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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.



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.“


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

  • Rebreather Forum 4


Low Cost DIY Rebreather Testing: Why and How

Third party rebreather testing is expensive and arguably provides limited data to users and or rebreather builders about scrubber performance. As such, it is rarely used by manufacturers to explore “What-Ifs” i.e., what would be the effect on scrubber duration if we insulated this or modified that? Fortunately, former American Underwater Products Chief Technology Officer Chauncey Chapman has developed a low cost, build-it yourself tester he calls the “Surface Normalized One-ATA Rebreather Tester,” aka SNORT, which can be built for less thaUS$3000-$5000. Download build instructions and a bill of materials. DIY testing anyone?




By Chauncey Chapman. Header image: Components of the Surface Normalized One-ATA Rebreather Tester, or S.N.O.R.T courtesy of C. Chapman

BioMarine developed the first modern rebreather—the MK15—in the 1970s for the US Navy. The critical component was the scrubber, and the Navy developed a method to test scrubber duration. The test method simulates human rebreather diving. 

This test method has six controlled elements:

  1. Water temperature: The Navy uses a standardized water temperature of  4° C/40° F. Researchers can maintain the temperature within a small range using a circulating refrigeration unit. 
  2. Depth: To assess performance at depth, researchers place the unit in a pressure chamber. The chamber has ports for chilled water circulation, depth control, temperature probes, pressure transducers, sample lines, and attachment to an external breathing machine. While the function of the depth chamber is standardized, the chamber designs in use differ significantly one to another.
  3. Breathing rate: External breathing machines can vary breath size and breathing rate, or breaths per minute. Standard breathing rates are measured in Respiratory Minute Volume (RMV). Forty RMV is used for baseline scrubber duration testing—this rate represents a heavy workload, and in testing where workload is simulated as a continuous value, no rest stops. Three standard RMVs used in rebreather testing are
    1. A 2 liter breath 20 times a minute (or 40 RMV) 
    2. A 2.5 liter breath 25 times a minute (or 62.5 RMV) 
    3. A 3 liter breath 25 times per minute (or 75 RMV)
  4. Heated & humidified gas in the breathing loop – The gas divers exhale is warm and humid. To simulate a diver using a rebreather, the gas in the test breathing loop passes through a heated and humidified chamber before being “inhaled” by the breathing loop. The gas entering the rebreather is controlled to be 32° C/89° F with a relative humidity of 80%.
  5. Constant injection of CO2 – In human respiration, CO2 production is relative to workload. The generally accepted value is 4% of the RMV equals the CO2 production in liters. At 40 RMV, 4% is 1.6 liters of CO2 per minute. In testing, the CO2 is injected into the breathing machine at a constant rate using electronic flow meters and a needle valve or an electronic mass flow controller. CO2 injected can be verified by using a gas totalizer (total liters injected vs. test time) or by weight loss of the CO2 source cylinder. 
  6. Measurement of CO2 in the loop at the point where the “diver” would inhale loop gas – A tap is placed in the breathing loop hose where the hose attaches to the DSV (or BOV). This extracts gas that the diver would be inhaling. That gas is piped through the depth chamber’s wall to an external CO2 analyzer. At 1 ATA, CO2 is referred to as being at the Surface Equivalent Value (SEV). Scrubber duration tests run until the breakthrough CO2 measured value is 0.5% (5 mbar) and then to 1.0% (10 mbar). The time from 0.5% to 1.0% must be more than ten minutes.

Scrubber duration results are based on several testing runs. A test requiring three runs—using stable test conditions—may have run times of 190 minutes, 205 minutes, and 197 minutes when tested with air diluent at 40 m/132 ft. To be safe, diving advice is to use the shortest test time—in this case, 190 minutes.

Keep in mind that this test method was designed for military divers: supremely fit and capable of maintaining a heavy workload for the entire dive without rest. This expectation might not be practical for the average civilian diver—can you ride a bicycle for 3+ hours at 26 kph/16 mph without rest?

  • Rebreather Forum 4

We also know that water temperature, diluent gas, depth, and workload all affect scrubber duration.

ISO 17025 accredited facilities charge around $3,000 USD per day for scrubber duration testing. CE certification to the 14143 standard takes seven scrubber duration runs; on a good day, a testing facility might squeeze in two runs. This expense keeps rebreather manufacturers focused on certification to a standard, but limits exploratory testing at more reasonable work rates or water temperatures for recreational rebreather divers.

Thus, the standard rebreather scrubber duration testing may not be a perfect guideline for a recreational rebreather diver quietly taking pictures of yellowhead jawfish (or being towed by a scooter). 

How can we enable low-cost, objective testing to allow exploration of how scrubber types, water temperatures, different diluent gasses affect scrubber duration? 

Divesoft co-founder and CTO Aleš Procháska with their “Big Bertha” rebreather testing facility.

Scrubber Duration Testing For Technical Divers

As rebreather divers, we do not want to exhaust our scrubbers while diving. Depending on our diving environment, this could (at best) create a horrible experience or (at worst) precipitate a fatal event.

Just as open circuit (OC) divers always plan on ending their dives with a useful volume of gas left over, rebreather divers should never plan to completely exhaust their scrubbers while underwater. In his book, Breakthrough, Dr. John Clarke recommends a minimum buffer of 5% instead of using 100% of the tested scrubber duration—for instance, a 100 minute scrubber should be retired at or before 95 minutes of use.

But, because testers assess scrubber duration using a “heavy” workload at 4° C/40° F, the resultant duration value (expressed in minutes) may not be the best guideline for a technical diver. However, because scrubber duration testing is very expensive, testing in warmer water and at lower work rates is not being done. (N.B., the military does test at elevated temperatures and workloads, diving in the Baltic Sea vs. diving in the Persian Gulf, but the test results are not made public)

Some tech rebreather divers routinely exceed their tested scrubber duration with no adverse effects. But, it simply isn’t safe to take the standard scrubber duration test and extrapolate a safe time for use in technical diving—there are too many variables that have not been tested. 

How does water temperature affect duration? Is the effect the same in every scrubber? Helium extends scrubber duration, but what % helium and by how much time? Depth must have some effect on scrubber duration, but how much? Some knowledgeable people advise that depth has a negligible effect on scrubber duration.

One way to rationalize longer scrubber use is to look at the scrubber as a CO2 sink. In standard testing, a 190-minute scrubber absorbed 304 liters of CO2 (190 X 1.6 = 304). If we are working half as hard as the test assumed, at a 20 RMV, our CO2 production would be 0.8 liters per minute. The 304 liters the scrubber absorbed would require 380 minutes to create, doubling the scrubber duration time. 

AP Chief Engineer – Terry Fisher – and Head of Product Design – Alex Wall – testing an INSPIRATION rebreather in the ANSTI 200m Test Chamber / Breathing Simulator at AP Diving’s dedicated R&D Rebreather Test Unit, Cornwall, UK.

Another rationale for longer scrubber duration is based on oxygen consumption. Depending on the individual, consumption (metabolization) of 1 liter of oxygen yields between 0.75 to 1.1 liters of CO2. If you use the conservative value of 1.1 liter of CO2 per liter of oxygen, you will consume 276 liters of oxygen to generate 304 liters of CO2.

But, neither of these rationales are based on objective testing. 

Using a 17025 accredited facility for testing is expensive—how can we objectively test scrubber duration for consumer applications at a much lower cost?

If we design a test system to only test scrubber duration at 1 ATA, we can bypass the most expensive elements. We need to design an inexpensive breathing machine, heat and humidity chamber, temperature-controlled water bath, accurate CO2 injection system, and accurate CO2 analyzer.

Introducing the Inexpensive Surface Normalized One-ATA Rebreather Tester, or S.N.O.R.T.

Chapman’s SNORT DIY rebreather tester

The Surface Normalized One-Atmosphere Rebreather Tester (SNORT) was conceptualized during discussions about the SCUBATRON Generic Breathing Machine (GBM), a chest-mounted rebreather, in the fall of 2021.

Like other scrubber testers, SNORT features five important subsystems:

●      Temperature controlled water bath

●      Breathing rate Breath volume and breaths per minute

●      Exhaling warm humid gas into the rebreather

●      Adding CO2 to the breathing loop

●      Measuring the level of CO2 in exhaled gas

GBM designer Gregory Borodiansky and Chauncey Chapman collaborated on the design of the sub systems, and fabrication began in late 2021 on a part-time basis. In the early summer of 2022, SNORT 1 was completed and verification testing began. SNORT 1 can test a sport rebreather under conditions closely mirroring EN14143 or NEDU TM 0194. CO2 injection is measured using an electronic flow meter, continuous breathing is achieved using a regulated air pressure piston, air entering the rebreather is heated and humidified, testing occurs underwater using ice to maintain the required temperature, and CO2 breakthrough is monitored in a Windows app using a GSS CO2 sensor.

SNORT achieves the standard 40 RMV used in rebreather testing, by means of two opposing 2 liter pneumatic cylinders—one drive cylinder and one breathing cylinder. The shafts are coupled to each other. The drive cylinder has two solenoids ported to the cylinder: one normally open solenoid and one normally closed solenoid. When one set of solenoids is energized, gas can flow into that end of the drive cylinder; this moves the piston pair to cause the active port on the breathing cylinder to either inhale or exhale. There is a double pole, double throw switch mounted to the shaft coupler. When the piston has moved to each end of the stroke, an adjustable stop throws the switch to its other position, causing the piston to move back and forth. Speed is controlled by adjusting the drive air pressure.

Warming and humidifying the gas before it enters the rebreather is accomplished using a rectangular aluminum box with breathing gas ports on either end. This box is partly filled with water and has a contact heater on the outside of the box. The heater is thermostatically controlled to maintain a temperature of 31 to 33° C/88° F to 92° F. This temperature is monitored on the SNORT control panel. The breathing gas dwells in this chamber during the inhale part of the breathing cycle. The breathing gas is warmed and humidified prior to entering the rebreather.

CO2 is added to the loop by injecting CO2 into the breathing cylinder. Just like breathing rates (RMV), CO2 injection rate is defined as being 4% of the RMV. So, at 40 RMV, the CO2 injection rate is 1.6 liters per minute. SNORT uses a SIARGO MEMS (micro-electro-mechanical system) mass flow sensor and a Swagelok needle valve to inject precisely 1.6 liters of CO2 per minute, a totalizer is included for validation.

A gas sample is withdrawn from the breathing loop at the point where scrubbed gas enters the diver’s shutoff valve—this is the gas that a diver will inhale. The sample gas travels through a water trap, a NAFION tube dryer, a metering pump, and a GSS 0-5% NDIR CO2 sensor. Sample flow is visualized in a bubble chamber on the control panel. The sensor output is displayed on a computer screen and logged as a .csv file.

The rebreather under test is immersed into a water bath; the temperature of the water bath is monitored on the control panel and maintained by adding store-bought ice. 

SNORT Verification Testing

SNORT verification testing: first we eliminated leaks, then explored how the CO2 sub-system worked, and verified that we had heated humidified air in the loop and could maintain 2 liter breaths 20 times a minute to achieve 40 RMV. In the initial testing, was done using:

1.      The SCUBATRON Generic Breathing Machine CM rebreather

2.      Performed initial dry runs

3.      Completed subsequent runs with the rebreather loop underwater

4.      Added runs testing lower temperatures, down to 0° C/32° F

5.      Tested using air and 10/60 Trimix (10% O2, 60% He, balance N2) diluent

Once we had the system verified, we knew it could duplicate all EN14143 test requirements (except one).

Test Method Validation Testing

To validate SNORT,  we tested another device—the Hollis Prism 2. We ran cold and wet tests, comparing our results to the EN14143 testing performed on the Prism 2 at the QinetiQ testing facility. We mirrored their testing protocols, simulating a 40 m/131 ft dive on air. Thanks to Nick Hollis and Hollis Rebreathers for granting access to the official Prism 2 test results.

Using the Hollis Prism 2, we compared the average scrubber duration from 3 tests done at 40 meters at the QinetiQ facility in the UK to 2 tests in our SNORT facility at a slightly colder temperature:

Breakthrough in testing is when the first faint trace of CO2 is detected (in minutes)

Note that he complete Hollis Prism 2 test results from QinetiQ can be found here:

In the QinetiQ testing, the three 40-meter scrubber duration test results (190 min, 197 min, 205 min) varied by 7.9% (i.e, min 190 to max of 205). In our tests, we used the same Sofnolime keg for both tests; in each test, the scrubber was packed with the same weight of Sofnolime to eliminate issues of sorb variation.

The takeaway may be that running a dive to the published scrubber duration time might not be the smartest thing to do, and arbitrarily extending a scrubber’s run time beyond the manufacturer’s published duration time is at least risky.

The low-cost SNORT system may answer some persistent questions about scrubber duration:

What is the impact of water temperature on scrubber duration? 

What is the impact of trimix vs. air on duration?

What is the impact on scrubber duration of lower workloads?

How does an Axial scrubber compare to a Radial scrubber with the same gram weight of absorbent? 

Does insulating an Axial scrubber extend its duration?

  • Rebreather Forum 4

A Call to Action

Build your own SNORT – materials list, suppliers and their item numbers , build notes, schematics, and operation checklists can be downloaded at SNORT SCRUBBER TESTER 

To reduce variables in testing, have the same person pack the scrubber for each test, and use a single keg of absorbent, weigh the scrubber before packing and after to record the weight of absorbent used in each test.

It is essential that the breathing loop and scrubber canister must be submerged in the water bath.

Developers, divers, and tinkers can then:

·       Research scrubber duration in different water temperatures

·       Duration at lower CO2 injection rates

·       Temperature effect in axial vs. radial

·       Can insulation extend scrubber duration? What works best?

·       How does scrubber shape effect duration – small diameter axial vs. large diameter? 

·       Duration using Air diluent vs Trimix Diluent

·       8 lb scrubber vs 5 lb scrubber, which is more efficient in absorbing CO2

NOTE: Rebreather scrubber testing must be conducted in situ in the complete rebreather breathing loop. While testing in SNORT mirrors “industry standard” testing, it does not duplicate all of the test conditions; SNORT tests at 1 ATA only. Depth may have a negative impact on scrubber duration. Water temperature and diluent also affect scrubber duration. But, Dr. John Clarke notes in his book, Breakthrough, that perhaps the most impactful variable is the human in the loop: “Randomness inherent in both physics and human physiology influences the lifetime of a CO2 scrubber.  Furthermore, since divers are not mechanical breathing machines built to specifications, a testing agency’s assumptions about diver physiology may not match your physiology on any given day. So, divers beware.”

Commercially available scrubber monitoring systems are available on some rebreathers:

· The xCCR gaseous CO2 sensor: Fires an alert when gaseous CO2 breaks through the scrubber

· AP Diving Temp Stick: “sees” the burn front in their axial scrubber and estimates time remaining

· Mares rEvo RMS temperature monitoring system: estimates time remaining based on thermal measurements

Dive Deeper

InDEPTH: The Secrets of Scrubbers by Jeff Bozanic (2023)

InDEPTH: InDEPTH’s Holiday Rebreather Guide: 2022 Update

InDEPTH: The Case for an Independent Investigation & Testing Laboratory by John R. Clarke PhD (2019)

Chauncey Chapman—I love being underwater. My career started with commercial & industrial diving, then became an early PADI instructor, dive center owner, resort manager, USCG boat captain, and PADI Instructor Trainer.  I thrived in all aspects of recreational diving.  I was hired by American Underwater Products (Oceanic, Aeris, Hollis) in 1985 and while working for Bob Hollis took on various roles of increasing responsibility culminating in Chief Technology Officer.  I was responsible for quality management, involved in product design, development, testing, manufacturing, regulatory compliance, and project manager for rebreather projects. Currently life 3.0 finds me diving as often as possible, volunteering as the treasurer for the Rebreather Training Council (RTC),  working  as a consultant to the recreational diving industry, and the designer of SNORT.

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