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Confessions of a Chamber Diver



Interview by Tim Blömeke. Images courtesy William Howell unless otherwise noted. Special thanks to Duke consulting professor and biomedical engineer Dr. Rachel Lance who suggested this story . All subjects provided express written consent for the use of their images and names for this article.

Gabriela Morales, Doug Kiesewetter, Kyle Habecker, Kelly Burns, JoAnn Haack, and Mike Winters prepare for an experimental chamber dive at Duke University.

Forty-one year old computer scientist and cave diver Mike Winters, has been a frequent lab rat, err test subject at Duke Center for Hyperbaric Medicine & Environmental Physiology, and has participated in most of their hyperbaric research studies over the last 15 years. The low profile, former special operations soldier, now a software researcher and endurance athlete, says he has always been interested in advancing hyperbaric science and medicine, as well as learning the limits and capabilities of his own body in various extreme situations.

Mike, you must be one of the most seasoned chamber divers out there. How does one get a gig like this? 

Chamber diver Mike Winters

I first got into chamber diving when I was an undergraduate university student. I was heavily involved in our SCUBA leadership program – helping to teach classes, maintain equipment, etc. It just so happened that my diving instructor, a the head of the University’s Scuba program, was a longtime friend of a man named Gene Hobbs.

At the time, Gene was in charge of the Duke Hyperbaric Chamber. I don’t recall his exact title or responsibilities, but he mentioned that the chamber was doing a research study and looking for candidates, and suggested I go try to participate. He also ran the Rubicon Foundation Research repository at the time and had an Explorer’s Club flag. Great guy, endless fount of knowledge. 

As a broke college student who was also very interested in the science behind it, I was extremely interested (this was maybe in 2007-2008). Then I got on the Duke chamber’s mailing list and have been getting invited to participate in studies ever since.

Anyone can contact their local hyperbaric chamber and inquire about current or future studies. For that matter, universities and hospitals are often running various research studies on a range of subject areas.

Do you log your chamber dives? How many have you done?

I’ve logged some of them, but I unfortunately have not been very organized or fastidious with my regular diving logs or my chamber dives. I did contact the research coordinator at Duke and got a list of most (All? I’m not sure) of the studies I’ve participated in over the years. See: “List of Studies” below.

Most of those involved multiple chamber dives and/or multiple days at the chamber. One of them involved living in the chamber for about three days. I would guesstimate I have about three dozen dives in the chamber at this point.

How would you describe the experience to someone who has never been in a hyperbaric chamber in their life?

If I had to pick just one word, I’d say interesting or perhaps surreal. It’s hard to describe the entire experience, and each study has been a bit different. Before you even enter the chamber area, you walk down a hallway lined with display cases full of old diving gear, photos of astronauts and researchers who have been here or been involved with the chamber over the decades. It’s an interesting walk through history.

Seeing the chamber control room area kind of feels like a smaller version of the Apollo 13 movie—it’s an area of maybe 20’ x 20’ [6m x 6m] with large banks of controls. Some of them are video monitors, but mostly it’s just large dials and gauges—a lot of old, manual analog equipment which sits next to the chambers themselves. It’s all in one very large room, with a second story “loft” area built above parts of it. 

Dr. Rachel Lance at the control panel for the hyperbaric chambers. Photo by Christopher Wilson.

How old is the chamber? 

I believe the chamber was originally built back in the 1960s or so, and has done some extreme dives. They even had three people live at ~2,250 ft/686 m deep for a month or so back in the late nineteen seventies and early eighties. Duke Hospital is also quite old; walking into it gives you a sense of history.

At no point does it feel rickety or unsafe; you can just tell that it has been here for a while. It was built and designed in a different era, and it was built to last. It’s well used—the paint is fading in some places or scratched and peeling in others, and there are things built into the walls of the chamber that feel decades out of date but are still functional.

The staff are all incredibly knowledgeable and very friendly. These are some of the people who have literally written the books on hyperbaric medicine, and they will joke around with you and tell stories. It’s a very friendly and casual atmosphere. At the same time, it’s immediately obvious that these people are very, very good at what they do.

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Please describe a chamber dive for us.

Doing a chamber dive is very different from a real dive, namely that you are (or can be) in a dry chamber most of the time, depending on the dive and chamber configuration. It can be fairly loud at times, as the pumps are pressurizing the large steel room you’re in. Sound carries and echoes well there. 

Rapid pressure changes also alter the temperature drastically: a fast descent can get quite warm, while a drop in pressure can get quite chilly. You’re also able to notice a difference in your voice because of the denser gas you’re breathing. Anyone who has used a full-face mask while diving may have experienced this before, but for most divers this would be a new sensation. For me personally, I sometimes feel nitrogen narcosis more acutely in a chamber than I do on a real dive—not so much the mental aspects, but the warmth and dizzying effect of it.

On dives where I’ve had to exercise in a wet chamber, that’s also a bit unique, as they have custom built a special underwater bicycle. In some configurations, you sit in it similar to a recumbent bicycle. In others, you lie down face first. On some dives I used a normal SCUBA regulator, on others, a full-face mask. Some dives have required the flying of a simulated aircraft or submarine while listening to radio commands and juggling other tasks, like managing fuel leaks in the craft. Each brings its own physical and mental challenges.

In addition to the time spent with the actual diving inside the hyperbaric chamber, there usually are some additional medical portions; for example, ultrasounds and ECGs. Many of the studies I’ve participated in required blood draws, arterial catheters, or a Swan-Ganz catheter, which is a small balloon that is inserted into an artery at the wrist or elbow and then run all the way into the heart, where it can be inflated to lock it into position, allowing researchers to draw blood from directly inside the heart. 

Many studies have also involved physical fitness tests, or a VO2 max test, which basically makes you exercise at increasingly high levels until you are unable to perform or endure the pain any longer.

Dr. Jeff Phillips of the Institute for Human and Machine Cognition (IHMC) provides his own head to test physiological monitoring equipment, designed by the IHMC team, in Duke’s Foxtrot chamber.
Duke biomedical engineering student Sarah Piper monitors the output of the underwater eye tracking device built by IHMC

Going back to your first ride, what was most unexpected to you from a subjective, sensory standpoint?

It’s been so long since my first chamber dive (15-ish years?), and I’ve done so many different studies at this point that I actually don’t recall my first experience. At the moment, I’d say the most unexpected thing was the way narcosis tends to feel so much more pronounced in a dry chamber than on a real dive. I’ve never really felt narked on a real dive, even when diving moderately deep or exercising hard at depth, but I’ve felt it on a number of different chamber dives, even while resting.

In broad strokes, could you describe some types of experiments that you took part in?

Some of them were cognitive tests – breathing various different gasses (several involved high CO2 content) and seeing what sort of cognitive decline they cause. This is measured with increasingly complex cognitive tasks and hand-eye coordination tasks. This is really important for rebreather divers.

Others were trying to determine the correlation between exhaled CO2 and the CO2 levels directly in the heart. Again, important for rebreather divers from a monitoring perspective, if you can put a CO2 sensor in the exhalation part of the loop. 

Some of the studies involved diving at altitude, or flying after diving, and trying to develop safer ways to do that. Some studies were testing new medications, or new applications for existing medications, and how they affected the body in various ways. One involved seeing if breathing low concentrations of carbon monoxide could increase or stimulate muscle growth and repair. Another involved seeing if a three-day ketogenic diet would enhance resistance to oxygen toxicity (which for me was a massive change, for others it had less of an effect). My most recent study was helping them to build an AI/ML system to detect bubbles in ultrasound videos.

Dr. Rachel Lance (right) inspects Duke’s Golf chamber with the IHMC team. Golf chamber was home to the extreme Atlantis series of dive experiments to study high-pressure nervous syndrome, the most extreme of which locked three volunteers inside at pressure for 40 days. Only 4.5 days were spent at the max depth of 2,250 feet of seawater, with the rest required for decompression.

Among these studies, which are the ones that stand out to you and why?

Some dives have required breathing high concentrations of CO2, which is not a very fun experience. I had one dive where I essentially blacked out, but was still conscious and trying to do the tasks. I can distinctly remember the change in taste when switching over to the mix with the elevated CO2 content, and thinking to myself “you only have to do this for five minutes; just tough it out”. 

As a military veteran who served in a special operations unit, and former marathoner and ultramarathoner, I’m used to pushing through pain and discomfort. That thought was the last thing I remember. A few minutes later, the chamber attendant was yelling in my ear that I could come off the gas now, and I believe she even had to take the regulator out of my mouth. I’m not sure. That five-minute window of time is completely lost to me.

However, the scary thing is that I was still exercising—underwater, at about 165 ft (50 m) of depth if I recall correctly—and trying to pilot the sub, basically a flight simulator video game, designed by NASA, I believe. I am sure I was doing a horrible job of it. I was conscious and active, but the brain just wasn’t there anymore; the lights were on but nobody was home, which gives me a very healthy respect for some of the dangers of rebreather diving. They told me I had the highest CO2 tolerance of anyone they’d ever seen, which is not a good thing, as it means I can get myself into trouble by working too hard on a deep dive, or by “enduring” a rebreather with a scrubber failure.

Sarah Piper (right) of Duke and Dr. Jeff Phillips (center) and Connor Tate (left) of IHMC watch equipment testing from inside Foxtrot chamber. Institutions who collaborate on research projects with Duke Hyperbarics are invited to leave a sticker above the chamber entrance.

Another really memorable one involved living in the chamber for three days with three other people. We were in an approximately 8 ft. (2.4 m) diameter sphere the entire time, living at high altitude, except for when we went into the adjacent chamber to do our wet dives. While there were some interesting physiological changes from living and diving at altitude, the big takeaway for me was a newfound respect for astronauts, who have to live in such a small space for an extended period of time. I’m not claustrophobic at all – I’m a cave diver and quite used to tight places, but the boredom and lack of autonomy from being confined in the chamber was interesting.

Another memorable study involved muscle biopsies. They essentially had to cut and drill into the thigh, insert a small tube about the size of your pinky finger about three inches deep, and then suction or vacuum out a small chunk of muscle. That was not particularly fun. I was numbed, so it wasn’t painful, but I could still feel the pressure of the procedure, which was a very strange experience.

Many of the studies I’ve done involved arterial catheters or Swan-Ganz lines. I’ve probably had 10-15+ of those over the years, which always shocks any doctor who discovers that. It’s not really an uncomfortable experience, although there is a bit of pain at the incision site, and it takes a few days or a few weeks to fully heal. But mostly it’s just an odd experience watching a device getting inserted into your heart on the monitors, or feeling your heart skip a beat sometimes because of the device partially blocking one of the valves or the blood flow. It feels a little “different” or “off”—you know something is there—but it doesn’t really affect you very much.

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What are the most important things you learned as a test subject in scientific experiments?

I learned a lot about my own body and its limitations, which is something I’ve always been curious about. As an athlete, all the VO2 max tests I’ve done over the years have been useful benchmarks for me. I remember using some of those as training aids when I was preparing for an Ironman.

But more than that, I’ve learned so much about how my body reacts to carbon dioxide, both physically and mentally. I’ve also learned about my susceptibility to oxygen toxicity and how much it can change when being in ketosis, which makes me feel much safer and more comfortable for my real-world decompression dives. 

In addition to all the above, I’ve learned more about decompression theory, hyperbaric medicine, and physics, as well as various aspects of medicine and anatomy. As a former combat medic and EMT, that part fascinates me. Most importantly perhaps, I’ve met some really great people over the years, and I consider myself fortunate for having had the opportunity to learn from them.

Dr. Rachel Lance explaining test procedures in front of the control console for the hyperbaric chambers.

Is there anything you learned during your chamber dives that you’ve been able to apply in your wet diving practice?

For me, the ketogenic study showed me that I can massively increase my own oxygen tolerance. While this is valuable information, I choose not to do that diet because I’m too lazy to be very precise with it. However, knowing my own O2 limits helps me stay within a safe window during my own decompression dives. 

The several studies involving CO2 probably had the biggest impact on me personally. I’ve seen how it affects my physical capacity, both in an exercise sense and a lingering fatigue sense, as well as a mental capacity. I even had the one experience where I essentially blacked out.

With all of that, I have a very healthy respect for rebreather failures, and even though I am a cave and technical diver, and some of my dives have been in the six hour range, all of my “serious” dives are open circuit. I view rebreathers as a useful tool, but not one that I need for my diving at the moment.

Savannah Ripley (right) and Connor Tate (wearing mask) of IHMC function check their own equipment in Foxtrot chamber in preparation for a study.

As a final question, what kind of diving do you do when you actually get in the water?

I’m currently a bit landlocked where I live, so most of my diving has been in a local rock quarry. But my biggest love in diving are caves. I love the mental and physical challenge, the planning and preparation, the excellence and focus that it demands, and the exploration aspect. It’s an incredible feeling to be in a place where, in the history of our species and our planet, only a small number of living things have ever been.

Mike, this has been excellent. Thank you very much for your time, and safe diving always.

You’re very welcome.

Special thanks to photographer Christopher Wilson for his help illustrating this story. Check out his amazing work at:


Other InDEPTH stories by Tim:

DAN Europe blog: Highlights from the first day of Rebreather Forum 4 by Tim Blomëke

DAN Europe blog: What You Missed from Day Two of Rebreather Forum 4 by Tim Blomëke

DAN Europe blog: Insights and Breakthroughs: A Recap of Day Three at Rebreather Forum 4 by Tim Blomëke

InDEPTH: When Easy Doesn’t Do It: Dual Rebreathers in Extended-Range Cave Diving by Tim Blömeke (SEP 2022)

Tim Blömeke teaches technical and recreational diving in Taiwan and the Philippines. He is also a freelance writer and translator, as well as a member of the editorial team of Alert Diver. For questions, comments, and inquiries, you can contact him via his blog page or on Instagram.

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What is Undeserved in “Undeserved Decompression Sickness”?

Divers still seek comfort in the notion of the “underserved” hit to explain unexpected incidents of decompression sickness. “Hey, my computer said I was fine.” NOT. Here diving physiologist Dr. Neal Pollock exposes the fault in this notion. While decompression algorithms take into account the divers’ profiles, i.e., time and pressure, there are a multitude of factors that can potentially impact divers’ decompressions, as the author explains. Once divers’ reject the escapism that accompanies the ‘undeserved’ label, they can get on with the important business of diving and giving adequate consideration in their deco planning.




by Neal W. Pollock, PhD

Photo by SJ Alice Bennett

Spoiler Alert: the most undeserved element in the title is the word “undeserved.”

Describing cases of decompression sickness as “undeserved” generally speaks more from an emotional perspective than a rational one. The driving factors are typically faith in imperfect tools and a desire (conscious or unconscious) to shift responsibility. 

Decompression algorithms rely almost exclusively on pressure and time data to predict effects. Enticing pictures can be painted on the authority of any algorithm, but the reality is that all rely on limited input to interpret complex situations for people who are not uniform. Modern decompression models are important constructs that can help us to dive safely, but the products are rudimentary from a physiological perspective, without sufficient sophistication to deserve unquestioned trust. 

The dive profile is almost certainly the most important determinant of gas uptake and elimination, but the truth is that we do not yet have sufficient data to quantify the impact of many of the variables that can influence outcomes (Pollock 2016). Instead, algorithms rely on simple measures and mathematical bracketing with the hope of covering the contributing factors. The problem is not in doing this; the problem is in being surprised when the outcome is not what was expected. 

Decompression safety is influenced by a multitude of factors, variably related to the dive and the diver.

Decompression Factors

Accepting that the dive profile is the most important determinant of decompression risk, there are additional factors that can also have dramatic effects. Exercise is one of these. Pre-dive exercise may have complicated effects on the subsequent diving exposure. Exercise during the descent and bottom phase will increase inert gas uptake and the resulting decompression stress. Mild exercise during the ascent and stop phase can promote inert gas elimination and decrease the resulting decompression stress, but excessive exercise can promote bubble formation and increase decompression stress. Post-dive exercise is likely to increase decompression stress in all cases. Practically, while the concepts are clear, the definition of meaningful thresholds for “mild” and “excessive” exercise is difficult at best, and quantifying real-time effects far exceeds current capabilities. 

Thermal state is another potentially dramatic factor (Gerth et al. 2007). Being warm during the descent and bottom phase can substantially increase blood flow and delivery of inert gas to the periphery and increase the subsequent decompression stress. Being cool during the descent and bottom phase can decrease inert gas uptake and decrease the subsequent decompression stress. Being cool during the ascent and stop phase will inhibit inert gas elimination and increase the subsequent decompression stress. Being moderately warm during the ascent and stop phase can promote blood circulation to the periphery and increase inert gas elimination, but excessive heating of peripheral tissues in this same phase can promote bubble formation as heating decreases the solubility of inert gas, effectively increasing the decompression stress.

Again, as with exercise, it is extremely difficult to identify meaningful thresholds for thermal state at different points in a dive, and quantifying real-time effects is not within current capabilities. It is certainly clear that the ambient temperature measured by a dive computer can have little correlation to the thermal status of the diver, and any thought that this information informs decompression models in a meaningful way is misplaced. 

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The wild card of individual (“predisposition”) factors further highlights the challenges unmet in current decompression models. Not only are these parameters not measured, it is unclear how the information could practically guide the risk assessment at this time if available. While the importance of these factors is hard to assess, it is also noteworthy that some, most often dehydration, may be used as scapegoats to explain away decompression sickness (DCS). 

A state of dehydration can adversely affect circulation, potentially impeding inert gas elimination, but this almost certainly has much less impact than the dive profile, exercise, or thermal state in many cases. The impact is also not as straightforward as making it a blame agent might imply. For example, if a state of dehydration impairs inert gas elimination during the ascent and stop phase to increase decompression stress, might it not also decrease inert gas uptake during the descent and bottom phase to reduce the decompression stress?

Sound levels of hydration are good for general health and probably for decompression safety, but a state of dehydration in no way guarantees an outcome of DCS, just like a good level of hydration in no way guarantees an outcome of no DCS. The blame directed to dehydration is probably related to the observation that DCS can be accompanied by clinically important fluid shifts. This, though, is more a consequence of the disease than a cause. 

Photo by SJ Alice Bennett

The rest of the predisposition factors offer similar challenges. Physical fitness appears to confer some protection against decompression stress, but the quantification of such effects is not yet possible. A history of DCS can go either way, with persons prioritizing blame shifting over understanding or behavioral changes having a higher risk of repeat events, and persons improving understanding and moderating risk factors having a lower risk of repeat events. Increasing age is a risk factor, but the partitioning of chronological vs physiological age still needs to be worked out, as does the interaction between age, physical fitness, and biological health.

Women might have a slightly higher physiological risk, particularly during the first half of their menstrual cycle, but this is likely largely (or more than) mitigated by sex-based differences in risk tolerance and practices. The norms and practices of a buddy can affect individual risk either positively or negatively. Circulation issues include state of hydration, the presence of a patent foramen ovale (PFO), and possibly old injury sites that disrupt circulatory pathways. The presence of a PFO is likely only able to become important in decompression stress if bubbles are present, which will depend on a host of other factors. Biological health elements are likely to offer interesting insights in the future, but the ability to assess and understand them exceeds current capabilities. 

The major point here is that there are a lot of unknowns and half-knowns that make it important to not expect any decompression algorithm to describe truth. They offer a first order approximation of risk. They provide what might be reasonable guidance within a wide swath of possible error. Staying within guidelines does not guarantee safety. The goal should be to plan for the possibility of suboptimal elements, perhaps several of them, that could influence outcomes.

Photo by Sean Romanowski

Conservative Settings

Divers often address the recognized shortcomings of dive-computer-based decompression models by altering conservative settings and/or practice. Those who feel they are bends-resistant may push the limits; those who prefer greater peace of mind may add buffers. One of the additional challenges is that not all practices put forward to enhance conservatism will actually act in that way. The best example of this is probably deep stops. The concept of stopping deep to minimize bubble formation was enticing, but flawed. Stopping too deep will certainly minimize the possibility of bubble formation at that point, but at a point when it would never reasonably be expected for bubble formation to occur. The problem is that the time at the deep stop depth allows any tissue that is not fully saturated to take up more inert gas. The additional uptake creates increased decompression stress as the diver ascends. The concept was well intended, but the impact was counterproductive. 

One of the conservative settings that is intuitively simple is gradient factors. The M-value describes a theoretical limit of supersaturation that a tissue can tolerate before problematic levels of decompression stress develop. This limit is another first order approximation of risk, with no guarantees of safety if staying within the limit. Gradient factors (GF) simply tailor limits to a different percentage of the M-value. GFs are typically presented as two values, GFlow and GFhigh, presented as GFlow/GFhigh. Those who believe in deep stops may choose a GFlow less than or equal to 20%. Those who do not believe in deep stops will likely choose a GFlow equal to or greater than 30%. Those who feel confident in their overall ability to tolerate decompression stress might choose a GFhigh in the 85% range. Those who want to add more buffers to protect against unknowns and surprises might choose a GFhigh less than or equal to 70%. 

Determining whether a case of DCS should be considered “deserved” or “undeserved” is problematic when prescribed limits are based on incomplete data and when they can be altered by a variety of settings. The argument, “My computer said it was okay!” holds little if any weight. A better approach is to focus on the fundamentals. The first fundamental is to consider when DCS is a possibility. As a rough rule of thumb, any dive within the traditional recreational range (40 m/132 ft) that is approaching half the US Navy no-decompression limit carries a non-zero risk of DCS. Similarly, pretty much any dive deeper than the traditional range carries a non-zero risk. “Non-zero risk” repudiates the claim of “undeserved.”

Photo by SJ Alice Bennett

Once the possibility of DCS has been accepted, the most productive deliberation includes an honest assessment of all of the risk factors that may have contributed to the outcome. While trying to pin the blame on one modifiable risk factor can be comforting, it probably does much less to ensure future safety. There are many effects that cannot yet be quantified, but the risk potential can be recognized. Focusing on any one variable can discourage a more honest appraisal of the possibilities.

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Assessing the Risk

DCS symptoms may develop due to frank violations of accepted practice, but many cases are shrouded in ambiguity. An honest and objective assessment will almost certainly improve understanding and future outcomes more than claiming an “undeserved hit” will. It is unlikely that any two exposures will truly be identical, either for two divers sharing one dive or one diver repeating a given dive. Subtle differences can accumulate to have a meaningful impact. These differences coexist with the probabilistic nature of decompression stress, effectively that a safe outcome experienced once or several times may not guarantee the same for all future exposures. 

Once all reasonable contributing factors have been considered, some room should be left for doubt. Appreciating the knowns, the unknowns, and the complexity of interactions can promote thoughtful practice without frustration. Practices can be optimized without guarantees. The first step is getting rid of the escapism that accompanies the “undeserved” label.


Gerth WA, Ruterbusch VL, Long ET. The influence of thermal exposure on diver susceptibility to decompression sickness. NEDU Report TR 06-07. November, 2007; 70 pp.

Pollock NW. Factors in decompression stress. In: Pollock NW, Sellers SH, Godfrey JM, eds. Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS Workshop. Wrigley Marine Science Center, Catalina Island, CA; 2016; 145-56.

InDEPTH: In Hot Water: Do Active Heating Systems Increase The Risk of DCI? by Reilly Fogarty

Neal Pollock holds a Research Chair in Hyperbaric and Diving Medicine and is an Associate Professor in Kinesiology at Université Laval in Québec, Canada. He was previously Research Director at Divers Alert Network (DAN) in Durham, North Carolina. His academic training is in zoology, exercise physiology, and environmental physiology. His research interests focus on human health and safety in extreme environments.

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