rebreather shoppers. Want to find out whose breathers are naughty or nice? Here is your chance.
Your Geeky Holiday Guide
The idea for this holiday guide was originally proposed to us by Divesoft’s U.S. General Manager Matěj Fischer. Thank you Matěj! Interestingly, it doesn’t appear to have been done before. Our goal was to include all major brands of closed circuit rebreathers in back mount and sidemount configuration in order to enable shoppers to make a detailed comparison. In that we have largely succeeded. We also included Halcyon Dive Systems’ semi-closed RB80 and more recent RBK sidemount unit, which are both being used successfully as exploration tools.
Absent are US-based Innerspace Systems, which makes the Megalodon and other models, as well as Submatix, based in Germany, which manufactures the Quantum and sidemount SMS 200, neither of which returned our communications. M3S, which makes the Titan, declined our invitation to participate, as they recently discontinued their TITAN CCR—they will be coming out with a replacement unit, the TITAN Phoenix CCR in the near future. We did not include the MARES Horizon, a semi-closed circuit rebreather that is aimed at recreational divers. No doubt, there may be brands we inadvertently missed. Our apologies. Contact us. We can update.
Update (22 Jul 2021) – French rebreather manufacturer M3S contacted us and sent us the specs for their updated chest-mounted Triton CCR, which are now included in the guide.
Update (9 Dec 2020) – Submatix contacted us and the Guide now contains their Quantum (back mount) and SMS 200 (sidemount) rebreathers. We were also contacted by Open Safety Equipment Ltd. and have added their Apocalypse back mounted mechanical closed circuit rebreather. We will add other units as they are presented to us by the vendors.
It’s The Concept, Stupid
The plan was to focus on the feature sets of the various rebreathers to provide an objective means to compare various units. But features by themselves do not a rebreather make. As Pieter Decoene, Operations Manager at rEvo Rebreathers, pointed out to me early on, every rebreather is based on “a concept,” that is more than just the sum of its features. That is to say that the inventors focused on specific problems or issues they deemed important in their designs; think rEvo’s dual scrubbers, Divesoft’s redundant electronics, or integration of open and closed circuit in the case of Dive Rite’s recently launched O2ptima Chest Mount. Shoppers, please consider that as you peruse the various offerings. My thanks to Pieter, who helped us identify and define key features and metrics that should be considered.
Though not every unit on the market has been third-party tested according to Conformitè Europëenne (CE) used for goods sold in the European Union, we decided to use CE test results for some of the common feature benchmarks such as the Work of Breathing (WOB), and scrubber duration. For vendors that do not have CE testing, we suggested that they use the figures that they publicize in their marketing materials and asked that they specify the source of the data if possible. As such, the guide serves as an imperfect comparison, but a comparison nonetheless.
Also, don’t be misled by single figures, like work of breathing or scrubber duration as they serve only as a kind of benchmark—there is typically a lot more behind them. For example, whether a rebreather is easy to breathe or not is a function of elastance, work of breathing (WOB) and hydrostatic imbalance. In order to pass CE, the unit must meet CE test requirements for all three issues in all positions from head down, to horizontal trim, to being in vertical position (Watch that trim!), to lying on your back looking upwards. It’s more difficult to pass the tests in some positions versus others, and some units do better in some positions than others.
The result is that some of the feature data, like WOB, is more nuanced than it appears at first glance. “The problem you have is people take one value (work of breathing for instance) and then buy the product based on that, but it just isn’t that simple an issue,” Martin Parker explained to me. “It’s like people buying a BCD based on the buoyancy; bigger is better, right? Wrong! It’s the ability of the BCD to hold air near your centre of gravity determines how the BC performs. With rebreathers you can have good work of breathing on a breathing machine only to find it completely ruined by it’s hydrostatic imbalance or elastance.”
Due to their design, sidemount rebreathers are generally not able to pass CE requirements in all positions. Consequently, almost all currently do not have CE certification; the T-Reb has a CE certification with exceptions. However, that does not necessarily mean that the units haven’t been third-party tested.
Note that the guide, which is organized alphabetically by manufacturer, contains the deets for each of their featured models. In addition, there are two master downloadable spreadsheets, one for back mounted units and one for sidemount. Lastly, I’d also like to give a shout out to British photog phenom Jason Brown and the BARDOCreative Team (Thank you Georgina!), for helping us inject a bit of the Xmas cheer into this geeky tech tome [For insiders: this was Rufus and Rey’s modeling debut!]. Ho, ho, hose!
With this background and requisite caveats, we are pleased to offer you our Rebreather Holiday Shoppers’ Guide. Happy Holidays!!
Note – Most prices shown below were specified by manufacturer before tax.
Download our two master spreadsheets, one for back mounted units and one for sidemount to compare rebreathers.
Special thanks to Amy LaSalle at GUE HQ for her help assembling the feature spreadsheets.
Michael Menduno is InDepth’s editor-in-chief and an award-winning reporter and technologist who has written about diving and diving technology for 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. He also produced the first Tek, EUROTek, and ASIATek conferences, and organized Rebreather Forums 1.0 and 2.0. Michael received the OZTEKMedia Excellence Award in 2011, the EUROTek Lifetime Achievement Award in 2012, and the TEKDive USA Media Award in 2018. In addition to his responsibilities at InDepth, Menduno is a contributing editor for DAN Europe’s Alert Diver magazine and X-Ray Magazine, a staff writer for DeeperBlue.com, and is on the board of the Historical Diving Society (USA)
Amanda White is the managing editor for InDepth. Her main passion in life is protecting the environment. Whether that means working to minimize her own footprint or working on a broader scale to protect wildlife, the oceans, and other bodies of water. She received her GUE Recreational Level 1 certificate in November 2016 and is ecstatic to begin her scuba diving journey. Amanda was a volunteer for Project Baseline for over a year as the communications lead during Baseline Explorer missions. Now she manages communication between Project Baseline and the public and works as the content and marketing manager for GUE. Amanda holds a Bachelor’s degree in Journalism, with an emphasis in Strategic Communications from the University of Nevada, Reno.
Kenzie Potter Stephens is a production artist for InDepth as well as part of the GUE marketing team. She earned her BS degree in Industrial Engineering and Marketing at the Karlsruhe Institute of Technology (KIT) in Germany, which assists her in using her multicultural upbringing to foster international growth within the community. In addition to her activities as a yoga teacher and an underwater rugby trainer, she has completed her GUE Tech 1 and Cave 1 training and is on her way to becoming a GUE instructor. Not letting any grass grow under her feet, she has also taken on a second major in biochemistry in order to create a deeper understanding of our planet’s unique ecosystems as well as the effect of diving on human physiology.