Equipment

Decompression Habitats Are Ascendent

Armed with reliable rebreathers, expedition-grade scooters, electric heating, helium mixes, high-powered dive computers, and those all-important P-valves, today’s cave explorers are giving our collective underwater envelope a hard shove (deeper and longer), all the while enduring increasing hours of long, cold, boring decompression. That’s the reason that the use of deco habitats—first pioneered by Dr. Bill Stone in the late 1980s—is on the rise. Here anesthesiologist-cum-cave explorer Andy Pitkin explains everything you need to know about modern deco habitats from their history, construction, and positioning to ensuring adequate, safe breathing gas flow.

Published

5 months ago

October 27, 2022

InDEPTH

By Andy Pitkin

Cold. Hungry. Uncomfortable. Bored. These adjectives can aptly be applied to the vast majority of divers during the decompression portion of advanced technical dives. The commercial diving industry, less concerned about divers’ comfort and more interested in safety and efficiency, has long incorporated decompression in a dry chamber for anything other than shallow diving operations. Unfortunately, with the notable exception of Bill Stone’s 1999 Wakulla 2 project, surface decompression in a pressurized chamber has been impossible for technical divers. The next best thing is a habitat.

*The ‘habitrough’ used by Sheck Exley at Cathedral Canyon during his exploration there in the 1980s. Photo by ?*

Habitats are gas-filled spaces underwater that allow a diver to remain at pressure while getting part or all of their body out of the water. The name comes from experimental living quarters such as the Sealab series where divers would remain underwater for a number of days for (usually) scientific purposes. The habitats used for decompression by technical divers are much more modest, and this article will discuss the theoretical and practical considerations of decompression habitats, some of which are obvious, and some have had to be learned through real-world experience.

Paul Deloach (left) and Sheck Exley (right) decompressing in the habitat designed by Bill Stone and funded by Rolex for the Wakulla Spring 1987 exploration project by the US Deep Caving Team. The 40m (120 ft) chain hoist allowed the divers to adjust the depth of the habitat as their decompression progressed. Photo taken by Wes Skiles and courtesy of Bill Stone/US Deep Caving Team, Inc.

Advantages Of Habitats

The benefits of a decompression habitat are so self-evident that they hardly need to be mentioned. The most obvious is warmth, because of the much lower loss of body heat in a gaseous environment compared with immersion in water. Even if only the diver’s head is out of the water there is a significant improvement in both subjective and objective thermal homeostasis. Being out of the water reduces both the risk of oxygen toxicity and the severity of the consequences of a seizure, which is likely to be fatal underwater but could probably be survived in a habitat.

Eating and drinking is much easier, and the ability to talk, listen to music, watch movies and pass the time in relative warmth and comfort makes a long decompression of many hours much easier to tolerate, as well as being considerably safer. A habitat also can be used as a makeshift on-site recompression chamber, which could at least allow a diver’s symptoms to be stabilized while arrangements are made to support the necessarily lengthy in-water decompression phase.

Andrew Pitkin illuminates the 15 m/50 ft habitat at Lineater Spring. This habitat is 700 m/2300 ft underwater from the cave entrance. The aluminum “floor” greatly assists entry and egress from the habitat. Photo by Kyle Moschell / Karst Underwater Research.

Securing the Habitat

Decompression habitats have occasionally been installed in open water; examples include Martin Robson’s exploration of the Blue Lake in the Russian Caucasus mountains in 2012 [1] and Michael Lombardi’s Ocean Space Habitat, also in 2012 [2]. The overwhelming majority have been used in cave diving, because underwater cave exploration often mandates lengthy decompression and the environment usually guarantees that decompression will occur in a specific location. The wide variety of underwater caves has resulted in many different approaches to construction, from sealing a natural airspace formed by a dome in the ceiling using a tarpaulin (“habitarp”), upturned rubbish bins (“habibin”) to large custom-designed and manufactured enclosures. A volume of gas large enough to be useful has considerable buoyancy, which must be restrained either from above by the cave ceiling or from below using the floor or wall of the cave passage. A 1000 liter (264 gallon) IBC container often used for this purpose has a buoyancy of 1000 kg (2204 lbs), and many habitats are larger.

Unless it is constrained by the cave ceiling, the anchoring system must be very strong and reliable. Natural anchors such as rock projections and large boulders are better for conservation, but they may not be available in the required location, necessitating placement of artificial anchors in the cave wall or floor. These are very similar to anchors used in vertical dry caving, and can be screw anchors, expansion bolts or even glue-in types, typically made of stainless steel (or titanium, if money is no object!).

Air-powered drills are much less expensive than battery-powered underwater drills but can use a large amount of compressed air. When our group (Karst Underwater Research or KUR) placed a habitat 2 km/6562 ft from the entrance of a cave in 2013 (when no suitable battery-powered underwater drill was available), the large volume of bubbles released from the air drill we used to make the holes for the anchors percolated so much silt from the walls and ceiling that the water visibility was reduced to almost zero for about a third of the exit distance. Whatever method of fixing the habitat is chosen, it needs to be very secure, as the consequences of an anchor coming loose could be extremely severe.

The 16 m/52 ft habitat in the Pearse Resurgence. Photo by Simon Mitchell.

The depth of the habitat may be a compromise between what is ideal for decompression and what is dictated by the location. Since the final decompression stop is the longest, the habitat is often targeted as close to 6 m/20 ft deep as possible. Some advanced projects, most notably the Wet Mules’ exploration of the Pearse Resurgence in New Zealand, have used multiple habitats at various depths because of the extreme maximum depth of more than 240 m/787 ft and cold water 6°C/43°F.

To maximize the air space, the habitat container needs to be as level as possible. In other words, the water level can be no lower than the highest point of any of the sides where gas can escape, and this consideration may be more important than installing it at the ideal depth. When a habitat is anchored from below, it is usually easiest to start a little deeper than the intended depth and then adjust to the correct depth before the container is completely filled with gas.

Our group typically uses polyester static caving rope (nylon lengthens about 5-10% on getting wet) with equalized double anchors at the bottom (double figure 8 or bowline on a bight) and ‘super Münter’ adjustable hitches at the top for easy adjustment of length. When the anchor points have been close to the bottom of the habitat, we have had a lot of success with appropriately-rated webbing ratchet straps.

The 30 m/100 ft and 40 m/130 ft habitats in the main shaft of the Pearse Resurgence during the Wet Mules’ exploration in 2021. The cable carried power for drysuit heating and a simple buzzer communication system. Photo by Simon Mitchell.

The Use of Containers

Many factors will influence the choice of a container for a habitat, but they can be reduced to two primary ones: location and cost. Inflatable habitats—for example modified commercial lift bags—have the advantage that they can be rolled or folded up to fit through narrow parts of the cave. We have found that a large golf club case works as a streamlined container for an inflatable habitat that can be swum or towed by a DPV.

A rigid habitat, typically an industrial or occasionally purpose-built container, is much more cumbersome to move into a cave, and these are typically installed close to the cave entrance, which obviously has to be large enough for it to fit through. Experience has shown that any modifications to the container (e.g. rings or hooks for hanging equipment) are vastly easier to perform out of the water before the habitat is installed, especially if any kind of adhesive is required. A reliable valve near or at the highest point in the habitat is very helpful for removing gas when the habitat needs to be adjusted or removed but, with a little practice, gas can be siphoned out by two divers and a short length of garden hose.

Unless the cave floor is close to the bottom of the habitat, the occupants will need either a floor or seats to keep them out of the water. The size and positioning of seats is a compromise between comfort and ease of entry into the habitat.

Breathing Gas

The easiest and most inefficient option is for divers to use a conventional open-circuit, second stage regulator, with the cylinder being hung in the water below the habitat. Using a conventional diving rebreather may be difficult because of space limitations, prompting some home-made designs which are usually of the chest-mounted (or ‘laptop’) configuration. They can also be suspended at any convenient place in the airspace, because there is no hydrostatic counterlung loading.

The most efficient and comfortable option is for divers to breathe the habitat atmosphere itself, which immediately presents three new considerations: oxygen addition, carbon dioxide (CO₂) removal, and gas monitoring. Let us look at each of these in turn.

Adding Oxygen

The above-mentioned 1000 liter IBC container, large enough for two divers, positioned at 6 msw/20 fsw, and filled with the surface equivalent of 1280 liters of oxygen and 320 liters of nitrogen, would entail an oxygen fraction of 0.8 and a partial pressure of oxygen (PO₂) of 1.28 ata. We can conservatively assume that a decompressing diver will have an average oxygen consumption of about 1 liter/minute, and therefore two decompressing divers would consume about 120 liters of oxygen per hour. After one hour, the oxygen fraction within the habitat would have dropped to 0.78 and the PO₂ to 1.25 ata, assuming the resulting CO₂ does not remain in the airspace. This simple calculation, which is supported by practical experience, shows that elaborate arrangements for maintaining habitat PO₂ are unnecessary and can be accomplished by simply purging an oxygen second stage intermittently within the habitat (e.g. every 30 minutes or more).

Removing CO₂

There are only two ways of removing CO₂ from an enclosed airspace: replacement by adding gas free of CO₂, and chemically removing the CO₂ from the atmosphere using a CO₂ absorbent (scrubber). The first method is often used in hyperbaric chambers—which share many of the practical problems of underwater habitats—because it is safe and simple. Unfortunately for technical divers, it is too inefficient to be practical in most circumstances. Going back to our example above, our two divers will have exhaled about 96 liters of carbon dioxide in the first hour, assuming a typical respiratory quotient of 0.8, resulting in an ambient CO₂ concentration of 6.5% (surface equivalent by volume). By this point, both divers would likely be feeling significant adverse effects.

If we assume that the CO₂ in the habitat atmosphere should be maintained below the 0.5% surface equivalent value commonly used for rebreather scrubber testing, flushing of the habitat would have to be started after less than 5 minutes. The rate of continuous flushing to keep the CO₂ in an enclosed pressurized airspace at a constant level is given by the following equation [3,4]:

where Q_gas is the rate of gas ventilation, P_amb is the ambient pressure, VO₂ is the total oxygen consumption of the divers, R is the respiratory quotient, F is a mixing factor (1 = ideal mixing) and PCO₂ is the desired ambient partial pressure of carbon dioxide.

For our two divers, the habitat would have to be flushed at a rate of 512 liters per minute or 19.5 cu ft per minute (surface equivalent) to maintain the CO₂ at a surface equivalent of 0.5%. Note that the amount of gas required is independent of the volume of the habitat. This is logistically unsustainable in most situations: a typical 80 cu ft (11 liter) aluminum cylinder would last less than 3 minutes. This shows how difficult it is to maintain low CO₂ levels with flushing of the gas space. Even if the CO₂ is allowed to rise to a surface equivalent of 2%, which would cause some breathlessness but might be tolerable, the same cylinder would still only last about 16 minutes.

For the Wakulla project in 1987, Bill Stone calculated a 32 cu.ft./min (906 liters/min) gas flow requirement for an exploration team in that habitat positioned at 60 ft/20 m depth [5]. Two industrial Ingersoll-Rand surface compressors were easily able to meet this demand via a 400 foot long, ¾ inch internal diameter hose with manual shutoff valves and check valves fitted at both ends to prevent inadvertent venting of the habitat atmosphere when the compressors were not running. No direct measurement of habitat CO₂ levels were made; the divers were able to purge the gas in the habitat whenever it seemed excessively ‘stuffy’.

The only other way to reduce the CO₂ in the atmosphere is to remove it chemically, turning the habitat into a giant shared rebreather. This is relatively a simple engineering task, using a sealed 12V motorcycle radiator fan to blow habitat gas through a scrubber bed, ideally with some form of speed control to allow the flow rate through the absorbent to be controlled by the diver(s). It can be powered from portable battery packs (such as those used for dive lights or undersuit heating) or a cable from the surface. Such a device needs to be transported to the habitat inside an appropriate container or designed into a pressure-proof housing (see picture).

A habitat scrubber built into a pressure-proof housing, with a venting valve on the left end cap. The white cylinder in the center is the axial scrubber basket, the space on the right is occupied by the fan, and the space to the left contains oxygen sensors and monitoring. The whole unit is neutrally-buoyant and measures 14 in/36 cm long with a diameter of 6 in/15 cm. Photo: Andrew Pitkin

Monitoring Your Gas

When KUR started building habitat scrubbers about 10 years ago, we used a prototype CO₂ monitor for a rebreather to help decide how fast to run the scrubber motor. The monitor, which used infrared absorption spectrometry to measure CO₂, was power-hungry and would exhaust all of its battery capacity in a few hours if left on continuously, so we would only switch it on intermittently. To pass the time while it was warming up, we would attempt to guess what the reading would be, and after a few iterations we became surprisingly good at estimating the CO₂ level subjectively by how ‘stuffy’ the habitat atmosphere felt. Switching on a habitat scrubber fan feels pleasantly like someone opening a window, but the insidious accumulation of CO₂ when the scrubber is off is much harder to notice. As an aside, I believe there is some potential for research into whether divers can be trained to recognize increasing levels of inhaled carbon dioxide from scrubber breakthrough. Handheld CO₂ meters are available, and we are currently evaluating some of these for use in our habitats. Many are not suitable for the environment or will not give accurate readings in the presence of 100% humidity.

Oxygen measurement is simple, as in any rebreather, and can easily be combined with the scrubber assembly so that the sensors sample the gas being circulated by the fan.

Ensuring Diver Safety

The limited space in most habitats often precludes the use of the divers’ main scuba system, in which case this must be removed when entering the habitat. When leaving for the surface (or a shallower habitat) this must either be redonned or a separate (often simple open-circuit) scuba used. These transitions present some hazards, especially if there is no solid floor beneath the air space with the potential for critical items to be dropped out of reach. A support diver is very valuable to assist a mission diver with entering and exiting the habitat and retrieving any items that are inadvertently released.

Explorer Matt Vinzant decompresses inside the 15m/50 ft habitat at Lineater Spring wearing his dual rebreather rig. This habitat is custom-built to allow two divers with redundant rebreathers to sit reasonably comfortably inside. Photo: Andrew Pitkin

As mentioned above, the positive buoyancy of a habitat can easily exceed 1000 kg (10kN) so all anchors, ropes, and connectors such as carabiners and maillon rapides should be appropriately rated for the application. The consequences of a habitat breaking loose in an uncontrolled ascent could be very severe and even fatal.

One concern, especially if the habitat atmosphere has a significantly elevated PO₂, is fire safety. With the bottom of the container open to the water, its atmosphere necessarily has 100% humidity, which has been shown experimentally to dramatically inhibit flame spread due to the latent heat of evaporation of water. While practical experience has been reassuring so far, the relative balance of fire-promoting conditions and humidity within a habitat has not yet been scientifically studied, so I would advise great caution with any potential ignition source, especially electrical switches, brushed motors (potential arcing), and dive lights (overheating).

Communications

The Wakulla 1987 project, pioneering in so many ways, introduced the use of habitat to surface communications with two phone lines, one of which was able to be used for long-distance calls, although the pushbutton phone used for the latter became unreliable after a time because of moisture ingress affecting the pushbuttons. Our group, like some others, has adopted single-wire earth-return telephones (also known as Michiephones) for communication with the surface. These are simple, robust, and require only a single wire to be installed to the habitat, although we sometimes use two-conductor military field phone wire with the conductors paralleled for redundancy. You can see them being used in this Alachua “habichat” video.

We have also used the combination of an LTE modem, power-over-ethernet switch, rugged ethernet cable, and a wi-fi access point in a pressure-proof housing to provide internet access within a habitat close to the entrance. While attractive, this option is not suitable for long-term installation and the effort of setting it up for each dive makes our dive teams generally prefer the single wire phone option. Other systems, such as two-wire intercoms for offices or door entry have also been used successfully. All these devices need to be able to function at elevated atmospheric pressure with 100% humidity.

Neville Michie (pronounced ‘Mickey’), an Australian caver, designed a simple and reliable earth-return telephone in the 1970s, although the concept is much older, dating back to the 19^th century. Many similar units have been constructed for use around the world by cave rescue teams. Picture: Andrew Pitkin

Is There A Deco Habitat in Your Future?

We have already seen one version of the future: the Wakulla 2 project’s surface decompression chamber system with a transfer capsule (“bell”) to transport the divers under pressure from the water into a dry decompression chamber on the surface. Unfortunately very few sites have the geography, and even fewer divers the financial means, to support it.

The ultimate habitat? An exploration diver entering the personnel transfer capsule (PTC) during the Wakulla 2 project in 1999. Because of the topography of the cavern, an angled Tyrolean of ¾” (19mm) rope was rigged along its roof to allow the PTC to reach the divers at 30 m/100 ft depth and get them out of the water several hours earlier than if the PTC was lowered vertically. Photo taken by Wes Skiles and courtesy of Bill Stone/US Deep Caving Team, Inc.

Some explorers have started experimenting with small one-person collapsible habitats which with advances in materials technology can be made more compact and lighter. I foresee more use of purpose-designed enclosures, especially collapsible ones that can be deployed in multiple locations. Underwater rotary hammers are now available which, although expensive, allow rapid placement of anchors in hard limestone. I also anticipate more habitats deployed in open water, like Michael Lombardi’s system-see below.

For deep cave exploration, habitats offer safety, some very welcome mouthpiece-free time, a chance to eat and drink, and even entertainment. More importantly, they allow the diver to warm up and stay warm at a critical phase of the dive, promoting (presumably) better perfusion and faster off-gassing. For these extreme dives, habitats truly change the game.

See Companion article: Portable Habitats—New Technical Diving Capabilities are Well Within Reach by Michael Lombardi

References

[1] Blue Lake: the habitat.

[2] Lombardi M. Portable Habitats: New Technical Diving Capabilities are Well Within Reach. InDEPTH V 4.11

[3] Nuckols ML, Tucher WC, Sarich AJ. Life Support Systems Design: Diving and Hyperbaric Applications. Pearson Custom Publishing, Boston, USA, 1996.

[4] Gerth WA. Chamber Carbon Dioxide and Ventilation. NEDU TR 04-46. Navy Experimental Diving Unit, Panama City, FL, USA, 2004.

[5] Stone WC. The Wakulla Springs Project. U.S. Deep Caving Team. January 1^st, 1989. ISBN-10: 0962178500. ISBN-13: 978-0962178504.

Andrew Pitkin learned to dive in 1992 in the cold murky waters of the United Kingdom and started cave and technical diving in 1994. His first exposure to exploration was in 1995 when he was one of a team of divers who were the first to reach the bottom of the Great Blue Hole of Belize at 408 fsw (123 msw). Subsequently he has been involved in numerous cave exploration projects in Belize, Mexico and Florida.

From 1996-2000 he was employed at the Royal Navy’s Institute of Naval Medicine, running a hyperbaric facility, treating decompression illness, participating in research into outcome after decompression illness, submarine escape and testing of new military underwater breathing systems. He is one of a handful of civilians to be trained by the Royal Navy as a diving medical officer.

He moved to Florida in 2007 and is currently on the faculty of the College of Medicine at the University of Florida in Gainesville. With Karst Underwater Research he has participated in numerous underwater cave exploration and filming projects. Like many explorers, he spends much of his spare time developing and building innovative equipment for exploration purposes.

Related Topics:decompression Habitat Technical Diving V 4.11

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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

3 weeks ago

March 1, 2023

InDEPTH

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 33^rd 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 increases. Note: 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 deeper—the 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

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.

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