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By Michael Menduno, original paper by Dr. John Heine
Do rebreathers work in ice-cold
“We had an increasing number of queries from researchers that wanted to use rebreathers in the Antarctic,” Heine, who is the Diving Safety Officer for USAP and a member of its Diving Control Board, explained. “The problem is we couldn’t answer the [fundamental] question: will they work or would it be too risky? So, we decided to evaluate a number of rebreathers to see how they performed.” The results of the study were published last year (see
Scientific divers, who operate under an exemption from the Occupational Safety and Health Administration’s (OSHA) commercial diving regulations, have been diving in Antarctica since the 1960s. However, the exemption requires that diving operations be approved by the relevant institution’s Diving Control Board (DCB), in this case USAP’s, which has limited diving to open-circuit scuba. Though there have been a few non-scientific rebreather operations conducted in Antarctica, including the Wes Skiles 2002 Ice Island Project with explorer Jill Heinerth, and a Disney wildlife filming expedition on the Peninsula, these have been poorly documented.
The performance of open-circuit scuba equipment in freezing water is well known. Relevant equipment is regularly tested by the U.S. Navy’s Experimental Diving Unit (NEDU) and within the USAP, and certain regulators, such as the Sherwood Maximus, that perform well in icy conditions, have been approved for use by scientific divers.
Not so for rebreather technology. NEDU presumably has extensive knowledge of the use of rebreathers in cold water—it’s known that Navy Special Forces divers lock out of submarines in arctic waters. However, according to Heine, they won’t(or are unable to) discuss their experience or share data. In fact, as I learned when I did a profile of the NEDU for Alert Diver magazine a few years ago, they don’t even like to acknowledge that sailors dive from submarines.
Putting Rebreathers To The Test
Due to their silence, lack of bubbles, and extended range, an increasing number of scientists have employed rebreathers in their research, albeit in warmer waters, over the last two decades. Not surprising, there are also numerous potential scientific applications for rebreathers in the frigid depths of Antarctica.
These include wildlife behavioral studies, under-ice collections and sampling, and use in the McMurdo Dry Valley lakes to minimize mixing of water layers and adding exhaled gases into the environment. There are also the potential benefits of extending divers’ time and depth underwater, and of course breathing warm, recycled gas as compared to open-circuit scuba.
However, there were many unknowns. Rebreathers are typically tested at temperatures down to 39.2° F /4° C for CE certification. But that’s a big difference with the sub-freezing 28.6° F /-1.8° C water temperatures found in Antarctica, where air temperatures typically average -20° F/-29° C. Heine, who made his first Antarctic dives in 1989, and has subsequently spent 14 seasons on the ice, was concerned about the impact of the cold on the scrubber’s CO2 absorption efficiency, as well as freezing in the loop due to moisture, battery duration and function, display irregularities, accuracy and precision of all readouts and sensors, and potential solenoid and regulator issues.
Heine and his team, which included Dr. Jeffrey Bozanic, author of several books on rebreathers, tested the performance of seven rebreathers, specifically the AP Diving Inspiration, Inner Space Megalodon Legacy and Megalodon 15, the Poseidon Se7en, the Hollis Prism 2 and semi-closed Explorer rebreather (see footnote 2), and Expedition One’s Titan. Their goal was to evaluate the overall performance of regulators, valves, batteries,
Heine’s five-person dive team conducted a total of 116 no-stop dives to a maximum depth of 130 ft/40 m on the seven rebreathers during the austral summer season in Antarctica (Oct-Nov 2016). The average depth of the dives was 85 ft/27 m, with an average dive time of 33 minutes, for a total of nearly 66 hours. They used air diluent in the rebreathers; low setpoints were 0.5 or 0.7, and the high setpoints were 1.2 or 1.3. Divers were equipped with 40 ft3/5.5 L bailout cylinders, which were also used for drysuit inflation. They also had a safety diver on open-circuit, and surface tender(s).
The dives were staged from a heated hut, with a temperature of approximately 60° F/15.5° C, and a water temperature of 28.6° F/-1.8° C. The rebreathers were pre-breathed in warm air, either in the dive locker or in the heated hut. Pre-dive checklists were performed on all of the units.
Most dives were conducted in no current and were characterized as “low activity level.” The scrubbers were only used one-half of the manufacturer recommended time (at 4° C) on the advice of Scientist Emeritus and Retired Scientific Director of NEDU Dr. John Clarke, who sits on the USAP Diving Control Board. “Our dives were rarely longer than 40 minutes,” Heine said. “The limiting factor was the cold, and in some cases decompression, not the scrubbers.”
In addition, they performed dry tests where the rebreathers were pre-breathed in a warm shed or in cold ambient air temperatures of 5° F (-15° C) and then left in the cold for a period of two to three hours. Temperature data from the various portions of the loop were recorded and analyzed, along with qualitative observations on the function of the units. “It is eye-opening how fast things freeze up in air,” cautioned Heine, who was first certified in 1976 in Laguna Beach, CA.
The Cold Facts
The good news was that the rebreathers performed better than expected, with the exception of the Hollis Explorer. One hundred eleven dives (96%) were considered “successful,” which was defined as a complete dive without cause for ending or aborting the dive, or switching to bailout. Five dives (4%) required aborting or switching to bailout and ending the dive.
The Se7en, for example, had a few problems with its (galvanic) oxygen sensors; the automatic diluent valve (ADV) on the Inspiration had probable “freeze-ups” on two occasions, and the Explorer had a number of issues with the electronics, including a “bad cell” warning.
These results compare
The batteries and displays functioned well, except in very cold air temperatures of 5° F/-15° C. In the dry test runs in cold air, scrubber temperatures stayed relatively warm, but temperatures in the lids near the oxygen sensors were below freezing, which is not recommended by the manufacturer. Mouthpieces also froze shut.
In-water evaluations were somewhat mixed. The Megalodon 15 showed temperatures in the lid approaching the ambient water temperature of 28.6° F/-1.8° C, while the inhalation counterlung temperature was about 10° F/5 °C above ambient, suggesting slightly warmed gas being delivered to the diver. In the Prism 2, both counterlung temperatures were near the ambient water temperature, while the temperatures in the lid (near the oxygen sensors) and the central tube of the scrubber remained around 40° F/4.4° C. The exhalation counterlung temperature was right at the ambient water temperature in the Se7en, while the canister temperature was 6-20° F/3-11° C above ambient, similar to results in other rebreather models.
In the Titan rebreather, the scrubber and the lid temperatures remained relatively warm during the dives, while the inhalation hose temperature was close to ambient. In temperate water trials, the inhalation hose temperature was also close to ambient, which suggests that the rebreather was not delivering warmed gas to the diver. The team was not able to measure inhalation gas temperature with the available technology, nor were they able to measure the CO2 in the loop (temperature served as a proxy for absorption efficiency), so these results are unknown.
Note that gas temperatures being delivered to the diver in open-circuit systems would most likely be less than the ambient water temperature, due to gas expansion and pressure drop from the second stage pressure of 150 psi above ambient to ambient. So, all CCRs delivered “warmer” gas to the divers compared to open-circuit, but generally not to an appreciable level. Heine believes that adding insulation materials to the canister and breathing loop hoses and/or counterlungs might help in keeping the breathing gas warmer.
As a result of the study, Heine is now incorporating the use of rebreathers into USAP’s diving standards. A first group of rebreather divers from the BBC, who will be filming seals, is expected next season. There will also likely be a project studying diatoms, which grow beneath the ceiling of ice and are easily disturbed by bubbles.
Note: Unfortunately, Global Underwater Explorers (GUE) divers planning to participate in GUE’s 2021 Antarctica Expedition will need to leave their rebreathers at home. The trip will be limited to open-circuit diving only, unlike Heine’s diving scientists.
1. Heine, J.N. and Bozanic, J. 2018. Evaluation of Closed Circuit Rebreathers for the National Science Foundation US Antarctic Scientific Diving Program Diving for Science 2018: Proceedings of the AAUS 37th Scientific Symposium. 40-58.
2. Huish Outdoors acquired Oceanic and Hollis in 2017 and discontinued the Explorer semi-closed rebreather.
3. Lang, M.A. and J.R. Clarke. 2017. Performance of life support breathing apparatus for under-ice diving operations. Undersea Hyper. Med. 44(4): 299-308.
John Clarke Online:
Authorized for Cold Water Service: What Divers Should Know About Extreme Cold: https://johnclarkeonline.com/tag/en-250/
Information on scrubbers and the cold:
Primer on Scrubbers:
Michael Menduno is InDepth’s executive editor and, an award-winning reporter and technologist who has written about diving and diving technology for 30 years. He coined the term “technical diving.” His magazine “aquaCORPS: The Journal for Technical Diving”(1990-1996), helped usher tech diving into mainstream sports diving. He also produced the first Tek,
The Thought Process Behind GUE’s CCR Configuration
GUE is known for taking its own holistic approach to gear configuration. Here GUE board member and Instructor Trainer Richard Lundgren explains the reasoning behind its unique closed-circuit rebreather configuration. It’s all about the gas!
By Richard Lundgren
Header photo by Ortwin Khan
Numerous incidents over the years have resulted in tragic and fatal outcomes due to inefficient and insufficient bailout procedures and systems. At the present time, there are no community standards that detail:
- How much bailout gas volume should be reserved
- How to store and access the bailout gas
- How to chose bailout gas properties
Accordingly, Global Underwater Explorers (GUE) created a standardized bailout system consistent with GUE’s holistic gear configuration, Standard Operating Procedures(SOP), and diver training system. The system was designed holistically; consequently, the value and usefulness of the system are jeopardized if any of its components are removed.
Bailout Gas Reserve Volumes
The volume of gas needed to sustain a diver while bailing from a rebreather is difficult to assess, as many different factors impacts the result— including respiratory rate, depth and time, CO2 levels, and stress levels. These are but a few of the variables. All reserve gas calculations may be appropriate under ideal conditions and circumstances, but they should be regarded as estimates, or predictions at best.
The gas volume needed for two divers to safely ascend to the first gas switch is referred to as Minimum Gas (MG) for scuba divers. The gas volume needed for one rebreather diver to ascend on open-circuit during duress is referred to as Bailout Minimum Gas (BMG). The BMG is calculated using the following variables:
Consumption (C): GUE recommends using a surface consumption rate (SCR) of 20 liters per minute, or 0.75 f3 if imperial is used.
Average Pressure (AvP or average ATA): The average pressure between the target depth (max depth) to the first available gas source or the surface (min depth)
Time (T): The ascent rate should be according to the decompression profile (variable ascent rate). However, in order to simplify and increase conservatism, the ascent rate used in the BMG formula is set to 3 meters/10 ft per minute. Any decompression time required before the gas switch (first available gas source) must be added to the total time. One minute should be added for the adverse event (the bailout) and one minute additionally for performing the gas switch.
BMG = C x AvP x T
Note that Bailout Minimum Gas reserves are estimations and may not be sufficient! Even though catastrophic failures are unlikely, other factors like hypercapnia (CO2 poisoning) and stress warrants a cautious approach.
Decompression bailout gas volumes are calculated based on the diver’s actual need (based on their decompression table/algorithm), and no additional reserve is added.
It should be noted that GUE does not endorse the use of “team bailout,” i.e. when one diver carries bottom gas bailout and another diver carries decompression gas based on only one diver’s need. A separation or an equipment failure would quickly render a system like this useless.
Common Tech Community Rebreather Configuration
- Backmount rebreather (note side mount rebreathers are gaining in popularity)
- Typically, three-liter oxygen and a three-liter diluent cylinder on board (each hold 712 l/25 f3)
- Bailout gas in one or more stage bottles which could be connected to an integrated Bailout Valve (BOV).
Containment and Access
Rather than carry bailout minimum gas (BMG) in a stage bottle, which is typical in the rebreather diving community, GUE has designed its bailout system as a redundant open-circuit system consisting of two 7-liter, 232 bar cylinders (57 f3 each) that are integrated into the rebreather frame, and called the “D7” system, i.e. D for doubles, 7 for seven liter. Note that GUE has standardized the JJ-CCR closed-circuit rebreather for training and operations.
These cylinders, each with individual valves, are linked together using a flexible manifold. This system holds up to 3250 liters of gas (114 f3), of which only about 10% is used by the rebreather as diluent. Hence, close to 3000 liters (106 f3) is reserved for a bailout situation. This gives a tremendous capacity and flexibility in a relatively small form factor for dives requiring additional gas reserves (when direct ascent is not possible or desirable).
The following advantages were considered when designing the bailout system:
- The D7 system is consistent with existing open-circuit systems utilized by GUE divers. A bailout system that is familiar to the user will not increase stress levels, which is important. A GUE diver will rely on previous experience and procedures when most needed.
- The system contains the gas volumes needed according to the GUE BMG calculations as well as the diluent needed for a wide range of dive missions.
- The system is fully redundant and has the capacity to isolate failing components, like a set of open-circuit doubles and still allowing full access to the gas.
- The overall weight of the system is less, compared to a standard system with an AL11 liter (aluminum 80 f3) bailout cylinder. In addition, it contains 800-900 liters/20-32 f3 more gas available for a bailout situation compared to the AL11 liter system. Weight has been traded for gas.
- The system does not occupy the position of a stage bottle which allows for additional stages or decompression bottles to be added.
- If the ISO valves on each side were closed, the flex manifold can be removed and the cylinders transported individually while still full.
Bailout gas can be accessed quickly by a bailout valve (BOV), which is typically configured as a separate open-circuit regulator worn on a necklace, consistent with GUE’s open-circuit configuration. However, some GUE divers use an integrated BOV. After evaluation of the situation, while breathing open-circuit from the BOV, the user can transition to a high-performance regulator worn on a long hose if the situation calls for it.
The long hose is carried under the loop when diving the rebreather. The chances of having to donate to another GUE rebreather diver is low, as both carry redundant bailout. Still, GUE maintains that the capacity to donate gas must be present. The process is more likely to involve a handover of the long hose rather than a donation.
Still, if needed, such a donation is made possible by either removing the loop temporarily or by simply donating the long hose from under the loop.
Bailout decompression gasses are carried in decompression stage bottles. If more than three bottles are needed, the bottles that are to be used at the shallowest depths are carried on a stage leash (i.e. a short lease that clips to your side D-ring to carry multiple stage bottles). Maintaining bottle-rotation techniques and capacity through regular practice is important and challenging, as this skill is rarely used with the rebreather.
Bailout Gas Properties
The choice of bailout gas is extremely important, as survival may well depend on it. It is not only the volume that is important, the individual gas properties will decide if the bailout gas will be optimal or not. As the D7 system contains both the diluent and bailout gas, both gasses share the same characteristic. The following gas characteristics must be considered when choosing gas:
The equivalent (air) gas density depth should not exceed 30 meters/100 ft or 5.1 grams/liter. This is consistent with the latest research by Gavin Anthony and Simon Mitchell that recommends that divers maintain maximum gas density ideally below 5.2 g/l, equivalent to air at 31 m/102 ft, and a hard maximum of 6.2 g/l, the equivalent to air at 39 m/128 ft. You can find a simple gas density calculator here.
Ventilation is impaired when diving, due to several factors which increase the work of breathing (WOB); when diving rebreathers, the impairment is even more so. High gas density, for example, when diving gas containing no or low fractions of helium, significantly decreases a diver’s ventilation capacity and increases the risk of dynamic airway compression. CO2 washout from blood depends on ventilation capacity and can be hindered if a high-density gas is used. The impact of density is very important, and the risk of using dense gases is not to be neglected. Note that this effect is not limited to deep diving. Using a dense gas as shallow as 30 meters/100 ft reduces a diver’s ventilation capacity by a staggering 50%.
The (air) equivalent narcotic depth should also not exceed 30 m/100 ft, or PN2=3.16. Rebreathers and emergency situations are complex enough without further being aided by narcosis.
The PO2 should be limited to allow for long exposures. GUE operating standards call for a maximum PO2 for bottom gases of 1.2 atm, a PO2 of 1.4 for deep decompression gases, and a PO2 of 1.6 for shallow decompression gases. GUE recommends using the next deeper GUE standard bottom gas for diluent/bailout when diving a rebreather in combination with GUE standard decompression gases.
Bailout gasses are not chosen in order to give the shortest possible decompression obligation. They are chosen in order to give the best odds of surviving a potentially life-threatening situation.
GUE’s D7 bailout system is flexible and contains the rebreather’s diluent as well as bailout gas reserves needed for a range of different missions. The familiarity the system, along with the knowledge that they are carrying ample gas reserves, gives GUE divers peace of mind. Choosing gases with properties that will aid a diver in duress while dealing with an emergency completes the system.
GUE did not prioritize the ease of climbing boat ladders or reducing decompression by a few minutes. These are more appropriately addressed with sessions at the gym, combined with finding aquatic comfort. Nothing prevents a complete removal of the entire system at the surface if an easy exit is needed.
Founder of Scandinavia’s Baltic Sea Divers and Ocean Discovery diving groups, and a member of GUE’s Board of Directors and GUE’s Technical Administrator, Richard Lundgren has participated in numerous underwater expeditions worldwide and is one of Europe’s most experienced trimix divers. With more than 4000 dives to his credit, Richard Lundgren was a member of the GUE expeditions to dive the Britannic (sister ship of the ill-fated Titanic) in 1997 and 1999, and has been involved in numerous projects to explore mines and caves in Sweden, Norway, and Finland. In 1997, in arctic conditions, he performed the longest cave dive ever carried out in Scandinavia. Richard’s other exploration work has included the 1999 filming of the famous submarine, M1, for the BBC; the side scan sonar surveys of the Spanish gold galleons outside Florida’s Key West in 2000; and the search for the Admiral’s Fleet, an ongoing project that has already led to the discovery of more than 40 virgin wrecks perfectly preserved in the cold waters of the Swedish Baltic Sea.
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