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Part Three: Bubble-wise, pound-foolish. Are deep stops dangerous?

In part three, of this four-part series on the history and development of GUE’s decompression protocols, GUE founder and president, Jarrod Jablonski begins by asking the question, “Are Deep Stops Dangerous?” He then goes on to discuss the early rise and experience with bubble models VPM, VPM-B and RGBM in the late 1990s, and the subsequent impact on the tech community, when the U.S. Navy Experimental Diving Unit (NEDU) released their deep stop study in July 2011. Their unambiguous conclusions? The redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives! Additional work followed calling deep stops into question. Jablonski assess what these all mean for decompression procedures, and presents GUE’s current approach. Feel free to DIVE IN and share your thoughts.

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by Jarrod Jablonski

Header photo from the GUE Archives. WKPP dives in Wakulla.

Have you read Part One and Part Two?

Through the 1990s and early 2000s, technical diving was becoming progressively more popular, and the exploits of explorers were being reported around the world. There were numerous reports of greatly adjusted decompression profiles, many of them attributed to the benefit of deep stops. During this period, any mention of dissolved gas algorithms was tantamount to talking about a flat earth. It seemed self-evident to so many people that controlling bubbles just made sense. How could all of those technical divers be talking about adjusting their decompression profiles if there wasn’t something to this deep stop/bubble control concept? Were they safely reducing decompression time, just getting lucky, or perhaps exaggerating their success, whether intentionally or not? 

Questions like this encouraged decompression enthusiasts Erik Baker and Erik Maiken to work with researchers like David Yount to refine Yount’s Varying Permeability Model (VPM), extending the early concepts to support bubble management during repetitive, mixed-gas decompression diving. During one planning session, Baker demonstrated to me that VPM mirrored the type of shortened decompression that was eerily similar to the schedules our team had evolved organically. Over time, the early enthusiasm for this new model gave way to more realistic appraisals. Short dives that formed the base of “typical” tech diving–around 75 m/250 ft for about 30 minutes–were resulting in very short VPM decompressions. In fact, the output looked troublesome, causing Global Underwater Explorers (GUE) to delay implementation of VPM for more than a year. When GUE did include VPM, guidance was codified in GUE’s Standards and Procedures document and required that all profiles continue to refer to the original Buhlmann as the reference standard. That requirement remains to this day.

Problems with VPM became more frequent, and decompression sickness (DCS) was being reported somewhat regularly, even among die-hard enthusiasts. Ironically, a calculation error relating to Boyle’s law was discovered, and VPM was adjusted and re-released as VPM-B; although most favored calling it VPM since it was intended as a replacement. VPM was included in a variety of decompression programs and competed with Bruce Wienke’s Reduced Gradient Bubble Model (RGBM). Over time, enthusiasm for the new bubble models eased somewhat with many divers picking and choosing depending upon a given dive. People still tended to believe in the idea of bubble models, albeit with a more cautious view of the application. 

WKPP dives in Wakulla. Photo from the GUE Archives.

Extrapolating theoretical bubble dynamics into real-world application is complex but also deeply intriguing. It also encourages divers to ask if such a paradigm shift might illuminate a deeper truth about the mechanisms at work. Physiologist Brian Hills (1934–2006) became deeply intrigued by the idea, being at least partly inspired by observing pearl divers’ successful decompression in one-third the time presented in commonly accepted U.S. Navy tables. This reduction in time was similar to the claims of some technical divers who also believed the result was influenced by their control of developing bubbles. Both the pearl and tech diver “results” require a great deal of context, which we will save for a more detailed review. These results may well foreground both a flawed process and a unique insight.  

Hills commented that: 

“Haldane’s calculation method did not say the same thing as the equations he used to formulate diving tables. Haldane and subsequent Naval tables were based upon the axiom that the bends-free diver must be bubble-free. This is demonstrated qualitatively by the diver who develops a case of the bends during ascent. Now knowing that he has bubbles, you would move him deeper as a treatment. On the other hand, if those bubbles had not become manifest as the bends, you would continue to take him shallower, assuming that he was bubble-free.”

Given the complexity, the early difficulty of modeling bubbles was probably to be expected. There are numerous variables involved in developing an effective bubble model. We might speak about micro bubbles that grow from seeds, and where we strive to limit a bubble’s critical radius and the critical volume of allowable bubbles. In making these assessments, modelers must work from lab experiments which strive to determine and then extrapolate what actually happens in the body. Even if they manage to get all the particulars correct, they still remain unclear about how a given bubble may or may not impact a diver. For example, where does the bubble go, and how does this create symptoms? Are the impacts from bubbles mostly or exclusively related to where they come to rest, i.e., when they stop and block blood flow and/or impinge upon a nerve and cause pain? Or, do bubbles cause problems by their presence, signaling the body’s immune response and resulting in collateral symptoms? Even a perfect model of bubble development might fail to develop consistent and useful decompression tables.

The uncertainty revolving around bubble models was nothing new in the technical diving world. Recall that many divers had been regularly modifying their own profiles for years with little certainty. Most divers seemed to believe bubble models had value, albeit more carefully structured than early assessments might have prompted. 

Planning World Record Dives

The shot that rang across the technical diving community

In July 2011, the Navy Experimental Diving Unit (NEDU) released a deep stop study with the unambiguous conclusion that “REDISTRIBUTION OF DECOMPRESSION STOP TIME FROM SHALLOW TO DEEP STOPS INCREASES INCIDENCE OF DECOMPRESSION SICKNESS IN AIR DECOMPRESSION DIVES.” 

There it was, in black and white for all to read. Deep stops not only did not help but they also actually INCREASED the risk of decompression problems. Lest one imagine the issue settled, the protests began almost immediately. The NEDU study did not model the type of decompression used by technical divers, most notably by forgoing oxygen-rich mixes as part of the decompression. Others complained about the use of air, an abomination in some tech circles, and enough for some divers to immediately discount the study. Still others disliked the ascent profile, which placed 44 minutes of decompression between 21 m/70 ft and 15 m/50 ft alone. Some argued that even a conservative use of gradient factors with deep stops would only result in 13 minutes over the same range, leading some to argue that such on-gassing would naturally outstrip any value to deep stops that are excessively long.

Jarrod Jablonski and George Irvine at the last stop in a Wakulla Springs habitat. Photo from the GUE archives.

The researchers conducting the NEDU study are exceptionally bright, capable, and well-informed experts. They had excellent reasons for the choices they made, and these have been well defended in various media, perhaps most eloquently in online discussion forums by Dr. David Doolette and Dr. Simon Mitchell , two giants in the fields of hyperbaric research and treatment, respectively. Both men are among the world’s foremost experts in their respective fields, and anyone of reasonable sense would carefully consider an opinion that challenges their conclusions. 

My intent here is not to argue for or against the NEDU study, or even deep stops in general. I will leave that discussion for later, and the final determination belongs to those for whom it is relevant. For now, I hope to summarize the particulars and leave the reader to review the substantial body of information available. What we can say now is that it is unlikely anyone study could convince those that perceive years of success with a given approach. For some, the NEDU study was missing many critical details. One can certainly argue that deep stop efficacy should be independent of these details, but technical divers care less about that aspect than they care more about using deep stops in conjunction with their normal practices. The NEDU study argues, many would say compellingly, that deep stops are not beneficial. We know that because a larger share of divers in the study developed problems while using deep stops than not using these stops. In fact, the research is more compelling given the number and severity of DCS cases. Moreover, there are additional studies that support the NEDU conclusion, while there have not been any studies that support the value of deep stops. 

The NEDU study appears to be empirically rational and logistically consistent, though deeply unsatisfying, at least for some. With so many differences between the NEDU study and typical technical profiles, resistance was to be expected. Change can be unsettling, and no doubt some of the resistance can be explained by this discomfort, something known as cognitive dissonance among those that like to label such things. Even a casual review of the discussion forums illustrates the emotional attachment we have to long-standing ideas: Some are gleeful about the news, enjoying the chance to deride those that followed this path, which is an understandable backlash to the uber-confidence of some deep stop advocates. Others are angry, blaming others for duping them, apparently absent a sense of personal responsibility. Between the growing anti-deep movement and the declining pro-deep camps resides a mostly cautious base, with some patient experts helping to channel the discussions. Despite a few unhelpful personal insults, we can be broadly impressed with the ways in which technical divers are processing this new information. Ideally, we would learn from the largely unsubstantiated rush into deep stops enacting a measured exit strategy, especially while managing a few other pesky details relating to the use of deep stops. For example, how should a person convinced by the science against deep stops treat fellow divers? How about the dive buddy relating anxiety over the change? What about those experiencing DCS symptoms in deeper water? The onset of these symptoms can include pain, numbness, and neurological problems. There is a known risk in deeper and longer dives, and the frequency is enough to encourage a standard requiring that surface-supplied dives in excess of 91 m/300 ft be conducted as saturation-only in the U.S. Navy, among others. [Ed. note: U.S.Coast Guard regulations require that commercial diving jobs deeper than 91m/300 ft be conducted using saturation diving. Meanwhile, many clients such as BP and Shell Global mandate saturation diving below as little 37m/120 ft]. 

George Irvine before a Wakulla Springs exploration dive. Photo from the GUE archives.

It is problematic to suggest that divers experiencing DCS symptoms at depth ascend. Meanwhile, some of these divers have established protocols, including some version of deep stops, that they believe help manage the problems they are confronting. Put simply, what do you say while in the water and managing a diver that reports decompression problems in deep water during the ascent? Do you tell them to have faith in the balance of currently developing research? More broadly, how should individuals, teams, and organizations manage the variety of competing strategies within their community? In considering this problem, we find unexpected complexity, even reaching beyond the relative simplicity of the deep stop vs. no-deep-stop debate.

In this video, GUE President Jarrod Jablonski discusses historical and practical decompression aspects with GUE Explorers Mario Arena and Richard Lundgren.

Are we in a post-deep-stop world?

During the late 1990s and early 2000s, deep stops experienced great popularity, but almost as quickly as they appeared, they become a black sheep in many circles. That a community can so quickly embrace and then reject an idea is, in many ways, a positive feature of rational humans. True science, when done well, represents the best of this ideal because it takes almost nothing for certain. One develops a hypothesis, tests rigorously, and informs upon that hypothesis. Other researchers hopefully pursue a similar effort, and, over time, we gain confidence in a given idea or we do not. Even longstanding ideas are not technically settled, even though the overwhelming weight of evidence supports that hypothesis. 

When considering details regarding decompression or deep stops or any of the variety of the semi-common modifications in the technical diving community, we should maintain some balance in our view. While the rush into deep stops exemplifies the desire of the technical diving community to push past historical barriers, the enthusiasm was likely too hasty, given the lack of evidence and clarity in execution. This kind of initiative, for better or for worse, defines our species. Now some are pushing to accelerate ascents from depth. While this may well be the correct approach, we should manage the transition with a bit more foresight than was previously demonstrated. There are still many unanswered questions in the search to better understand decompression problems.

For the moment, it can be said that deep stops likely do not represent a clear value in accelerating one’s decompression and that they may actually present problems. It is obvious that the deep stop profiles, such as the one tested at NEDU, are not useful and can be dangerous. There are many compelling arguments that these results are directly correlated to the lack of utility in deep stops themselves, and other studies support this view. These individuals are sometimes frustrated by what they perceive as an outdated and unsupported view of evolving decompression science. At the same time, a research study that tests deep stops in a way that appears to be totally removed from their practical use is bound to elicit suspicion. Regardless of our personal conviction for or against an idea like deep stops, we should take the experience of our peers into consideration.

This backdrop of uncertainty requires some accommodation on two primary fronts. First, those engaged in technical dives must weigh the available evidence and make an informed decision about the best way forward. Second, one should respect the experience and choices of those with whom they choose to dive. There are no certainties in decompression, and the divers actually in the water doing the decompression maintain the ultimate responsibility for an associated plan. For GUE, and others, these aspects require careful balancing. 

GUE was founded and is managed by leading explorers, regularly conducting aggressive diving projects where lengthy exposure can become a notable liability. Changing weather, thermal problems, or other developments can force a diver to get out of the water as quickly as possible. Meanwhile, GUE is a training organization and maintains the need to establish a conservative approach in support of new technical divers. These new divers must determine, through experience, their individual susceptibility to decompression sickness. All divers should begin this process slowly, adjusting toward more aggressive profiles only if it makes sense based upon need and experience. However, in most cases, it will not make sense for divers to manipulate their decompression to be more aggressive. 

Jarrod Jablonski after a dive in Wakulla Springs during the 1990s. Photo from the GUE archives.

Evaluation of what constitutes an aggressive profile is a big part of what gradient factor methodology hoped to illuminate. To what extent that goal was realized remains an open question, but the use of gradient factors remains extremely common for both deep-stop and anti-deep-stop groups. Those favoring a move away from deep stops favor more aggressive ascents with higher gradients. Meanwhile, some divers resist rapid adjustment to what they perceive has been working. GUE policies regarding gradient factor strive to balance these factors while leaving the ultimate decision in the hands of experienced teams. It is very reasonable to act with consideration to prevailing research but we should also remember that most of the details remain unclear, leaving each diver with a burden to determine the best course of action. I would like to assert that these choices look more significant than they are in most cases, as I will detail in later sections. For now, we might ask if using low gradients can be dangerous , which is related but somewhat different from the removal of deep stops.

A deep stop profile may or may not be less efficient in terms of ascent time, but should its inclusion be strongly resisted? How about during an ascent where divers are experiencing or have experienced problems? If you feel greatly disadvantaged in terms of efficiency, then I would like to create some context. A typical dive to 45m/150 ft for 30 minutes while using a gradient of 20/85 produces a total of two minutes more decompression as compared to a 60/85 profile. Of course, the “problem” with low gradients becomes much more relevant with deeper depths and/or longer bottom times. In this case, adding deeper stops might result in a growing disparity between the total decompression times though this largely depends upon the model and safety factors utilized. Yet, these longer profiles are not conducted by new tech divers or students, and modifications to long and deep profiles ultimately rest with experienced divers making these choices for themselves. 

Jarrod Jablonski and Bill Main in 1992. Photo from the GUE archives.

As an organization founded by explorers and with wide-ranging expeditions conducted annually, GUE has always provided notable latitude to experienced divers but has also guided new divers toward conservative decompression exposure. Our experience over 30+ years demonstrates that a diversity of decompression profiles can be “successful.” Yet, we should always push to better define what success looks like. That question,  along with some of the more aggressive experiments in our community, highlight an interesting, if not blasphemous, possibility: Are we making progress toward understanding the underlying issues guiding decompression, or are we merely accumulating data? If we are making progress, do we appreciate the nuances enough to properly contextualize the outcomes? 

It is clear that advances in decompression knowledge have been significant, and that most individuals can dive with relative assurance that they will not become injured. I do not intend to suggest otherwise. However, I would like to ask if we are certain enough that we should push others toward our own beliefs. Are the details too vague for there to be the best solution that works for everyone? I hope you will join us for the final, part four of our series where we explore some unique, often under-discussed aspects of decompression development. 

Note: I hope the reader is able to appreciate my intent in this writing. I am not pushing any agenda, save the idea that open dialogue and respect for the experiences and reports of others is an important part of evolving practices. GUE is strongly committed to standard practices, although an often unappreciated aspect of this commitment is the understanding that some adjustment is natural. The idea is not to create a rigid, unthinking policy but a set of common tools, useful in large part because of their standardization within a community. Those standards can and do evolve, although they should not be changed carelessly unless a meaningful value is established. 

Tell us what you think. Should the industry immediately abandon all forms of deep stops? How hard should we push resistant dive buddies? How should we manage those experiencing problems during ascent but finding resolution with the inclusion of deep stops? We welcome your thoughts and want to hear about your experiences. 

Additional Resources

1. Blatteau JE, Hugon M, Gardette B. Deeps stops during decompression from 50 to 100 msw didn’t reduce bubble formation in man. In: Bennett PB, Wienke BR, Mitchell SJ, editors. Decompression and the deep stop. Undersea and Hyperbaric Medical Society Workshop; 2008 Jun 24-25; Salt Lake City (UT). Durham (NC): Undersea and Hyperbaric Medical Society; 2009. p. 195-206.

2. Spisni E, Marabotti C, De FL, Valerii MC, Cavazza E, Brambilla S et al. A comparative evaluation of two decompression procedures for technical diving using inflammatory responses: compartmental versus ratio deco. Diving Hyperb Med 2017;47:9-16.

3. Gennser M. Use of bubble detection to develop trimix tables for Swedish mine-clearance divers and evaluating trimix decompressions. Presented at: Ultrasound 2015 – International meeting on ultrasound for diving research; 2015 Aug 25-26; Karlskrona (Sweden).

4. Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. Technical Report. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. 53 p. Report No.: NEDU TR 11-06.

5. Fraedrich D. Validation of algorithms used in commercial off-the-shelf dive computers. Diving Hyperb Med 2018;48:252-8.


Jarrod is an avid explorer, researcher, author, and instructor who teaches and dives in oceans and caves around the world. Trained as a geologist, Jarrod is the founder and president of GUE and CEO of Halcyon and Extreme Exposure while remaining active in conservation, exploration, and filming projects worldwide. His explorations regularly place him in the most remote locations in the world, including numerous world record cave dives with total immersions near 30 hours. Jarrod is also an author with dozens of publications, including three books.

Cave

An Underground Perspective on GUE’s Cave Curriculum

It’s difficult to objectively compare and contrast technical courses from different training agencies, and many agencies are less than enthused about the prospect. So when I learned that NSS-CDS cave instructor and training council member, Chris Brock had completed GUE’s cave curriculum, I was eager to get his perspective. On the record of course!

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by Michael Menduno

It’s difficult to objectively compare and contrast technical diving courses from different training agencies, and the agencies themselves rarely encourage the juxtaposition. Each has its own particular philosophical bent, focus, and of course, what the organization deems its secret sauce, which can be hard to quantify. As a result, thoughtful, informed perspectives are hard to come by, though they can be illuminating when you’re trying to grok the depth and breadth of our tech community’s education ecosystem.

Needless to say, I was excited to learn that my former cave instructor, National Speleological Society Cave Diving Section (NSS CDS) instructor Chris Brock, had taken GUE’s Cave 1 and Cave 2 courses—the equivalent to the CDS Cave Diver course with decompression procedures and its Stage Cave Diver course. Note that Brock is also a member of the CDS Training Council and an Instructor Sponsor. Accordingly, I reached out to him, hoping to query him on the record, of course, about his experience. 

The circumstances of the 52-year-old cave instructor’s participation in GUE classes were somewhat unique.  Brock had agreed to assist his friend and fellow cave diver Meredith “Mer” Tanguay, who was completing her GUE Cave 2 Instructor Exam with Instructor Examiner (IE) Daniel Riordan in late 2020, in the midst of the global pandemic. 

Tanguay presciently acted to “bulletproof” her IE class—Mer managing Mr. Murphy—by making sure she had qualified ‘students’ at hand in case there was a dropout. Brock had previously completed Cave 1 under Tanguay’s tutelage in preparation for that eventuality, so when she received a cancellation a week before Cave 2, he was ready to go. What a guy!

I should note that Tanguay, principal of Wet Rocks Diving,  is one of GUE’s ardent, hard working recreational, technical, and cave diving instructors, a Fundamentals Instructor Trainer (IT), and an IE for Rec 1 & 2. She has been diving with GUE for 18 years. She was the first female to successfully complete her GUE’s Cave 1 and Cave 2 instructor rating, and is currently the only one teaching both GUE Tech and Cave, and GUE’s only female Cave 2 instructor, which until now had long been the sole province of the boys. 

As luck would have it, I was able to connect with Brock, a former public school teacher who is also a TDI, IANTD and PADI tech and cave instructor, to ask him about his experience with the GUE cave curriculum. Here’s what the man had to say. 

Let me ask, if you had to pick one word or phrase to describe your experience taking Cave 1 & 2, what would it be?

Chris Brock: Procedural! Everything you do in GUE has a set procedure. For me, having been away from GUE for 10 years when I took my “Fundies” class, it was hard at first to be procedural when I have so much muscle memory from just doing the dives all these years without thinking about it. It was difficult for me. It was very procedural and I had to slow down. 

Danny had a memorable piece of advice for me. He said, “You just have to slow down because your mind is thinking so fast about it and the way you do it. You have to just take it step by step and slow back down.” So, my takeaway at this point is that it was ‘highly procedural.’

Ha! GUE IE Guy Shockey, describes the work of GUE courses is teaching divers “how to do simple things precisely.”  What did you like most about the courses?
Line drills. Photo courtesy of M. Tanguay.

What I liked most about the course was seeing different ways of teaching the same concepts that I teach. I found a lot of value in how GUE does gas switches. I plan to do that from now on and incorporate it into my teaching. 

Now, will I encourage my students to dive in a Hogarthian configuration? I don’t know that I can do that fully, but I think we can incorporate Hogarthian principles into every configuration, even if they choose to use a chest strap and things like that. I think there is simplicity there as well as some tips and tricks that I’ve picked up from Danny and Meredith that will be helpful to students. 

Danny also addressed some of the things that I’ve seen as a weakness to it, to the Hogarthian configuration. He showed me a lot of little tricks that I had not seen before that I will also incorporate. There’s a lot of stuff that I’m going to try to add into my courses as well.

What did you like least about the courses?

I told Meredith and Danny this too: I think the strategy of GUE from what I saw over the two weeks of class, is a whole lot of push, push, push, push procedure. And they push the intensity level so high that some divers don’t have time to assimilate it and let it sink in before the instructors push students further. 

I think you should push a diver only so far, let them relax for a little bit and understand what they’re doing, then push them again. And in that sense, I don’t think there’s enough time for assimilation because there was constant intensity on all 10 dives. There was a push on all of it. So that’s one thing I did not like. 

Anything else?
Chris Brock labeling his cylinders. Photo courtesy of C. Brock.

The second thing I did not like was that in some ways I found it too procedural to where they were stressing valves, stressing lights, stressing and breaking everything rather than teaching these people how to cave dive. [Ed. Note: GUE tech and cave classes focus heavily on scenario i.e. failure-based training.

Teach me proper technique in this type of tunnel, so that I can transfer that learning on how to do it in that tunnel over there. Or teach me proper technique for diving in flow, so I can transfer it to how to dive in a high flow tunnel, that sort of thing. 

There wasn’t enough of that modeling, in my opinion. There wasn’t enough of that kind of modeling from the instructor showing me how to cave dive. I mean the instructor would correct things. She would say, “Your frog kick needs work or has some weaknesses in it.” But sometimes, and I do this—I did it in your class—I’ll say, I want you to follow me and do what I do. I think instructors should do that. Those were the two things I mentioned in my debrief with Meredith and Danny. I told them that that’s not what I would choose to do.

Ah yes, I remember. You had me do that at the mud tunnel at Ginnie Springs, “Follow me and do exactly as I do.” I was going to ask you about that because it really struck me when I worked with you and Reggie [Former NSS-CDS training director Reggie Ross, who passed away in 2019] that your course focused on the cave, the cave environment, and dealing with the cave. 
GUE tech courses are typically scenario-based. I was curious about what your take was on that. I talked to Meredith and she told me that GUE’s focus is 50% on the cave and 50% on what one would call problem-solving. So, you’re saying that it seemed to you it was less on the cave and more on problem solving.

From my point of view, yes. They told us, “The first half of the dive is yours, the second half of the dive is ours,” and they offered some coaching, and modeled and tried to correct our technique. The problem with it was this: Everybody knows—or learns quickly by living through the first few dives—[that] it’s hard to relax during the first half of the course. In the first three and a half or four days of the six-day course, you’re not enjoying those dives. Because it’s all about breaking you down and stressing you out. 

I understand why Meredith says 50-50. Because they do coach on that 50% in. But there was never that time, and maybe it’s because I was there, that she didn’t have to do as much modeling. She could always put me in front, and there you have it. And she could stay out of the team by doing that. Yeah, that’s just my feeling on it; we did have a conversation about it. I think there should be more teaching cave diving techniques and not just breaking every valve under the sun.

Right. You mentioned that you thought there should be a bit more time for students to absorb the lessons between high stress periods. And you have a unique perspective because you are already a cave instructor. But let me ask, do you think that the focus on problem solving—that is, dealing with continual multiple failures—helps students build confidence and capacity? 

That’s a tough one to answer. The answer is yes and no. I think that managing these situations within the team and getting everyone out safely definitely ups the student’s capacity. Where I think it’s a no is that sometimes a set procedure of doing things is not necessarily the only or even—in my opinion—the best way of dealing with certain failures. But it does focus problems back on the team rather than on the individual and the prescribed procedure for dealing with them. 

I do find the whole thing very interesting and very challenging for those making the standards for GUE education. I mean writing procedures that can easily be utilized across all types of diving is a challenging undertaking.

You raised a topic I was going to ask you about, the team. Obviously GUE is known for its team focus versus focus on individual diver. Was that different? How did that come through in the class and how does that compare with CDS training? Did you find more focus on the team in this course than you would in a CDS class? 

Yes, there was much more emphasis on the team because in a CDS course we can teach a single diver and we put ourselves as the dive buddy with them. After going through this process I do believe having that second teammate is very important. Learning how to work together as a team is sometimes more difficult than learning to cave dive.

“After going through this process I do believe having that second teammate is very important. Learning how to work together as a team is sometimes more difficult than learning to cave dive.”



Ha! Like a lot of things, it’s a learned skill, or at least can be improved with training. 

Exactly! In the GUE course, you’ve got to have at least two student divers. Now, in both of these courses we were teams of three. It was interesting, on the first course (Cave 1) we had one guy that had a difficult time fitting in with the team. That’s not to say he was a bad person or anything like that. His personality was just different than the others and it was a difficult mix. The differences in how we looked at things did create some friction in the course as all of us had strong opinions. It was interesting.

In the second course (C2), I was paired with a husband and wife and they were fantastic. Just fantastic. I thoroughly enjoyed being teammates with them and we worked well together. Not to say there wasn’t friction on occasion, but never anything where you felt like the team was fragmenting. I enjoyed that piece of it quite a lot. 

Because you learned something?

I actually enjoyed it a lot because for me, it was a different way of thinking about stuff. I’m so used to doing most of these things on my own with one or two students. It forced me to be more conscientious of others around me and I had to make some changes in how I communicated. Having worked with Reggie, I am used to very direct communication. And I had to temper that on occasion with certain teammates because they did not respond well to that. So, I had to adapt. 

Brock tweaking his DPV with an onlooker. Photo courtesy of C. Brock.

That was helpful. Meredith would actually tell me, “Chris, you’re a little too direct; you’re being a little too direct underwater.” And I said, “Well Meredith, they’re doing X, Y, and Z.” And she said, “I know they are. I know they’re making mistakes. But you need to realize that these aren’t people that have done 1000 cave dives. You are raising their stress level with the way you are communicating.” 

So, I said, “You know what? You’re right. I need to work on this.” And so, I enjoyed that piece of it. In the CDS, we rarely teach teams of two and three, so it was good to participate in that.

I know that you did the classwork on Zoom beforehand and then got together for the diving. I have two questions here. First, I am guessing that with the pandemic still raging, more and more instructors from all agencies are conducting Zoom classes these days. How did you find that? 
Second, I’d be interested to get your take on the knowledge content, how it compared to a CDS or TDI cave course. Did you feel it was complete and well presented, that kind of thing?

I think Meredith does a fantastic job. She has gone to great lengths and spent countless hours putting her courses into an online learning environment. She has put a lot of time into lecturing online, presenting PowerPoints online, and providing supplemental resources online, and I found that very, very helpful. 

Tanguay conducted her cave class lectures and meet ups on Zoom.

We were running at eight in the morning until around seven at night, every night. And if you were to throw another hour and a half lecture on that, it would’ve been impossible. I have been using the TDI online course for my students, and I do like it. However, it is a supplement to my own lectures. The difference being that currently most of my presentations are done in person.

I worked through the TDI online full cave module a few years ago as part of my research for a story. It’s very different from a live lecture over Zoom. 

Yes, Meredith does active instruction because there is no packaged online course module for Cave One or Two. So, it’s active lectures and she answers questions and it was very helpful. I learned some things.  There are also some videos that I want to use, and I plan to incorporate GUE.tv in my own courses. So, I found it very helpful. I think the way that she is approaching online learning is the wave of the future and I think a lot more instructors should incorporate that into their teaching.

No doubt, things are going to remain crazy for a while, so this is a good impetus for instructors to think about online learning. I know many GUE instructors have gone that route.

Here’s what it does. As a student you get this very high level of instruction ahead of the course, and then you then have the opportunity to review that material as you’re getting closer to the course. You have an opportunity within the course to go dive and experience it, and there’s the opportunity to have much deeper conversations than you normally would because now you’ve experienced it three times before you ever really get into it. So, you’re already at a state of being able to formulate really relevant questions at the conceptual level versus just rote knowledge. 

I like what she’s doing with this. She’s raising the bar on getting the content across. It’s very good, it’s very good.

The class you took was Meredith’s Instructor Examination (IE) to become a Cave 2 instructor. I know you have participated in CDS Instructor Institutes. Talk to me from an IE perspective. How did the instructor examination come across? How did it compare to CDS’ instructor development process?

It was completely different. In the CDS, we can do our IEs in several different ways. Yes, if a candidate has a group of students, then we can just observe them teaching their students and that can be their IE. So, we can do that. But most of the time, candidates come to us. They do classroom presentations for us, they do land drills for us, they do in-water work for us. So we see all of that as instructor evaluators. 

In this case, Meredith had to take care of all of the details and she was being evaluated on everything: from logistics and pacing of the course, dealing with events that happened within the dive, the debrief, corrections that she would have to make in the dive regarding safety, or whatever. We had a couple of instances, two that I can think of. One student had had an ear problem on a dive and then we had a problem with a student that just wasn’t feeling it and he thumbed the dive early. Danny was of course evaluating everything.

It was completely comprehensive. I mean the whole thing was being looked at. That’s a lot different than what we do at the CDS. Meredith was really put through the ringer, without a doubt. 

All of my GUE instructor friends, most of whom have other agency certs as well, tell me that GUE sets a very high bar for their instructors: it’s hard to become one, they have to do a partial requalification annually, and a full requalification every four years.

They do. Meredith had to do her requalification too. She had to do the swims and climb stairs wearing her doubles. The whole enchilada.

So from an instructor’s perspective, how would you compare and contrast GUE’s overall approach versus the CDS’s?
Chris Brock stowing gear after a dive. Photo courtesy of C. Brock.

If you get a good instructor, a good, tough instructor in any agency, you can get a good cave class. But if you get a burned-out instructor, in any agency, you can get a bad cave class. 

What I saw was certainly the cream of GUE rising to the top. I saw Meredith’s best work; her preparedness was point-on because of what she was going through. And I’ve been around her in other situations where she’s teaching and she does set the bar at a very high level for cave instruction. I’m very impressed with her. Very impressed.

I would say GUE’s courses are well suited to people that are very motivated, high-level achievers who love that team camaraderie. And if that personality type is who you are, I think GUE is a fantastic route for you to go. If that’s not the kind of personality that you have, then you might struggle within it. 

“I would say GUE’s courses are well suited to people that are very motivated, high-level achievers who love that team camaraderie. And if that personality type is who you are, I think GUE is a fantastic route for you to go. If that’s not the kind of personality that you have, then you might struggle within it.”

So to say that it’s better than TDI, CDS, any of that, I can’t say that it’s better. Is it more intense? I would think, by and large, it is more intense. I do a lot of stressful stuff in my courses, but I don’t raise it to that level. I don’t know how I would’ve handled that if I hadn’t a lot of experience already. It wore on me over the days. 

In my experience, GUE courses tend to be fairly intense. Heck, GUE is fairly intense. Ha!

Yeah, they are. But here’s one thing, I will say this. One of the debrief questions at the end of the first course was this: Do you consider yourself a GUE diver? And I said, no, I’m not a GUE diver, I’m a cave diver. I’ll dive with anybody, anytime, who’s safe and looks out for their buddy. 

Now, at the end of the second course they didn’t ask that question so I told them, you should’ve asked me that question again. And Meredith said, “Okay, what would you say?” And I said, “I would’ve said, yes, I am.” Because there is something rewarding about being tested that hard, and receiving that credential. You earned it. You really earned that certification. 

“Because there is something rewarding about being tested that hard, and receiving that credential. You earned it. You really earned that certification.”

I don’t know that I am a GUE diver, but will I dive with GUE guys? Sure. Do I think there is some of the bullshit that hangs over from the old days? Absolutely. There is still some of that under the surface that I see, but by and large I have met a lot of really good people.

Chris Brock doing what he loves best. Photo courtesy of C. Brock.
I find your take on the type of diver who might find GUE appealing interesting. In my younger years, I was like, 150% pushing all-out, all of the time to improve my diving. And now, I find that mindset just doesn’t fit me quite as well. I’m more like, “Yeah, yeah, yeah, can we just go diving?” Maybe it’s just old age, I don’t know. I think it may actually be a bit healthier for me to not to push all the time. You know what I’m saying? I’m just a little more chill.

I’m the same way, and that’s kind of why I would challenge them. I don’t know if Meredith spoke to this. She could’ve had a much easier IE if I hadn’t been there. I was the guy that was challenging her on every point while she was getting evaluated on her responses to every challenge I made, by Danny. 

Ha! Right, some friend you are! Making it harder.

And Danny spoke about that in our debrief together. He said, “Chris, you brought a whole other perspective and Meredith had to defend what we do. That was good for her to have to defend it. Because you understand the various aspects of cave diving and are coming in and asking, “Why?” Tell me why because I’m not going to believe it on face value like a normal student who listens to the instructor say, “Do X, Y and Z,” and says, “OK,” instead of, “Why do you do it this way and not that?”

And I’m with you—I think that there is a personality type that fits GUE and there’s a personality type that doesn’t. And there’s a personality type somewhere in the middle that fits GUE sometimes but maybe not all the way. Perhaps that is me. Overall, I think it is a pretty solid program. I really do.

So you think there may be other GUE courses in your future? 

I do. I’m looking forward to my next professional development course with GUE. Either Tech 1 with Meredith or possibly a DPV course with Danny. As an instructor in both of those areas I am curious as to how that content is presented as well.

“I do. I’m looking forward to my next professional development course with GUE. Either Tech 1 with Meredith or possibly a DPV course with Danny. As an instructor in both of those areas I am curious as to how that content is presented as well.”

Thanks Chris! Thanks for being willing to share.

Footnote by Meredith:

Meredith Tanguay in her element. Photo by Annika Persson.

Appropriately challenging both “normal-track” Cave 2 students and a student with over 1000 cave dives in the same class most definitely added another layer of difficulty to my Cave 2 Instructor Exam class. It was certainly a challenge, and I like challenges, so I thank Team FIYD FIYD for that! [Ed.—Inside joke] Our job as instructors is to provide challenges to improve on each diver’s starting set of skills and help them become better and more efficient, even the ones that are already solid divers. This makes every GUE class different and customized for that unique team of students.

Additional Resources:

Meredith Tanguay, principal Wet Rocks Diving

Chris Brock, principal, Cavediving.com


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 DeeperBlue.com. He is on the board of the Historical Diving Society (USA), and a member of the Rebreather Training Council.

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Planning World Record Dives

Bubble models and the deep stops that resulted were very popular through the early 2000’s though problems with these models became more and more evident with divers reporting a growing number of problems with decompression sickness. Early imprecision in the development of VPM even required development of a replacement algorithm, originally known as VPM-B. Meanwhile, it appeared progressively more likely that whatever benefit might be obtained from use of deep stops required more precision than had become the norm. This was the backdrop the author confronted when the second of my long-time dive buddies’ retired. Our Woodville Karst Plain Project (WKPP) team would restart exploration activity after a long pause due to unfavorable conditions. 

The new exploration dives would be conducted with Casey Mckinlay and we set out to reconsider all aspects in preparation for a new round of world record cave dives. We were expecting dives of at least 12 hours at depths up to 90 m/300 ft and in-water times of nearly 30 hours. During previous dives, I had seen a modest frequency of low-level symptoms among our small group. For example, pain during ascent that resolved, reappeared and resolved again and did not persist at the surface. Another complaint related to minor pain in the joints, described by some as being like very fine crushed glass. And of course, there had been some incidents over the many years of aggressive exploration and these coming dives would be longer those ever done before. Should we use bubble or dissolved gas models? What about deep stops? 

In the broader community, a variety of deep stop strategies were used but most sought to reduce gas pressure below the maximum value (M-value) by some margin, usually by conducting the first stops around 75% of the maximum depth of the dive. One aspect that can vary greatly relates to the time spent in deep water which, in many cases, was not tracked carefully. I felt divers had become progressively sloppier about the execution of deep stops. Being able to relax after hauling large payloads of equipment encouraged divers to stay longer and not properly account for the time spent. GUE had already moved to better clarity with ascent protocols, instituting a “run time” approach with deep stops and a “stop time” approach for middle and shallow stops, effectively encouraging their divers to move efficiently in deep water and not to overstay missed time from deeper stops. In considering an approach for the upcoming WKPP dives, I proposed elimination of the deepest of our decompression stages and efficient movement to more shallow stops at 57 m/190 ft. We still engaged in what one can call deep stops, albeit with less emphasis and greater attention to detail. 

Ultimately our teams successfully conducted many dozens of very aggressive dives with numerous setup, exploration and cleanup dives toward the discovery of a connection between Wakulla Springs and the Leon Sinks cave system in 2004. The longest of these dives involved more than 12 hours at depths approaching 90m/300 feet, requiring nearly 30 hours total immersion. We had used similar procedures to those in use over many years and changes to our use of deep stops might not appear significant, but they had marked a subtle though persistent reappraisal of deep stop methodology which continues to this day.

Dr. Doolette is an active technical diver whose extensive research experience and role among the principle investigators at NEDU make his contributions to any discussion particularly relevant and useful.

Personal Profile

Dr. Mitchell is a technical diver and gracious contributor to diving conferences and public forums. His general knowledge and experience treating decompression sickness are highly valuable contributions to the industry and this discussion of deep stops.
Personal Profile

1. Blatteau JE, Hugon M, Gardette B. Deeps stops during decompression from 50 to 100 msw didn’t reduce bubble formation in man. In: Bennett PB, Wienke BR, Mitchell SJ, editors. Decompression and the deep stop. Undersea and Hyperbaric Medical Society Workshop; 2008 Jun 24-25; Salt Lake City (UT). Durham (NC): Undersea and Hyperbaric Medical Society; 2009. p. 195-206.

2. Spisni E, Marabotti C, De FL, Valerii MC, Cavazza E, Brambilla S et al. A comparative evaluation of two decompression procedures for technical diving using inflammatory responses: compartmental versus ratio deco. Diving Hyperb Med 2017;47:9-16.

3. Gennser M. Use of bubble detection to develop trimix tables for Swedish mine-clearance divers and evaluating trimix decompressions. Presented at: Ultrasound 2015 – International meeting on ultrasound for diving research; 2015 Aug 25-26; Karlskrona (Sweden).

4. Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. Technical Report. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. 53 p. Report No.: NEDU TR 11-06.

5. Fraedrich D. Validation of algorithms used in commercial off-the-shelf dive computers. Diving Hyperb Med 2018;48:252-8.

US Navy Manual 14-4.19 Decompression Sickness in the Water. Decompression sickness may develop in the water during surface-supplied diving. This possibility is one of the principal reasons for limiting dives to 90m/300 fsw and allowing exceptional exposures only under emergency circumstances. The symptoms of decompression sickness may be joint pain or more serious manifestations such as numbness, loss of muscular function, or vertigo.

USN procedures for in-water onset of DCS include recompressing up to 20 fsw and lengthening stops.

USN Diving Manual, Rev 7 Change A, Ch 15-12.7.1

During the early 2000’s, GUE developed a reference standard based upon Buhlmann algorithms. The intent was to ensure divers reference profiles with the most successful history. From this base, consideration for team control and unity during ascent, as well as the potential utility of bubble control were considered. Balancing these factors resulted in a gradient factor of 20/85. These settings will not only result in deeper stops but will also account for these stops with additional decompression time.

Current research challenges the value of deep stops, suggesting they may be less efficient. GUE has been slow to adjust parameters for dives conducted during training because the relatively short profiles of students and their need to gain proficiency with a controlled ascent speaks against the value of faster ascents and/or shorter total decompression time. Balancing the experience in our community, while considering the most useful priorities for students, supports a deeper gradient than might be otherwise encouraged by developing research.

GUE protocols maintain a 20/85 reference gradient for training dives where the priority is ascent training and team refinement and where a slight increase in additional decompression time is not problematic. As divers gain experience. they are free to adjust gradients in a way that is suitable to the team while considering personal experience, team preference, mission objectives, and evolving research.

Low gradients such as 20/85 should not be confused with studies like that done at NEDU. The NEDU research greatly delayed the ascent, adding 3.4 x’s the stop time in the first three stops alone, as compared to a similar 20/85 profile. More importantly, the NEDU study did not account for the additional decompression time these low-gradient stops develop because it was testing whether the value of deep stops in controlling bubbles was enough to overcome the increased on-gassing at these deeper depths. Yet, a 20/85 profile is very different, since it will increase the decompression time as a consequence of the lower gradient at depth while actually “insisting” upon a relatively low compartment pressure upon surfacing. One can certainly argue that these deep stops are not useful or that they delayed the ascent unnecessarily, but it is difficult to argue that they are more dangerous, unless the diver ignores the resulting shallow decompression time. The problem with low gradients is mainly an issue of decompression time, with risk accruing when divers add deep stops while ignoring the consequences of that gas loading. This reduced time was the hope and advertised benefit with deep stops, but shortening shallow time is not necessary with the inclusion of deep stops.