by Reilly Fogarty
Header photo by Jong Moon Lee.
For nearly two decades a vocal minority in the diving community has been gathering their pitchforks and protesting against deep stops, and anyone who would commit the thought-crime of considering them. The data has appeared to legitimize this pursuit, with several studies failing to confirm benefits or indicating negative outcomes with the addition of deeper decompression stops. Endless debates among academics and divers alike ultimately made it taboo to promote deep stops in the North American dive community.
Across the pond the backlash against those who would perform their decompression marginally deeper has been similarly negative but rather less dramatic. Whether that contributed to the ongoing research in deep stops is hard to say, but some theories have indicated that deep stops may yet provide value. Data from some of the biggest names in diving research has indicated a potential decrease in decompression risk with some profiles, and while the data is far from conclusive, it warrants a closer look.
Dr. Costantino Balestra and JP Imbert are two proponents of continued research, and their research seems to show a correlation between the addition of deep stops and a decrease in post-dive bubbling in some cases. Here’s what we know:
DIY, This Is Not
Most discussions about deep stops veer into the realm of specific practices. All too often technical divers see promising research results and apply derivations of the latest hypotheses immediately. In the case of deep stops, this usually manifests in the manual addition of deeper decompression stops either during their dive planning or on the fly. It is theoretically possible that Balestra’s and Imbert’s hypotheses are correct, and the added deep stops might happen to coincide with their recommendations in some fashion, but the typical outcome is an increase in bottom time and, potentially, in decompression risk.
On-the-fly implementation of theoretically suggested practices was common enough that a paper from 2011 aimed to study the models divers were modifying rather than finding the ideal application for deep stops (Cronje, 2011). Research to determine if deep stops have any benefit whatsoever is still being performed, so the range and specific applications of those stops are still a ways off.
Even just the three papers cited at the end of this article show three separate deep stop protocols, and those are carefully calculated protocols designed for research purposes. The reality is that we don’t yet know enough for divers to be adding these to their profiles. Confounding the issue further, there is so much about decompression science that we don’t yet understand that any benefit or trouble from the addition of these stops might be caused by factors that appear unrelated.
Crunching the Numbers
Broaching a topic that many consider to be put to rest long ago is tough, and it requires excellent data. Balestra and Imbert point to several papers as background and supporting documents for their ongoing research. Each of these focused on the addition of a decompression stop significantly deeper than the first indicated by their decompression model of choice and looked at the rate of DCS incidence in the participants. The first used a stop added at half the maximum depth (Cronje, 2011), another added a stop at 50 feet following a test dive to 25 m/82 feet (Bennett, 2007), and a third used a range of ascent rates and compared a each to a range of deep and shallow stop protocols following a 25 m/82 ft dive. (Marroni, 2004).
The Cronje paper was actually written in response to end-user modification of dive tables to add deep stops at half the maximum depth with anecdotal or Doppler evidence to support them. Significant debate over the efficacy of the modifications at various depths ensued, and the aim was to determine whether those modifications put divers at risk for spinal DCS. An animal model was applied; it involved compressing groups of rats to 3.5-6.0 atm for one hour and then using a 7-minute decompression schedule with and without a 5-minute stop at half the maximum depth.
Interestingly, this profile is known to cause spinal DCS in anesthetized rats, but no rats displayed symptoms of DCS. Thus another trial was conducted to determine the threshold for DCS in the subject participants. This trial involved compressing 11 animals to 4.93 atm (without results) and another 14 animals to 5.4 and 5.9 atm with and without deep stops, also without results. Across all models there were two deaths and two breathing abnormalities (both in the group compressed to 5.4 atm without deep stops) and zero instances of spinal DCS or other symptoms. It’s difficult to point to a reason for the apparent fortitude of these rats, but the lack of symptom evolution in any subject — using a proven DCS-causing profile — illustrates the wide variability in DCS onset.
The Bennett and Marroni papers had somewhat greater success with their subjects, and their work provides a foundation for an ongoing interest in deep stops among researchers like Balestra and Imbert. The Bennett paper compounded on prior work indicating a correlation between deep stops and a decrease in precordial Dopper-detectable bubbles. This paper focused on optimizing the stop times from the initially applied 5-minute stop at 15 m/50 ft following a dive to 25 m/82 ft with an ascent rate of 9 m/30 ft of seawater (fsw) per minute.
Subjects were asked to perform 20- and 25-minute dives to 25 m/82 ft with an ascent rate of 10 msw/33 fsw/min and apply one of 15 profiles with stop times ranging from 1 to 10 minutes at 15 m/50 ft and a second shallower stop performed at 6 m/20 ft. Decompression stress was estimated with the use of precordial Doppler bubble counts. Data indicated that deep stops of 1 minute actually increased bubble counts, leading to the greatest bubble evolution of any profile tested, but a 2.5-minute deep stop followed by a shallow stop of 1-5 minutes led to the lowest bubble counts.
Increasing time at the shallow stop did not measurably decrease bubble count. The results led the researchers to recommend a deep stop of “at least 2½ minutes” at 15 m/50 ft in addition to a stop at 6 m/20 ft for 3-5 minutes following dives to 25 m/82 ft, but the authors noted that they could not extrapolate those recommendations beyond those profiles without further study.
The Marroni paper also relied on precordial Doppler bubble counts to measure decompression stress, although its focus was more specifically on spinal and neurological DCS. The authors of the paper hypothesized that introducing deep stops would reduce bubble formation specifically in fast tissues and result in a decreased risk of neurological DCS, measurably by a lower bubble count. 181 dives to 25 m/82 ft were performed by 22 volunteers, with bottom times of 20 and 25 minutes and 3.5-hour surface intervals. Eight ascent profiles were utilized with ascent rates of 3, 10, 18 msw/min (10, 33 and 60 fsw/min) combined with no stops, a shallow stop at 6 m/20 ft, and a deep stop at 15 m/50 ft plus a shallow stop at 6 m/20 ft.
The greatest bubble counts were found in divers utilizing the slowest ascent rate, while the lowest were found in divers using the 10 msw/min (33 fsw/min) ascent rate with a 5-minute stop at both 25 m/50 ft and 6 m/20 ft. These divers showed nearly half the bubble load of the control group at 5 minutes post-dive and 70% of the bubble load at 10 minutes postdive. As with the Bennett paper, the researchers could not extrapolate the data to other profiles but did find the use of deep stops correlated with a reduced overall bubble load.
Decompression research is a tough field because of the variability inherent to individual subjects and symptom onset, but that’s no surprise given the breadth of factors involved. It’s disheartening to wade through a decade of research just to discover that we have, at best, anecdotal data on the application of deep-stops, but that’s where things currently stand. There is significant data that indicates that deep stops in a small range of researched profiles may decrease bubble load, and more research is warranted.
What we don’t yet know is how the deep stop profiles compare to profiles using the same decompression time or tissue gradients but at shallower depths, how those deep stops affect decompression at technical depths, or how they can be applied in the field. The nature of the work makes it difficult and time-consuming to collect research subjects and data, and it’s unlikely that we’ll have answers to these questions in the immediate future, but the possibilities are fascinating to consider.
Decompression, Deep Stops and the Pursuit of Precision in a Complex World:
- Cronje FJ, Meintjes WA, Bennett PB, Fitchat S, Marroni A & Hyldegaard O. (2011). Analysis of clinical outcomes of linear vs. deep stop decompression from 3.5 to 6 atmospheres absolute (350 – 600 kpa) in awake rats. Undersea Hyperb Med 38, 41-48.
- Bennett PB, Marroni A, Cronje FJ, Cali-Corleo R, Germonpre P, Pieri M, Bonuccelli C, Leonardi MG & Balestra C. (2007). Effect of varying deep stop times and shallow stop times on precordial bubbles after dives to 25 msw (82 fsw). Undersea Hyperb Med 34, 399-406.
- Marroni A, Bennett PB. (2004). A deep stop during decompression from 82 fsw (25 m) significantly reduces bubbles and fast tissue gas tensions. Undersea Hyperb Med 31, 233-243.
Reilly Fogarty is a team leader for risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, Mass. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.
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.