Connect with us

Equipment

The Secrets of Scrubbers

Dr. Clarke’s geeky new monograph on the inner workings of rebreather scrubbers represents the culmination and synthesis of more than three decades of Naval research, as well as the personal research of the retired scientific director of the Navy Experimental Diving Unit (NEDU) into stochastic systems. As such, it is arguably a MUST READ for serious rebreather users. Here marine scientist and rebreather instructor Jeffrey Bozanic dives into some of the important details of Clarke’s book and the operational questions it seeks to answer. Feed your head!

Published

on

By Jeffrey Bozanic PhD. Header image courtesy of Bernie Campoli.

Let me begin this book review of Breakthrough: Revealing the Secrets of Rebreather Scrubber Canisters by John R. Clarke with my conclusion. This is one of the most important works available to rebreather divers. It dispels many unsafe practices and provides a base for making informed decisions. This monograph is a “must read” for all serious users of rebreathers, all rebreather designers, and all rebreather manufacturers. 

I began diving rebreathers in 1988, and since that time have accumulated in excess of 2,500 hours in a wide variety of open water conditions. Early on in this time, I began wondering about the truth of “best practices” that were taught to me (and that I have since been teaching), as access to quantifiable and repeatable data was lacking in our civilian/scientific diving communities. 

I often wondered about scrubber canisters, one of the intrinsic critical components in any rebreather.  How well do they work? What can make them fail? How long will they really last in the environmental conditions I use them in, as opposed to how they are tested? What is the best method for packing canisters? Can canisters be safely used on a dive, stored, and reused? If so, how long may they be stored? Should we modify use time based on such storage? What are the impacts if the canister becomes partially flooded? Are there practices which will extend absorbent efficacy, either during or between dives? Can I modify canister times based on my expected work rate, compared to the work rates simulated during testing? Does my personal size (240 pounds/110 kg) and fitness level suggest that I modify canister use time compared to test standards? What are the implications of using one absorbent grain size versus another? How much does ambient water temperature impact absorbent efficacy? How can I modify my personal use time based on that? And this is but a partial list of questions I have had.

Rebreather Forum 4

Sure, we all have stock responses to many of these concerns. “Well, we know that absorbent will last longer in the 84oF/29oC water in which I dive, so I just double the manufacturer’s suggested time.” Or “If you bag and label your scrubber after use, you may use it on a future dive.” Most of these responses are based on anecdotal experience or data, so they may not be completely without merit. And most seem to make sense if we think about them hypothetically. But none have been adequately tested or modeled from a scientific perspective.

US Navy SEAL Delivery Vehicle Team Two launch an SDV with rebreather divers from Los Angeles-class submarine USS Philadelphia

It’s The Data, Stupid

I mentioned earlier that, “access to quantifiable and repeatable data was lacking in our civilian/scientific diving communities.” In much of the research that I have done, there were indications that such data may have existed in the military community, specifically at the US Navy Experimental Diving Unit (NEDU), where such questions are typically researched to support military divers in completing their missions successfully. However, as a civilian, I did not have or had only limited access to such research documentation. I found this to be extremely frustrating. And it left me curious as well: What do they know that I do not?

Enter John Clarke. John was the Scientific Director at NEDU for 28 years. A physiologist by training, he found himself working with divers, engineers, and mission planners on projects or research designed to validate options for classified operations. Much work revolved around rebreathers, including scrubber canisters. In fact, before this book was published, the manuscript had to be reviewed and approved by the Department of Defense, an uncommon process for most publications, to ensure that it contained no classified information. While there were minor edits made, they allowed the publication to go forward.

You see, John had developed most of the model upon which much of this book is based during his personal time, after hours and at home outside the workplace. While he used the fruits of his labor at NEDU, it was not a NEDU project. But the reality is, John’s work benefitted from his NEDU projects and experience, and the research at NEDU benefitted from John’s extracurricular labor.

Computer simulation of calcium carbonate deposition in a scrubber canister as a result of carbon dioxide absorption. The colors purple, red, and yellow, respectively, indicate increasing concentrations of carbonate.

To understand the benefits (and limitations) of Breakthrough, you must first understand the differences between some basic concepts: hypotheses, anecdotal information, experiments, and modeling. 

Hypotheses, Anecdotes, Experiments, And Modeling

A hypothesis is an idea or thought that you believe could be true. There are two basic types of hypotheses, those that you believe may be correct (true), and those that you set up as “straw men,” hoping that by proving them incorrect that the obverse must then be true. Most people inherently understand the first type of hypothesis. In fact, some people look at some of these hypotheses, and because they seem so inherently reasonable, often consider them to be “true.” An example of this might be the statement that, “The more absorbent you use in a rebreather, the longer you can remain underwater.” This seems reasonable, right? Yet, what about the case in which the ability to dive longer is limited by oxygen supply, and not by absorbent? Or when, even though you have more absorbent, the resultant increase in breathing effort caused by the increase in gas travel path through the absorbent bed leads to increased breathing effort, increased carbon dioxide generation, and consequently reduced dive time? We should not confuse a hypothesis with “truth.” Unfortunately, this is relatively common in the civilian diving world. It requires validation to prove a hypothesis. 

Many people believe they can validate a hypothesis with anecdotal data. This is information which may have been observed but may not have been rigorously structured. A common argument heard in the recreational diving sector is, “Well, I did it, and I’m fine.” That argument is often used to justify further deviations from recommended guidelines. This normalization of deviance may continue until a significant adverse incident (such as a fatality) occurs. Even short of such an event, using anecdotal evidence is skewed reasoning.

  • Rebreather Forum 4

Consider the example of a rebreather diver using a specific rebreather, the manufacturer’s use guidelines state that their rebreather may be used for three hours in 40oF/4oC water at a VCO2 (metabolic carbon dioxide production rate) of 1.6 LPM (liters per minute). The diver recognizes that their typical workload during a dive produces much less carbon dioxide than that, and that they dive in the Caribbean Sea at much warmer temperatures. So they hypothesize that they can dive longer than three hours, and “test” that hypothesis by conducting a dive of four hours. Surviving that dive, their next dive is five hours, and so on until they are diving their rebreather for ten hours on the scrubber. Obviously, the manufacturer must have been overly conservative in their guidelines, correct? 

The fallacies with using this type of anecdotal evidence to “prove” their hypothesis are many. The diver’s workloads are not consistent during the dives, and results from one dive will not match another. Water temperature varies seasonally, with depth, and often with location, due to currents and other factors. Depths were almost certainly not constant, impacting gas density and scrubber efficacy. Finally, there was no means of objectively determining the partial pressure or percentage of carbon dioxide in the breathing loop at any given time, other than, “I felt OK.” 

My personal research has shown that divers will not notice a 5% CO2 level in a breathing loop over a short time, where 0.5% is considered a “safe” limit. All we learn from the body of anecdotal evidence from our diver is that he survived what may have been hundreds of hours of “testing” and “proof” of his personal guideline. 

Two Navy saturation divers working at depth during physiological studies in the Man-Rated Chamber Complex at the Naval Medical Research Institute, Bethesda, MD. Divers: Frank Stout and Tom Brisse, left to right.

The only way to prove a hypothesis is through experimentation. Experiments are tests in which all variables are tightly constrained, so that the impact of any single variable (like the time a scrubber will last) is well understood and defined. If conducted properly, the results are consistent and repeatable. In the previous example, multiple trials of duration would be conducted, constraining water temperature to 84oF/29oC, depth to 130 fsw/40 msw, VCO2 to 1.2 LPM, and having a way to actually measure when PCO2 reaches 0.5% SEV, as one example. We would then need to run this same set of constraints multiple times to see that we are getting consistent results, before changing one of the independent variables and repeating the experiment again.

The problem with experimentation is that it is time consuming, and expensive. Also, some observations we might wish to make (like seeing the inside of a scrubber real time) is not possible. Modeling allows us to circumvent these issues by establishing a set of mathematical equations that simulate what occurs in real life. This would allow us to predict results if we dove in those actual conditions. Modeling is limited by how well we can establish the mathematical formulas that we use for predictions, the granularity (fineness) of the model, and the processing power and time available. Models are validated and revised based on experimental results and occasionally anecdotal data.

Temperature profiles in a simulated axial-flow canister immersed in frigid water during active carbon dioxide absorption. Absorbent bed temperatures range from cold (black) to white (hot. The diver’s exhaled breath flows from left to right.

Breakthrough uses all of the above approaches for considering scrubbers. However, much of the predictive information and discussion arise from models that John has built, which have consequently been refined and improved upon using the experimental data from his years of research at NEDU. While this information is new and interesting, it did not answer all of the questions I asked above, and may leave you wondering with your own specific questions. That said, I still found the book illuminating, and in some cases downright scary. The remainder of this review I am going to spend highlighting a few concepts and results from the text that I found particularly engaging.

Before I begin, one last observation: John’s goal was to provide relevant information, not just for divers, but also for theorists, engineers, and physicists. The book is based in large part on math. I have had several years of calculus and statistics in the course of my academic studies and still found some of the math to be above my head. Do not let that dissuade you from reading it. Even if you do not understand the math, John’s discussion generally highlights the importance of what he is trying to explain with the math, and the diagrams help as well. If you get to a part of the manuscript where the math is daunting, just skip over it and resume your reading when you get past the numbers.

Mission-oriented divers. Photo courtesy of JFD Global

A Study In Variability

In many respects, the book is a study in variability. How does the real world vary from theory? How do variations in constraints impact scrubber performance? What role does environmental variability play in scrubber efficacy? When may small, seemingly inconsequential variances have a large impact on final results? How does physiological variability impact scrubber performance? And what should we do to increase dive safety based on this information?

Revelations began on the very first page of the text. Examples here, and on pages 3 and other locations, discussed problems with the actual absorbent products provided by different manufacturers. Contaminated absorbent, mislabeling, grain size variations, and extrusions could or were all impacting scrubber performance for divers. I had never considered that there could be defects in the actual absorbent that were undetectable by cursory visual examination.

  • Rebreather Forum 4

An engineering concept that John stressed in the book and that I had seen before—but was not really familiar with—was propagation of error. While mentions of it are made throughout the book, the most concise discussion was on pages 45-52. In this example, the potential impact of physiological uncertainty and conversion of metabolic oxygen consumption to carbon dioxide by any given diver, means that breakthrough could occur much sooner than the published limits, in this case, it’s likely to be a 13% shorter duration. What John is saying is that during manned dives, breakthrough can occur well before unmanned test limits indicate.

Dirty Dozen divers in Chuuk Lagoon. Photo courtesy of Dirty Dozen Expeditions.

This is scary! Putting it another way, in 17 out of 100 dives, the PCO2 would exceed 2% SEV, (a generally accepted “safe” value  when diving a helmet, but not α rebreather) within the published time, a value four times than expected. In actual field practice, we make the exact opposite assumption, based on differences between test water temperature of 40oF/4oC and our dive environment temperature (warmer), and anticipated VO2 and VE being less than test standards. The point here is that I have always regarded published canister limits as conservative and to be regarded as a starting point for determining a reasonable dive time on a scrubber. This may not be true. Later in the same section, using a different set of covariance data, he demonstrates that a diver may reach a 2% SEV level as often as one dive in five (about 20% of the time) due to propagation of error effects.

  • Rebreather Forum 4

Having worked with statistics to evaluate my own research data on many past projects, I am quite familiar with confidence intervals and prediction limits. However, I had never considered them with respect to scrubber performance. The same figure indicated a reduction in time of 20% at some temperatures, when a 95% prediction limit was considered. Page 50, Table 4, provides data on high and low COV and indicates that there is a 1 in 42 chance of breathing 6.3% CO2, when aiming to conclude a dive at 0.5% CO2. I found this astounding. I think John summarized it best on Page 54, “Being able to safely dive to  a single published canister duration, such as 180 minutes, is a fantasy.” This discussion also indicates to me that we in the diving community would be much better served if the manufacturers providing us with canister limits also provided confidence limits with those figures.

Respiratory exchange ratio (RER) is a measure of how much CO2 is produced based on the amount of oxygen consumed. I had always considered a value of 0.8 to be typical for this, although I was aware that it varied, particularly with diet. However, the Navy uses a value of 0.9 for their standard, and some experiments conducted by the Navy resulted in a RER of 1.07 (Page 16, “VCO2 was 1.4-1.9 LPM, and VO2 was 1.21-1.77 LPM.”) In other words, the divers were producing more CO2 than they were consuming oxygen! This has obvious implications for the rebreather user.

The Power of Modeling

In Chapter 5, John begins the discussion of the model he has developed for predicting scrubber performance. He explains the parameters and how the model evolved as he used it. The diagrams are quite interesting and are an important stepping stone to understanding the discussion in Chapter 6 on cold soaked scrubbers and other applications.

Simulated canister temperature tracings. The canister starts at 40°F and is immersed in 34°F water. The brown and orange tracings are from sensors located near the beginning of the canister, in the center, and near the outside edge of the cylindrical canister. The other tracings represent the average granule and average gas temperature.

One section titled “Decoupling Work and Ventilation” on Page 75 considers hypoventilation. This is a common practice as many experienced open circuit divers often train themselves to hypoventilate (“skip breath”) as a technique for conserving gas. This practice may be contraindicated when using rebreathers, but the habit is so ingrained due to muscle memory that the diver is unaware of it. In the experimental setting discussed on the following page, the practice directly resulted in the diver passing out while under water from high end tidal CO2. This may, in fact, be related to “deep water blackout” that has led to fatalities in some deep water cave dives.

One of the things I needed to keep in mind was that these chapters dealt with modeling, not actual dive operations. I found it enticing to think of applying this information to my personal diving habits. For example, on Page 95, he states, “The simulation ran about 3.5 times longer in the warmer temperature before canister breakthrough.” It would be easy to use this as justification of extending canister use to 3.5 times the manufacturer recommended limit. However, John then goes on to caution the reader that these are model results, and should not be used to plan real world dives. That warning should be kept in mind while evaluating all of the modeling results. Modeling indicates possibilities that then need experimental validation prior to adopting for actual dive applications.

Page 97 begins a discussion of Macroscale Variability. Translated, John is referring to whether the canister is completely filled with absorbent. Practically, what is being discussed is how well the canister is packed with granular (loose) absorbent, a critical skill in any rebreather training class. Unfortunately, we have no means of objectively determining if we have accomplished this vital task adequately. Perhaps weighing the final packed canister is the best alternative, but few divers do this on a regular basis. However, John shows in Figure 70 that with only a 99% fill compared to a full fill, there is a possibility of as much as a 35% loss in duration before breakthrough. 

This is an astounding number, and emphasizes how important proper canister filling is to the real world diver. It is also a strong argument, in my opinion, for the utilization of prepacked canisters (like those used with Poseidon rebreathers), or solid absorbents (like Micropore’s ExtendAir cartridges). 

The first real-world problem replicated by the simulation. After a canister warms up during a pre-breathe, cold-saturated canisters can begin functioning, but then fail permanently. However, simulated canisters stored at warmer temperatures before being dived in frigid water, do not fail prematurely.

I have always believed that long pre-breathe times (>30 seconds) were unnecessary as part of pre-dive preparations, given water temperatures of 50oF/10oC or warmer. The data plotted on Page 120, Figure 88, seems to support that hypothesis. What I did not expect, however, was his discussion on Pages 119-122 highlighting the initial scrubber failure in cold soaked canisters, followed by an initial effectiveness in absorbing carbon dioxide, followed minutes later by complete scrubber failure. This highlights the importance of managing cold soaked canisters effectively, including pre-breathing protocols. In Antarctica, we used a 5-minute pre-breath protocol, with no noticeable failure in canister efficacy once the dives began, while diving in 28.6oF/-1.8oC water.

Antarctic divers Christian McDonald and Steve Rupp prebreathing their CCR within the confines of a relatively warm dive hut placed over the dive hole in thick sea ice at McMurdo. Photo Credit: Mike Lucibella, National Science Foundation.

The section on Absorbent Granule Size Distributions (Pages 124-130) returned to a theme introduced on Page 1, variability in absorbent as provided by the manufacturer. You do not always get what you think you are getting. Two other points struck me as particularly important here. The first dealt with friability, or the tendency of absorbent granules to break into smaller particles. I have personally found this to be problematic operationally with some absorbents, including Draeger’s DiveSorb. The other point made in this section was the use by some recreational rebreather divers of absorbents other than those recommended by the manufacturers. This issue has been a long-held concern of mine. In this discussion, John pointedly discussed the use of Spherasorb (a medical absorbent used in anesthesia machines) in place of Molecular Products’ Sofnolime, the manufacturer recommended absorbent for use in their rebreathers (Page 128). Experimentation by Duke and New Zealand researchers showed that time to breakthrough using medical grade Spherasorb was significantly less than when using Sofnolime. 

The section labeled Pre-Dive Decision Making is especially important to rebreather users (beginning Page 133). The variables discussed in this section have a direct bearing on real world diving decisions. Topics covered include selection of a fine grain versus a larger grain absorbent, breathing resistance, pressure drop across an absorbent bed, breathing resistance limits, and other physiologic considerations in rebreather use. I will just say to the divers in the audience… “Read it!”

  • Rebreather Forum 4

As I read the discussion on gas density and physiologic interactions, I was left with a single question: “Why am I still alive?” My deepest dive depth on air was 86 mfw/280 ffw (not planned—chasing another cave diver who was impaired from narcosis, and who had exceeded the planned depth of 67 mfw/220 ffw). I consciously avoided any work at all, relying on overweighting to bring me down and lift from my BC to bring me up again. This section highlighted that I have been extremely lucky in some of my past dives.

On Page 149, John repeats a question posed to him, “Which is more lethal for rebreather divers, breathing resistance or CO2?” His response: “It’s both!” This statement pointed out the fact that, as rebreather divers, we cannot focus on any single aspect of threat or danger while using this equipment. To focus on one to the exclusion of others engenders risk that may be excessive. We have to focus on multiple variables and manage multiple variables, continually, while diving.

Nixie expedition diver surveying newly discovered passageway.Photo courtesy of Rannva Joermundsson

In Conclusion

Conclusions are presented in Chapter 8. This is a concise summary of the major findings in the book. While “skipping to the end of the book” to read these findings gets some of the major concepts across and may give you a feel for some of the material presented, it is not a substitute for reading the manuscript. The discussions, developmental arguments, and conclusions placed in the framework of the modeling and experimentation are well worth reading.

The remainder of the book is a series of Appendices that provide additional supportive data and information, including additional advanced math. John has promised to provide an even deeper review of some of the material in the form of an eBook Supplement that should soon be available and linked to on his website.

These were the sections that I found of interest, but  probably because they were based on my personal experiences. I have no doubt that different readers will find other points John makes that engage them more  than they do me. There is a wealth of information here, and it is difficult to grasp it all in a single reading.

Overall, I feel this is an excellent book with well-considered points based on experimental data, as well as modeling. It is a valuable addition to any rebreather user’s library.

Dive Deeper

InDEPTH: Too Much to Absorb: What You Need to Know About Your Scrubber by Reilly Fogarty (2020)

InDEPTH: Estimating Your Scrubber Duration

InDEPTH: InDepth’s Holiday Rebreather Guide: 2022 Update

InDEPTH: How Deep Is Your Library?


Jeffrey Bozanic PhD is a technical diving instructor and research scientist. Based in southern California, Jeff provides consulting and training services in the diving market. Specializing in rebreather use, he is probably best known for his seminal textbook on the topic, Mastering Rebreathers, and his work as senior Technical Editor for the 6th Edition of the NOAA Diving Manual.

A marine scientist, Jeff has participated or lead over 60 diving expeditions during the past 40 years, to places like Palau, Africa, Tonga, and Mexico. From 1989-1992 he oversaw scientific diving operations in Antarctica for the U.S. Antarctic Program, including co-authoring the Antarctic Scientific Diving Manual. In 2016 he was part of a research team that tested and evaluated rebreathers in Antarctica for the National Science Foundation. 

Jeff has served on many Boards, including as NSS-CDS Chairman, NAUI Vice Chairman, and AAUS Treasurer. He has been honored with the NAUI Lifetime Achievement Award, DAN/Rolex Diver of the Year, and AAUS Conrad Limbaugh Award for Scientific Diving Leadership.

Subscribe for free

Equipment

Hydrogen, At Last?

In February, Dr. Richard Harris aka Dr. Harry and the Wet Mules conducted the world’s first hydrogen rebreather dive to a test depth of 230 m at Pearse Resurgence, New Zealand. The purpose of the 13.5 hour dive was to determine the practicality and efficacy of using hydrogen to improve safety and performance of über-deep scuba dives. Harris will present the work up and details of the dive at Rebreather Forum 4 in Valletta, Malta on 22 April. In preparation, we thought it useful to review some of the history and research regarding hydrogen diving. Here’s what you need to know.

Published

on

By

By Michael Menduno. Header image: Dr. Richard Harris and Craig Challen at the entrance of Pearse Resurgence . Photo courtesy of Dr. Simon Mitchell.

🎶 Pre-dive clicklist: Hydrogen Song by Peter Weatherall, the singing scientist 🎶

Harris’s recent dive is the latest of an estimated 54 experimental hydrogen dives that have been conducted over the last 80 years by military, commercial and, yes, technical divers. It was the first reported hydrogen dive made on a rebreather, in this case, dual Megalodons connected by the bailout valve (BOV)—one charged with trimix diluent, the other with hydreliox (O2, H2, He). It was also the first hydrogen dive conducted in a cave.

The idea of using hydrogen to improve diver safety and performance on deep dives has been a long time in the making. The Swedish Navy began a program led by 26-year-old diving inventor and engineer Arne Zetterström (1917-1945)—think a young Lamar Hires, Bill Stone, or perhaps Jona Silverstein—who was the first to experiment with hydrogen as a possible deep diving gas during World War II. At the time, the United States had the only known helium reserves. And, because of its use for military “barrage balloons” filled with non-flammable lighter-than-air gas, the US had instituted the “Helium Control Act of 1927,” which forbade the export of helium. Zetterström’s mission was to determine if hydrogen was a possible alternative.

Zetterström, whose father was a Swedish naval architect, developed a method for producing hydrogen from ammonia. He had worked out the flammability issues (hydrogen gas is explosive when mixed with more than 4% oxygen), and calculated a rudimentary decompression plan for hydrox. Ultimately, he conducted six surface-supplied dives on hydrox 4/96 (4% O2, 96% H2) to: 40m/131 ft, 70m/230 ft, 110m/361 ft and 160m/525 ft, with one case of decompression sickness (DCS). Bottom times ranged from 10-25 min with 140 min of air and oxygen decompression. Tragically, Zetterström was killed on the 160 m dive in 1945, when tenders mistakenly pulled him up from depth without decompression.

It was nearly 40 years before interest in hydrogen as a diving gas was rekindled. At the time, commercial diving was getting deeper as North Sea oil production boomed, and commercial diving contractors were pushing the limits of heliox (an oxygen helium mix) diving as depths approached and exceeded 306 m/1000 ft. They believed that hydrogen might offer a solution. Hydrogen is half the molecular weight of helium, reducing breathing gas density, and research by Duke University’s Dr. Peter Bennett on trimix suggested that the narcotic properties of hydrogen might ameliorate the effects of High Pressure Nervous Syndrome (HPNS), which is a major limiting factor on heliox dives. 

Commercial Divers Do It Deeper

Pioneering French commercial diving contractor Comex (Compagnie maritime d’expertises) launched its hydrogen program dubbed Hydra in 1982. Beginning with animal experiments, over the next decade, Comex conducted a series of manned wet and chamber complex dives, Hydra II-Hydra X, to determine the efficacy of hydrogen diving. 

In February 1987, the Undersea Hyperbaric Medical Society held its 33rd workshop in Wilmington, North Carolina, titled “Hydrogen As A Diving Gas,” which included presentations by Comex. The following year, during Hydra VIII, four Comex divers and two French Navy divers successfully performed six days of work at 530 m/1738 ft on an offshore platform near Marseille with dramatically improved diver performance when compared to heliox diving. 

Comex saturation divers working on an offshore platform at 530m depth breathing hydreliox.

In 1991, Comex conducted a final record chamber dive to 701 m/2300 ft at its Marseille headquarters. Serendipitously, as then publisher and editor-in-chief of aquaCORPS Journal, I was invited to witness the dive, which was in progress in the Comex chamber complex. Heady stuff indeed! Special thanks to decompression engineer, JP Imbert, who worked at Comex at the time, and diving safety officer George Arnoux who made that possible.

Following the UHMS Hydrogen Meeting, the US Navy’s Naval Medical Research Institute (NMRI), in Bethesda, Maryland, began a program that lasted more than a decade (1990-2001) researching hydrogen under the direction of research physiologist Dr. Susan Kayar. Kayar and her team showed that breathing hyperbaric hydrogen had no ill biochemical effects on mammals that had been previously overlooked.

Next they demonstrated the feasibility of “biochemical decompression,” aka biodec, which reduced the incidence of DCS in in animal test subjects following hydrogen dives by roughly half. With a human biodec protocol, divers would take a capsule of special mammal-friendly, H2-eating gut bugs—think probiotics—which would consume the H2 gas and convert it to methane that would then be released to the atmosphere through the path of least resistance. Small wonder that Dr. Kayar, who measured flatulence from pigs and rats decompressing from hydrogen, became known as the “Queen of Farts.”

Susan Kayar aka the “Queen of Farts”. Photo courtesy of S. Kayar

Comex and the research by the US Navy demonstrated that hydrogen could significantly improve the safety and performance of working divers, and in doing so extend their range, much as the introduction of helium breathing mixes has been able to greatly improve diver safety and performance over deep air diving. Unfortunately, hydrogen was very expensive to mix and handle in a saturation diving environment, and it arguably arrived on the scene too late.

By the late 1980s, the commercial diving industry was already transitioning to robotics, especially for deep diving, due to the economics—it was far less expensive and safer to put a remotely operated vehicle (ROV) at depth for many tasks, instead of a diver. As a result, hydrogen was never “operationalized” as a diving gas, and has remained an exotic vestige of hyperbaric research in search of an application.

Bring on The Tekkies

Hydrogen was on the minds of tech pioneers during the emergence of the “technical diving revolution” i.e., the adoption of mixed gas technology, in the early 1990s. Legendary cave explorer Sheck Exley cited hydrogen in an interview I did with him for aquaCORPS,“Exley on Mix,” in the fall of 1991. Exley had already made several sub-260 m/852 ft dives using trimix at a time when the majority of the sport diving industry would have been hard pressed to spell N-I-T-R-O-X, let alone know what it was used for. Ironically, that was the year that Skin Diver magazine editor Bill Gleason ingloriously dubbed nitrox the “Voodoo Gas,” a moniker that could arguably better be applied to hydrogen, whether hydrox or hydreliox.

Sheck Exley at Zacatón in the early 1990s. Photo from the aquaCORPS archives

As Exley explained, “From what I’ve been learning, there seems to be some real potential for hydreliox, though no one has looked at its use for deep bounce dives. With the kind of compression rates that I was dealing with at Mante, you need the heavy nitrogen to avoid HPNS, but I’d rather have the hydrogen.” 

You may be surprised to learn that Harris and the Wet Mules are not the first technical divers to conduct an experimental dive with hydrogen. That distinction goes to diving engineer Åke Larsson, and six tech diving colleagues. Fortuitously, Larsson worked at the Hydrox lab Swedish Defense & Research Institute under Dr. Hans Örnhagen. 

In 2011, in collaboration with the Swedish Diving History Society (SDHF) and Royal Institute of Technology Diving Club, they formed the Hydrox Project led by fellow techie Ola Lindh, with the goal of revisiting the work of Zetterström. Specifically, they focused on developing a mixing station, procedures and measuring system, and investigated flammability suppression, in the hopes of recreating Zetterström’s 40m dive.

Åke Larsson and colleague prepare for a 42 m hydrox dive. The yellow cylinder contains hydrox 4/96. Photo courtesy of Åke Larsson

In July 2012, the seven Hydrox Project divers each completed a single hydrox open circuit dive to 42 m/138 ft in an isolated quarry. The divers descended on air back gas, switched to the hydrox 4/96 stage cylinder at depth and breathed for five minutes, switched back, and then began a padded deco schedule on air, no oxygen for operational simplicity, using an extrapolated ZH-L16 algorithm. According to Larsson, “Breathing hydrox was a surprisingly pleasant experience! We planned the dive using air, so I was slightly narked but feeling well at 42 m. When we switched to hydrox gas, it felt cold but almost slippery and very, very easy to breathe compared to air. My brain cleared up in less than a minute and the nitrogen narcosis was gone.” 

Where Art Thou H2?

Though the Hydrox Project dives were technically not tech dives, and not very deep, there is reason to believe that hydrogen could improve diving safety and performance on über-deep untethered dives. First, and most importantly, hydrogen can improve the work of breathing (WOB), which is a major risk factor on deep dives.

The gas density of trimix 4/90 (4% O2, 90% He, balance N2) at 250 m/816 ft is 7.3 grams/liter, considerably above the 6.0 g/l threshold where the risk of a negative outcome dramatically increasesNote: it is believed that a high WOB killed Dave Shaw who suffered respiratory insufficiency during a 270 m/880 ft dive at Bushmansgat. With half the molecular weight of helium, hydrogen could reduce gas density to safe levels resulting in lowered work of breathing. For example, the density of hydreliox 4/30 (4%O2, 30% H2, balance He) at 250 m is 4.56 g/l, equivalent to breathing normoxic trimix 21/35 at 40 m.

Hydrogen, which is narcotic at deep depths (PH2≥ 15-20 bar), has also been shown to ameliorate HPNS, which is another major limiting factor at depth. And with market interest in hydrogen as a fuel source on the rise, the gas is plentiful, and cheap relative to helium, though mixing costs could be pricey.

However, there remains a HUGE caveat. Though technical divers were successful in adapting helium-based mixed gas technology to scuba diving, the situation with hydrogen is completely different. Helium diving was well established in the military and commercial diving communities by the late 1980s/early 1990s, and the technical community was able to draw on that experience. That’s simply not the case with hydrogen diving, which has not been operationalized, and most of the research has dealt with saturation diving.

In fact, there are significant challenges that must be addressed if self-contained hydrogen diving is to be possible, let alone successful. These include the real risk of fire and explosion, both above and below the surface, respiratory heat loss—hydrogen has about 3x the specific heat of helium and could lead to acute respiratory heat loss (RHL), followed by dyspnea, cough, and hyper-secretion; its high specific heat also impacts scrubber efficacy, particularly in cold water. Then there’s H2 to He isobaric counter diffusion, hydrogen narcosis. Oh, and the issue of decompression, which according to one expert may be the least of one’s worries {Ed. note: Because the other factors would likely kill you first!], and then there’s the matter of a suitable breathing platform. It’s a formidable list.

The Wet Mules at Pearse Resurgence for their February 2023 expedition. Photo courtesy of Simon Mitchell

Dive Like A Mule

The Wet Mules have been unrelenting innovators in their quest to explore Pearse Resurgence, a project that Harris first became involved with in 2007. Ironically, in 2012, a few months before Larsson and his team made their historic 42 m hydrox dive, Dr. Harris was explaining to the delegates at Rebreather Forum 3.0, in Orlando, Florida, why he and team mate Craig Challen were giving up on open circuit bailout at Pearse—it required some 28 cylinders each to bailout from a 220 m/718 ft dive with 30 minute bottom time. Instead the pair of push divers planned to dive dual back-mounted Megalodon rebreathers connected at the BOV—a configuration they still use today. They rejected sidemount bailout rebreathers due to poorer work of breathing (WOB). They have also worked with O’Three to adapt suits and electric heat to stay warm in the chilly 6ºC/43ºF Pearse waters, and created a system of decompression habitats in the cave to make the decompression safer, and more manageable.

One could surmise, it was only a matter of time before ‘Dr. Harry,’ who is a  deep diving anesthesiologist after all, would feel compelled to hit the H2. Feeling lucky today Punk?

Craig Challen helps Dr. Harry gear up. Note the dual Megalodon. Photo courtesy of Simon Mitchell

Harris will be presenting his report on his historical 230 m hydrogen rebreather dive at Pearse at the closing banquet for Rebreather Forum 4, in Valletta, Malta 22 April. His talk will be videotaped. He and Dr. Simon Mitchell, who was present with the Mules for the dive, is also preparing a paper for the Journal of Diving and Hyperbaric Medicine, and Harris will prepare a paper for The RF4 Proceedings, which will be issued in the fall.

Hydrogen Dreamin’

It is uncertain whether we’ll see a team of NIXIE tech divers—you know who you are— exploring a newly discovered 460 m/1500 ft deep shipwreck, supported by diving bells and an ROV in the next 10-20 years—think Britannic expedition on steroids! However, as tech dives continue to get deeperthe ten deepest cave dives today average 284 msw/926 fsw (adjusting for altitude and freshwater) or 75m/246 ft deeper than those of the 1990s thanks to rebreathers, while the deepest shipwreck dives today average 176 m/576 ft or 55m/180 ft deeper than those of the past it is possible that we will see limited use of hydrogen for special, well-funded tech projects.

It is perhaps more likely that one day soon, clandestine military divers will be locking out of a subsea platform wearing dual or triple H2-enabled rebreathers to carry out some über-deep mission—though we will surely never know.

Then again, hydrogen may well just remain another geeky diving dream that proved to be a dead end, albeit one that inspired us to continue think big and boldly about underwater exploration, and helped increase our understanding and knowledge of what is required for humans to safely breathe underwater, and dive deeper, and stay longer. Isn’t that what diving is all about?

Below please find a selection of curated hydrogen diving articles and resources. We hope you find them useful. And do consider coming to Malta to hear Dr. Harry’s talk. It will be historic.—M2

Rebreather Forum 4

Playing with Fire: Hydrogen as a Diving Gas

As every tekkie knows, helium is essential for deep diving due to the fact it’s non-narcotic and offers low breathing gas density. But it’s conceivable that hydrogen may one day become a part of the tech tool kit for dives beyond 200 m/653 ft, by virtue of the fact that it’s light, a little narcotic and offers the possibility of biochemical decompression. Diver Alert Network’s Reilly Fogarty has the deets.


High Pressure Problems on Über-Deep Dives: Dealing with HPNS

If you’re diving beyond 150 m/490 ft you’re likely to experience the effects of High Pressure Nervous Syndrome (HPNS). Here InDepth’s science geek Reilly Fogarty discusses the physiology of deep helium diving, explains the mechanisms believed to be behind HPNS, and explores its real world implications with über-deep cave explorers Dr. Richard “Harry” Harris and Nuno Gomes. Included is a list of sub-250 m tech diving fatalities.


The Case for Biochemical Decompression

How much do you fart during decompression? How about your teammates? It turns out that those may be critical questions if you’re decompressing from a hydrogen dive, or more specifically hydreliox, a mixture of oxygen, helium, and hydrogen suitable for ultra-deep dives (Wet Mules, are you listening?). Here the former chief physiologist for the US Navy’s experimental hydrogen diving program, Susan Kayar, aka the ”Queen of Farts” gives us the low down on biochemical decompression and what it may someday mean for tech diving.


Operation Second Starfish: A Deep Diving Novel of Submarine Rescue, Science, and Friendship

You read about the science of hydrogen diving last month in a “A Case for Biochemical Decompression,” by former U.S. Navy researcher, Susan Kayar. Now read her science fiction: where experimental diving may one day take us! Retired scientific director of the Navy’s Experimental Diving Unit (NEDU), John Clarke reviews Kayar’s new novel, “Operation Second Starfish.“


DIVE DEEPER

NDRI: Arne Zetterström and the First Hydrox Dives by Anders Lindén and Anders Muren. Swedish National Defence Research Institute. 1985

H2HUBB:  The History of Hydrogen

InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties by Michael Menduno and Nuno Gomes.

InDEPTH: Maintaining Your Respiratory Reserve by John Clarke

John Clarke Online: Hydrogen Diving: The Good, The Bad, the Ugly (2021)

Hydrogen Diving – A Very Good Year for Fiction (2018)

Diving with Hydrogen – It’s a Gas (2011)

Undersea Hyperbaric Medical Society: Hydrogen as a Diving Gas: Proceedings of the 33rd UHMS Workshop Wilmington, North Carolina USA (February 1987)


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. 

  • Rebreather Forum 4
Continue Reading

Thank You to Our Sponsors

  • Fathom

Subscribe

Education, Conservation, and Exploration articles for the diving obsessed. Subscribe to our monthly blog and get our latest stories and content delivered to your inbox every Thursday.

Latest Features

Trending