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Divesoft Is At It Again with New Tools for Tekkies

Czech Republic-based Divesoft upended the helium analysis market with their patented accoustic-based helium/oxygen analyzer, which they launched in 2013. What are they up to now?

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by Michael Menduno

Divesoft was born when Czech Information Technology (IT) entrepreneurs Ales Prochaska and Lucie Šmejkalová, who ran a successful online banking software business for 20 years, decided to forgo the corporate life and apply their considerable expertise to their shared passion for diving. Beginning with an innovative helium-oxygen analyzer, the “He/O2 analyzer,” which uses a patented (2012) acoustic helium sensor that Prochaska created and built for himself and his friends, the pair of brainiacs went on to create the Liberty Rebreather, which is arguably one of the most fault-tolerant systems on the market, along with their line of Freedom dive computers. 

After launching the company in the spring of 2013, Prochaska, Šmejkalová, and their team exhibited at the Diving Equipment & Marketing Association (DEMA) in 2014. I was there and remember the crowd of tech divers huddled around their booth, peppering the Divesoft staff with questions about the unique fault-tolerant equipment on display. I was one of them. 

Divesoft’s first helium-oxygen analyzer, the “He/O2 analyzer”.

Since that time, Divesoft has grown to become a serious contender in the global rebreather market, offering both back mount and sidemount versions of the Liberty. The company has attracted numerous high-profile users like Canadian filmmaker and educator Jill Heinerth; Polish deep cave diving record setter Krzysztof Starnawski, award-winning U.S. cinematographer Becky Kagan Schott, Italian explorer Edoardo Pavia, UK’s man in the Yucatan Steve Bogaerts, and many more. This summer, the innovative equipment makers introduced their “Analyser Solo,” a new lightweight, easy-to-use helium-oxygen analyzer. We caught up with Prochaska over the summer. Here is what the banking-software-guru-turned-tech-diving-inventor had to say. 

InDepth: You describe your He/O2 analyzer as the “foundation stone” for the company. What motivated you to create this analyzer? Of all the diving equipment you could create, why a He/O2 sensor?

Ales Prochaska: When I started diving helium blends, there wasn’t an easy-to-use analyzer that could show me the complete blend composition in a single step. So, I created the analyzer mainly for my own use. When I commissioned the electronics board, I had ten of them made, in case any fellow divers were also in need of an analyzer. Turns out they were, and the whole stock of analyzers sold within two days.

I understand you use a standard electrochemical oxygen sensor in the analyzer. How did you come up with the idea for an acoustic helium sensor versus a chemical sensor? It seems like a brilliant solution and, in retrospect, an obvious approach in some ways. Divers are, of course, very aware of helium’s acoustical properties from listening to our Donald Duck voices!

I researched all the physical properties of helium I could think of. There were many pages to go through. I studied them and tried to imagine a sensor based on each. And the acoustic principle won, being easy to implement and very accurate. 

How does it work?

The helium content is determined based on measuring the speed of sound in the analyzed mix. The speed of sound depends on the content of helium and oxygen, and the temperature of the mix. The dependence of the speed of sound on pressure is small and can be disregarded under normal atmospheric pressure.

Did your educational background help you create the analyzer?

Yes, in the case of the analyzer, I found my previous studies in engineering, electrical engineering, and software engineering to be very useful. Skills from all three fields were needed to design and manufacture the analyzer.

What was your and Lucie’s diving background? Are you both tech divers?

From my first diving course, I realized that technical diving was the direction I wanted to follow and explore intensively. Lucie, on the other hand, is not as attracted to technical diving or caves. She prefers a dive in the warm tropical seas among the coral fish!

How long did it take you to build that first analyzer? 

The first analyzer was sold in 2004, but the development began a little earlier.

And you have the patent. Brilliant! What benefits does the acoustic sensor bestow on users compared to other chemical-based He analyzers?

Yes, the sensor is patented. The acoustic principle used allows fast and efficient measurements, even on flowing gas. It’s also as easy to use as a conventional nitrox analyzer, and I think that was the biggest benefit of our analyzer.

Divesoft’s new analyzer SOLO.

Divesoft just introduced the “Analyzer Solo.” How does it differ from its original He/O2 Analyzer?

The Solo Analyzer is a lighter and more simplified version of the He/O2 Analyzer. However, the original is still produced because it allows the attachment of additional equipment such as a pressure sensor and others alike. The Solo has no plug-in connector.

What came next; the Liberty rebreather or the Freedom dive computers or both? 

They were supposed to launch simultaneously because the computer and its software were developed at the same time as the Liberty control unit (with which it has a common base), but we managed to release the dive computer a little earlier.

Liberty Rebreather by DiveSoft. Photo courtesy of Divesoft.

What year was that?

We launched the company in 2013!

How did you go from a He/O2 analyzer to a rebreather, the flagship of your company? What was your motivation to build a rebreather, or was that always the goal?

The motivation to develop Liberty was similar to that of the analyzer. We wanted to dive with a rebreather, but none of the devices on the market at that time had all of the features we found important. We had a clear idea of the qualities a rebreather should have and knew we could design it. This was in and of itself a great motivation to try. 

Necessity is the mother of invention! What made you think you could build a rebreather in terms of expertise and experience? How did your earlier work in IT inform your design ideas and implementation?

At first glance it doesn’t appear as such, but the fault-tolerant rebreather is an extensive software masterpiece above all. We had a lot of experience with fault-tolerant systems through our IT and even our underwater backgrounds. At one point, we had developed a software that was supposed to be perfectly resistant to failure. When Prague was hit by the huge flood in 2002, it destroyed the main data center. The system we designed, however, transferred the activity to a backup center and continued working as if nothing had ever happened. Since then, we knew that we wanted to pursue a system that could be sunk underwater deliberately, not just during a flood. 

Wow. Very cool. Clearly “fault-tolerance” is part of the DNA of your rebreather. Was that your starting point, then, for your design? 

Yes, that was a clear goal from the beginning. We knew that other rebreathers were not completely fault-tolerant. Their standard solution was to go into restricted safe mode or report an error and wait for the user to deal with it. Our intention was to develop a rebreather that, in the case of a control electronics failure, anything from discharging the battery to interrupting the solenoid coil, would continue to operate without any restriction. In technical diving, there may be situations where several problems gradually accumulate and prohibit divers from emerging in less than a few hours. In those cases, a rebreather that remains working despite having a defect may be necessary. 

Were dive computers just a logical add-on?

Yes, once we had mastered the rebreather control software, including the decompression model and the waterproof rebreather handset, it would have been a shame not to use it to design an independent diving computer. All we needed was to cut the rebreather cable, refill the battery and the computer was (almost) finished. (Ales smiles.)  

Talk to me a little about having four O2 sensors in the unit? How does that work?

The number of sensors is closely related to the fault-tolerant properties. The standard number of sensors is three, sometimes the ‘3 + 1’ or ‘3 + 2’ arrangement is used, where the added sensors serve as an additional check on the functioning of the three main sensors. But Liberty has two complete, full-featured control units, each with its own sensors. That’s why we used four sensors, because this number is easily divided by two [Ales smiles]. Of course, this does not mean that each control unit works with only two sensors. The units keep communications with one another, and both are aware of the measurements of all four sensors. 

Photo courtesy of Divesoft.

I remember from Rebreather Forum 3.0, Nigel Jones, who worked with Poseidon, said that three sensors in a “voting logic” algorithm do NOT offer true redundancy, and in some cases offers much less redundancy than divers imagine. Does your algorithm offer something stronger? Please explain.

The degree of redundancy depends on the evaluation algorithm i.e. “voting logic.” If properly designed, more sensors will always be more secure. At Liberty, we monitor not only the instantaneous values of the sensors (plus the elimination of the obviously defective, for example, flooded sensors), but also their course over time. We know when the system adds oxygen and how much and the depth and volume of the breathing circuit; from that we can calculate how much the sensor should measure. From the reaction of the other sensors, we can also manually add oxygen or diluent. We also know how and how fast the sensors should respond to depth changes. From all of this, we can identify the faulty sensor and exclude it from the measurements. And, of course, the system always informs the diver of such an event, as they have the final say in how to proceed. 

Fascinating! It sounds like the Liberty uses some of the same principles that Bill Stone used in designing his “Active Validation” approach in Poseidon rebreather, that is comparing instantaneous sensor values with what you expect them to be.

But what Nigel Jones said, of course, still applies. The diver should never assume the control unit will solve everything. The control unit solves only what it is programmed for, so, for example, the quality of the sensors and their replacement must be supervised by the divers themselves. There was a case where a diver had two faulty sensors and one good sensor in a rebreather, and the control unit ignored the values of the correct sensor. The unit decided based on two faulty sensors, which ‘outvoted’ the right sensor.

Why two helium sensors? The FHe is not going to change during the dive, is it?

Helium concentration changes during the dive because the oxygen concentration changes. The more oxygen, the less diluent and thus less helium. This logic could also be reversed and the oxygen concentration could be calculated by measuring the helium concentration (with a known diluent composition). Thus, the helium sensor not only serves for the correct calculation of decompression based on the actual He content, but could be used in emergencies to measure the oxygen concentration under certain conditions. There are two He sensors, with each control unit having its own.

Ah of course. In the loop! Clever! Dual computers as well, right?

Of course, a duplicate computer is necessary for fault-tolerant devices. Each computer has its own battery and is connected to the other only by a data bus. The data bus is, of course, waterproof and short-circuit-proof, so no conceivable failure of one computer could affect the other. 

I believe your back mount CCR came first, yes? What motivated you to create a sidemount unit as well?

Back mount CCR came first because it was and still is the main type of rebreather arrangement. Sidemount was the logical successor of the back mount. It uses the same body and a variety of other parts as the back mount but is horizontally mounted on a special rack. The need for sidemount originally came from cave divers, so we wanted to provide them with the opportunity to use Liberty in this arrangement as well. 

Becky Schott wearing the sidemount Liberty rebreather. Photo by David Schott.

Is a sidemount CCR a specialized niche product, or do you think it will replace or at least equally compete with back mount CCR?

Liberty sidemount is a complete rebreather, including bottles with diluent and oxygen. It can be used not only for cavers, but also as a backup rebreather, handed to another diver without having to switch hoses from off-board gases.

The number of divers who use sidemount as the main unit during diving is increasing. The sidemount’s arrangement has some advantages over the back mount and there are now known and proven procedures as well as technical equipment for diving with sidemount. Its growth is inevitable. While I do not expect the back mount to be replaced altogether, it seems the usage of sidemounts is increasing and many divers will choose it as their main unit.

What would you say are the critical issues right now in CCR diving?

The biggest problem is divers pursuing rebreather diving without adequate training. They see the usage of rebreathers increasing and think that diving with a rebreather is a common thing. But the fact is that diving with a rebreather is still more demanding in terms of skill and self-discipline than an open circuit, and divers who dive with a rebreather without undergoing any type of training are putting themselves at risk.

Would you say CCR diving is getting safer?

It is a little safer, especially since divers, instructors, and rebreather manufacturers are taking lessons from past accidents to try and avoid them. (Similar to the efforts of those in the auto and aviation industries.). 

I see you have just added Synchrony Bank financing option for purchases of Divesoft gear. How did that come about? Has financing been a problem for dive consumers?

Synchrony Bank has been funding our products since the beginning of August solely for the US market where rentals are popular and widespread. We wanted to accommodate customers purchasing our goods by credit card.

Where is Divesoft headed? Do you have a collective vision for the future? What’s on the horizon?

We want to be innovators in the field of equipment for technical divers with a significant overlap in the field of recreational diving, of course. In addition to analyzers, our main domains are still rebreathers and diving computers. In these areas, we are already in development and planning stages for major improvements and innovations. We are currently preparing several innovations to be launched in November 2019 at DEMA, followed by BOOT 2020 in Dusseldorf. Stay tuned for those. [Ales smiles]

Divesoft has been sponsoring the exploration of Hranice Abyss. Are there other projects that Divesoft is sponsoring as well?

We sponsor the Greek Seahorse Rescue Station Hippocampus Marine Institute, as well as Czech police divers during mine clearance operations in the waters of the Sava, Una, and Drina rivers in Bosnia and Herzegovina. Last but not least, the Hranice Abyss.

Thank you so much. You have taken a unique approach, and it’s evident that it is really paying off for you. Congratulations!

My pleasure. 

Read about their team’s exploration of the Hranice Abyss

Additional Resources:

Aleš Prochoaska’s patent: Device for measuring oxygen concentration in gas mixtures containing helium and/or hydrogen


Czech Republic-based Divesoft upended the helium analysis market with their patented sonic-based helium/oxygen analyzer, which they launched in 2013.

Michael Menduno is InDepth’s executive editor and, an award-winning reporter and technologist who has written about diving and diving technology for 30 years. He coined the term “technical diving.” His magazine “aquaCORPS: The Journal for Technical Diving”(1990-1996), helped usher tech diving into mainstream sports diving. He also produced the first Tek, EUROTek, and ASIATek conferences, and organized Rebreather Forums 1.0 and 2.0. Michael received the OZTEKMedia Excellence Award in 2011, the EUROTek Lifetime Achievement Award in 2012, and the TEKDive USA Media Award in 2018.

Additional Resources:

https://www.deeperblue.com/divesoft-unveils-solo-helium-analyzer/

Forbes Magazine article (In Czech):

I bez moře. Manželé, kteří v Roudnici vyrábějí potápěčskou techniku

In English:

“Even without the sea. Spouses who produce diving equipment in Roudnice

Equipment

Will Open Circuit Tech Diving Go the Way of the Dinosaurs?

Closed circuit rebreathers have arguably become the platform of choice for BIG DIVES. So, does it make any sense to continue to train divers to conduct deep, open circuit mix dives? Here physiologist Neal Pollock examines both platforms from an operational and physiological perspective. The results? Deep open circuit dives may well be destined to share the fate of the spinosaurus. Here’s why.

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Text and illustrations by Neal W. Pollock, PhD. Header image: SJ Alice Bennett

Evolution is an important force in both the natural and technological worlds. Fundamentally, new features emerge, compete, and the champions face off against the next challengers. The process can be complicated with technology. New products emerge to a reception ranging from enthusiasm to suspicion; a trial period—often long—results in a consolidation of opinion; and successful products gain an increasing market share, although not for long if unacceptable issues or compelling new challengers emerge.

Compressed gas diving has always been reliant on technology. The critical early steps were the effective storage of a pressurized gas supply. Open circuit diving was facilitated by the creation of demand regulators, with the version developed in 1943 by Emile Gagnon and Jacques Cousteau acknowledged as the milestone of modern development. Open circuit diving technology made its way into the civilian community following World War II, and a series of innovations followed to improve utility and safety. J valves were introduced in 1951, offering a simple strategy to hold some of the gas supply in reserve, but imperfectly since divers could fail to set them, they could be bumped into the off position unknowingly, and the rapid increase in inspiratory resistance when they were working could be stressful. 

Submersible pressure gauges appeared in 1958, providing much more information and increased confidence in supply monitoring. Buoyancy compensators appeared in 1961, reducing weighting concerns and improving surface safety. Automatic drysuit dump valves appeared in the early 1980s, simplifying buoyancy control. The line of advanced capability dive computers began in 1983, providing increased information and computational power to simplify dive planning, monitoring, and logging.

Closed circuit oxygen rebreathers also have a long history, with Henry Fleuss credited for developing the first commercially viable one in 1878. World War II provided the impetus for the creation of an array of new oxygen rebreathers, and a growing recognition of the need for equipment to enable safe diving in the range beyond that possible with oxygen systems. 

Electro-galvanic oxygen sensors were developed in the 1960s, expanding the possibilities of mixed gas rebreathers. The Electrolung rebreather was released commercially in 1969, but a high number of fatalities stopped sales within two years. Development through the 1980s was mostly for extreme use in commercial, military, and specialized applications, including science, cinematography, and exploration. The combination of high cost, high maintenance burden, and high training demands made them most appropriate to military and scientific commitment. 

More affordable and user-friendly technology became available in the late 1990s. Draeger released a semi-closed circuit rebreather in 1995. Semi-closed systems conserve the gas supply by allowing some expired gas to be rebreathed while some is lost overboard. They rely on a single gas supply, and the oxygen fraction varies with ambient pressure as it does with open-circuit systems. This technology will not be considered further here.

Peter Readey was developing the closed circuit Prism in the same mid-1990s timeframe, but the watershed event was the release of the Ambient Pressure Diving Inspiration rebreather in 1997. A review of rebreather use in scientific diving from 1998-2013 indicated that Ambient Pressure systems were used for almost 60% of the 10,200 dives logged on 17 different rebreathers by American Academy of Underwater Sciences members.1

Many of the improvements in control, monitoring, and planning helped divers gain comfort in reaching beyond the traditional limits of recreational diving. Open circuit systems provide an open architecture that can be easily expanded. Independent cylinder/regulator/gauge components can be added to provide various travel, bottom, and decompression mixes. The practical limitation becomes the number and bulk of components that a diver can effectively handle—a number that can increase with training, planning, and practice, but only so far.

Are open circuit tekkies staring extinction in the face?
Are open circuit tekkies staring extinction in the face? Image courtesy of SJ Alice Bennett

The term “technical diving” was coined by then aquaCORPS Journal publisher Michael Menduno in 1991 to reflect the complex equipment configurations and practices evolving in the community to expand the diving range. Most of the early efforts were with open circuit configurations, largely due to availability, reliability, and flexibility of the platform. While complex configurations can test diver limits, managing them effectively can also serve as a marker of achievement that is compelling in its own way. 

Perceived Strengths of Open Circuit Systems

Closed circuit technology is inherently more complex than open circuit technology, but the complexity of units designed for the most extreme exposures can provide a misleading point of reference. The design sophistication, reliability, and simplicity of use has continued to advance, particularly for units designed for less extreme applications. The maintenance and operation burden have been substantially reduced, many high-risk and user error failure points have been engineered out or substantially minimized, and the forgiving nature of the units enhanced. It is harder to put units together incorrectly, component reliability has improved, the work of breathing reduced, and internal backups and checks increased.

Fans of open-circuit technology may value the inherent simplicity, but this is compromised by the number of pieces required to accommodate technical diving. The simplicity of individual components may remain, but the collective complexity can be quite high, and the number of individual high-risk failure points substantial. Differences in points of attachment, materials, marking, and mouthpieces can all help to ensure that a switch is made to the right gas, but the possibility of making errors increases as components are added. Every extra pressure line and o-ring also represents an additional point of potential failure.

Additional cylinders add complexity.
Additional cylinders add complexity. Photo by Derk Remmers.

The cost of closed circuit equipment is a barrier, but this too can be misleading. While the initial cost of rebreathers is high, it should not be compared to that of a basic open circuit system, but to the cost of all of the components needed to achieve the desired, if not comparable, capability. This can include multiple cylinders, regulators, harnesses, manifolds, gauges, and the maintenance burden of all.

Closed circuit systems do require time to properly setup and test equipment pre-dive, and a meaningful share of attention throughout dives for monitoring. However, neither the preparation nor monitoring time is out-of-line with that required for complex open circuit technical setups. The ability to check and rely upon a smaller number of pieces of equipment has advantages, particularly as dives become more demanding.

Advantages of Closed Circuit Systems

Closed circuit technology offers some clear benefits to divers. The most obvious is operating cost. While money will be spent in replacing oxygen cells and carbon dioxide scrubber material, a great deal of money can be saved on breathing gas. Gas consumption during open-circuit breathing increases proportionately as a function of ambient pressure, while gas consumption with closed circuit breathing is unchanged by depth. The cost of compressed air for shallow open circuit dives may not be problematic, but the cost of nitrox is high in some places, and the cost of helium for open circuit mixed gas diving is staggering. Divers operating in the depth range of heliox or trimix can see tremendous cost-savings with rebreathers.

A badass-looking Fathom Mk2.5 CCR diver.
A badass-looking Fathom Mk2.5 CCR diver. Photo by SJ Alice Bennett
Gas use in open-circuit systems increases linearly with ambient pressure; gas use in closed-circuit systems depends on metabolic function, which is largely independent of depth.

One of the challenges in diving is that many of the greatest hazards are invisible. While graphic predictions are sometimes provided by dive computers, divers cannot see their actual inert gas uptake or elimination rates or their proximity to decompression or oxygen toxicity limits. Rebreathers do not change this reality, but they can materially change both patterns and hazards. 

Mixed gas rebreathers continuously monitor, and in the case of electronic systems, automatically regulate oxygen levels in the breathing loop in accordance with the setpoint, which is usually diver-designated. A typical setpoint will moderate inert gas uptake through much of the diving range during the descent and bottom phase, and will dramatically augment inert gas elimination and reduce decompression stress during the ascent phase. 

Cave diver sporting the Dive Rite Choptima.
Cave diver sporting the Dive Rite Choptima. Photo by Fan Ping

For example, a setpoint of 1.3 bar/1.3 atm equates to breathing air at a depth of about 52 meters of seawater (msw)/170 feet of seawater (fsw). Using a rebreather with this setpoint at any point shallower favors decompression safety over open-circuit air breathing. The difference is greatest in the shallowest water, which accelerates inert gas elimination during ascent. Considering air as the diluent gas in a rebreather, at 9 msw/30 fsw the nitrogen content would be 0.61 bar/0.6 atm, less than that breathed in air at sea level. At 3 msw/10 fsw there would be no nitrogen in the breathing mix at all. This compares to a PN2 of 1.02 atm breathing open circuit air, which represents a massively less favorable gradient for eliminating inert gas.

The figure depicts change in the partial pressure of oxygen (PO2) and nitrogen (PN2) in open-circuit (OC) with air and a closed-circuit (CC) system running a setpoint of 1.3 bar/1.3 atm. Closed-circuit systems reduce inert gas at shallow depths to optimize decompression. Inert gas loading will be greater at depths where the oxygen setpoint is less than the partial pressure of oxygen in open-circuit gas.

The oxygen setpoint is chosen to balance the risks of decompression stress and oxygen toxicity.2 Electronic rebreathers make continual adjustments to maintain the setpoint, which can reduce physiological stress. Open circuit gas concentrations vary strictly as a function of ambient pressure, which limits the range through which a given gas mix should be used. 

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Switching breathing gases in open-circuit configurations can control oxygen and inert gas levels, but in a very inefficient manner. The need to limit the number of gas switches means that gas fractions are rarely optimized, and can easily approach or exceed accepted safe limits at least transiently, and potentially much more so if the dive profile does not follow the plan. While the research evidence is understandably limited, gas switches may also increase the risk of oxygen toxicity and inner ear decompression sickness. 

The PO2 seesaw, with low PO2 on the left and high PO2 on the right. High PO2 offers both benefit and risk to divers. A reduction in decompression stress is balanced against an increased risk of oxygen toxicity.

A lesser but still important benefit of closed circuit rebreathers is the fact that the gas breathed is warmed and humidified. The warming, a product of the chemical reaction in the carbon dioxide scrubber, can reduce thermal stress, and the humidification both reduces respiratory heat loss and improves comfort. 

Points of Discussion for Closed Circuit System Use

There are several rebreather-related hazards that are not typical concerns of open-circuit divers. Substantial water volumes entering the breathing loop can react with the carbon dioxide scrubber material to produce a caustic foam that cannot be breathed. If oxygen injection into the loop stops, a hypoxic state can develop. If oxygen injection into the loop continues unchecked, a dangerously hyperoxic state can develop. Engineering has reduced the risk of all of these events. Effective water traps make it less likely for substantial volumes to reach the scrubber, and release valves make it easier to clear water from the loop. Oxygen monitoring and control systems are increasingly resistant to failure and provide continuous real-time information to divers to inform divers. 

While some emergency situations can develop quickly, many problems advance slowly with closed circuit systems, allowing divers time to consider options before taking action. Gas supply efficiencies offer clear advantages over open-circuit systems. Real-time warnings can also provide a cushion. For example, not only can divers see current values at any time, dedicated hypoxia warnings are typically activated at 0.41 bar/0.4 atm, almost twice the normal oxygen concentration breathed. This means that the physiological hazard is still a future event. In many cases, modern rebreathers provide the luxury of time to make necessary corrections or, if appropriate, to bail off of the loop and onto a backup breathing system. 

A Divesoft Liberty diver perusing the reef.
A Divesoft Liberty diver perusing the reef. Photo by Martin Strmiska

System engineering has solved many, but not all, issues with rebreathers. Oxygen monitoring technology is reasonably robust, but imperfect, which demands ongoing attention of divers. Carbon dioxide monitoring is still inadequate. While it is generally more difficult to configure systems incorrectly, divers do need to take responsibility to change scrubber material at appropriate intervals. 

Closed circuit rebreathers provide an array of enabling technologies. The economical gas use can make deeper and longer dives much easier to complete, and technical diving computers provide huge flexibility in dive planning and on-the-fly adjustments in plans. The safe range expansion is not unlimited, however. One critical soft limit results from the fact that the decompression algorithms used for deep exposures are developed as extrapolations3 from shallower computations with little or no physiological testing. Mathematical extrapolations from limited shallow water data are unlikely to provide perfect predictions for deeper exposures. They may be conservative, but they may also be liberal. It is critical to remember that math does not equal physiology—ever. A critical hard limit is work of breathing, which increases with depth and gas density. Recent discussion of gas density issues has increased awareness,4 but more effort is needed to ensure that rebreather divers consistently consider both narcotic potential and gas density in dive planning to choose appropriate gases and depth limits.

Arguments have been made that divers should learn open-circuit technical skills before learning closed circuit technical skills. While there certainly has to be knowledge of open-circuit to manage bailout to open circuit situations, it does not follow that one skill must precede the other. Divers can be trained safely in closed circuit techniques from the outset of their diving. This is similar to drivers learning to drive automatics with no manual transmission experience, or pilots learning precision instrument landing approaches without non-directional beacon approach experience. Learning a wide range of skills can be useful, particularly when it reflects a breadth of experience, but it is more myth than truth to say that training in one mode requires foundations in another for safety.

Where is Rebreather Diving Going?

Rebreathers are not a good choice for all divers. They require care in setup and constant monitoring during use. Divers who are not willing to commit the time and effort should stick to the most uncomplicated open circuit diving. A lack of commitment should also discount open circuit technical diving.

Diving is best when it is conducted smartly and safely. While chasing records will always appeal to some, there is probably a lot more pleasure and productivity to diving within skill and comfort zones that are well within the nominal functionality of any piece of equipment used. Rebreathers can offer substantial benefits in reducing decompression stress throughout what we think of as the normal recreational range. They can be used to expand the dive range more efficiently than can open circuit systems, but not without risk. Distance from the surface is important and increasingly unforgiving. A modest expansion of range can provide the best compromise of new experience and safety.

Divers who wish to prioritize gas supply conservation, decompression stress minimization, operational flexibility, and reliance on a single primary platform (with appropriate bailout capability) may wish to consider closed circuit. Those who like technology and value the insights of tracking their status throughout dives will get an extra bonus. 

Those who want to expand their diving range in depth or time should consider the relative merits of investing in and diving with large amounts of open circuit equipment versus potentially more compact closed circuit systems (again, with appropriate bailout equipment). Open circuit technical diving can allow some expansion of the range over non-technical open circuit diving, but operational demands will quickly force a complexity of setup and management obligations that can be problematic. Open circuit technical diving provided an important stepping stone in the development of our diving range,  and will remain important for uncomplicated recreational range activities, but closed circuit technology offers a tool with benefits in the traditional recreational range and clear superiority in the technical diving realm.

Is deep open circuit tech diving destined to share the fate of the spinosaurus? Complete our short OC vs CCR survey to help us find out.

See companion story: GUE and the Future of Open Circuit Tech Diving by Ashley Stewart

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References

  1. Sellers SH. An overview of rebreathers in scientific diving 1998-2013. In: Pollock NW, Sellers SH, Godfrey JM, eds. Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16-19, 2015 Workshop. Durham, NC; 2016: 5-39.
  2. Pollock NW. Oxygen partial pressure – hazards and safety. In: Cote IM, Verde EA, eds. Diving for Science 2019: Proceedings of the AAUS 38th Scientific Symposium. American Academy of Underwater Sciences: Mobile, AL; 2019: 33-38.
  3. Balestra C, Guerrero F, Theunissen S, et al. Physiology of repeated mixed gas 100-m wreck dives using a closed-circuit rebreather: a field bubble study. Eur J Appl Physiol . 2022;122: 515–522.
  4. Anthony G, Mitchell SJ. Respiratory physiology of rebreather diving. In: Pollock NW, Sellers SH, Godfrey JM, eds. Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16-19, 2015 Workshop. Wrigley Marine Science Center, Catalina Island, CA; 2016; 66-76.

Additional Resources

InDepth: Electrolung: The First Mixed Gas Rebreather Was Available to Sport Divers in 1968 by Walter Starck

aquaCORPS N12: Designing a Redundant Life Support System by William C. Stone (1995) 

InDepth: What Happened to Solid State Oxygen Sensors? by Ashley Stewart

Alert Diver: Do You Know What You’re Breathing? by Michael Menduno

Shearwater Blog: BENEFITS AND HAZARDS OF HIGH OXYGEN PARTIAL PRESSURE


Neal Pollock, PhD, holds a Research Chair in Hyperbaric and Diving Medicine and is an Associate Professor in Kinesiology at Université Laval in Québec, Canada. He was previously Research Director at Divers Alert Network (DAN) in Durham, North Carolina. His academic training is in zoology, exercise physiology and environmental physiology. His research interests focus on human health and safety in extreme environments.

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