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Oh Deco, Oh Doppler, O’Dive: Assessing the World’s First Personal Deco Safety Tool

Retired French Naval officer Axel Barbaud estimates that his company’s new product dubbed “O’Dive”—an innovative personalized safety tool—has the potential to reduce the diving community’s risk of DCS by a factor of five! Never mind that the exact incidence of DCS is unknown. What is known, is that O’Dive’s smart ultrasound sensor potentially represents a breakthrough in Doppler technology that has been sought after for more than 30 years.

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By Michael Menduno

“If I asked you where the hell we were,” said Arthur weakly, “Would I regret it?” Ford stood up. “We’re safe,” he said. “Oh good,” said Arthur. “We’re in a small galley cabin in one of the space ships of the Vogon Constructor Fleet,” said Ford. “Ah, said Arthur, “this is obviously some strange usage of the word ‘safe’ that I wasn’t previously aware of.” Douglas Adams, The Hitchhiker’s Guide to the Universe

Every so often, someone invents a paradigm-shifting product that changes the way we view the world, and consequently, the way we behave. In 1983, ORCA Industries launched the “EDGE” Electronic Dive Guide, the first computational1 personal dive computer. In doing so, ORCA created a new approach to managing multilevel dive decompression, which arguably revolutionized sport diving and improved diving safety. 

Similarly, in 1970, Dr. Merrill P. Spencer introduced the Spencer Precordial Bubble Detector, the first Doppler ultrasonic bubble detection device for divers, which enabled researchers to measure venous gas emboli (VGE) circulating in divers’ blood vessels. Though never a consumer item due to their expense and the expertise required to use them, Doppler monitoring technology has deepened our understanding of decompression sickness (DCS). It has also been a powerful tool for calibrating decompression tables and algorithms, including the algorithm developed by Karl Huggins for the EDGE.

Now a French company, Azoth Systems is hoping to achieve a similar distinction with a unique, innovative product called O’Dive, “The First Connected Sensor for Personalized Dives,” that couples statistical decompression analysis with Doppler ultrasound bubble detection, in order to improve diving safety. The company had a soft launch of the product last year and exhibited at the Paris International Dive Show in January, and also at the BOOT show in Düsseldorf, Germany. As of summer 2020, Azoth said it had about 1,000 O’Dive users in more than 25 countries.

Azoth Systems Didier Draguiev and divers from Team Alp Tech at the DiveTec show in Switzerland in 2019

The device and its associated phone or tablet app purportedly enable a diver to measure the “quality of their decompression” on a scale from zero to 100%, based on an analysis of their dive profile along with the results of the post-dive bubble measurements taken with O’Dive’s subclavian Doppler monitor. Designed for divers to take their own measurements, the smart, hockey puck-sized Doppler sensor that uploads bubble measurement data to the cloud where it is automatically graded, hopes to provide a breakthrough in Doppler monitoring, one that has been sought for more than 30 years. Until now, taking a post-dive Doppler measurement required a trained technician to take an accurate reading and interpret the results. 

But O’Dive goes further. With their decompression quality index on screen, the diver can query O’Dive’s predictive “what-if” model to determine if—and by how much—they could improve their decompression, by adjusting various factors of the dive, including the fractions of oxygen and helium in their gas mixes, the set point (in the case of a CCR dive), low and high gradient factors (GFs), addition of an extra decompression stop, and/or an adjustment of the length of the last stop. The diver could then incorporate preferred changes on subsequent dives, enabling them to improve their diving safety. Or so the theory goes. 

“The primary goal of the tool is to help divers adjust their practice to progressively lower their bubble scores,” explained 50-year-old Azoth founder and CEO Axel Barbaud, who says that his key insight was to combine the risk assessment of a dive profile with post-dive bubble measurements to create a new predictive approach. He and his team estimate that O’Dive has the potential to reduce the community’s overall risk of DCS by a factor of five. 

That may be tricky given that the exact incidence of DCS is unknown. However, it is a relatively rare occurrence with incident rates ranging from about 0.01%-0.1% per dive within the recreational, technical, scientific, military, and commercial diving communities.2 The problem is that many incidents occur in divers who reportedly followed their table or dive computer correctly—optimistically called “unprovoked hits.” These incidents might be partially explained by the variability of dive computer algorithms with user-adjustable settings even for the same dive (see Figure 1), but it also demonstrates that procedures do not account for individual susceptibility. That is the problem that Azoth Systems hopes to address with O’Dive.

Figure 1:  The variation in depths of first stops and decompression time depending on GF for a trimix dive of 25 minutes at 70m/230 ft.

Whether the product will be able to live up to its claims and actually improve divers’ safety is yet to be determined, and more evidence and experience will be required to demonstrate that O’Dive’s measurement and predictive systems are effective. “While I am aware of the O’Dive system, and think it important to develop these devices, I believe that optimism regarding its performance is premature. The validity of the tools must be assessed,” cautioned Neal Pollock, an Associate Professor in Kinesiology at Université Laval in Québec, Canada.

The Word on Dive Street

RAID president Paul Toomer is enthusiastic. Photo by Jason Brown.

Almost everyone I spoke to who was familiar with O’Dive thought that the concept was brilliant. Diving educators, most of whom had obtained and had a chance to use O’Dive just before the “lockdown,” were clearly enthusiastic. “Azoth has created a phenomenal tool. I think it’s the biggest game changer since the personal dive computer,” exclaimed Paul Toomer, president of RAID who said that his agency will soon offer a training course for users. ”I was pretty much blown away by it,” admitted his colleague, instructor trainer Edd Stockdale, who is currently writing the course. “As a scientist, I just geek out on this stuff.”

RAID’s course will not be the first. The International Association of Nitrox and Technical Divers (IANTD) launched its own O’Dive training program in June, led by President of IANTD Indo-Pacific and IANTD board member Christian Heylen, who has conducted and supervised more than a hundred dives using O’Dive prototypes and the current unit over the last year. “O’Dive is the missing link in diving research, particularly in the absence of human experimentation. I think it’s the next step forward in revolutionizing diving safety and decompression,” he said, referring to the large amount of user data that Azoth will accumulate, which may aid future decompression research.

CCR Divers pre-breathing before a dive during the APOCOLYPTRIP in the Red Sea. Photo by Olga Martinelli.

While excited about the product, veteran explorer and educator Phil Short expressed some concern that divers might misuse the device out of ignorance or willfulness. “The O’Dive concept is extraordinary. I just worry that the average diver may not know enough to interpret and understand numbers and do something safe with the information. Some might even use its predictive technology to see how far they could push the limits. On the other hand, if all O’Dive did was provide bubble readings, I don’t know if divers would buy it,” Short said referring to O’Dive’s what-if capability.

Interestingly, while most diving researchers and a dive computer executive I spoke with lauded O’Dive’s Doppler monitor, they were noticeably uncomfortable with the app’s underlying predictive model, which seemingly offers a high degree of precision in a field historically awash in uncertainty. One diving physiologist turned tech diver who was clearly skeptical about Azoth’s claims, called O’Dive a brilliant “marketing” concept: “It’s exactly the kind of techno-babble that techies buy into,” he said.

Divers Alert Network’s (DAN) US Research Director Frauke Tillmans is conducting one of the first comparative studies of three different Doppler devices including two precordial 2D-ultrasound devices and O’Dive’s subclavian sensor. The study was temporarily put on hold due to the pandemic. She refers to O’Dive as a black box. “I think Azoth’s subclavian approach is valid. It appears to be a reliable measurement technology. But we don’t know if safety is improved,” Tillmans told me.

Divers taking O’Dive measurements during DAN’s recent APOCALIPTRIP. Photo by Olga Martinelli.

DAN Europe’s Vice President of Research and Education, Professor Costantino Balestra, tested O’Dive’s bubble detection capabilities side-by-side with conventional Doppler during a recent DAN field trip i.e., “The APOCALIPTRIP” to the Red Sea in March and came away with positive impressions. “I trust the system for detecting bubbles. It works just fine. Data is still needed of course, and the DAN collaboration will help with that,” he said. A first paper published with Peter Germonpré on recreational Maldivian dives called,  “First impressions: use of the Azoth Systems O’Dive subclavian bubble monitor on a liveaboard dive vessel,” will appear in an upcoming issue of Diving and Hyperbaric Medicine.   A second paper that addresses technical deep rebreather dives (100m/328 ft) during several days (the Apocaliptrip) comparing O’Dive and echocardiography as bubble detectors will soon be published as well.

When I asked him what he thought about the efficacy of O’Dive’s predictions, Balestra paused for a moment and smiled and offered up his hands. “No one can say it’s wrong. No one can say it’s right. Though given years of development and proper bubble detection, it is unlikely to be totally wrong,” he said. 

One thing that people do seem to agree on is that Azoth’s O’Dive has the potential to help educate divers or rather help divers educate themselves.

The Quest to Improve Decompression

O’Dive—the O stands for “optimize”—is the brainchild of Barbaud and research engineer Julien Hugon, who heads the biophysics and computation department at Azoth Systems. The product has been under development for more than a decade. 

The original idea for the product dates back to mid-2000s when Barbaud was the officer in charge of diving safety for CEPHISMER—the French equivalent of the US Navy Experimental Diving Unit—and also served as a test diver. At the time, CEPHISMER was developing new decompression tables for Explosives Ordinance Disposal (EOD) divers in conjunction with hyperbaric doctors using Doppler ultrasound and 2D echocardiography technology. 

Barbaud was fascinated by the tools and their potential to improve decompression practices. “Given the large number of military dives performed every year, I repeatedly saw people who followed the procedures perfectly and still got bent. It was clear to me that divers needed procedural changes. Not just Navy divers but divers all over the world,” recalled Barbaud, who learned to dive at the Presqu’île de Giens in Southeastern France while at the Naval Academy studying naval operations.

Axel Barbaud in foreground with fellow French EOD diver following a heading. Photo courtesy of Axel Barbaud.

In 2010 Barbaud, then a Commanding Officer of a Naval Amphibious Squadron, decided to take an early retirement from the French Navy to form Bf Systémes in partnership with several labs involved in ultrasound technology and statistical analysis. He was joined by Hugon, who had recently completed his thesis on biophysical decompression modeling using Doppler monitoring. 

Axel Barbaud in uniform (2008). He is a retired Commanding Officer of a Naval Amphibious Squadron.

They soon changed the name of the company to Azoth Systems to better reflect its focus. Azoth is the name of the universal medicine or universal solvent sought in alchemy, while azote is French for nitrogen, to wit: the cure for too much nitrogen? Barbaud was also able to secure grants from the French government to continue Azoth’s research.

The company began by providing consulting services to commercial and military divers, including re-engineering decompression tables using Doppler bubble detection, and began collecting various diving databases for their research. “It was an opportunity to develop a deeper understanding of DCS from an operational viewpoint. I was always thinking about the what-ifs,” Barbaud said. However, their overall goal was to develop a technology that could help divers improve their decompression.

Building an Automated Sensor

Azoth’s initial approach was to focus on precordial Doppler monitoring with the hopes of developing an automated heart sensor. As mentioned above, precordial measurements require a trained ultrasound technician both to place the sensor correctly over the heart in order to get a good reading, and most importantly to interpret the audio signal, i.e., counting the levels of VGE returning to the heart and rendering a score or bubble grade, which requires training and expertise, and good ears due to the complexity of the precordial signal. Take a listen to the difference: Signal 1: Precordial ultrasound without bubbles, Signal 2: Precordial ultrasound with bubbles,  Signal 3:  Subclavien ultrasound without bubbles, Signal 4:  Subclavien ultrasound with bubbles.

Though several scales exist, bubble signals are typically graded on a five-point scale based on the original Spencer Bubble Scale which ranges from 0: No bubble signal, to IV: Bubble signals sounding continuously throughout the systole and diastole and obscuring normal cardiac signals. 2D echocardiography machines, which rely on visual data, use similar but expanded scales.

However, after three years trying to create  an automated precordial sensor without success, Barbaud was ready to close the company. Even today, researchers and engineers still haven’t been able to successfully automate the process of grading precordial bubble data.3 As Doppler measurement pioneer Ron Y. Nishi, lead researcher at Canada’s Defence and Civil Institute of Environmental Medicine (DCIEM) explained in a seminal 1993 article titled, “Tiny Bubbles, A Primer on Doppler Bubble Detection,” aquaCORPS #5 BENT“. Although considerable research has been conducted into automatic systems for bubble detection and analysis, none have been successful because of the complexity of the Doppler signal. Bubble identification and classifi­cation are still best done by the human brain.” 

A day in the life at Azoth Systems.

Instead, Barbaud and Hugon turned their attention to the work of DCIEM, which used post-dive bubble measurements to develop their decompression tables. DCIEM specifically used precordial Doppler, capturing blood returning to the heart in the vena cava; in addition, they took measurements at the left and right subclavian veins (SC) that lie just below the collar bones and bring blood back from the arms, and included these as part of their bubble data. 

DCIEM’s use of SC Doppler readings provided the insight that Barbaud and his team were looking for. The audio signal from SC Doppler probe was significantly cleaner and easier to grade than precordial data due to the absence of noise caused by heart valves, which meant that automated grading might be possible. In addition, the collarbone served as a useful reference point to correctly position a probe. 

Azoth’s Julien Hugon with decompression pioneer Ron Y. Nishi from DCIEM (2012)

However, a key question for the team was whether measuring SC bubbles provided sufficient information to accurately characterize the level of bubbling in the diver. While precordial measurements capture data on VGE returning to the heart from the entire body, subclavian data only measure VGE returning from the arms. Focusing on that question, they found that subclavian measurements were better statistically correlated with incidents of DCS than Azoth’s precordial measurements, and the correlation improved when the severity of the dive profile was included. They later published their findings in a 2018 paper authored by Hugon, Barbaud, Nishi and others.

As a result, they decided to develop a subclavian Doppler sensor. The plan was to build a device that would be used by divers instead of a medical device built for doctors, which would be subject to government regulation. “I was always thinking of measurement and how divers’ bubbles correlated with the risk of DCS,” Barbaud explained.

The O’Dive Tek for technical and rebreather diving

By 2015, Azoth Systems had their first prototype, just as the concept of the Internet of Things (IoT)—i.e., Internet-connected devices—was gaining traction. According to Barbaud, the prototype was ugly and not very reliable but it worked. Two years later, they had improved the unit enough to test it with some of their professional diving clients and conduct additional proprietary studies.

As noted above, Azoth officially launched recreational and technical versions of the product at the Paris International Dive Show this year. Each one had its own sensor and associated app. 

However, going forward, they have decided to offer a single SC sensor with separate apps for different types of divers: Sport, Advanced, CCR, Technical, and an app for instructors, dive centers, and clubs, enabling divers to share a measurement unit. Azoth will offer existing users an upgrade path. Prices will range from about €530-710 Euros excluding VAT or about US$600-800 before taxes. 

Biometrics for Divers

I was intrigued when I opened the waterproof case containing the O’Dive sensor with a wireless charger (with a European power adapter), ultrasound gel, and a compact mirror. I downloaded O’Dive’s Tek app to my iPhone, and set up an account. For a diver who measures and tracks his vitals, heart rate variability, daily activity levels, hiking, swimming workouts, and sleep cycles, the idea of measuring and tracking the quality of my decompressions in the hopes of improving them seemed like an obvious thing to do.

Phil Short in his J2s checking for post-dive bubbles

The first step in the process is taking a post-dive Doppler reading. Azoth recommends taking two readings. Each involves taking a 20-second measurement at both the left and right subclavian veins. For helium and rebreather (CCR) dives, they recommend the first reading be taken immediately after the dive, and a second 60 minutes later f. For nitrox and air dives, the readings should be taken 30 minutes after surfacing and then again at 60 minutes post dive. The idea is to catch post-dive VGE levels at their peak. 

O’Dive’s smart sensor and app are every bit as sophisticated and user friendly as you would expect from a Garmin or Polar sports tracker, and seem to make the process of obtaining an accurate measurement and bubble grade score easy to do. The app even includes a short built-in demonstration video. You turn on the sensor which pairs with the app via WiFi, and then apply a dab of ultrasound gel to the hockey puck-shaped sensor, which has a groove designed to fit against the collar bone, and a mirror to aid in the placement. 

The O’Dive app helps the diver to position the Doppler sensor

You place the sensor over your left subclavian vein, and the O’Dive app lets you know via the graphic display if the positioning is sufficient to get a good signal. Once the signal is confirmed, you press continue and the app visually directs you to breath in and breath out for 20 seconds while the sensor records the data. When the measurement is completed the app directs you to repeat the process on the right side. It even reminds you when the second post-dive reading needs to be taken. 

After taking the two bubble readings, you press synchronize and upload the data along with your dive profile imported from your dive computer or entered manually to the Azoth cloud, hosted in France, via Wi-Fi or a cellular connection. Currently the app communicates with both Shearwater and Suunto dive computers via Bluetooth, and others will likely be added in the future as the number of users grows. Of course, if you are diving in a remote location, your diving data is stored in your device until you have connectivity. 

How Good was Your Decompression?

O’Dive communicates with the Shearwater Perdix

Within a few minutes, depending on your connection, Azoth’s servers perform a comparative statistical analysis of your dive data based on their extensive diving databases. In the process they calculate a decompression quality index (QI) for the dive ranging from 0-100%, which purports to measure the relative safety of the dive from a decompression perspective. The index and associated data are then transmitted back to the user and displayed in the “My Results” section of the app.

The user can now select the dive and see the results in the O’Dive simulation screen, which represents the business end of the unit enabling the user to not only assess their deco quality, but if needed, determine what could be done to improve it. For those who were around in the day, you think of it as crossing the ingenious EDGE tissue saturation graphic with an Excel spreadsheet. “Gee, I wonder how my decompression would look like if my gradient factors were a bit more conservative and I added another 10 minutes of oxygen?” That’s exactly what O’Dive’s predictive model is designed to do.

As shown on the left-hand “Results” side of the simulation graphic, Azoth characterizes a dive with three parameters: the QI, and two components that act to reduce decompression safety. Ideally, divers strive to attain a QI of 100%, which Azoth has benchmarked as the relative risk or probability of DCS of five incidents per 10,000 dives or 0.05%. According to Azoth’s database, this equates to the relative risk of a diver who conducted a no-decompression air dive and has no measured post-dive VGE.

However, as shown, the QI can be reduced by a severity component (Sc), which can vary from 0-100% and represents the relative risk of DCS based on the severity of the dive profile itself in terms of gas loading. This relative risk would be the same for any diver following the specific profile and deco procedures.

Figure 2: O’Dive Simulation Screen 

The QI can also be reduced by a bubble component (Bc) reflecting the relative risk of DCS based on the diver’s production of bubbles post-dive, which ranges from 0%-40%—the 10% increments i.e., 0%, 10%, 20% etc. theoretically correspond roughly to five bubble grades. Unlike the severity index, the bubble component (Bc) is personal to the diver. Together, the three components, which are mathematically interdependent, add up to 100%. In mathematical terms, QI=1-Sc-Sb. 

For decompression management purposes, Azoth segments the QI index into three color-coded “risk” zones which are shown on the left-hand side of the dive quality graphic:

75%-100%: A good quality procedure, where optimization is still possible

50%-75%: Intermediate quality procedure, with significant margin for improvement. Note that a QI lower than 75% is associated in Azoth’s database with a risk higher than two incidents per 1000 or >0.2%

0%-50% The risk is significant and the diver’s procedure needs to be improved. An index lower than 50% is associated with a risk greater than one incident per 100 dives or >1% (the lower bound of the model). 

Note that the way that Azoth has scaled their index, every 33% reduction in QI corresponds to a 10-fold increase in risk in the model; in other words, a dive with a QI of 67% is 10x riskier than one with a QI of 100%. Again, Azoth’s intent is to help divers optimize their procedures from one dive to the next.

What If, What Now, What Then 

Once the user has gotten their post-dive results, the fun begins. Assuming their QI was less than ideal—okay, it sucked—they can determine what dive factors, if any, could be adjusted to improve their decompression next time around. This is done by adjusting the relevant sliders shown below the dive quality graphic. 

The simulated results are then shown in real-time on the right-hand side of the simulation graphic. Geeky divers like me will no doubt find this facility remarkable, though the question remains, how valid is the analysis?

For illustrative purposes, consider a “square” profile air dive to 32 m/105 ft for 56 minutes that a diver conducts using gradient factors of 85/90. More on square profiles later. The diver makes three decompression stops on their “air” back gas: 1 minute at 9m/30ft, 2 minutes at 6m/20ft, and 9 minutes at 3m/10ft. Note, this demo dive profile would NOT be recommended for numerous reasons.

Figure: 3 Demo Air dive to 32 m for 56 min)

According to O’Dive, the severity (gas loading) component would be 21%, and in this case, the diver has a relatively high bubble grade, which might be expected, resulting in a bubble component of 30%. The resulting QI would be 49%, making it a risky dive with a relative risk of DCS of greater than 1%. What’s to be done?

In the era of nitrox, it’s questionable why the diver would be breathing air in the first place (Reference meme: “Compressed air is for tires.”). Accordingly, the first question to ask is, “What if the diver were breathing nitrox?” The answer can readily be seen immediately by adjusting the slider marked “% O2 bottom gas,” to a standardized 32% mix. Note that the what-if model automatically sets a maximum partial pressure of oxygen (PO2) of 1.4 atm for the working phase of the dive, and a maximum of 1.6 atm for decompression.

Figure 4: Demo dive adding nitrox 32

The simulation shows a new QI of 75% along with a severity index of 6% (nitrogen loading is reduced) and a Bubble component of 19%. O’Dive calculates that the diver’s safety has been increased—i.e., the risk of DCS has been decreased—by an estimated 6.1-fold factor, clearly showing the benefits of nitrox. Note that the model has used the new what-if data to predict a reduced bubble grade.

In addition to nitrox, the diver might also consider adjusting their gradient factors (GFs) via the GF Low, and GF High slider. Instead of the current 85/90, what if the diver used GFs of 50/70? Adjusting the sliders, the updated QI index is now 84%, with a severity of 1% and bubble component of 15%, which O’Dive estimates to be 11.5-fold increase in safety over the original dive. Note that the model may limit certain low GF choices, or at least not use them in what-if calculations (the user is notified when this occurs) in order to limit certain “deep stops” which Azoth deems ineffective.

Figure 5: Demo dive with nitrox and more conservative gradient factors.

The diver might be satisfied with the results of their decompression at this point. However, they also may consider, “What if they added oxygen decompression?” Usually not a bad thing!

Accordingly, the diver selects the “With Oxygen Decompression Stops” button and moves the “O2 decompression stop %” slider to 100%, which simulates the effect on the dive of breathing O2 on their 6 m/20 ft and 3 m/10 ft stops. After making the adjustments, the model shows that the diver’s QI is now 95%, with 0% severity component and almost zero bubble grade resulting in an estimated 24.2-fold increase in safety over the original decompression. 

Figure 6: Demo dive with nitrox, gradient factor and oxygen decompression.

The diver would presumably then make these adjustments to their dive plan, i.e conduct the dive on nitrox 32 with 50/70 gradient factors and use O2 for decompression, on subsequent dives and thus improve their decompression safety. “In the past, people adjusted their gradient factors and other aspects of the dive, in some cases from ridiculous to sane, on what amounted to a wing and a prayer,” observed RAID’s Toomer. “Now they have an actual basis to adjust their procedures.” Providing of course that the predictions proved accurate. 

O’Dive performs similar calculations with complex mix or rebreather dives, offering divers the relevant set of adjustments. As noted above, divers can perform what-ifs by adding trimix, adjusting the fractions of helium (FHe) in their bottom mix, adjusting the set point in the case of a CCR dive, adjusting low and high GFs, adding oxygen decompression, adding an additional decompression stop, and/or adjusting the length of the last stop. Note in the case of CCR diving, O’Dive uses the set point, not the diver’s actual PO2. Similarly, the Sport version of the app (designed for no-stop recreational air and nitrox dives) enables the diver to adjust the O2 content in their nitrox mix, change GFs, and add a safety stop of varying length.

It’s easy to be wowed, distracted, or even a bit concerned by the seeming precision of O’Dive’s predictions, i.e., Conducting the dive on nitrox is 6.1 times safer? 6.1?!? Paraphrasing Arthur with a bit of literary license, “This may be some strange usage of the word ‘precision’ that we weren’t previously aware of.” O’Dive’s decimal-place accuracy is a result, primarily of the math, and may be greeted with some skepticism. However the overall idea of presenting divers with an estimate of the “relative risk” of their procedures makes a lot of sense.

Not surprising, Barbaud insists that the apparent precision of Azoth’s algorithms and whether they reflect physiological reality are not the issue. “The QI index is not perfect. The question is does O’Dive help divers to progressively adjust their practice to lower step by step their bubble production?” he proffered to me in an email, and then replied, “The answer, of course, is yes. This can be easily and practically confirmed by everyone using the technology.” 

Nevertheless, the fundamental questions remain: how are these calculations made, what do they mean, and how valid are they? For that we take a moderately deep dive into Azoth’s model. 

It’s a Math, Math, Math, Math World

I spent considerable time in discussions with Barbaud and Azoth executives trying to understand the logic and mathematics behind their model, and also spoke with several decompression researchers for clarification. (Full disclosure: I have a M.S. degree in Mathematics from Stanford.) Though I don’t pretend to understand the ins and outs well enough to be able to perform the calculations, I came away with a general understanding of the principles behind their work. Here’s what I learned.

In essence, Azoth has worked to mathematically characterize DCS risk based on a number of large databases of dive profiles, to which they have access. Specifically, these are:  

A database of air and nitrogen-oxygen dives in research trials conducted by the United States, United Kingdom and Canadian militaries from 1944 to 1997. This is a collection of more than 8000 exposures and more than 400 DCS cases allowing the analysis of the relationship between diving profile and DCS occurrence. 

Defence Research and Development Canada (DRDC, formerly DCIEM) database that was built from a number of studies conducted by Defence and Civil Institute of Environmental Medicine (DCIEM) over a period of about 40 years. Over 8000 man-exposures that generated 100 DCS have been monitored for bubbles detection and included in this DCIEM/DRDC Doppler ultrasound database. The database includes a large number of open circuit and closed circuit, trimix and heliox dives.

• A subset of French Navy database with more than one million air, nitrox, and trimix dives carried out over the period 2000-2015. The dives recorded in paper format (and do not include dive profiles or ultrasound assessments) have been collected, scanned and checked, including the information of dive parameters and DCS status.

COMEX Heritage mixed gas diving database which includes a large set of empirical and experimental data on heliox dives shallower than 100 m/330 ft, in addition deeper mixed gas bounce dives. 

Having access to large diving databases, characterized by a wide range of DCS risk, has enabled Azoth to make a detailed statistical study of the association between the DCS risk and dive parameters, which they’ve used as the basis of their model. They are not the first. The US Navy, of course, has long used statistical-based algorithms to calculate its decompression schedules, though the algorithms tend to be more complex and consider many more variables.

In contrast to the US Navy’s probabilistic algorithms, Azoth’s predictive model makes a simplifying assumption and only deals with square dive profiles. As a result, when conducting its analysis, Azoth software first converts the dive profile in question into an equivalent square profile. It does this by detecting the start of a diver’s decompression (and speed of ascent), and uses that data to compute an average dive depth and bottom time from leaving the surface. It then proceeds to calculate the risk inherent in the dive profile itself in order to derive the severity Component (Sc) as discussed above. 

For air dives, Azoth found that the risk of DCS increases with the depth and the duration of dive, and decreases when the total decompression time increases. Accordingly, Azoth defines a severity index (Is) modified from a formula originally developed by Royal Navy physiologist H.V. Hempleman to measure decompression stress on square profiles as follows: 

Is = P √t/dt, where P is the depth in pressure, √t is the square root of bottom time, and dt=total decompression time. As such, Is can be thought of as the ratio between a measure of gas loading and release.

Azoth software then calculates the risk or probability of DCS (pDCS) with the following probabilistic equation: the parameters “a” and “b” are fitted by means of statistical regression analysis using the three defense dive databases. 

pDCS=1/(1+exp(-a-b*Is))

The results are shown in the diagram below.

Figure 7:  The probability of DCS (pDCS) as a function of severity index (Is) taking into account the depth, the duration of dive and the total decompression duration.

Using the DRDC and COMEX databases, Azoth was able to extend the severity index (Is) to a multi-gas dive, which considers helium mixtures as well the use of nitrox and O2 for decompression. In addition, the Is index is modulated by a coefficient that increases the risk ratio by up to a factor of two, based on the actual ascent rate as compared to the maximum acceptable rate as defined by the COMEX data. This ratio was established indirectly, from a dedicated study on the impact of the ascent rate on the production of bubbles.

More Tiny Bubbles

In Azoth’s world, the presence of measurable venous gas emboli (VGE) circulating in divers’ blood vessels after the dive is an essential element of O’Dive’s risk predictions. Unfortunately, the existence of the VGE has a poor positive predictive value for DCS, meaning that the presence of bubbles doesn’t mean that the diver will suffer DCS. Particularly on the scale of a single dive, the risk ratio of a dive with no bubbles vs. the same dive with many bubbles is unknown, however, according to Azoth, at a scale of many dives the ratio becomes meaningful. 

A diver’s 2D echocardiography showing lots of tiny bubbles Photo by Olga Martinelli for DAN Europe.

Conversely, VGE has a high negative predictive value for DCS. That is, the risk of DCS is lower in the absence of detectable bubbles. That’s the reason that the O’Dive tool focuses on helping divers to adjust their decompression procedures to progressively lower their bubble scores. Of course, the challenge is that the presence of VGE is the result of numerous physiological factors and is highly variable both between individuals and for a given individual over time.

Nevertheless, based on an analysis of the DRDC database, Azoth theorizes that the presence of bubbles acts an amplifier of risk associated with the severity of the dive profile. In fact, they calculated that the observed risk ratio between a dive without or with few vascular microbubbles and a dive with bubble grade of IV is about 10. Specifically, when the severity of dive profile is considered, Azoth’s model is calibrated so that the DCS risk ratio is close to 2.0x when moving from one bubble grade to the next. The data used in coming to this conclusion is summarized in Table 1. 

Grade SCN observations%N DCS% DCS
066561.3 %40.6 %
116315 %31.8 %
21049.6 %21.9 %
314012.0 %117.9 %
4131.2 %323.1 %
Total1085100 %232.1 %
Table 1: DRDC air database characteristics used to build pDCS=f(Is, Grade) function.

Combining their severity index and bubble grade, Azoth calculates the risk or probability of DCS (pDCS) by means of statistical regression analysis, where again, the parameters a, b, and c, are fit to the data. The resulting risk curves are shown in Figure 8.

 pDCS=1/(1+exp(-a-b*Is-c*Grade))

Figure 8: DCS probability as a function of the dive severity index (Is) for five grades of post-dive VGE

Note that O’Dive’s use of subclavian bubble data, which offers a much cleaner signal than precordial Doppler, enables Azoth cloud software to essentially count the number of bubbles in the diver’s post-dive readings using spectral analysis, thus automating the scoring process. The software then converts the highest measurement to a five-point bubble grade scale, similar to the Spencer scale, used in the equations above. As noted above, this potentially represents a significant breakthrough in Doppler monitoring, insofar as accurate measurements can be made by the diver themselves. No small feat.

Once the diver’s profile, including decompression procedures, and bubble scores are uploaded, Azoth software calculates the QI decompression quality index, which now can be understood as the logarithm of a product of risk ratios, specifically the risk of the dive in question from a severity perspective compared to Azoth’s baseline no-decompression air dive, amplified by the diver’s bubble grade. This enables the index QI to be expressed as a sum of probabilities. In mathematical terms:

QI=

MAX (1-0.33*LOG{[PDCS(Is)/PDCS(Is_Air_NoD_limit)] *[PDCS(Grade, Is)/

PDCS(Grade=0,Is)]}

Equivalently:

QI=1-Sc-Bc where Bc=.1*Bubble Grade 

and Sc= MIN(0.33*LOG{[PDCS(Is)/PDCS(Is_Air_NoD_limit)];1-Bc).

In addition to the QI, the software also calculates various risk ratios between the risk of the underlying dive in question, and the risk of the dive when relevant what-if factors are activated. For example, using the above case of the air dive to 32 m/105 ft for 56 minutes, the software calculates the range of risk of the same profile if it had been conducted on a range of nitrox mixtures i.e., air (21%) through nitrox 33 (at which point the PO2=1.4, the maximum permitted for bottom mix by the model). These ratios are then transmitted to the diver’s device along with the QI and other relevant data, enabling the user to perform what-if calculations locally on their device.

As noted above, O’Dive’s model also predicts the diver’s bubble grade for the simulated what-if dive. In fact, the model progressively adapts its predictions based on diver’s past bubble history, which it characterizes into one of three classes of diver bubbling dynamics: low, medium, or high, identified through the DRDC database. Think of it as a kind of moving average based on the diver’s history. 

Researchers have long known that some individuals, so-called “bubblers,” consistently bubble while others rarely do. Azoth argues that one of the strengths of O’Dive is that it actually takes the individual’s history into account, offering a personalized what-if.

“Again, do the sliders reflect the perfect reality of gas effects on every diver in the world?” Barbaud asked me rhetorically in an email in response to my questions. “Of course not!” he answered.  “Nor are they built on pure supposition with no basis at all. The truth is in between these two positions; they are not perfect BUT they reflect a good reality,” he wrote. “They are simulations of the next dive, and provide indications that are more accurate than pure intuition, a supposition-based idea or no idea at all. This approach was designed to help divers reduce their bubble production and consequently improve their practice.”

Barbaud compared the situation to the first introduction of the dive computers. “Can we say that dive computers brought perfect information to divers? No. But they did bring a huge step forward to diving practice!” Did I mention that Barbaud and team are swinging for the bleachers?

What Works, Works?

As might be expected with a product as intriguing and complex as O’Dive, the diving and hyperbaric medicine scientists I queried had plenty to say about Azoth’s approach and procedures. Decompression physiologist David Doolette who is intimately familiar with the US Navy’s statistical approach to decompression raised several issues about Azoth’s model itself. The first question out of his mouth when we spoke? “How do they validate any of this mathematically?” Azoth’s answer: The model has been tested against the DRDC, French Navy, and COMEX Heritage databases from which they were derived. 

One of Doolette’s concerns about Azoth’s model was that it treats post-dive venous gas emboli (VGE) as an input to the model and not an output. “They have VGE on the wrong side of the equation,“ he explained. ”The problem is that the VGE measurement doesn’t really cause DCS, it’s a parallel or surrogate outcome to DCS that is closely related to the dive profile and consequently Azoth’s severity index (Is).” Note that the model predicts future VGE based on the severity index and current VGE. The result according to Doolette is that there is collinearity in the model which could make it unstable and lead to poor predictions in the field. “If the model worked, it would be great. But I question whether it’s ready for primetime,” Doolette said. Two other researchers shared his assessment.

David Doolette, PhD preparing for a North Florida cave exploration dive

“If this were a [peer-reviewed] scientific paper I would ask them to show me an external calibration set such as DAN Europe’s DSL dataset or DAN US’s Project Dive Exploration and see if they could predict it,” he said. Azoth has or is about to enter into a collaborative agreement with DAN Europe, so it’s possible that may come about. Currently the company does not have access to either of DAN’s diving databases.

For his part, Barbaud respectfully acknowledged the limitations of VGE, but questioned the physiologist’s premise. “There is room for being imperfect, while still adding interesting value. The fact that the information contained in bubble grades and dive profiles is not the same is reflected by the significant improvement in both the likelihood and the information criteria when bubble grades data is added to the dive profile DCS model. This prevents it from being unstable,” he said. However, this does not address Doolette’s concern with the accuracy of O’Dive’s predictions with new data. 

To his credit, Barbaud said that Azoth is always open to suggestions for improvement. “If someone can prove there’s a better way to do it, we’ll listen. We are focused on improving safety and enhancing our tool. That’s what counts.” 

A final question for Doolette: Whatever its accuracy, could O’Dive’s predictive algorithm actually make a diver who wasn’t trying to abuse the model less safe? “No, I don’t think so,” he said. “Assuming all the equations I have not seen are correct.”

JP Imbert, a French diving engineer involved with designing decompression procedures for the offshore industry suggests a larger view of the issue. Imbert has worked with Hugon in the past, and was the lead author of a recent scientific paper with Balestra and others that offers a new model on bubble formation.  “There is more to decompression safety than just bubbles. We have measured an oxidative stress after deep and long saturation decompressions in the commercial diving world. Inflammation could be the second dimension of the decompression stress, he explained. ”Inflammation is related in the scientific literature to free radicals and microparticles but we do not know yet if there is a link between microparticles and VGE. High oxygen levels and pre-existing microbubbles could be responsible for it.”

DAN America Director of Research Frauke Tillmans taking a O’Dive measurement. Photo by John McCain.

DAN’s Tillmans also had questions about the model. While impressed with O’Dive’s sensor, which she called “ingenious,“ Tillmans pointed out that decompression stress encompasses many factors besides bubbles including temperature, age, sex, sleep, BMI, and more. “A lot of science is ignored by only considering time, depth, gradient factors, and bubbles,” she said. “I believe Azoth has done everything they possibly could do, but I don’t think it’s the full-scale picture. They may prove me wrong.”

Pollock NW. Factors in decompression stress. In: Pollock NW, Sellers SH, Godfrey JM, eds. Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS Workshop. Wrigley Marine Science Center, Catalina Island, CA; 2016; 145-156.

Barbaud acknowledged her points while at the same time pointing out the associated research challenges. “Yes, we know bubbles are not all there are. There are a lot of factors like sleep, temperature, activity levels, and others. But how do we take these into consideration? What is good sleep, or bad sleep and how do you quantify this? Wait for 20 years and try to count the number of DCS in various situations? No. Instead, we can compare various situations in terms of bubble production. Our goal is to educate divers to lower their bubble scores dive after dive, which we know can only increase their safety.”

In the Field

Another issue raised by scientists who have spent time conducting field research is that Azoth’s bubble measurement protocol of taking only two measurements is inadequate to reliably capture peak bubble data, i.e., the reading with the highest bubble grade. As Pollock, who has been using ultrasonic monitoring to evaluate decompression risk for more than three decades, explained, “The goal of course is to catch the bubble peak, but taking only two measures will almost certainly miss it in some cases. The consensus recommendation of the ultrasound workshop conference was that a minimum sampling of every 20 minutes for two hours is required to achieve research quality data,” he explained.

“If self-monitoring misses peak bubble scores the predictions would be fundamentally flawed, and it is clear that the time to peak grade can vary. Collecting high quality data is often not convenient or easy,” Pollock said. In the case of trying to catch peak bubbles, more data is better. In addition, field researchers typically measure subjects both at rest and after doing knee bends or arm flexes. 

Neal Pollock, PhD taking a post-dive precordial Doppler measurement

Barbaud said that Azoth’s two measurement protocol was determined based on the observation of their databases. “If the diver properly follows these time slots, they will catch the peak in 90% of the time we will not miss the bubble peak,” he said and referred me to DAN’s APOCALIPTRIP trip data that showed a good correspondence between O’Dive’s and Balestra’s echocardiographic readings.

Barbaud also noted that the system was not developed for a single use, but rather was designed to adapt progressively to a user’s practice over three to five dives. “We may miss the bubble peak on a single dive but not every dive, and so the global trend for the diver (in terms of bubbling dynamic) will quickly appear. Regarding the knee bend and arm flexes, we know these practices very well of course. But we’ve observed in our databases that DCS is better correlated to bubble peaks at rest than after an exertion. What’s more, it doesn’t require any specific action from the diver, which is welcome if we want our product to be widely used,” Barbaud said.

IANTD’s Christian Heylen helping a student with his O’Dive measurement

One of the questions surrounding Azoth is what they will do with the volume of diver data that are accumulating from users, and whether it will be helpful to further decompression research. O’Dive users will of course have access to their diving data including their bubble scores much in the way they do now with their Shearwater computer via the Shearwater cloud app. Presumably, the data will help Azoth to improve their model and Barbaud said that they would also make the data available to legitimate researchers. 

However, on the question of whether the data will be useful for decompression research, the scientists I spoke to were not encouraging.

The first issue is that of the quality of the data as discussed above, which may be impacted by Azoth’s protocols, as well as the uncontrolled nature of the data collection. “The ability of a monitor to closely track a professional precordial monitor under controlled conditions is very different from a device able to monitor effectively under dynamic conditions, and still interpret the findings in a meaningful way,” Pollock said. 

The second issue is the same problem inherent in the two DAN dive profile databases. According to Doolette, “Lots of ‘safe’ data doesn’t tell you much and can become a data management nightmare,” he said. “You can’t pick and choose which bits you look at either, because you lose the bulk probability context. You do not, for instance, learn anything by choosing a subset that resulted in DCS and say 50% of these dives had high VGE. Maybe 50% of all dives had high VGE.”

Feed Your Head

RAID’s Edd Stockdale making his O’Dive stop

Given those caveats and possible limitations, almost everyone I spoke to agreed that O’Dive had the potential of being a powerful learning tool for divers, if only to measure how their own bodies respond to their diving practice. As discussed above, the ability for divers to now self-monitor their post-dive VGE is arguably a significant breakthrough in its own right.

I reached out to Stockdale, who has been working with colleagues and testing the product under a variety of conditions including cold water exposures, to get his perspective. “I can understand why the medical professionals are freaked out,” he said. “Many of them are passionate divers themselves, and they worry that divers could misuse the product, like seeing how far they could push the limits before getting bent. Ever since Doolette and [Simon] Mitchell’s 2013 paper on deep stops, everyone has become a deco theory expert,” he chuckled. 

Stockdale’s take on the product? “I think O’Dive is an effective education tool for divers to help them understand what works for them, and also for instructors who are teaching divers about decompression principles. I think that’s where its strengths are. It could also be a powerful tool for specialists like researchers and hyperbaric docs, and expedition divers. It’s the biggest game changer since mixed gas dive computers,” he said. 

The Orca EDGE helped change the diving sport diving world. Photo by M. Menduno

His outlook was seconded by IANTD’s Heylen. “Even for a basic diver, it’s a great tool to become more aware of the realities of diving; not just the false belief that nothing can happen to you. It will help them increase their knowledge, and get divers interested in knowing more.” Arguably, education can only be good for diving safety!

Like the introduction of dive computers to sport diving, it will likely take time, additional studies and more experience to determine whether O’Dive is the next EDGE, more akin to first analog dive computer, the SOS Automatic Decompression Meter, affectionately known as “The Bendomatic,” introduced by the Torino, Italy-based company Strumenti Ottici Subacquei in 1959, or somewhere in between. The ultimate test, as Barbaud and team have proffered, will be whether O’Dive is able to actually improve divers’ safety.

The SOS meter aka “Bendomatic” was an early forerunner of modern dive computers. Photo by Jon Council

In the meantime, divers are advised to remember that like dive computers that came before it, O’Dive’s models should be taken for what they are, mathematical representations of what we perceive as reality. It’s a message that Pollock has been delivering in his talks and papers for many years. “Developing new tools is important, particularly those that can be used by divers in their own practice. But it’s crucial for users to understand the limitations of any device to avoid confusing the information they provide with truth,” Pollock said. “While O’Dive may prove useful helping divers consider their options, we still have a long way to go in both physiological monitoring and data analysis to truly assess personal risk.”

Azoth System’s O’Dive is arguably a bold and innovative new step in that direction.

Footnotes:

1. Note that the Divetronic AG DecoBrain, which was introduced the same year in Europe, was a table-based look-up decompression meter.

2. Scientific diving may have the lowest incident rate of the various diving communities.

3. There have been some promising developments in recent years using portable 2D echocardiography and frame-based bubble counting.

Additional Resources

NETVA 2012 interview Axel Barbaud – BF Systèmes (En Francais)

La France bouge : Axel Barbaud, fondateur d’Azoth Systems January, 2020

O’Dive Video Tutorials

O’Dive Scientific References

Additional Papers

Carturan D, Boussuges P, Vanuxem P, Bar-hen A, Burnet H, Gardette B. Ascent rate, age, maximal oxygen uptake, adiposity, and circulating venous bubbles after diving. J App Physiol. 1999; 93: 1349-1356. 

Pollock NW. Use of ultrasound in decompression research 2007: Diving Hyperb Med. 2007: 37(2):68-72 

Dardeau MR, Pollock NW, McDonald CM, Lang MA. The incidence rate of decompression illness in 10 years of scientific diving. Diving Hyperb Med. 2012; 42(4): 195-200.

Balestra C, Germonpre P. The use of portable 2D echocardiography and ‘frame-based’ bubble counting as a tool to evaluate diving decompression stress. Diving Hyperb Med. 2014: 44(1):5-13

Møllerløkken A, Blogg SL, Doolette DJ, Nishi RY, Pollock NW. Consensus guidelines for the use of ultrasound for diving research. Diving Hyperb Med. 2016; 46(1): 26-32.

Doolette DJ. Venous gas emboli detected by two-dimensional echocardiography are an imperfect surrogate endpoint for decompression sickness. Diving Hyperb Med. 2016: 46(1): 4-10 

Papadopoulou V et al. Variability in circulating gas emboli after a same scuba diving exposure. Eur J Appl Physiol, 2018. 118(6): p. 1255-1264.

Hugon J, Metelkina A, Barbaud A, Nishi R, Bouak F, Blatteau JE et Gempp E, Reliability of venous gas embolism detection in the subclavian area for decompression stress assessment following scuba divingDiving Hyperb Med. 2018 Sep; 48(3): 132–140.

Imbert JP, Egi SM, Germonpre P, Balestra C. Static metabolic bubbles as precursors of vascular gas emboli during divers’ decompression: a hypothesis explaining bubbling variability,” Front. Physiol., 11 July 2019 

Balestra C, et al., Diving physiopathology: the end of certainties? Food for thought. Minerva Anestesiol, 2019. 85(10): 1129-1137.

Menduno M. It’s the metabolism, stupid: a new model for bubble formation. DAN Europe’s Alert Diver 2020


Equipment

Electrolung: The First Mixed Gas Rebreather Was Available to Sport Divers in 1968

While today mixed gas rebreathers are a standard tool for tech divers, it’s easy to forget that this enabling, underwater life support technology has been more than 50 years in the making. In fact, the first commercially available unit was available to sport divers in the late sixties! Here inventor and marine biologist Walter Starck dives into the deets of his Electrolung’s innovative design.

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By Walter Starck

Header image: W. Starck diving with first production run of the Electrolung on a drop-off in San Miguel, Cozumel, September 1968. The 180º Fisheye image used an optical dome port also developed by Starck in 1964.  Photos courtesy of W.Starck

This article was first published in serial form on the Rebreather Email List in July 1998, at the suggestion of Dr. Peter Heseltine, a researcher in diving and hyperbaric medicine.

Development of the Electrolung came about through my chance meeting with John Kanwisher aboard Ed Link’s diving research vessel in the Bahamas in early 1968. Ed was trying out his new diver lock-out submarine Deep Diver and had invited along several researchers with relevant interests. I was there to do some deep biological collecting, and John was there to do heart rate/respiration measurements on divers using some new acoustical telemetry equipment he had developed.

Lock-out dives from Deep Diver were done using hose-fed OC Kirby Morgan helmets. Gas for this purpose and to pressurize the lock-out chamber was supplied from a large, high-pressure sphere carried by the sub. The large amount of gas required for a single dive severely limited the number of dives which could be made and involved substantial cost and logistic considerations. The need for more efficient utilization of gas was clearly apparent.

It turned out that John and I had both been considering the feasibility of a mixed gas CCR using electronic sensors to control PPO2. We both knew in general terms what was needed, but John wasn’t a diver or a machinist, and I didn’t know that much about electronics. However, I had been diving for 15 years and had built a wide range of underwater equipment. John, in addition to being a physiologist, had invented the first polarographic oxygen sensor and held a dual appointment at Woods Hole Oceanographic Institute and MIT where he lectured on electronic instrument design.

The author in 1998 explaining the operation of the Electrolung

When we returned to our homes John started putting together the sensors and control circuit, and I started getting together the hardware and machining all of the necessary bits. Six weeks later we both had our respective parts together. John sent me the board and sensors; I installed them, and it worked. The overall configuration and design was basically as described, but there were, of course, numerous details to clean up. The electronics for example were wire connected on a breadboard, and the solenoid valve I had hand made and actuated with a solenoid scavenged from a battery operated coo-coo clock.

Although the prototype was put together quite quickly, it was far from a “first thing which comes to mind” effort. Quite a few years’ experience and thought had led up to it, so that when actual construction began we both knew pretty clearly what needed to be done and how to do it. Later, at Beckman, I had the opportunity of working with a whole group of specialists on improving the same device. The outcome was some tidying up of details but no fundamental improvement. 

A young Walter Starck about to dive in the Coral Sea with his Electrolung in 1971

The first production units with printed circuit boards and commercial pneumatic control valves were produced in August 1968 at my company, Oceanic Equipment Company in Miami. The price was $1995 and it was the first commercially available mixed gas electronically controlled rebreather. Production and sales were continued by Oceanic Equipment for two years and was then continued with further refinement of details by Beckman Instruments in Los Angeles.  

Buyers included commercial diving companies, the National Aeronautics and Space Administration (NASA), for use in their weightless simulation facility), the central Intelligence Agency (CIA) for unknown purposes, the Israeli Defense Forces, and the US Navy.  At Beckman they heard that the Navy had used the Electrolung at 1000-foot depth/306 m in a lockout dive from a submarine in the Arctic, but no other details. Then, last year declassified information revealed that the Navy had developed a nuclear sub with deep lockout capabilities at that time and had used it to tap into the Russian military communication network on a submarine cable running from Vladivostok across to the Kamchatka Peninsula in the Arctic. It appears that this must have been where the Electrolung was used; but what role it may have played is unknown.

At Beckman, the biggest problem was to prevent the creation of problems which didn’t previously exist but could be introduced through changes made by specialists who were unaware of consequences outside of the narrow area of their expertise. The experience gave me a real appreciation of both the power and the limitations of specialist expertise and the importance of systems analysis in coordinating and integrating the input of specialists.

Polarographic Oxygen Sensors

The Electrolung used three polarographic oxygen sensors. The sensors were robust hand-made ones we constructed ourselves. They had a central platinum cathode about 1/4″ in diameter surrounded by a concentric silver anode about 3/8″ Diam. In between was an annular groove for the KOH electrolyte. A .001″ teflon membrane held in place by a thick silicone rubber boot retained the electrolyte. Although the sensors would run for weeks before desiccation of the electrolyte became limiting, our SOP was to make them up fresh and calibrate them for each day’s diving. At the end of the day’s diving, the membranes would be removed and the sensors washed with distilled water. Making up and washing off the sensors only took a few minutes and assured that we always had fresh sensors.


Polarographic oxygen sensors. Left, bare sensor.  Middle, sensor with silicone rubber boot pushed half-way down over teflon membrane.  Right, sensor and boot in sensor holder.   Boot O.D. and hole in holder are tapered for snug fit.

Sensors of this type don’t wear out, so they are hardwired into the circuit. Unlike galvanic sensors, they don’t use oxygen but rather just respond to its presence. They work equally well submerged, so the effect of any condensation is negligible. A drop of water fully covering the end of the sensor would only slow the response time. In practice, we never had any condensation in the sensor area ,as this came immediately after the canister, so the gas was at its warmest and driest point in the circuit. Thick plastic walls probably also helped in avoiding condensation on cold surfaces.

The chief advantages of the sensors were that they were always fresh, and condensation wasn’t a worry. The disadvantage is that in making them up with fresh electrolyte we could screw up by contaminating the sensor via sloppy technique. Any significant change in calibration after a fresh makeup would be an indicator, and determining why should be mandatory before proceeding further. Still, there are the black box mentalities who will simply crank the trim pots until they get the reading they want and then assume all is well.

Cathode (center) is platinum. Anode (outer) is silver.  Electrolyte groove is between.  Body is epoxy.

There were two trim pots for calibrating each sensor. One for zero. The other for gain. Zero was checked each time before the sensors were made up. Gain was calibrated initially with air, and then the unit was put together, and a check with pure O2 was done. The permeability of teflon to O2 varies with temperature. The sensors were of potted epoxy construction with the electrodes embedded in the epoxy. A thermistor in contact with the underside of the cathode was also embedded. This thermistor had a similar response curve to the teflon and compensated for the temperature effect keeping output linear over the desired range.

Our chosen set point was 0.5 atm partial pressure of oxygen (PPO2).

The bottom line was that with proper care the sensors were very reliable. Enough so, that they could be hard wired in, and I know of no case where one ever had to be replaced.

The Electronics: Control, Readouts, Alarm

Unlike galvanic sensors, polarographic electrodes don’t generate electricity. Rather, the conductivity of the cell varies in the presence of oxygen. A bias potential from an external source is applied between anode and cathode, and the resulting flow of current is a function of the molecular concentration of oxygen present.  The current involved is very small, so an Op Amp is used with each sensor to boost power to a level useful for control and monitoring. Hermetically sealed trim pots which incorporate an O-ring seal around the adjustment screw provide for zero and gain adjustment of each Op Amp, thus enabling calibration.

Electronics section.  Solenoid and sensors at left, wrist display above.  Leads to wrist display are inside hose covered by SS braid connecting to the electronics section at ambient pressure.

The amplified signal is read out to a wrist display consisting of a stack of three edgewise panel meters. We used 100 microamp meters in conjunction with high resistance to prevent a possible short in this circuit from affecting the solenoid control. Mil Spec, so-called “shock resistant” meters were used. These resist minor bumps but still they won’t stand up if you drop the display on a steel deck or concrete. In practice it wasn’t a significant problem, but occasionally a meter did require replacement. This was quick and easy to do.

The big advantage of this type of analog display is that you can tell at a glance everything you need to know. In use, all you need to verify is that all of the readouts are in line with one another and at or a bit above the set point which was exactly mid-scale. This kind of meter is also precise enough for calibration purposes. If I were doing it today, I would look at bar type LCD or LED readouts for monitoring and perhaps a separate switchable numeric display for calibration. I would also seriously consider a miniature heads-up display in the mask instead of one on the wrist. I don’t like numeric displays for monitoring, as they entail reading and mentally comparing numbers which requires much more attention than just noticing if position and alignment are where they should be.  Possibly some of the commercial RBs have already done all this.

Main circuit board.  Front modules contain amplifiers for each sensor including zero and gain trim pots on front.  Module behind controls solenoid.   Silica gel canister and double throw power switch is at top.  Push button, momentary-on switches at right were a later addition enabling pre-dive battery check under load using the wrist display meters to read.  Previously this was done using an external meter.

The amplified signals from all three sensors were fed into a fourth Op Amp which in effect averaged them and used the resulting value to control the solenoid set point via a switching transistor. We used a fixed set point of 0.5 atm PPO2, but it would be simple to add a trim pot to provide an adjustable set point. Clipping circuits limited the input to the control Op Amp from each sensor to values corresponding to 0.25 and 0.75 atm PPO2. If any one sensor began to read drastically different from the others, its effect on automatic solenoid control was thus limited. Clipping came after the meter display, thus they would continue to read true output even if the input to the control Op Amp was clipped. Clipping also activated an audible alarm. If the alarm sounded, a glance at the meters would tell you what the situation was. If only one was off, the other two would continue to exercise control. If all were high, low, or different from one another, you could use manual control while aborting the dive.



The Op Amps require a + and a – voltage power supply. This was supplied by a pair of 9V Manganese Alkaline transistor radio batteries. Bias to the sensors was provided from the same source via a voltage dividing resistor circuit. A second pair of the same batteries provided switchable backup power. A third pair used in parallel provided separate power for the solenoid. The solenoid did not have backup, as this is non-critical because manual control of O2 is easily affected.  The snap terminals used for this type of battery were securely attached to a bulkhead. A screw-adjusted base plate held the batteries firmly in place, and against the terminals, avoiding any possibility of a loose battery connection.

Battery holder and base plate (sponge rubber pad has come loose from base plate in photo).  Audible alarm is at right.  This is on the opposite side of the longitudinal bulkhead from the electronics.  Connection for O2 to solenoid is at lower left.

All the electronics were incorporated on a single circuit board about 4”x5″/10-13 cm. This was mounted on one side of a longitudinal bulkhead in the electronics housing with the batteries and audible alarm on the other. This longitudinal bulkhead was itself mounted on a transverse bulkhead which separated the electronics compartment from a plenum above the absorbent canister. The solenoid and sensors were mounted on the opposite side of this transverse bulkhead, thus everything electrical other than the wrist display was immediately adjacent to each other.

In the units I made, all of the electronic components were on a printed circuit board. After assembly, the boards were coated with a spray-on waterproofing compound as is widely used for marine electronics. At Beckman, the components were assembled into 4 micro-welded, epoxy potted modules which plugged into gold plated sockets on the circuit board. In theory, this is a better way to go, but in practice, it didn’t make any noticeable difference.

With respect to the reliability of electronics in this kind of application, recently someone posed the question, “When was the last time your TV failed?” to which Robert made the wonderful reply, “The last time I took the bastard underwater.” Both comments reflect important points. Electronics in themselves can be extremely reliable. In terms of MTBF, they are far more reliable than most mechanical devices. Enough so that they can be trusted for things like passenger aircraft control systems where thousands of systems are in everyday use and a single failure means the loss of hundreds of lives. But Robert is right too. If you flood them with water, they fail.

The problem then is really a mechanical one. Can electronics be reliably enclosed so as to prevent flooding in underwater use? If it were solely a matter of constructing a watertight pressure-proof housing for the electronics, that alone wouldn’t be too hard. Unfortunately there is also the matter of connections for sensors, displays, a solenoid, and a switch, plus keeping all these external devices themselves dry.  The possibilities for leaks begin to multiply. With a great deal of care in construction and use, high reliability is achievable, but I think there is a much easier way to reliably keep out the water.

Wrist display of three edgewise panel meters.  Construction is teflon coated cast aluminum with o-ring sealed 1/4″ acrylic face plate held in place by a large circlip.

The key to the solution is pressure. Keeping things watertight under 100-200 psi is difficult. Doing it under 0.5-1 psi is easy. In the Electrolung everything was at ambient pressure. The electronics compartment was vented via a small canister of silica gel with the rest of the system. A standpipe for the vent orifice prevented any accumulated moisture in the canister plenum from being pushed into the electronics compartment. In anything but a head-down position, the electronics were above the counterlung; thus, any leak would normally result in gas escaping rather than water coming in. In practice, with the kinds of seals involved and the very low pressure differentials, leakage anywhere in the electronics section was never a problem.

Humidity and condensation were also not problematic. Plastic construction probably helped in avoiding the latter, and the waterproof coating seemed to be quite sufficient for the former, as is well attested by a wide array of complex devices and vast usage experience in the marine electronics industry.

The only practical way to get your TV underwater with this type of system is to flood the entire system. This is inherently no more likely nor any more or less disastrous than it would be with any other rebreather, regardless of type.

Gas Supply

Two, 9 cf at 2100 psi (1.75 ltr @ 145 bar) steel gas cylinders were used for O2 and inert gas. These were lightweight models FAA certified for use in aircraft. Initially we used chrome plating to protect them; later we went to teflon coating. Beckman liked the more military look, and there was some concern over possible hydrogen embrittlement from the chroming process.

Standard, old style, “K” valves were used as cylinder valves. On the inert side, a SCUBA regulator-type yoke was used to mount a high pressure 1/8″ NPT needle valve operated by rotary action of a T-shaped handle. Inert gas was valved in manually directly from the tank as needed using this valve. In use, it had a very smooth precise feel. Inert gas was valved into the plenum at the bottom of the absorbent canister so that some mixing would take place before it got to the sensors.

Jon Kanwisher wearing the Electrolung
 

On the O2 side, a piston-type first stage of a U.S. Diver’s single hose regulator was used to reduce tank pressure to about 60 psi. This is somewhat lower than such first stages normally delivered and was achieved by using a weaker piston spring. The normal hose to the second stage was used to connect the O2 supply to the solenoid valve. The octopus port of the first stage was used to attach an O2 bypass valve. This was a spring action, lever activated low pressure valve and it was protected by an enclosure which required opening a spring closed cover to get at the valve. The manual bypass valved O2 directly into the sensor compartment, so the result was immediately readable.

I will digress briefly on O2. In addition to the physiological risks recently discussed in some detail on the list, there is also the danger of fire and explosion. Valves, regulators, fittings, and any other equipment used for O2 have to be thoroughly degreased of any petroleum based lubricants. If lubrication is required, as for example with the o-ring seal of a regulator piston, non-combustible silicone based lubricants must be used. Be aware that even a fingerprint oily with suntan lotion can start an explosive fire with O2. Once an O2 fire starts, all sorts of things you might not ordinarily think of as combustible burn ferociously. I have heard stories of chamber fires in which everything inside, including the occupants, was reduced to ash.

My partner Kanwisher was on one of the advisory panels to National Aeronautics and Space Administration (NASA) in connection with the Apollo program. Although he recommended using a mixed gas atmosphere in the Apollo capsule, he was overridden by the engineers who felt that monitoring the PPO2 was too difficult. John knew better, as he had been doing it for several years in conjunction with his work on respiration, but the engineers prevailed. The result was the fire that killed three astronauts.

The solenoid valve we used was a miniature 12-volt one made for pneumatic control. We equipped it with a miniature screw-adjusted needle valve outlet. When the setpoint is reached and the solenoid is triggered, it takes perhaps three or four seconds for the sensors to respond and rise enough to cut it off again. The solenoid needle valve was adjusted so that the O2 injected raised the PPO2 to a peak pulse of about 0.75 atm and would usually trigger a couple of beeps from the audible alarm. Within a couple of breaths, mixing brought the level back to perhaps 0.65, after which it dropped more slowly, as it was consumed by metabolism until the setpoint was reached again after about a minute or so. That would be for moderate activity such as easy swimming. At complete rest, it would of course take longer to drop back to the set point and less time if you were actively swimming.

If the needle valve was adjusted to a lower flow rate, solenoid activation would be more frequent and of longer duration, placing an unnecessary drain on the solenoid batteries. If much higher flow was adjusted for the O2 spikes would be too high, and the alarm would be sounding much of the time. I think there are now smaller, more power-efficient solenoid valves available.

The solenoid and manual bypass valves were of the downstream type so that if high pressure leakage from the regulator occurred, it would release when it reached the level where it overcame the spring tension which normally closed the valve. This is important to prevent either valve lockup or blowing out the supply hose in the event of a high-3wqaz¸pressure leak. In the event of O2 leakage from either valve, the cylinder valve could be used to cut it off.

High pressure needle valve for control of inert gas supply.

Oxygen regulator and bypass valve.  Spring loaded cover over bypass valve actuator is being held up.

The gas cylinders were mounted on either side of the central larger cylinder containing the absorbent canister and electronics section. This assembly was worn as a backpack with the valves at the bottom at hip level. Inert gas had to be added several times on descent and at other times if you lost any from nasal exhalation. Manual O2 was normally only used in decompression. The inert gas valve was therefore on the diver’s left side leaving the right hand free for more complex tasks. Swapping sides for southpaws would have been easy, but I don’t recall anyone ever raising the question. It was no big thing either way.

The Breathing Circuit

The Electrolung mouthpiece was originally constructed of PVC. The main body of the mouthpiece assembly was made in three parts glued with PVC solvent cement. At Beckman, this part was manufactured as a teflon-coated aluminum investment casting which made a nicer looking part. A rotating drum-type valve operated by a small lever permitted closure of the mouthpiece if you wanted to take it out of the mouth underwater. Two check valves directed gas flow to the proper hoses for inhale and exhale cycles.

Mouthpiece valve assembly viewed from front.  Mouthpiece mounts at top back.  Close-off is actuated by lifting the lever at right.  Bottom hose goes to counterlung.  Right and left hoses go to and from the absorbent canister.

Three breathing hoses were attached to the mouthpiece. One went straight down to the counterlung which was worn on the chest. The other two were routed to either side over the shoulders to the inlet and outlet of the absorbent canister/electronics section which was worn on the back. Initially, hoses from the old style twin hose scuba regulators were used. At Beckman we found a manufacturer in L.A. who made high quality fiber reinforced hoses for the O2 breathing systems in military aircraft. The price was only a little more than the scuba hose, and they could be made to order in small quantities in various lengths. Scuba hoses tended to start to leak after a year or two. The others are still usable after a quarter century in tropical conditions.

Hose clamps may seem mundane but are worth considering. A hose coming off with scuba is an inconvenience. With an RB, it is a life-threatening disaster. Initially, we used the spring steel ratchet-type clamps used on scuba regulators, but I didn’t fully trust them, and they rusted. We tried chrome plating them and found they would then break unexpectedly due to hydrogen embrittlement. In the end, we went to good quality heavy duty SS worm screw type clamps. Some SS clamps have ordinary mild steel worm screws which rust, electrolyze, and break after a while in marine use. Lightweight SS clamps can also break for no apparent reason due to stress fatigue. SS is prone to this. SS fitting used in sailboat rigging are notorious for letting go at the most inopportune times.

Mouthpiece parts. Body is teflon coated cast aluminum, other parts PVC.   Threaded parts are sealed with non-acidic silicone sealant.  Inlet check valve is in the PVC part at left.  Outlet check valve is inside the body just above the hose connection at middle right.  Close-off valve is at top.

A lot of people seem to now use the nylon ratchet type ties for clamping hoses, but I wouldn’t trust these either for critical applications. Nylon is subject to cold flow under stress, and after a while they become looser. Using them as a backup next to a good SS clamp however, might not be a bad idea.

The counterlung was a bag made of two pieces of plastic material electronically welded together over a 1/4″ surface around the entire perimeter about 1/2″ in from the edge which was then sewn together and wrapped in a heavy edging tape. Originally, I used clear vinyl for the material. This worked well enough, but later I found a lighter, more flexible, translucent fiber reinforced plastic material which was better.

Being clear or translucent offers two advantages. It lets light in and so reduces the growth of microorganisms. It also makes it easy to see if any water has accumulated and get rid of it before it gets to a level where it might be drawn into the absorbent canister. Places which make awnings, boat covers, etc. or those who make plastic zipper bags, folders, and the like can easily and inexpensively do this kind of work. All you need is a paper pattern of what you want. One-offs for prototyping are not expensive and with a $100 or so for a die for the welding, they can pop out small quantity runs dirt cheap. Usually they also have samples and catologues of all sorts of material to choose from. Teflon coated nylon is now available and might be very good for this application. I mention all this because some homebuilders may be interested.



The counterlung had a single hose attached about 1/3 of the way up from the bottom on the front (away from the body) side. A drain plug was at the bottom near the lower left corner. At Beckman we added an overpressure relief valve near the top. The fittings all used a flange and threaded collar-type attachment similar to a kitchen sink drain. The flange incorporated a groove and o-ring in its face which ensured a firm grip and seal with the counterlung material. The fittings were machined from PVC, except for the small drain fitting, which was SS or chromed brass with a 1/4″ plug on a short lanyard. It was basically the same as a control gland used in underwater camera housings.

The overpressure relief valve released at somewhere around 0.75 psi. Its practical use was only the prevention of possible counterlung rupture if gas was accidentally valved in with the mouthpiece shut-off valve closed. It was introduced at the suggestion of experienced OC divers who, not being used to getting rid of excess gas via the nose during ascent, tried to exhale against a full counterlung and couldn’t.

Counterlung.  Hose connection is near center, drain at lower right.   Grommets for attachment at corners.  Material is a fiber reinforced translucent plastic.  Probably vinyl.

The counterlung was attached by grommets at each corner which mated with twist studs mounted on the shoulder straps at the top and on short adjustable straps paralleling the backpack waist strap on each side at the bottom. The counterlung volume we used was about 4 L.

There has been some discussion on the list recently re: the relative merits of chest mounted (resistance on exhale) vs. back mounted (resistance on inhale) counterlungs. In an earlier post, which has been quoted in the recent discussion, I opted for chest mounting as preferable because the mechanics of breathing musculature is such that the power available for exhalation is greater than that for inhalation. The counter argument is that resistance on exhale reduces the volume of exhaled gas leading to CO2 retention.

First, we need to keep in mind all this is somewhat hypothetical, and in the real world both configurations have been used successfully. With sustained high level exertion where any advantage might be important (and in the continuing absence of any proper comparative testing) I would opt for the chest mount for two reasons: It’s less tiring to put a bit of extra effort into exhaling than it is into inhaling, and the bottom line at the extreme for ventilation lies with how much gas you can move in and out in a given time. Given equal resistance in either direction, the more powerful exhalation cycle will move the greater amount of gas. In the end, over a few breaths, expiration and inspiration must be equal. Restriction of either sets the limit, so if there has to be a restriction I would rather it be on the side which can best handle it.

We sold a number of Electrolungs to commando-type users. One of their prime concerns was breathing resistance in sustained hard swimming. After trying it, all gave it their thumbs up in this respect. In out-of-water chamber tests at 1000 fsw/306 msw pressure, breathing resistance during exercise was encountered. This was in the breathing circuit itself and could of course be relieved somewhat by bigger hoses, larger absorbent bed cross section, etc., but as the market for that capability was effectively nil, it was never pursued.

Counterlung details.  Bar on inner collar of hose fitting is to prevent  possible stoppage of gas flow by the opposite side of the counterlung covering the opening while gas still remained in the upper part of the bag.  Note the welded seal near the edge of the bag.  All hose fittings on the Electrolung had a groove like this one.  The groove is beneath where the hose clamp goes, and hose material is squeezed into the groove by the clamp making for a much more secure attachment than a smooth surface.

One memorable experience with the commando types took place in the Bahamas. A British Royal Marine Commando attached to the Canadian Navy flew down to join me on my vessel and try out the Electrolung. He was a big, bullet-headed guy, built like a fridge with a head. After a couple days’ instruction diving, he wanted to do a long, hard swim with it, and as there was no one else to do it with him, I ended up going along. He went for a couple of miles virtually, flat out. Luckily the water was crystal clear, so I managed to at least keep him in sight. When we came to the surface, the bastard wasn’t even winded. He was satisfied it could do the job and just wanted a smoke. It was flat calm, and the water was only about 30 feet deep, so I had a skiff following us with his smokes and we could ride back.

Although I still don’t know for absolute certainty whether chest or back mount is optimal, I do know that chest mount is good enough. What I really do like about it is that it is easy to see if there is any water in it and easy to pull the plug, squeeze the bag, and expel it.

We did have a couple of experienced open circuit (OC) divers, new to the Electrolung, let water leak in around their mouths until it was gurgling away with each breath. They continued until they had largely flooded the absorbent canister and eventually got a mouthful of absorbent cocktail. They were quite irate about all this and swore it was the fault of the Electrolung. This kind of thing is a recurrent problem with rebreathers (RBs). Experienced OC divers have habits which don’t go with RBs. They also tend to think of themselves as expert divers rather than as novice RB users. As a result, they often don’t really listen and don’t take advice well, and they tend to blame the device if anything is not right, rather than realizing that they have to learn to use it right.

The actual breathing circuit for the Electrolung was: Exhale directly to counterlung via bottom mouthpiece hose. Inhale draws gas from counterlung back out the same hose into the left mouthpiece hose then to the bottom of the canister via a central tube inside it. At the bottom, the gas emerges into a plenum which distributes it over the inlet surface of the absorbent column. After passing through the absorbent, it emerges at the top into the chamber where the sensors and solenoid are located. From here, it continues via the right mouthpiece hose into the mouthpiece itself.

The Absorbent Canister/Electronics Housing

In the Electrolung the CO2 absorbent and electronics were housed in a transparent acrylic (Plexiglas) cylinder which, together with the gas bottles, was worn as a backpack. This cylinder was 24″ long x 4″ I.D. with 1/4″ thick walls. It was divided into two pieces. An 18″ section contained the absorbent bed with a 4″ space at the top which accommodated the sensors and O2 solenoid. Above this, a further 6″ section separated by a bulkhead contained the electronics. A small knob on the electronics section operated a double throw, center off switch controlling power to the electronics via either set of batteries. This was reachable behind the head if development of your biceps didn’t prevent it. Actually, geeks do make the best rebreather divers anyway, but out of respect for the temporary cease fire, I will refrain from further comment along that line.

Backpack viewed from behind.  Inert gas cylinder on left, O2 on right, absorbent canister between.  Electronics section at top.  Breathing hoses attach on the opposite side just below the electronics bulkhead.
Bottom of absorbent canister.  Note spring loaded bulkheads and screen to retain absorbent, central gas inlet tube from above and tie rod which threads into top of electronics section holding everything together.

The ends were sealed by 1/2″ thick PVC O-ring sealed plug-type closures. A thick PVC double plug-type bulkhead joined and partitioned off the electronics section from the absorbent section. The entire assembly was held rigidly together by a central 1/4 SS tie rod running from top to bottom. It had a large external knob at the bottom for tightening and loosening which was affected by screwing into a metal socket on the top closure. O-ring seals were used to seal the tie rod penetration of the bottom closure, the metal tie rod socket on the top closure and where the tie rod passed through the electronics bulkhead. This last was only to prevent capillary action from possibly drawing any water from the absorbent section into the electronics section. The electronics section atmosphere was vented to the absorbent section through a small canister of silica gel via a separate bulkhead penetration and a small standpipe as mentioned earlier in the description of the electronics.

Top of absorbent canister looking into space where sensors and solenoid fit when assembled.  Inhaled gas comes in on right, down the central tube to the bottom, then back through the absorbent, past the sensors, and out to the mouthpiece via the opening on the left.  Note top absorbent bulkhead in background and threaded top end of central tie rod in foreground. The shoulder in main cylinder is because acrylic tubing varies somewhat in size and we machined it out to a standard I.D. so the electronics bulkheads and end closures could be made to a standard size.

The O-ring seals for the end closures and the join at the electronics bulkhead were all radial type seals which automatically affect a proper seal when they are plugged into the cylinder. Sealing is effectively independent of how tightly or loosely things are clamped together. With this type of seal and operating at ambient pressure, the possibility of leakage around a seal is vanishingly small.

Front side of Electrolung backpack showing shoulder and waist strap arrangement.  Adjustable tabs carrying the twist studs for attaching the counterlung at the shoulders and waist provided for its positioning adjustment.

A brief aside for homebuilders: Although O-rings are marvellously effective seals and are universally used in all types of underwater equipment, it is remarkable how often manufacturers use them improperly. O-ring suppliers have various free pamphlets and data sheets on proper application of O-rings which includes data on the correct shape and tolerances for the grooves which accommodate them. It is well worthwhile to avail yourself of this information.

Common errors in O-ring usage often seen in marine equipment are: Grooves too deep, resulting in inadequate sealing pressure. Too shallow, resulting in too much compression of the seal, leading eventually to fine, radial cracking of the O-ring itself and consequent leaking. Too narrow, which interferes with proper compression and sealing at low compression and distortion toward a square cross section under full compression, this leading again to radial cracking of the seal. Finally, and most ignorant of all, is the use of rounded U-shaped grooves which defeats the whole principle and advantages of the circular cross section and turns it effectively into a flat gasket.

The absorbent canister portion of the cylinder was a 12″ section toward the bottom defined by two 1/4″ thick acrylic or PVC internal bulkheads perforated with an array of holes. Plastic screen was used to keep the absorbent from falling through the holes. The top bulkhead was fixed in place. The bottom one was free to move but held in place against the absorbent column by a large spring. This served to keep the absorbent compacted without channelling despite any minor settling of the granules after filling the canister.

We used Baralyme as an absorbent. This is a National Cylinder Gas trade name for Barium Hydroxide. It was widely used in hospitals and came in hermetically sealed one quart cartons of the type used for milk. The Electrolung canister held two cartons which would be sufficient for six hours of moderate activity. We changed them after four hours. Baralyme came with a color indicator—pink when fresh, blue when expired. It is less caustic than soda lime and worked well for us.

The author in the Electrolung showing the counterlung

As described earlier, gas was drawn from the counterlung to the space at the bottom of the canister down a central 1″ I.D. tube leading from the inlet hose attachment down to the bottom end of the canister section. From there it passed back up through the absorbent into the sensor/solenoid chamber and on via the inhale hose to the mouthpiece.

The gas supply cylinders were mounted on either side of the absorbent canister/electronics cylinder using spacer blocks conforming to the curvature of the respective cylinders. The three cylinders were secured rigidly in place by two large SS hose clamps. One was adequate for the purpose, so there was backup in the event of one breaking. The spacer blocks also served as attachment points for the harness, which was a U.S. Divers, one of the types widely used before B.C.’s took over this function. There was a wide vinyl strap for each shoulder plus a waist strap. The overall configuration of the Electrolung backpack was similar in many respects to that of the small triple tank OC rigs favored by the French at that time. It rode well on the back and was quite comfortable. All up, the weight of the Electrolung was about 30 pounds/14 kg.

Some people expressed concern about the use of acrylic, fearing the possibility of breakage. This is one of those things which is more apparent than real. In this case, it is protected on one face by the wearer’s body, on either side by steel gas cylinders, and at top and bottom by thick PVC ends. The only real exposure to any possible impact was the curved surface of a 4 1/2′ O.D. ¼”- thick cylinder, which would be extremely hard to break. We did look at using polycarbonate (Lexan) which is literally bullet proof but found it crazed and crumbled into small pieces when exposed to hydroxides. 

It would of course be easy to make the whole thing out of PVC, but I feel the advantage in being able to see condensation, water, and the condition of your absorbent more than outweighs the non-problem of smashing heavily into things while going backwards. Beckman offered a fiberglass fairing for those who might be concerned with this, and of course some then bought it because they liked the way it looked and others did so because they felt that if it was offered they probably should get the complete setup.

Note the absence of open circuit bailout. Limited open circuit bailout was available by manual O2 and inert gas control valves combined with use of a 50% Helium 50% air mix [trimix 10.5/50] for inert gas. This provided manual control of a breathable inert mix permitting ascent to a depth of 50 ft/15m above which manual control of oxygen could be used.—W.S.

A final aside for homebuilders: The Electrolung was really a homebuilt which became a commercial product. It was built entirely with a drill press, lathe, and jigsaw, plus a bench grinder for shaping and sharpening lathe tools as the only power tools. For anyone attempting to build any kind of underwater equipment, a metal lathe is really a must. You can easily make all kinds of cylindrical housings, O-ring sealed fittings, ports and closures, and any kind of threaded fitting you might need with one. 

A small lathe with a five or six inch swing over the bed will enable you to make housings and ports up to 10-12″ in diameter. Good quality, Chinese-made lathes suitable for this kind of work are now available for about US$1500. For another few hundred dollars, you can also get a milling attachment as well, which is a useful addition. Teaching yourself how to use it is not hard. Good textbooks which cover this kind of machine tool work are readily available and easy to follow.

Here is the Electrolung patent. It contains much more detail including various drawings.



Reflections and Speculations

Although development of the Electrolung was interesting, even exciting, in itself it was just an interesting incident in a bigger, far more interesting, and significant picture. Like most historical events, I suppose, what was happening to the participants at the time didn’t appear to be so remarkable as it later does with the broader perspective of hindsight. The larger perspective on what is taking place right now tends to be somewhat obscured by the ordinary events of living. Except for rare instances, whatever we are doing, however interesting and exciting it may be, tends to still feel like life, not like history in the making.

In retrospect however, I have come to realize that from the mid 1950s through the mid 70s, something really remarkable was taking place in diving. During that period, diving grew from the obsession of a small group of generally impecunious young people mostly in Florida, California, France and Italy to a global industry catering to well-to-do hobbyists. Remote tropical islands all over the world began to sprout airports and dive operations and diving became strongly oriented to travel to exotic locations. Though all this was in itself remarkable, something truly unique was at the real heart of what was happening.

For the first time in history, humans could freely enter, explore and personally experience the oldest, richest, most beautiful and exotic communities in nature, tropical coral reefs. Coral reefs are truly remarkable places. Nowhere else can one experience such an abundance and diversity of life. Nowhere else is it so colorful, exotic and so easily experienced at close range.

Diver with Electrolung.  Note pink Baralyme absorbent.  The acrylic bubble was being used for a shark cage.

Diving on a reef is like a trip in a time machine to a time before humans existed and nature ruled in primeval pristine abundance. Fossils of many reef creatures from as much as 60 million years ago are essentially the same as those on reefs now. In fact, some Pacific reefs have existed as reefs for that period of time.

For a biologist, being among the first to dive on reefs was a most extraordinary experience. In a way, it was a bit like landing on another planet. On nearly every dive, we were going where no human had ever been before. The discovery of phenomena of life, as well as strange and beautiful creatures whose existence we never even suspected was an everyday occurrence. At the time this kind of experience was so commonplace, tropic seas so vast and remote, and so few people were doing it, that it began to seem as if this was just the way things were, and this kind of experience would continue on indefinitely.

Already, however, this era has become history. Although there are vast amounts still to be discovered about the details and inner workings of reefs, still undiscovered species are getting harder and harder to find and remote locations are becoming less and less remote. The experience of being among the first to explore the richest realm of nature has come and gone, not to be repeated.

On reefs, one niche still remains. Actually it is a really big one. The zone below 200 ft/61 m, down to the deepest limits of what you might call part of the reef community at about 600 ft/184m, is still largely unexplored. Although it is not as rich in life as the shallower regions, it is still incredibly rich in life and is an area about which we know very little.

As far as I am aware (in 1998), the only person on the planet regularly exploring this zone is icythologist Richard Pyle. What he is doing is really exceptional, and he is doing it essentially on his own. While discussion on the Rebreather List is largely restricted to the technology and physiology as an end in itself, what Rich is doing goes well beyond this. As well as making more deep free dives than anyone ever has before, he is coming back with knowledge and specimens from every dive. What he is doing is a permanent contribution to knowledge which will stand long after any of today’s diving records are broken and forgotten. I have never met him personally, and am commenting only out of recognition of something exceptional.

Over the past 25 or 30 years advances in diving technology have been almost entirely small and incremental. The only real exception I can think of is the development of dive computers. It appears we are up against the realities of human physiology. With every increase in depth and bottom time, the cost, complexity, effort, and risk increases exponentially while the return of useful achievement remains more or less linearly related to bottom time.

The future, it seems, lies in other directions—especially robotics. Here the advances have been impressive, and future development promises to become even more so. Already we are at a point where more and more functions which previously required a diver can be effectively carried out by ROV’s. It is not hard to foresee that in a few years most of what we do at great effort and risk by diving can and will be done by nerds at consoles. In fact, right now in the Gulf of Mexico, Shell and BP are drilling in 5000-6000 fsw/1532-1839m and all work at the wellhead is done by ROV’S.

If you find this kind of scenario uncomfortable, don’t let it worry you. Long term prediction, no matter how well reasoned and seemingly inescapable, has a way of almost always being wrong. So much so that I have often wondered if beneath the facade of Newtonian certainty of our universe, somewhere in the iffy probabilistic realm of quantum mechanics, there is not a relationship which dictates that the very act of prediction sets in motion forces which generate a different outcome. So, if you don’t agree with my prediction, the good news is that I may well have avoided it by predicting it.

Fortunately, the real outcome is usually even more interesting than any of the predictions.

How  did Starck and his colleagues manage their Electrolung decompression? Stay tuned for Starck’s follow-up story: Decompressing on The Electrolung in a coming issue of InDepth.

Additional Resources:

Alert Diver (2017): Oxygen Sensing in Rebreather Diving by Michael Menduno

Popular Mechanics (July 1965): Half A Mile Down With Scuba by C.P. Gilmore

Reprinted with permission from the Historical Diving Society USA, The Journal of Diving History Volume 23 #85 4Q 2015.

SDM’s Publisher/editor Paul Tzimoulis’s account of diving to 300 ft/92m on the Electrolung (1970)
A review of the developments before the Electrolung including the work of Alan Krasberg

To become a member and access our treasure trove of dive history, visit us at: www.HDS.org. We are also on Facebook: Historical Diving Society USA

Download aquaCORPS Journal #7 C2 (DEC93): aquaCORPS’ rebreather issue See “Electrolung,” by Walter Stark p. 6 & 8. Proudly sponsored by Divesoft (Original magazine with two Divesoft inserts).


Walter Starck is one of the pioneers in the scientific investigation of coral reefs. He grew up in the Florida Keys and received a PhD in marine science from the University of Miami in 1964. Since 1978, his home has been in north Queensland, Australia. Throughout his career in marine biology, participating in expeditions around the world,  Dr. Starck has been extensively involved with development of the technology required to facilitate his activities. In several instances patented inventions and commercial products have resulted. In addition to the optical dome port and the Electrolung other noteworthy achievements in this area have been: The Bang stick, a hermetically sealed underwater firearm for hunting and defense. Underwater housings for numerous cameras and instruments. Underwater lighting systems. A multipurpose commercial waterproof electrical connector.  Design of the unique research vessel El Torito, a 9 meter high-speed diving launch, a 24 passenger eco-tourism vessel, and the Oceanic 8000 Longboat.  The longboat was a long narrow high efficiency powerboat inspired by the efficiency of the log canoes of the Solomon Islands.  He has also built and flown an amphibious aircraft of advanced canard wing design using high technology composite materials. Recently (Aug 2017) he was senior author on an extensive update on the Alligator Reef study that brought the total species list for that locality up to 618 species.

Dr. Starck has authored over 100 articles and books, which include numerous technical and peer reviewed scientific studies as well as many articles in leading popular publications. His photography has been widely published in conjunction with his writing, and he has produced nearly 20 films and videos. Throughout his extensive career, he has managed to inspire not only admiration, but also the ire of some detractors who have taken umbrage at his efforts to inject what he believes to be “a rational perspective on human ecology into the eco-mania that has become epidemic in our struggling Western economies.” His criticisms of the “poor science and blatantly false claims widely used to support various environmental agendas” have earned him some criticism.

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