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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!

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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).
Divers on the AP Diving Inspiration rebreather in typical backmount configuration. Photo by Martin Parker.
Cave diver in the DiveSoft Liberty sidemount rebreather. Photo courtesy of Marissa Eckert.

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.

Photo by Kirill Egorov.

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. 

Photo by Jesper Kjøller.

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:

Density

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%.

Narcosis

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.

Oxygen Toxicity

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. 

Two GUE CCR divers in California. Photo by Karim Hamza.

In Summary

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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The Joys and Challenges of Teaching Kids To Dive

We all lament the fact that we don’t see more young people getting into diving. British instructor and content creator Victoria Brown
is doing something about it, and finding joy in the process.

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by Victoria Brown
Header Photo by Leo Grower, Ocean Leisure

The sound of escaping air whooshing through purge valves tears through me. I consider the impending hours I will be standing in front of the compressor, filling the cylinders as my group of divers is using their air supply as ammunition stocks for their new air guns. Sorry, I mean regulators. 

I am dressed in my wetsuit, with my diving instructor hat on, loudly addressing the rowdy crowd of teenage boys, attempting to gain silence and get their attention.  Futile, as you can imagine. In a bid to appear in control, I try again in a firmer voice, this time individually name checking each member of the group, none of whom are paying attention to me over the buzz of activity. I walk around the buddy pairs conducting final checks, repeating the pre-dive safety mnemonic to the group (that seems to slip their minds from week to week), ensuring the strict sequences are executed, sorting tangled hoses, and tightening the cam bands of BCDs worn by the next generation of divers.  

The crowd of excited boys yells gas pressures to the instructor on surface support duty, and they waddle carefully in their fins toward the edge of the pool to get ready to make their giant stride entrances. This is Kid’s Club, and it is more than fun. Once I’m happy, I step into the water and all eyes are on me.  I mutter the weekly joke about how cold it is in the 28°C/82°F pool and go on to instruct the divers to enter the water one by one.

In the next moment we are sinking into the magnificent azure of a hidden world in search of adventure, treasure, and marine life that we discussed just an hour ago during the dive briefing. In reality, the azure comes from the colored tiles that are creating the turquoise hue, outlined by darker navy blue bordering the lanes along the 25 m/82 ft length of the pool. The only marine life are the clumps of hair, rouge plasters and lost sparkly earrings missing their pair, glinting in the bright glow of the strip lights in the roof.  

Ages 13-18, and amongst their school peers, the kids are concerned with looking cool and in control during their brave pursuit of the underwater world. The group is competitive and this helps them to up their game and achieve mastery in their skills. Any slight feelings of apprehension are put aside as they try to out hover each other in sweet trim and give the OK sign every few moments to their buddy.

Colour Me Child Diver

My first experience diving with children was when I began diving.  As the youngest, keenest, and last member of the Brown family divers to experience diving, my family members were my trusted companions. In hindsight, I learnt the importance of looking after other divers from them. Of course, as a new diver I erred on the wild side with little regard for or understanding of safe diving practices. I would laugh until my regulator fell out and use my air up trying to gain some sort of semblance of neutral buoyancy as I darted around discovering new species of fish and corals. I was never left alone to my own devices. They kept me safe and away from danger, they always had my back (and my SPG in view), and they never constrained my exploration through underwater play. 

Victoria Brown underwater. Photo by Leo Grower, Ocean Leisure.

As I edged out of my teens and into my 20s, I was teaching recreational courses every week. My first diving job  was at a dive centre in London that ran a whole variety of kid friendly diving experiences, from “bubblemaker” parties to junior open water courses, via the PADI Seal Team.  I immediately began instructing the kid’s courses, and I adored it. As a young female instructor working in a busy London dive center, I was relieved to have an audience that never questioned the level of experience I had in comparison to my age.

Teaching groups of kids brought a whole world of imagination and joy to diving that reminded me of my early days learning to dive. I felt a great sense of achievement every time I had the opportunity to lead a diver underwater for the first time and reveal the secrets of the deep. This feeling was heightened by the great enthusiasm of youth. 

Hogwarts On Helium?

Nowadays, I run a scuba club at a south London boys’ school. It is magical. The school is mega, it feels like Hogwarts, and is set in stunning grounds. The facilities are excellent too, a large heated pool, hot showers and free cups of tea in the staffroom. 

Energetic kids during a pool session. Photo courtesy of purpleturtlediving.com.

Each child has a unique personality, and it is this powerful individualism of each of my students that defines our communication in lessons. There are the loudest characters who want to catch me out and interrogate me about non-diving related issues in front of their buddies. Then there are the divers who ask a thousand questions, the quiet ones who listen, and the ones that do not!  From the less sporty ones who persist in mastery of the skills we conduct in training to the ones who appear to already be born to dive, they all come together to share this extreme experience.

Despite the liveliness of some groups, as the time nears for open water qualifying dives the groups tend to knuckle down as their thoughts turn toward cold water, and they consider the severity of possible situations if things go wrong.  Concentration and attention are increased tenfold during the final confined water training sessions. The divers are all there to learn now, their natural competitiveness subsides, and the team begins to work together.  

Occasionally diving instructors are not keen on teaching children, for they are uninterested in what is perceived as ‘babysitting’. I find I have endless patience for the wee diving ones and find their spirit enchanting. Kids are not for the faint hearted.   It is not the same as teaching adults, although I can recall a few occasions that maturity might be confused with age. 

It is important to consider that they are not just smaller adults. They have shorter attention spans, and more quizzical questions arise; tactical explanations are a necessity if I want to successfully guide kids into diving. I learn so much from interacting and diving with the kids and appreciate their vivid originality.  I also enjoy the challenges posed by some wonderful questions in class that spark awesome discussions.  

Inspiring the NextGen

Sometimes teaching theory can be interrupted by the excitement of getting in the water, but it can also be more rewarding than the diving itself.  Delivering a lesson in a firm, precise, and comprehensive manner can help. So can shorter, fun bursts that include a clear explanation of value, repetition where possible, and a lot of patience.  I have a cool story to drop into at every moment, so when daydreams replace concentration, and students drift out of the classroom, sharing a gripping tale of an underwater adventure that I have had certainly grabs their attention

And another thing: when I hit waterside, ensuring that the kids have well-fitted equipment is essential. Smaller cylinders, kid-sized BCDs, well-fitted masks and smaller mouth pieces on regulator second stages and snorkels are critical.

One thing you learn when you spend time with children outside the water is anything that can happen, will happen. Introduce water, depth and equipment to the mix and it really gets exciting.  There is not a moment you can safely take your eyes off the young students. You need eyes everywhere. Topside rules are check, check, and check again. 

Taking the first breath underwater. Photo courtesy of purpleturtlediving.com.

To me this is not a chore, it is part of the fun. The intense excitement is infectious, and when a group goes underwater to take their first breaths, it feels great watching the wonderment behind their eyes. I love asking the kids about how they feel and what it was like, and what they thought about.  

Being a role model for my groups is incredibly important. I want to demonstrate how to be safe, have fun, and share experiences and adventures with their friends.  Enabling the children of today to take the reins of tomorrow is going to change the future.  

Imagine the possibilities as future underwater explorers and ocean advocates that learnt under your instruction go on to discover places where no one has ever dived before, or reverse the effects of climate change, or make more divers to protect our water sources.


Victoria Brown is a PADI Diving Instructor, Commercial Diver-Surface Supplied, and Technical Diver.  Based in London, UK, she works as a freelance Content Creator for Suunto, Deeperblue.com, and Midlands Diving Chamber.

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