By Reilly Fogarty
Header photo by Reilly Fogarty
Be sure to check out the following stories:
InDepth: Increasing The Probability Of Surviving Loss Of Consciousness Underwater When Using A Rebreather by Paul Haynes
InDepth: A Mouthpiece Restraining Strap Just Might Save Your Life by Andrew Fock
We surveyed CCR divers from around the world. Here are the results.
Rebreathers are excellent tools for extending dives and pushing the limits of human exploration. Unfortunately, discussing how best to set up your rebreather in any public forum can push the limits of human patience. The cost, the training, and the experience required to dive a rebreather can lead to strong opinions, and the debates about the commercially available options are endless. In these debates, consideration of important safety features can be lost.
Among these features are mouthpiece retainers sometimes referred to as “gag straps.”.The concept is neither new nor revolutionary—most are simple rubber straps designed to hold the circuit in a diver’s mouth in case of loss of consciousness. Some designs feature a half or a full face mask. The purpose of the feature is to keep water out of the airway of a diver who has lost consciousness. Beyond a few configuration options, there are few differences between most of the units available to recreational divers.
Research has shown that mouthpiece retainers offer a real benefit in terms of safety, so regardless of how you choose to configure your rebreather, a loop-retaining device is crucial.
Understanding what kills rebreather divers is complicated by the small number of participants and the problem of separating anecdotes from statistically relevant data in small incident sample sizes. Estimates of the total number of rebreather divers globally range from 6,000–20,000 divers; neither manufacturers nor training agencies provide their sales or certification numbers to the public. A safe estimate of active rebreather divers is generally considered to be around 14,000.
The military and to a lesser extent the commercial diving community (commercial Saturation divers use rebreathers for bailout) represent other groups of users. Both of these communities use slightly different equipment and adhere to much stricter operational protocols. Military/commercial use and fatality numbers are largely unknown to the public as well, but there are some exceptions that have proven enlightening. Based on incident reports, DAN Annual Diving Report analysis, and a series of DAN Rebreather Forum meetings, the single largest killer of rebreather divers is drowning.
Based on incident reports, DAN Annual Diving Report analysis, and a series of DAN Rebreather Forum meetings, the single largest killer of rebreather divers is drowning.
While this may sound unsurprising or even obvious, it illustrates that what kills rebreather divers is not freak accidents or the increasing number of health issues we see in the larger general population of recreational divers. This shows that rebreather fatalities can largely be attributed to some kind of drowning—generally subsequent to loss of consciousness via hypoxia, hyperoxia or hypercapnia. It’s in addressing this cause of mortality among rebreather divers that diver supply valve (DSV) retainers come into play. These devices aren’t a catch-all designed to save divers from their own mistakes or drown-proof undertrained, would-be explorers. What they do is to provide a measure of safety in loss-of-consciousness events and to measurably decrease fatalities.
These mouthpiece retainers come primarily in two variations. The first is a retainer strap, like a mask strap, that attaches to a rebreather mouthpiece and is positioned behind the head. This strap attaches to a device that will seal around the lips (such as the lip seal on a Drager Safety Strap). The second option is a full face mask designed for CCR use, like the Draeger Panorama, or a partial mask design like the Kirby Morgan M48. There are now several commercially available options for rebreather divers looking for full or partial mask options; as long as the device is intended for rebreather divers and adequately minimizes dead space, it can reasonably be considered for use. Excessive dead space in a full face mask can lead to unintended CO2 retention, which poses a greater hazard to rebreather divers than it does to open-circuit divers.
Both options function similarly, sealing the rebreather circuit to the lips and allowing the diver to continue breathing. It’s important to note that losing control of the circuit while underwater will also result in the loss of loop volume and a commensurate decrease in buoyancy, exacerbating any issues that led to the initial loss of control. This type of compounding incident is common among new rebreather divers and difficult to recover from.
Given the size of the market and the limited availability of military research, it’s not surprising that statistical studies of mouthpiece retainers are difficult to find. While it’s true that case studies abound, and many provide valuable information, the compounding nature of rebreather accidents makes it difficult to determine a single incident catalyst rather than guess at the most likely contributing factors to a fatality. What we know about mouthpiece retainers comes primarily from two studies. The first reviews 54 loss-of-consciousness events in military rebreather diving (Haynes, 2016), while the second reviews 153 diving injuries among French military rebreather divers (Gempp, 2011).
The Haynes paper begins by reviewing the range of issues created by the initial adoption of rebreathers by recreational divers. The early modifications, protocol creations, and fatalities illustrated incident statistics surprisingly close to what we see now, with “inappropriate gas” causing more than half of all rebreather fatalities in Haynes’ data analysis. This category (in this analysis) indicated a rebreather-delivered gas causing hypoxia, hypercapnia, or hyperoxia rather than a tank of mislabeled open-circuit gas. This holds true with what we now know about rebreather fatalities and serves as the motivation for Haynes’ review of mouthpiece retainers to minimize the fatalities caused by the resulting loss of consciousness in these incidents. While the Haynes paper goes on to cite military adaptations of mouthpiece retainers, case reviews, and expert testimonial, the most educational data cited is taken from the Gempp paper.
Descriptive Epidemiology of 153 Diving Injuries with Rebreathers Among French Military Divers from 1979 to 2009 confirms both the Haynes and industry data, with gas toxicity causing 68% of injuries. More importantly for our purposes, it reviews 104 cases of gas toxicity with 54 of those resulting in impairment or loss of consciousness in the water. Of these, the outcome was “always favorable” if the diver could be retrieved to the surface. Among the loss-of-consciousness events, only 3 fatalities were recorded. The paper goes on to state that “gas toxicities are frequently encountered by French military divers using rebreathers” but that the low fatality rate can be attributed to strict safety protocols, specifically the “mouthpiece strap, buddy team with link, and diving instructor with open circuit to lend assistance if necessary during training” (Gempp, 2011).
Both papers are clear about the use of mouthpiece retainers saving lives among the population studied. As with any research, it’s important to note that this may not apply directly to your diving. It’s true that mouthpiece retainers showed very promising results in case studies as well as incident reviews, but the subjects do not represent most recreational divers. Not only do both studies focus on military divers in better physical condition and with better equipment maintenance, but their subjects adhere more strictly to safety protocols and receive significantly more training than recreational divers.
The lack of training in particular is what brings issues like bail-out protocols to light. It’s true that bailing out with either a mouthpiece retainer or full face mask becomes somewhat more complicated, and the addition of a bailout-valve adds additional opportunities for user error. With adequate training and emergency protocols it seems unlikely that any of these concerns would notably increase risk, but that’s not something we have the data to confirm just yet.
For now, consider the research, and consider how you dive: do you have enough experience and training to add a mouthpiece retainer to your configuration? If the data applies to your diving, a mouthpiece retainer might just save your life.
Haynes, P. (2016, December). Increasing the probability of surviving loss of consciousness underwater when using a rebreather.
Gempp, E. (2011). Descriptive Epidemiology of 153 Diving Injuries with Rebreathers Among French Military Divers from 1979 to 2009.
When he’s not working with DAN on safety programs, Reilly Fogarty can be found running technical charters and teaching rebreather diving in Gloucester, Mass. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.
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.
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.
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.
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.
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  and Michael Lombardi’s Ocean Space Habitat, also in 2012 . 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 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 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.
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.
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).
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 . 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).
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.
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).
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.
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.
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
 Blue Lake: the habitat.
 Lombardi M. Portable Habitats: New Technical Diving Capabilities are Well Within Reach. InDEPTH V 4.11
 Nuckols ML, Tucher WC, Sarich AJ. Life Support Systems Design: Diving and Hyperbaric Applications. Pearson Custom Publishing, Boston, USA, 1996.
 Gerth WA. Chamber Carbon Dioxide and Ventilation. NEDU TR 04-46. Navy Experimental Diving Unit, Panama City, FL, USA, 2004.
 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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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...