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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, gives us the low down on biochemical decompression and what it may someday mean for tech diving.
by Susan R. Kayar, PhD
Header courtesy of A. Tocco Comex
Thirty years ago, the Naval Medical Research Institute (NMRI) in Bethesda, Maryland, hired me for what at the time I thought was the coolest job I could ever be asked to do. I still think so. I was hired to be the physiologist for their experimental hydrogen diving program. Why dive with hydrogen? A recent InDepth article by Reilly Fogarty, “Playing with Fire: Hydrogen as a Diving Gas”, does an excellent job of explaining this subject. The short answer: because hydrogen is the smallest molecule.
One might think that in an era with excellent one-atmosphere hard suits, and multiple forms of submersibles and robotics, there is no need to send bare-naked divers to the sorts of depths involved in hydrogen diving, as will be described shortly. If these alternatives to divers are so great, why do we still use commercial divers at all? One needs to ask an operational person this question, rather than a scientist like me. But I think the words “logistics”, “costs”, “safety,” and “the direct human touch” would figure in the answers.
Once a diver dives deep enough to exceed safe limits with regard to nitrogen narcosis, the usual gas switch for the diluent to oxygen is helium. However, if a diver keeps on going into the range of 1000 to 2000 feet of seawater (roughly 300-600 msw), a helium-oxygen gas mixture becomes dense enough that the work of breathing becomes difficult. Divers fight to move this dense gas into and out of their lungs, making the effort to breathe a serious source of fatigue and a distraction to their assigned jobs. (See “Maintaining Your Respiratory Reserve,” by John Clarke). Hydrogen is a diatomic molecule (i.e. H2) with two protons and no neutrons, and is therefore half the molecular weight of helium, a monatomic molecule with two protons and two neutrons. Therefore replacing helium with hydrogen, eases a diver’s respiratory distress i.e. work of breathing.
There is also a phenomenon of ultra-deep diving known as High Pressure Neurologic Syndrome, or HPNS, (also known as High Pressure Nervous Syndrome) which is evidently a function of high pressure interfering with the transmission of signals in the nervous system. Symptoms of HPNS can range from tremors to confusion to psychosis and are highly variable in depth at onset and from diver to diver. For unknown reasons, hydrogen at high pressure is narcotic and can suppress HPNS. Past a very high pressure that again varies with the diver, but generally on the order of 23 atmospheres partial pressure of hydrogen, its narcotic properties can become overwhelming and have their own psychotic effects.
There are also serious issues involving the explosivity of hydrogen in combination with oxygen, but these issues are manageable with the care one always uses in handling oxygen and other combustible and hyperbaric gases. Hydrogen and oxygen can be combined safely if the oxygen content is less than 4% of the gas mix, with dive operations usually opting for 2% oxygen as their safe upper limit. A 2% oxygen mixture is breathable if the total pressure is 10 atmospheres (roughly 90m/295 f) or more. This is normally accommodated by starting a pressurization with helium and then switching to hydrogen after 10 atm. As a final consideration, the price of helium is rising, and may make hydrogen substitution increasingly attractive. Consequently, for a variety of practical reasons, hydrogen has a potential place in ultra-deep diving beyond 10 atmospheres of pressure.
Investigating Biochemical Decompression
As the physiologist to the hydrogen diving program at NMRI, my assignments were two-fold: first, to determine if there are any dangerous biological effects that had been previously overlooked of breathing hyperbaric hydrogen, and second, to look into something that NMRI was calling “biochemical decompression,” or “biodec,” a term they had coined themselves.
The unknown dangerous biological effects portion of the research was addressed first. The short answer to that was “none”. We found no evidence that inhaled hydrogen could participate in any unwanted biochemical reactions in the body, discounting whatever reactions eventually make hydrogen narcotic. We still do not know exactly why hydrogen becomes narcotic, but it is unlikely from the physical properties of hydrogen that its narcotic effects are permanently harmful post-dive.
Then we got to the really exciting part of the hydrogen research program at NMRI: biochemical decompression. A few years before I was hired in 1990, a biochemist at NMRI, Dr. Lutz Kiesow, heard it was possible for divers to use hydrogen as a breathing gas. He knew there were many microbes that possessed a hydrogenase enzyme allowing them to consume hydrogen gas as a metabolic source equivalent to the consumption of oxygen as a metabolic source for most land organisms. End products for hydrogen metabolism can vary with the microbe, but is often methane (CH4). Hence, as a class, such microbes are called “methanogens”.
Dr. Kiesow proposed that NMRI establish a research project to isolate the hydrogenase from a methanogen, and insert it somewhere in the body of a diver to effectively create a chemical scrubber unit for hydrogen. If a diver could continuously scrub out some of the hydrogen going into solution in his body during the dive, the diver would have a reduced body burden of inert (to the diver) gas, and could subsequently decompress more rapidly with lower risk of decompression sickness (DCS).
What a cool concept! I loved it from the moment I heard it. But the real challenge was to resolve Dr. Kiesow’s “somewhere in the body” requirement into a safe, readily reachable, functionally useful body location. The director who hired me understandably warned me that divers would be opposed to receiving routine injections, or any sort of biological implant making them Bionic Men, permanently different from their former selves or from other divers. So what was left?
On my first musings with the scientific head of NMRI when I was hired, I wondered if we could perhaps encapsulate the hydrogenase enzyme, or better yet just whole methanogens, and swallow the capsules down for delivery to the large intestine as the working location for this scrubber unit. The scientific head instantly responded he had been thinking the same thing, but had not wanted to bias my thinking by saying it first. The approach met all our criteria. Taking capsules by mouth is as easy and as non-invasive a way to get things into the body as there can be. The large intestine has many microbial species living there safely and performing many jobs that we are slowly realizing are important to our health.
Trust Your Gut?
Methanogens typically are anaerobic organisms that would die quickly if exposed to oxygen, and the large intestine is the only part of the body that provides an anaerobic environment. Some species of methanogens are even a normal part of our intestinal flora, where they consume traces of hydrogen manufactured by other intestinal microbes. We were therefore confident that adding more methanogens should do no digestive harm. The amplified population of methanogens in the intestine would be likely to stay high only for as long as the divers breathed hydrogen, and return to baseline shortly after the exposure to hydrogen ended. The methane end product of this hydrogen scrubbing has a safe means of escaping from the intestine.
The methane-releasing issues were the only parts of this research that got a little weird at times. I was very carefully coached by Navy people to use lengthy euphemisms such as “the methane is released to the environment following the path of least resistance,” or “methane has an obvious means of egress from the intestine.” I was warned never to use what I have come to refer to as “the four-letter f-word” for methane release. But the euphemisms never helped. All audiences instantly understood the euphemisms as such.
Indeed, I came to consider it a sign that my audience was truly listening to me and following the science when they suddenly started squirming in their seats and trying with greater or lesser success to cover their laughter when I started explaining the fate of methane. Jokes followed. One Navy brass listener asked me if the implementation of hydrogen biochemical decompression meant a negation of the stealth intended for Navy SEALs when they used closed-system (i.e., non-bubbling) breathing rigs. The only sensible thing for me to do was laugh along with the room.
An interesting phenomenon happened as soon as people got over their initial laughter at this childishly scatological word that I did not say but that they obviously thought of themselves. They started thinking about the physiology and the gas transfer physics I was describing, and they liked it. No more laughter after that moment of enlightenment arrived. So go ahead and laugh now. “Better out than in” applies to laughter also. I got a million of ’em. I am known in some circles as the “Queen of Farts” with good reason.
Measuring Flatulence err Farts
I retired from Navy civilian service years ago, so I can say whatever I wish. I measured farts. Measuring farts is funny. And measuring farts in rats and pigs is exactly how my NMRI team and I succeeded in demonstrating the feasibility of hydrogen biochemical decompression to reduce the incidence of DCS following hydrogen dives by roughly half. As far as we know, methane release rate is the only variable that can be biologically manipulated with a measurable effect on DCS incidence following any kind of dive. There is nothing humorous about reducing DCS incidence.
The methanogenic species we chose has a rather grand first name but oddly mundane last name: Methanobrevibacter smithii. It is native to the intestinal flora of many mammals, including humans and pigs, and thus does not cause digestive issues when added to the intestines. The metabolic equation for M. smithii is the following:
4H2 + CO2 = CH4 + 2H2O
To speed things along in the lab, we surgically injected M. smithii cultures into the upper end of the large intestines of our lab animal models of divers, which were initially rats and later pigs. The animal-divers were then placed in a hyperbaric chamber which we pressurized with hydrogen and oxygen. Some hydrogen and oxygen breathed by an animal-diver dissolves in the blood for transport throughout the body. When the blood circulates through the vasculature of the intestinal wall, some hydrogen diffuses down its partial pressure gradient into the intestinal cavity, where the M. smithii are housed.
Oxygen is taken up by the cells of the intestinal wall and aerobically metabolized to carbon dioxide (CO2), some of which also diffuses into the intestinal cavity. M. smithii metabolizes the hydrogen and carbon dioxide to methane and water. The animal-diver safely absorbs the water. It is a real scientific benefit that the methane exits the body as easily as it does. Since no mammalian cell manufactures methane, we could track the metabolism of our methanogens inside our animal-divers simply by measuring the rate of release of methane from them to the surrounding environment by gas chromatography. As the hydrogen pressure in the chamber increased, we measured increasing quantities of methane in the chamber gases
When we then decompressed our animal-divers, on average, the animals with supplemental methanogens had approximately half the incidence of DCS as those without supplements. As the volume of methane they released during the dive increased, their incidence of DCS decreased.
Knowing from the metabolic equation above that four hydrogen molecules are consumed for each methane molecule manufactured, we could easily estimate the rate of hydrogen-scrubbing inside our animals. Based on the solubility of hydrogen in body tissues (which we guesstimated as being similar to water), and the time at pressure of the dive, we could estimate how much hydrogen would dissolve in an animal of a given body mass by the end of the bottom time, and what fraction of that body burden of hydrogen had been eliminated by our process. We computed that when M. smithii eliminated approximately 5% of the hydrogen dissolved in our animal-divers’ bodies, DCS incidence was reduced by 50% (Fahlman et al, 2001).
Human Biodec
Having succeeded in demonstrating hydrogen biochemical decompression in a small animal model, the rat, and a larger animal model, the pig, we are at least scientifically prepared to extend this work to human divers. A diver would make a saturation dive (commonly abbreviated to “sat”, meaning a dive sufficiently long i.e. 24 hours or more, to saturate the diver’s tissues with the breathing mixture) using a hydrogen-oxygen blend we usually call “hydrox”, or a hydrogen-helium-oxygen trimix which goes by the awkward name of “hydreliox”, depending on practicalities.
Dive operations may even opt for a quad-mix including nitrogen. The ultra-deep diving trials at Duke University found the narcotic properties of nitrogen helped to suppress HPNS, which was so problematic for their divers breathing heliox. However, the interaction is complex. Since we are still working out the exact mechanisms that make nitrogen and hydrogen narcotic under pressure, it remains to be determined if combining nitrogen and hydrogen for deep sat dives makes narcotic issues better or worse. The issue deserves testing.
Regardless of the other gases in the sat diver’s mix, if there is hydrogen, then hydrogen biochemical decompression could be considered. A couple of days before the end of the bottom time, the diver would prepare to biochemically decompress as a supplement to the physical decompression. The basic process would be identical to that for our animal models, except for a gentler way of delivering the methanogens to the diver. We would freeze-dry cultures of M. smithii and pack them into oral-delivery capsules designed to dissolve only under the conditions inside the large intestine. It would take around 24-36 hours to have a capsule arrive in the intestine, dissolve, and re-activate the methanogens. We would know that the M. smithii were on site and sufficiently active by chemically analyzing the sat chamber gases for methane output. Then we would get to watch the diver not bend as he decompressed faster than divers in other hydrogen diving operations without biochemical decompression. As I said, coolest job ever, or what?
But wait!
There is one more really exciting finding to report. We have evidence that even the quantity of methanogens native to the intestinal flora of a pig can provide sufficient hydrogen-scrubbing activity to reduce DCS incidence from a hydrogen dive (See Fig. 4 below). Humans and pigs are similar in many respects, including basic intestinal flora. It may well be that any human divers on a hydrogen dive, such as those at COMEX , have already benefited from hydrogen biochemical decompression without realizing it. They have only to test for methane in their chamber gases to know.
Skeptics have argued that the relatively small percentage of hydrogen scrubbing we have computed may be far too little to have any impact on DCS risk in human divers or to make a worthwhile reduction in decompression times. In addition to pointing to our DCS incidence data, we note that all divers are familiar with how important small differences in gas loads can be in DCS risk. If we dive within the time at depth limits of our chosen algorithms, we are confident to a very high level of probability that our dive will end safely. But exceeding our planned no-decompression limits by even a few minutes, and thus adding only a relatively small percentage increase in our inert gas load beyond what we think of as safe, makes our dive profile much riskier. [Ed. Note: These are computational risks not necessarily operational ones i.e. small changes in times/depths are unlikely to result in DCI] Likewise, we are all in the habit of making what we term a “safety stop” in 3-5m/10-15 ft even from a low-risk, no decompression time-requiring dive.
Sat dive operations currently using heliox and contemplating a shift to adding hydrogen will be dismayed to realize that hydrogen is considerably more potent at inducing DCS than is helium (Lillo R.S., E.C. Parker, W.C. Porter, 1997 Decompression comparison of helium and hydrogen in rats. J. Appl. Physiol. 82(3) 892-901). This would mean that costs saved by substituting relatively inexpensively manufactured hydrogen (by electrolysis of water) for increasingly expensive imported helium could be overwhelmed by the costs added in significantly longer decompression time. This is where hydrogen biodec may provide its greatest advantage: in shaving down the extra time needed for safe decompression from a hydrogen dive to something closer to that of a heliox dive. Until someone takes the step of testing hydrogen biodec in human subjects, we will not know to what extent operational decompression times could be reduced.
Nitrogen Biodec?
What comes next? In an ideal scientific world, our research in animal models would be followed by equivalent studies in human divers. However, for the time being in the post-Russian Cold War Era, the US Navy has expressed no further interest in hydrogen diving and has not offered to support human studies in hydrogen biochemical decompression. To assuage my disappointment, I wrote a novel in which hydrogen biochemical decompression is used to help save the day in a submarine rescue scenario. The novel is entitled “Operation SECOND STARFISH, A Tale of Submarine Rescue, Science, and Friendship,” available as a paperback and Kindle e-book on Amazon.
But I am still dreaming bigger than that. Since hydrogen biochemical decompression works, why not shoot for something everyone in the diving world could use? Nitrogen biochemical decompression! There are nitrogen-metabolizing microbes native to our intestinal flora. But the problems of experimentally making nitrogen biochemical decompression work are staggeringly complicated. One of many is that in nitrogen metabolism, usually referred to as nitrogen fixation, the end-products are molecules such as nitrites, nitrates, and ammonia, which are not gases that would just come bubbling out for us to measure.
These fixed nitrogen compounds would stay dissolved in the fecal material and join many more such molecules already there from protein digestion. (If you think the fart jokes are bad, consider the fecal jokes. “No shit!”-Ed.) The presence of fixed nitrogen products in feces (also known as “fertilizer” under other circumstances) suppresses the nitrogen-fixing microbes from fixing even more, since the process is energetically expensive to the microbes and done only by necessity. It would take some genetic manipulation of the microbes to get them to work for us, and some form of special molecular labeling to measure how much end products they are making. I leave those problems to future scientists to solve, while I enjoy my retirement in New Mexico, the Land of Enchantment, and go on dive vacations to Hawaii, Papua New Guinea, Tahiti, Fiji, and Raiatea to keep my vestigial gills damp. I may even write another novel.
Dive Deeper
Operation SECOND STARFISH, A Tale of Submarine Rescue, Science, and Friendship
References
Bennett, P.B., R. Coggin, M. McLeod, 1982. Effect of compression rate on use of trimix to ameliorate HPNS in man to 686 m (2250 ft). Undersea Biomed. Res. 9(4)335-51.
Fahlman, A., P. Tikuisis, J.F. Himm, P.K. Weathersby, and S.R. Kayar, 2001. On the likelihood of decompression sickness during H2 biochemical decompression in pigs. J. Appl. Physiol. 91:2720-2729.
Imbert, J.P., C. Gortan, X. Fructus, T. Ciesielski, and B. Gardette, 1988. Ch. 13. Hydra 8: Pre-commercial Hydrogen Diving Project. Advances in Underwater Technology, Ocean Science and Offshore Engineering, Vol. 14, pp 107-116.
Kayar, S.R., M.J. Axley, L.D. Homer, and A.L. Harabin, 1994. Hydrogen gas is not oxidized by mammalian tissues under hyperbaric conditions. Undersea Hyperbaric Med. 21(3):265-275.
Kayar, S.R. and M.J Axley, 1997. Accelerated gas removal from divers’ tissues utilizing gas metabolizing bacteria. U.S. Patent No. 5,630,410.
Lillo R.S., E.C. Parker, W.C. Porter, 1997 Decompression comparison of helium and hydrogen in rats. J. Appl. Physiol. 82(3) 892-901
Kayar, S.R., T.L. Miller, M.J. Wolin, E.O. Aukhert, M.J. Axley, and L.A. Kiesow, 1998. Decompression sickness risk in rats by microbial removal of dissolved gas. Am. J. Physiol. 275 (Regulatory Integrative Comp. Physiol. 44):R677-682.
Kayar, S.R., A. Fahlman, W.C. Lin, and W.B. Whitman, 2001. Increasing activity of H2-metabolizing microbes lowers decompression sickness risk in pigs during H2 dives. J. Appl. Physiol. 91:2713-2719.
Kayar, S.R. and A. Fahlman, 2001. Decompression sickness risk reduced by native intestinal flora in pigs after H2 dives. Undersea Hyper. Med. 28(2)89-97.
Valée, N., Weiss M., Rostain JC, Risso JJ, A review of recent neurochemical data on inert gas narcosis. Undersea Hyper. Med. 38(1)49-59
Susan grew up in the St. Louis, Missouri, area. An early fascination with the films of Jacques Cousteau inspired her to become certified as a scuba diver while still in high school. Her diving in Missouri was confined to artificial lakes with sunken rowboats, lost Coke bottles, and a few carp as the thrills. She persevered in her interests in marine sciences and attended the University of Miami as a biology major, remaining at that institution all the way through to a doctorate. After graduation, it did not take long to realize she would starve if she insisted on a job in marine biology, so she moved into studying physiology in extreme environments and exercise stress. Postdoctoral research appointments sent her from Colorado to Switzerland to New Jersey. Her dream job finally materialized in an appointment with the US Navy in the Washington, DC area, where she studied decompression sickness risk in animal models of ultra-deep diving.
Susan was inducted into The Women Divers Hall of Fame in 2001 in recognition of her Navy diving research. When funding for her Navy program ended, she managed research funding efforts for the National Institutes of Health (NIH), Defense Advanced Research Programs Agency (DARPA), and the Office of Naval Research (ONR). Now in retirement, she has written a diving-themed novel, “Operation SECOND STARFISH.”
Cave
N=1: The Inside Story of the First-Ever Hydrogen CCR dive
This Valentine’s Day, Dr. Richard Harris, aka ‘Dr. Harry,’ and the Wetmules made the first reported hydrogen (H2) rebreather dive to a depth of 230m/751 ft, in The Pearse Resurgence, New Zealand. The 13 hour dive, which was nearly two years in planning, was a field test to determine the efficacy of using hydrogen to improve safety and performance on über-deep tech dives. Harris’s dive was the deepest “bounce” dive in approximately 54 experimental H2 dives—the majority SAT dives—that have been conducted over the last 80 years by military, commercial and, yes, a group of technical divers. Now in this first published account, InDEPTH editor Ashley Stewart details the inside story behind the dive, a dive that will arguably be remembered 100 years from now!
By Ashley Stewart. Images courtesy of Simon Mitchell unless noted.
On March 11, a little more than three weeks after completing what is believed to be the first-ever rebreather dive with hydrogen as a diluent gas, Dr. Richard “Harry” Harris convened the group of scientists and researchers who had spent years helping to plan the attempt.
He started with an apology. “All of you had the sense that you were party to this crime, either knowingly or suspecting that you were complicit in this criminal activity,” Harris, an Australian anesthesiologist and diver known for his role in the Tham Luang Cave rescue, told the group.
The apology came because the dive was dangerous—not just to Harris who was risking his life, but for the people who supported him were risking a hit to their reputations and worried their friend may not return home. Harris and his team put it all on the line to develop a new technology to enable exploration at greater depths.
A significant challenge to deep diving is an increased work of breathing and CO2 buildup as breathing gas becomes more dense at greater depths. This can not only culminate in fatal respiratory failure but also increases the risk of practically everything else divers want to avoid, like inert gas narcosis and oxygen toxicity. For this reason, helium is favored by divers for its low density and non-narcotic effect. However, at such great depths, helium increases the risk of tremors and seizures from High Pressure Nervous Syndrome (HPNS). This can be ameliorated by keeping a small amount of narcotic nitrogen in the mix. The problem is that even small amounts of nitrogen makes the mix too dense past 250 meters.
Harris’s experiment would determine if divers can turn to an even lighter gas: Hydrogen, the lightest in the universe. Hydrogen is about half the density of helium. It’s also slightly narcotic and hence thought to ameliorate HPNS, thus allowing elimination of nitrogen from the mix.
The addition of hydrogen into a breathing gas, however, comes with one small technical uncertainty—the extremely explosive nature of hydrogen. History confirmed this reality with the 1937 Hindenburg disaster in which the hydrogen-filled dirigible airship burst into flames. As Harris tells it, he set out to dive hydrogen in his diluent gas while avoiding the nickname “Hindenburg Harry.”
Hydrogen in the Mix
Why would anyone attempt to breathe hydrogen? Harris and his colleagues have spent more than a decade and a half exploring the Pearse Resurgence cave system in New Zealand. This extremely challenging, cold water cave system (water temperature is 6ºC/43ºF) has been explored by Harris and his team, who call themselves the Wetmules, to a maximum depth of 245 meters/803 feet in 2020. Their gas density at depth was 7.2 g/l, significantly above the recommended hard ceiling of less than 6.2 g/l.
Diving past this point introduces increased risks, not only of CO2 buildup, but narcosis, decompression sickness, HPNS, cold breathing gas, having adequate gas supply or bailout, and isobaric counter diffusion (ICD) in which different gasses diffuse into and out of tissues after a gas switch causing bubble formation and related symptoms, cold breathing gas, and having adequate gas supply or bailout.
Divers have been examining hydrogen as a breathing gas for decades. The Swedish Navy was the first to experiment with hydrogen as a possible deep diving gas during World War II. The U.S. Navy in a 1965 paper proposed replacing helium with hydrogen due to projected helium scarcity. Later, beginning in 1991, researchers at the Naval Medical Research Institute (NMRI) in Bethesda, Maryland spent a decade studying hydrogen’s potential physiological impacts and biochemical decompression. French commercial diving contractor Comex (Compagnie maritime d’expertises) launched its hydrogen program in 1982, and the Undersea Hyperbaric Medical Society (UHMS) held a workshop “Hydrogen as a Diving Gas,” in 1987.
Even technical divers considered hydrogen. Legendary cave explorer Sheck Exley considered hydrogen in the early 1990s to mitigate HPNS symptoms, which are ultimately believed to have contributed to Exley’s death at Zacatón in 1994. Nearly all of the experimental hydrogen work up until this point used surface-supplied systems and saturation diving versus self-contained diving, and none of it, as far as we know, has been done with a rebreather.
The primary objective of Harris’ hydrogen experiment was to address the issue of increased work of breathing. Harris’s team had previously encountered CO2 incidents at the Pearse Resurgence. In one incident, while at 194 meters/636 feet, explorer Craig Challen—Harris’s primary dive buddy since 2006—lost buoyancy but was unable to find his buoyancy compensating button quickly. He kicked up a couple of times to stop his descent and immediately got a CO2 hit. Challen was able to grab the wall, calm down, slow his breathing, and survive. Based on such incidents, it’s clear to the team that they have reached the limits of the gas. “I feel we are on the knife edge all the time,” Harris said, in terms of physiology and equipment.
While hydrogen in the diluent breathing mix was expected to address increased work of breathing, the rest of the issues associated with deep diving were “major unknowns,” and some (such as respiratory heat loss) were potentially even made worse by hydrogen.
“At what depth do the risks of introducing this new technology outweigh the risks of carrying on with trimix?” Harris said. “That’s a very difficult question to answer. At some point we are going to have to consider different technologies and, at this point, hydrogen is perhaps the only one available to us.”
H2 Working Group
In 2021, the year after Harris completed his deepest dive at the Pearse Resurgence, InDepth editor-in-chief Michael Menduno was taking a technical diving class and reading about the government looking at hydrogen as a diving gas again. “Technical divers should be at the table,” Menduno said he thought to himself at the time, “our divers are as good as anybody’s.” He called John Clarke, who had spent 27 years as scientific director of the U.S. Navy Experimental Diving Unit (NEDU), and discussed setting up a working group. Menduno’s next call was to Harris, who had shared his troubles with gas density at the Pearse Resurgence. Harris had also, separately, been thinking about hydrogen.
The so-called H2 working group met for the first time in May 2021 and included many of the top minds in diving medicine and research, including Clarke, NEDU’s David Doolette and Greg Murphy, research physiologist Susan Kayar who headed up the US Navy’s hydrogen research at the Naval Medical Research Institute (NAMRI), along with her former graduate student Andreas Fahlman. There was diving engineer Åke Larsson who had hydrogen diving experience, deep-diving legend Nuno Gomes, decompression engineer JP Imbert who had been involved in COMEX’s Hydrogen diving program, and anesthesiologist and diving physician Simon Mitchell. The group was later joined by Vince Ferris, a diving hardware specialist from the U.S. Navy, and explorer and engineer Dr. Bill Stone, founder of Stone Aerospace.
The working group met regularly with the goal of figuring out how one might possibly operationalize hydrogen for a deep technical dive using the Resurgence as an example. During one of their meetings, Clark used a breathing system simulator built for the Navy to predict how hydrogen would affect gas density in a closed circuit rebreather at depths to 300 meters/984 feet.
To Doolette, who has known Harris for decades and supervised his Diploma of Diving Medicine project in 2001, it was immediately clear this was not a hypothetical discussion. “Unlike some of the scientists, I was under no illusion that the question before the working group was fiction, I knew that Harry was likely to try a H2 technical dive in the Pearse Resurgence,” said Doolette, a cave explorer in his own right, who has laid line in the Resurgence.
By fall of 2022, it was clear to many in the group that Harris was going to attempt the dive. The group had mixed feelings ranging from cautious optimism to comments like, “My friend is going to die.”
Doolette was concerned Harris and Challen would not survive the dive due to either ignition of hydrogen—in the worst case, inside the rebreather at depth—or a serious adverse response to respiratory heat loss (the latter was especially if Harris attempted diving beyond 245 meters/803 feet as he had originally planned) he said. “I have known Harry for longer than most in the group. I encouraged him to take up cave diving, so I felt a personal responsibility toward him,” Doolette said. “I have a lot of experience in operationalizing new diving technology. My goal was, if unable to discourage him, to force him to focus on the important issues.”
Leading up to the dive, Menduno scheduled Harris to give the banquet talk about the expedition at the Rebreather Forum 4 industry meeting in April. The outcome of the dive, of course, was uncertain, and the two had to make an alternate plan in the event that Harris did not return. “We had to say we were going to talk about your dive one way or another,” Menduno said. “If you don’t make it back, Simon Mitchell is going to have to give a presentation about what went wrong. Harry made some typical Harry joke like, ‘Well, as long as you don’t stop talking about me.’” Harris’s lighthearted tone betrays how seriously he took the dive and its preparation, people close to him said.
While no one involved was taking as big a risk as Harris and Challen, they were risking a hit to their professional reputations by being associated with a controversial dive, especially in the event of a tragic outcome.
“At heart, I’m an explorer, and that was pure exploration,” Mitchell, who was the diving supervisor on Harry’s dive, said when asked why he would take such a risk. “Exploration in the sense that we were pioneering a technique that hadn’t been used for quite some time and never in technical diving, not deep technical diving.” He also emphatically added, “I was more worried about my mate dying than about my professional reputation.”
Later, in planning Harris’s trip to the RF4 event, Menduno had occasion to speak to Harris’s wife, Fiona who brought up the dive.
“She said to me ‘I hope Harry is going to be OK’,” Menduno said. “I had no idea how much Harry told her, what she knew and didn’t know. All I could say was he’s got the best people in the world on his team, and if anybody can do it, he can.”
“We all held our breath and waited,” Menduno said.
‘Hydrogen Trials’ at Harry’s House
Ahead of the dive, Harris was preparing at home. The first thing Harris said he had to get his head around was—no surprise—the risk of explosion, and how to manage the gas to mitigate that risk. The potential source of explosion that Harry was most concerned with was static ignition within the CCR itself, plus other potential ignition sources like electronics, the solenoid, and adiabatic heating. Industrial literature—or “sober reading” as Harris calls it—suggested that the tiny amount of static necessary to initiate a spark to ignite hydrogen is .017 mJ, 400 times less than the smallest static spark you can feel with your fingertips and several hundred times less than required to ignite gasoline. “It ain’t much, in other words,” Harris said, noting that counterlung fabric rubbing against itself could generate just such a spark.
Ultimately, Harris came across research that suggested that static decreases with humidity. “I started to feel like there was no source of ignition inside a rebreather, but then again I said to myself, ‘Harry you only need to be wrong once’.”
The other concern was whether he could actually fill hydrogen safely while decanting, or filling one tank from another at the same pressure, and boosting the gas to reach higher pressures.
“I decided there is only one way to actually resolve this and that is to retire to the shed, order a sneaky bottle of hydrogen, and without telling my wife what was going on down the back of the house, start to actually have a bit of a play with this,” Harris said.
First Harris had to make his own DIN fitting (though not out of the ordinary for the anesthesiologist who built and tested his own rebreather before buying a commercial one in 2002) to decant the gas. Next he took his dual Megalodon rebreather with 100% hydrogen in one diluent cylinder and 100% oxygen in the other to the “test bed” in his backyard—his pool—and started to introduce hydrogen into his rebreather.
“Putting an explosive device into water was perhaps not the most logical approach because it becomes more like a depth charge than a bomb, but I thought, ‘Well, at least it might contain the blast somehow into the pool.’ I knew if I broke the back windows in the house or worse, my life wouldn’t be at risk just from the hydrogen. There would be bigger trouble afoot,” Harris said. “I left the lid of the rebreather unclipped in the vain hope it would spare me and the pool and the dog, who was helping with this experiment.”
He pressed the button of the Automatic Diluent Valve (ADV) on his rebreather, introducing hydrogen to the loop, and finally activated the solenoid before he started breathing from it. The first breaths were pleasant, he said. “It did feel very light and very slippery, and the hydrogen voice is even sillier than the helium voice, as you would expect,” he said. “I don’t want people to rush away thinking this is a safe and sensible thing to do. I’m under no illusions I’ve produced any evidence for you to see, but this is an honest account of the hydrogen trials at my house.”
The unit had not exploded with a fill of oxygen from zero to 70%, and very low humidity. “Harry, dog, and CCR survive,” as Harry wrote in his report of the trials. “Nothing bad had happened, so it was reasonable to move to the next step,” he said.
The Expedition
Harris, Challen, and other members of the Wetmules, arrived at the site of the Pearse Resurgence on New Zealand’s south island in February 2023. The cave system is so remote they needed around 10 helicopter trips to transport the team and all of its equipment. Mitchell, the diving physician, ran surface operations with “mixed feelings,” as Harris put it.
The group stayed for two weeks at a campsite, complete with a gas-mixing station, an electronics shelter for charging gear, and a “big green army tent where we meet and drink a lot of coffee and try and put off going back into the water each day,” Harris said.
The expedition was plagued with an unheard of number of problems, Harris said, “Every time we got in the water, something popped or blew up or failed.” The campsite is where Harris boosted hydrogen for the first time, from 100 to 150 bar. He flushed the booster and all the whips with hydrogen prior to boosting to make sure no oxygen was left in the system, but it was an anxious moment.
On dive day, Harris and Challen set out on what would be a 13 hour dive to 230 meters/754 feet—a “comfortable depth,” as Harris put it. Due to some problems during the expedition, it was decided that Harry would dive hydrogen, while Craig would dive trimix. At 200 meters/656 feet depth, Harris pivoted the switch block to introduce hydrogen into the loop. “The first cautious sip of hydrogen just to activate the ADV was satisfying,” he said. Gas density was not subjectively improved, but Harris noticed an obvious benefit—the HPNS-induced hand tremors he typically experienced after 180 meters/590 feet disappeared. Harris kept his setpoint at .7 during the descent and working portion of the dive, careful not to reach a fraction of oxygen above 4% which would make the mix explosive, and proceeded to the 230-meter test depth.
After completing their time at 230 meters, the team began their ascent. Harry shut off the hydrogen feed to the active loop of his dual Megalodon rebreather back at 200 meters, and then conducted a diluent flush every 10 meters/33 feet to remove the hydrogen from the loop until reaching 150 meters/492 feet. At that point, Harris boosted his PO2 to 1.3 from his set point of 0.7 (Challen remained at 1.3 throughout the dive), and they continued their ascent decompressing on a trimix (O2, He, N2) schedule, treating hydrogen as if it were helium. The complete technical details of the dive will be published in a forthcoming paper in the Diving and Hyperbaric Medicine Journal.
As soon as the team were helicoptered back to civilization, Harry called Michael from the road. “Michael, we did it!,” Harris said.
“Harry, you’re alive!,” Menduno responded.
N=1
At that March meeting with the H2 working group, Harris presented his findings from the dive. “I’m not sure what to conclude to a highly scientific, analytical, and evidence-based audience like yourselves,” he told the group. “Conclusions: N=1,” meaning it had been successful one time.
Doolette, who had been the most vocal in the group about his concerns, suggested Harris could add to his conclusions “the probability of survival is greater than zero.” Doolette, whom Mitchell contacted as soon as they reached civilization, said he “was relieved to hear that Harry survived this test dive” but remains disappointed with some aspects of the experiment, and concerned about possible future attempts. “For instance, I imagine among the engineers he consulted would have been someone with the ability and resources to do a computational fluid dynamic analysis of the Megalodon rebreather to establish the ignition risk, but instead Harry filled his rebreather up with hydrogen in his backyard.”
Overall, Harris said his findings are that hydrogen can be handled and boosted, hydrogen and CCR diving are compatible, a strategy to introduce hydrogen on descent was successful, a decompression dive was successful, a low setpoint at depth did not practically affect total dive time, strategy to reintroduce a high PO2 on ascent was successful, and HPNS and narcotic impacts were subjectively favorable.
“In introducing hydrogen we have addressed the issue of gas density, but we certainly have not established it is safe to use in terms of explosion risk, decompression of the thermal hazards,” Harris said.
Among his conclusions, Harris pointed out that he also managed to evade the nickname “Hindenburg Harry.” “Fortunately that was avoided,” he said, “but remains an ever-present risk.”
The Future of H2
Harris warns not to read too much into what his team achieved—a single data point that should in no way encourage others to repeat the dive. “David Doolette’s comment should be heeded,” Harris said. “All we have shown is that we got away with it on one occasion.”
Provided it can be safely proven and built upon, Harris said he thinks of his hydrogen dive as a window into the future that would enable tech divers to continue exploring into the 250 to 350 meter/820 to 1148 feet range. “Imagine the wrecks and caves that lay unvisited around the planet,” Harris said.
DIVE DEEPER
YouTube: Wetmules 245m Cave Dive in the Pearse Resurgence, New Zealand (2020)
InDEPTH: Hydrogen, At Last by Michael Menduno
InDEPTH: Density Discords: Understanding and Applying Gas Density Research by Reilly Fogarty
InDEPTH: Playing with Fire: Hydrogen as a Diving Gas by Reilly Fogarty
InDEPTH: High Pressure Problems on Über-Deep Dives: Dealing with HPNS by Reilly Fogarty
InDEPTH: The Case for Biochemical Decompression by Susan Kayar
John Clarke Online: Hydrogen Diving: The Good, The Bad, the Ugly (2021)
InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties by Michael Menduno and Nuno Gomes.
Undersea Hyperbaric Medical Society: Hydrogen as a Diving Gas: Proceedings of the 33rd UHMS Workshop Wilmington, North Carolina USA (February 1987)
InDepth Managing Editor Ashley Stewart is a Seattle-based journalist and tech diver. Ashley started diving with Global Underwater Explorers and writing for InDepth in 2021. She is a GUE Tech 2 and CCR1 diver and on her way to becoming an instructor. In her day job, Ashley is an investigative journalist reporting on technology companies. She can be reached at: ashley@gue.com.
