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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.

Dive Deeper

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.

Equipment

Decompression Habitats Are Ascendent

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

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By Andy Pitkin

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

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

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

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

Advantages Of Habitats

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

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

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

Securing the Habitat

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

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

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

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

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

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

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

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

The Use of Containers

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

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

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

Breathing Gas

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

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

Adding Oxygen

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

Removing CO2

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

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

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

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

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

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

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

Monitoring Your Gas

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

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

Ensuring Diver Safety

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

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

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

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

Communications

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

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

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

Is There A Deco Habitat in Your Future?

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

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

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

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

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

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References

[1] Blue Lake: the habitat. 

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

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

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

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


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

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

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

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