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Building A Fault Tolerant Rebreather: Our Path to Simplicity

Divesoft’s factory instructor trainer Jakub Šimánek presents the design philosophy and considerations behind the creation of its family of Liberty rebreathers, which feature fully-redundant electronics that keep the rebreather operating despite one or more electronic failures.

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by Jakub Šimánek
Photos courtesy of Divesoft unless otherwise noted. Illustrations by Aleš Procháska. Header photo by Martin Strmiska.

Full Disclosure: Divesoft is a sponsor of InDepth.

With open circuit diving, there is a general consensus that what is simple is safer and more functional, and the community has adopted simple, yet sufficiently redundant configurations. However, as rebreathers have come to the fore, it is time to ask what is necessary and simplest, and what can no longer be simplified.

The simplest way is not always the best way

The simplest, functionally ingenious, and—in principle—trouble-free rebreather is undoubtedly the oxygen rebreather. Simplicity itself. Breathing bag, CO2 absorber, oxygen bottle, oxygen regulator, breathing hose, and directional mouthpiece is all you need. I wish this was the final solution! Unfortunately, as we know, with only oxygen we can not safely dive very deep and it is necessary to complicate the device quite a bit to do so. 

Photo by Martin Strmiska.

Rebreathers have been developed through semi-closed, electronic closed circuit (eCCR), manual closed circuit (mCCR, and passive semi-closed (PSCR). We already know that if we want a safe device that allows us to dive deep, we cannot do without electronics, because all mechanical solutions have their limitations. SCR wastes too much gas, PSCR is much more economical, but decompression is not ideal. Manual CCR is as powerful as electronically controlled closed circuit in terms of gas savings and decompression efficiency, but has depth limitations due to the blocked reference ports in the first stage regulator, which sense ambient pressure. As a result the intermediate and ultimately the low pressure delivered by the regulator does not increase with depth. 

While an open circuit regulator valve delivers a fixed pressure of gas above ambient pressure, a mCCR constant flow valve, aka a leaky or trickle valve or fixed orifice, delivers a fixed pressure of gas, in this case oxygen, independent of depth. Accordingly, the valve will deliver oxygen until the ambient pressure is equal to the pressure of gas exiting of the flow valve. So, for example, if the flow valve delivers a pressure of 10 bar, the depth will be limited to 90 m/294 ft or 10 bar of pressure ATA.

Photo by Daniel Válek.

This is not the only thing that is problematic on mCCR. mCCRs have been designed with maximum simplicity and with the elimination of “dangerous” electronics—excluding oxygen sensors—in mind. This moves the most critical part, i.e., the control of the partial pressure of oxygen, to the constant flow nozzle with manual addition by the diver. However, the diver is a human who can suffer from bad moods and bad concentration. They can underestimate the situation, have limited ability to concentrate on multiple things at once, especially in ‘critical’ situations and are impacted by limited visibility, cramped space, inert gas effects and great depths etc. 

We want to avoid electronics and take control, but we must acknowledge that the weakest link in the whole chain is ourselves. How does our human error rate compare with electronics? How accurate are our oxygen injection calculations vs the machines? Aren’t human factors the hottest topic of recent times in the diving world? The answers are obvious when we consider that one makes three to six mistakes per hour.1 

Photo by Petr Slezák.

Personally, not counting the testing of pre-production prototypes, I have not had a dive computer fail underwater since 1996. There are no statistics on underwater electronics failure, but I would argue that the ratio of human error rate vs. the error rate of electronic dive computers is quite high. And what is the difference between a dive computer and a rebreather control electronics? Only that the control electronics of the rebreather processes information from multiple sources and has a software algorithm for controlling the solenoid.

Yet some people choose mCCR anyway. Research2 has even shown that people are more willing to trust other people and forgive inevitable human mistakes rather than trusting computers.

Machines can’t think, and we can’t do that for them. We have to think for ourselves, but we can entrust straight forward calculations to computers. They are much better at it than we are.



Important Things Must Be Redundant

Electronic devices have become an integral part of our lives. We wake up with them, we move with them on the ground, in the air, in space, and underwater. We spend the whole day with them, and we fall asleep again with them in the night. Some electronic devices work 24 hours a day, 7 days a week. 

Electronics help us in critical situations. When we are driving a car (ABS and other electronic systems), we let it navigate us from point A to point B. We entrust our lives to it when we travel by airliners, which today cannot function without electronics at all. The development of electronics is constantly advancing, but we must admit that electronics, like any other part of the device, can fail. If it is a mobile phone or a television, it is not usually a tragedy, but if it is a hard drive on which you have your family photos, or the results of your many years of work, for example, or a device on which human life depends, the data or device must have a reliable back up. 

Photo by Daniel Válek.

We all know it from both diving and everyday life, that those who do not back up their data will be sorry when they lose it. Those who do not have a backup plan in diving, a backup source of gas, can lose their lives. We are talking about redundancy.

Redundancy when diving with open circuit means having two first and two second stages, a buddy team, enough gas for the diver and their partner to surface, a buoyancy compensator that is backed up by a dry suit and any measuring device, such as a computer, sufficiently backed up by a second measuring device. So there is partly a kind of team backup. CCR failures cannot neccesarily be solved by team backup; a complete OC bailout ascent should be the last solution when there is no other option left. CCR cannot be backed up by a team member; CCR cannot be shared by two divers. The rebreather must contain its own back up.

Photo by Petr Slezák.

What is a Fault Tolerant System?

A fault tolerant system is a feature that allows a system (often a computer system) to continue to work properly even if one of its components fails. Fault tolerance is desirable for systems with high availability or providing vital functions such as a space shuttle or an aircraft, so why not a rebreather? Life depends on it just as much.

As already mentioned, a rebreather is a complex system. What does such a system consist of? The electronic control unit receives information from several sensors, evaluates the data, and calculates the appropriate next action such as firing the injector, adjusting the decompression calculation, etc. The input systems are as follows: a pressure sensor, oxygen sensor, RTC (real time counter), and possibly additional helium or CO2 sensors. We also need a power source, i.e., a battery, and a user interface in the form of a handset or a simple display of PO2 values. Those are the basics.

Fig. 1 A generic eCCR.

Therefore, in order to appreciate what a fault tolerant rebreather is, let’s first look at a standard eCCR as shown in Fig. 1. The system is very vulnerable. A single error can cause us to either execute special skills or go straight to bailout. Try to imagine a failure of a battery, a single sensor, a solenoid, or a control unit.

The next diagram illustrates a fault tolerant rebreather schema—namely the Divesoft Liberty CCR (Fig.2)

Fig. 2 The Divesoft Liberty Rebreather.

We start the dive with a fully functional rebreather. The loop is not very different from a conventional rebreather, except that all elements are doubled (a manual add valve (MAV) and automatic diluent valve (ADV), an oxygen MAV and two solenoids). In addition, the electronics are doubled: two control units, two main displays, two secondary displays (HUD and buddy display (BD), solenoids, batteries, and all sensors are doubled. These are actually two self-sufficient systems that are interconnected and can communicate with each other.

Let’s phase out the individual components and observe how robust and resilient the fault tolerant system is. Let’s say I just cut off the handset cable in the wreck. Nothing is happening, I still have a second handset that shows me all the data and I am able to control the whole device. In addition, water does not leak into the device, because the cables are protected by watertight partitions on all inputs and outputs.

Photo by Daniel Válek.

Oops, I broke the second handset against a rock. Still, nothing is happening. All the sensors work and I am able to read the ppO2 from the HUD. One of the batteries is exhausted/failed. This means the loss of one CU. But, I still have two oxygen sensors and therefore two O2 sensors, depth sensors, a solenoid. I can still continue on the unit and return safely to the surface without any need to intervene.

Fig. 3.

How To Make A System Fault Tolerant

The fault tolerant rebreather design consists of three basic parts:

1. A robust software solution

2. Hardware redundancy

3. A fault detection system

Photo by Daniel Válek.

Complex software for modern CCRs consists of individual software modules. The software modules (tasks) are: ppO2 measurement, ambient pressure measurement, ppO2 regulation, decompression calculation, and the user interface. Which of these components probably experiences the most errors? Of course, every software engineer sees it right away; it’s the user interface—the most complex part of almost any software. What’s with that?

The solution is simple. We separate the user interface into another hardware-separated unit, i.e., a handset. It communicates with the rest of the system, i.e., with the control unit via the bus, in a fixed, formal, precisely defined manner. In the event of a fault, it only stops working, but the vital core of the system goes on (Fig.4).

Fig.4. Graphics by Aleš Procháska.

Because the control unit (CU) does not include a user interface, it can be programmatically divided into small, simple modules that are easier to verify.

The second principle is hardware redundancy. For hardware redundancy to really work as fault tolerant, it requires a sophisticated system.

Figure 5 shows a primitive rebreather without redundancy. One mistake is enough and it is inoperative (Fig.5).

Fig.5: A control unit, sensor and solenoid

We improve the system by placing three oxygen sensors. In addition to these,  we connect a backup monitoring system, and the battery is doubled (Fig.6).

Fig. 6.

Figure 7 shows another improvement: a separate oxygen sensor for backup. As in the previous case, the backup is short-circuit-proof on the measuring bus (Fig.7), all of which combine to form a fail-safe system. If any one element fails, it may not necessarily remain functional, but we will immediately learn about the problem and go to the bailout.

Fig. 7.

Further improvements: everything is completely doubled. In case of any one fault, the system remains functional (Fig.8). In this case, we can already talk about a fault tolerant system.

Fig. 8.

Third principle: a fault detection system? If I only have two oxygen sensors (Fig. 9) in each part of the system, it is not possible to automatically evaluate which one is defective (but a diver can decide manually because he or she sees all four sensors). How do we solve the problem?

Fig. 9.

Add three plus three sensors. This will, of course, double the cost of sensors. (Fig.10)

Fig. 10.

Alternatively, enable each system to see all four sensors. The problem is that it is not at all easy to prevent a short circuit on one system (ex: flooding with salt water) not to short the other system at the same time (Fig.11).

Fig.11.

Or, simply connect both systems via a bus, through which they can send the measured values from the sensors to each other. The bus is much easier to disable in the event of a short circuit thanks to analog measurement.

Fig.12

And, other sensors (depth, temperature, etc.) are treated in the same way, effectively doubling them.

Fig.13.

Internal complexity does not mean complexity for the user.

If we look at the fault tolerant rebreather scheme, one may think that the system is too complex, but that’s just on the inside. We have already shown that an increased number of components does not lead to a greater risk; on the contrary, in the case of a completely redundant system, it leads to greater safety. 

Externally, on the other hand, the control of such a system is very simple and the user does not feel the internal complexity at all. It’s like driving a modern car full of the latest cutting-edge technology, and all we need to do is step on the gas and turn the steering wheel and enjoy the d(r)ive.

Sources:

  1. Edkins G., Human Factors, Human Error and the role of bad luck in Incident Investigations, May 23th 2016
  2. Andrew Prahl and Lyn M. Van Swol; “The Computer Said I Should: How Does Receiving Advice From a Computer Differ From Receiving Advice From a Human,” presented at the 66th Annual International Communication Association Conference, Fukuoka, Japan, 9-13 June 2016.
  3. Procháska Aleš; Principles of Fault tolerant rebreather“ 2016, powerpoint presentation

Additional Resources:

aquaCORPS #12 Survivors: Designing a Redundant Life -Support System by William C. Stone (1995) 


Jakub Šimánek graduated as a biology and physical education teacher from Charles University in Prague. Thanks to his father, he has been diving since childhood. He transferred his experience from teaching, biology, diving and sports to his instructor activities. Since 2012, he has been working in Divesoft, where he participates in the analysis and development of diving equipment, mainly rebreathers. He has been working as a Factory Instructor Trainer since 2014 and is an author of training procedures for this device. He is currently actively involved in developing and diving with bailout rebreathers.

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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