RebreatherPro is a free source of information for aspiring and experienced rebreather divers. Launched in 2007, resources have recently been moved to this site. Please be patient as we repopulate the archives with lots of great information and posts.

What Lies Beneath

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Blog # 4 / Dr. Keene Haywood / Dec. 6, 2016

Keene Haywood is the director of the exploration science program at the University of Miami’s Abess Center for Ecosystem Science and Policy.  The program offers a Master’s degree in Exploration Science through the Master of Professional Science (MPS) program at the UM-Rosenstiel School of Marine and Atmospheric Science (RSMAS).  He holds a PhD in Geography and MFA In Science and Natural History filmmaking.

Driving south down the main high way on Abaco, the slightly rolling terrain of pine trees and low vegetation makes for a somewhat hypnotic drive as early morning light filters past the long slender trunks and across the green expanse.   Some twenty miles south of the town of Marsh Harbor, one turns off the highway onto a rocky, bumpy road that leads west across the island.   Another world exists parallel to this forest of whispering pines.   Below is a labyrinth of caves, the likes of which are only beginning to be fully understood and mapped.   Dan’s Cave and Ralph’s Cave are two entrances into this otherworldly realm that few people have entered.  Named after the hunters who originally found the caves decades ago, one project goal is surveying and mapping these complex and beautiful water filled passages, exploring the edges of what is known and unknown. But the project has other dimensions of exploration.

Exploration – the very word tends to mean different things to different people, but it seems to always seems to elicit the same emotion – wonder.   What is out there?  Why?  What is around the next bend, the next passage?  What are the social and ethical implications of revealing the unknown? Who ‘owns’ the intellectual property and economic benefits that may be revealed? The list goes on. Trying to encapsulate this wonder and the moral and practical questions into a discipline is what exploration science seeks to do.  As the director of the exploration science program at the University of Miami (, I often am often asked just what is this discipline?  Broadly, the approach uses elements of observation, documentation, and communication to bring together this wonder and pull it together into new knowledge about our world. This program seeks to ground students in all three areas, encouraging them to embrace new technologies, follow their curiosity and pull together multi-disciplinary approaches to answering what is out there and why, all while considering the historical and ethical context of exploration.

For this project, key components of exploration are strongly supported. Through video and photography, the caves are being observed and documented in both scientific and journalistic ways to convey different aspects of the wonder of Dan’s Cave. Through mapping, the surveying team is bringing back data to provide a permanent record of past exploration of the cave using new tools and software to understand distances, depths, and intricacies of this maze of nature.  In addition, uses of emerging technologies such as 3D printing of artifacts and photogrammetry work yield compelling new ways to persevere and communicate the wonders of Dan’s Cave to a wider public.  In this case, this includes through direct communication with local communities both at the site and remotely.

More direct communication approach is taking place daily for five days this week with groups of school children from Abaco.  These children get a chance to experience some of the wonder of Dan’s cave directly by coming to this area with their teachers to interact with the expedition team and go through a series of hands-on experiences ranging from crawling through simulated cave squeezes to science experiments showing how groundwater picks up pollutants to making bush medicine teas with local elders to coring trees to determine their age. While the data and images will go far beyond the island of Abaco, it is the direct impact of experiential learning first hand by the younger generations of Bahamians that is most gratifying aspect of the project for many of us. It is in seeing the kids discovery and wonder in action that exploration science ceases becoming an abstract idea and begins to be a concrete experience not just for the school kids visiting Dan’s Cave, but for all of us.

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Expedition Files Post 3 from the Abaco Blue Holes Cave Diving Expedition with National Geographic

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Chris Milbern / Dec 6, 2016 / Dive Safety Volunteer

Chris Millbern is the 2016 Our-World Underwater Scholar. Trained in ecology and evolutionary biology at UCLA, Chris found his passion in dive safety and volunteered to join the National Geographic Blue Holes Project.

Hey everyone, my name is Chris Millbern and I’m here with Nat Geo representing the Our World-Underwater Scholarship Society!  A few months ago I was chosen as the 2016 North American Rolex Scholar, a title that comes with it a year’s worth of research and opportunity in all things diving designed to help young people like myself pursue their dream careers.  I’ve been in the water from the Arctic to the Antarctic (just got back last week!) and everywhere in-between hoping to develop the skills, knowledge, and experience necessary for a life of exploration.  It’s with a huge smile I get to thank National Geographic, Dr. Kenny Broad, and the entire team here for having me aboard!

As a diver medic and hyperbaric technologist, I’ve managed to treat scuba diving injuries in all sorts of situations: hospitals, boats, parking lots, and the occasional cave as well.  But while I’m passionate about treating injured divers, the best job for me is one where everyone comes out safe and happy- which is often a factor of preparation, attitude, and experience.  So you can imagine that when I was invited to this expedition, it was like being invited to dive safety nirvana; some of these divers wrote the books I made a job out of!

But no matter who you’re with, cave diving is not without its risks.  It’s Day 4 of the expedition here and the site is busy with documenting camp-life (pro-tip: coffee made in a bucket tastes better), hosting live stream learning with classrooms from around the world, and teaching local students about their environment.  It’s easy to get caught up and forget that our exploration efforts- specifically those of our divers – are a matter of life and death.  So what are those risks, and what can we do to mitigate them?

The biggest problem with caves is that they have a ceiling, and more importantly that we can’t breathe ceilings.  If you’re a diver that runs out of air in the ocean, you’ve always got the option (though not always a good one) of swimming to the surface and sipping some of that sweet, sweet air.  Not so in caves!  As cave divers, we have to understand that anything we bring down with us is all we’ve got; and that means redundancy is key. Leaving at least a third of our air unused for emergencies, bringing backup lights and equipment, and staging sources of air and pure oxygen strategically throughout the cave all help to mitigate risk and ensure happy divers. There are many other rules and the #1 rule is:


As if running out of air isn’t bad enough, using the wrong diving gas at the wrong depth can kill you.  Nitrogen, a major component of the air we breathe, has a tendency under pressure to dissolve into the liquids and tissues of our bodies.  If left unchecked, this dissolved nitrogen causes all sorts of damage- think cracking open a soda bottle and watching the bubbles rush out.  The deeper we go and the longer we dive increases this effect. Even oxygen at the wrong depth can cause central nervous system issues like seizures that lead to drowning. Thus, we have to pick the proper diving gasses for the depths we plan to explore and this may be a mix of different percentages of oxygen, nitrogen, and sometimes helium.

Caves are often in remote places, outside the range of helicopters and cell towers.  With no access to emergency healthcare here in the jungle, a real problem means driving to the nearest city, waiting for a medivac (available only in daylight), and hoping for the best as the diver is flown to Nassau or Florida for recompression therapy.  It’s because of this that our responsibility lies in preventing injuries before they happen.

If this all sounds scary, good!  A healthy dose of fear is what keeps us paying attention, and what keeps cave divers the world over safe.  And while I don’t anticipate any good stories on my end, I’m proud to be working with some of the best people in the world at keeping cave diving boring! (Medically, that is.)  Cheers!


Abaco Blue Holes Project Expedition Files #2

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Expedition Blog 2 / Dec 5, 2016 / Tom Morris

Team member, Tom Morris, has been exploring caves around the world the world for almost fifty years. Tom Morris is a biologist and diver who lives in Gainesville, Florida. Today, Sunday, is his 70th birthday. His birthday present was a passport that he left in his car before heading across the Florida Straits by boat to join the team. He can now leave the Bahamas when the time comes, although he would rather remain in the Bahamas, where the pines sing, the bracken is tall, and every other plant is an aphrodisiac.

Most people who visit the Bahamas end up on New Providence Island, home to Nassau and all its tourist amenities. I appreciate Nassau’s attractions, but I am drawn by the wilder side of the great archipelago, and by its rich geological and natural history. So, when my friend Kenny Broad invited me to join this expedition to Abaco Island’s newest nature preserve, I jumped at the chance.

Most everyone on the expedition team flew over from Florida, but Kenny and I needed to ferry a ton of expedition gear on his boat. Crossing the Gulf Stream is always a great adventure. After loading gear we left Miami in the dark AM and motored into the confused and rough seas of the Gulf Stream. As the sun rose we were treated to the visually stunning deep blue of the Straits of Florida, caused by two thousand foot depths and nutrient-poor clear water. Fortunately, Dramamine worked its magic, and my usual tendency to seasickness was no problem.

Hours later, the taller buildings of Grand Bahama Island came into view, and the water gradually changed to the nice turquoise color of shallower Bahama Bank waters. We worked our way along the coast, recently ravaged by hurricane forces, keeping a close eye for dangerous rocks, and entered the main Grand Bahama Port of Entry. An overrated problem of my missing passport was overcome and we fueled up and headed over to the infamous Lucayan Waterway to anchor in protected water for the night. The Waterway is a more or less 100-foot wide canal dredged across the entire width of Grand Bahama Island to create valuable waterfront property. The canal was a horrible idea, and drained part of the island of billions of gallons of scarce fresh water, and allowed salt water to contaminate a significant portion of the islands rocky aquifer.

The next morning we cruised in the calm protected waters of the Little Bahama Bank. The Bahama Banks are a carbonate factory, and corals, calcareous algae, and chemical processes have, over millions of years, deposited an incredible nineteen thousand feet of limestone sediments. The sediments were all formed in shallow marine conditions, just like today, with subsidence matching the rate of production. The many islands of the archipelago, large and small, are just the tiniest tip of this carbonate “iceberg.”

A few hours later found us dockside in Marsh Harbor. Here the generally unrecognized part of every expedition began in earnest. All the thousands of pounds of equipment had to be unloaded and taken to our vehicles. Everyone gets to enjoy the work, even the expedition leader (Kenny). Fortunately for Kenny, his labors were interrupted by a necessary visit to the harbor master. This was the second of many times that the expedition members will delight in, moving the gear. Expeditions are a great weight loss program.

It was great to be back in Marsh Harbor. This small town is reputed to be the third largest town in the Bahamas. It is full of friendly people, all driving on the wrong side of the road. I have heard a number of people remark that most of the Bahamas is like Florida used to be one hundred years ago. I believe it, and I like it.

We drove a mile or so to the Friends of the Environment compound, where we moved all the gear once again (third time). This excellent organization, along with the National Museum of the Bahamas (Antiquities, Monuments, Museums Corp.) is helping sponsor the expedition and is letting us use their new guesthouse. The rest of the day and much of the night found us putting together SCUBA and other gear.

The next day found us moving (fourth time) much of the equipment out to Dan’s Cave and setting up shelters and prepping for the arrival of primary school students on Monday. The site is beautiful. The setting is picturesque Caribbean slash pine forest (Pinus caribaea var bahamensis), known locally as pineyards. The bracken fern (Pteridium aqualinum), usually about two feet high in Florida, is head-high in the pineyard shrub layer. Poison wood (Metopium toxiferum), is scattered throughout. Winter migratory birds from the continent are moving about in the bushes. Forestry burns the pine forests about every three years to keep combustible fuel at safe levels. The trade winds sing in the pines.

The December temperatures are pleasant. Perfect for moving gear.

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Expedition Files Abaco Blue Holes #1

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BLOG POST 1 / DEC 4 / Kenny Broad

4am on Dec 4, officially the second day of our Nat Geo cave diving expedition to Abaco, The Bahamas. My umpteenth trip to Bahamas, but my first blog, so I’ll keep it relatively short, sentimental, sardonic, informational, and therapeutic for my insomnia:

Quick summary of expedition start: rough seas crossing the tip of the Bermuda triangle (did not find Atlantis, by the way); overloaded boat with unwanted sloshing water water beneath the deck from broken water line – soaking critical gear; electronic navigation system gone haywire, thus forced to actually navigate with a compass (you can get a picture of these ancient devices on the interweb for the younger readers); key expedition members out w/ bizarre infections in ears and lungs; forgotten passport by unnamed team member (Tom, last name and address available upon request); failure of some critical dive equipment; two broken bones by my son, Jasper, on his birthday the day after I left (which I missed and promise to make up for); waiting hours for visit by prime minister which was cancelled at last minute; and I forgot my hair care products. But we have coffee. A lot of it.

My first expeditionary trip here was over 25 years ago, led by the late Wes Skiles, an exploration and filmmaking pioneer of the subsurface, and a larger than life character who died in 2010. It was my first true cave exploration, and as I surfaced at 2am from an ocean cave with an empty reel in my hand after laying out almost 1000 feet of line – in what are known here as blue holes – I held up the reel in a display of shameless hubris to Wes and the others on the back deck of the boat we were living on. His response: “Where’s the data?” I had laid the line in this untouched world, but brought back no survey information, no pictures, no water samples, only images in my head. It was my lesson in the difference between adventure and exploration and I’ve been trying to make up for it ever since.

In many ways this project is the culmination of the lessons learned by many of us on the team. We’re mapping this cave system that could prove to be the most extensive island cave system in the world. In doing so, we’re testing new underwater technologies, and hoping to create the first virtual reality / 3-D room of an underwater cave system to be used for education and developing an interpretive trail above the cave system. But the most rewarding part of the expedition is an extensive outreach and education component involving ethnobotany, water experiments, and surface exploration games, with school kids at the site for all sorts of hands on activities. Beyond the local impact, we’re live satellite broadcasting to classrooms in The Bahamas, North America and Canada, where the kids can interact with the explorers, scientists, and other kids across the ocean.

Our project, funded primarily by the National Geographic Society, is a collaboration between the The Bahamian Government, local NGOs, the University of Miami and supported by many local organizations. Over the next weeks, there will be blogs on topics ranging from cave diving, water resources, 3-D technologies, and speleology from team members who come from The Bahamas, United States, United Kingdom, Mexico, Italy, Canada, and France.

To state the obvious, exploration is not without it’s risks. Wes died underwater, almost to the day that his photos made the cover story of the August 2010 issue of National Geographic Magazine, bringing to life these same caves that we’re diving, with many of the same team members from that project with us on this trip. The article brought to life the otherworldly beauty and the scientific treasure trove these windows into the underworld hold – vast networks of limestone passageways that are time capsules with cave formations (e.g., stalagmites) that allow us to understand ancient and thus modern climate patterns; extremophile forms of life can live in darkness without oxygen and in toxic (to humans!) hydrogen sulphide; providing genetic clues to the biogeochemistry (great scrabble word, by the way) of how life evolved on earth billions of years ago and may survive now in the far reaches of space. The anoxic (no oxygen) deeper salt waters preserve all sorts of fossils, giving us tantalizing clues to animal and human migrations and disappearances in this low lying, carbonate platform, separated by deepwater canyons.

Beyond the scientific value, the reservoirs of lighter fresh water supplied by rain that sits atop the denser salt water in these coastal systems – often called aquifers – hold most of the planets’ fresh water that is not locked up in ice. They are out of sight, out of mind and subject to all sorts of pollution and near the coast, to salt water intrusion caused by sea level rise and overconsumption for human uses. Ok, so that is our rational justification for exploring these places – science and human health. Truth be told, we’re drawn to the them because beneath our feet, often off the side of the road, you can jump into a muddy hole and enter an innerspace as alien as your imagination can see and challenging, mentally and physically to explore.

The area we’re working in which is primarily inland pine forests dotted with windows into this karst (limestone) network of caves has recently been designated a conservation area by the Bahamian government. Many thanks go to National Geographic and the dedicated explorers and conservationists who have brought this invisible innerspace to the public’s attention, particularly the policy makers who are protecting the future generations’ water resources and the rare forms of life that live in these hidden worlds beneath our feet. Our hope is that this project provides the data and the experience for the local and faraway folks to appreciate and protect this finite resource. Stick with us the next few weeks for videos, blogs and honest insights into the good, the bad, and the ugly of exploration.

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By | All Posts, Bell Island, Newfoundland, Rebreather Diving, Underwater Photo and Video | No Comments

Both of my Grandfather’s served their country in the “Great Wars.” My husband served in the US Navy Seabees. These men survived their ordeals and came home appearing to be in one piece. Yet, the unseen wounds of their sacrifice are difficult for those of us that did not serve to understand. For this Remembrance Day I will reflect on those that did not come home as well as those that left a piece of their body or soul in a place of conflict. Their sacrifices are unimaginable.

There are many people that made sacrifices that are little known to Canadians; some dying in explosive sinkings on our Canadian shores in 1942. This week, my diving friends at Ocean Quest Adventures in Newfoundland will place a wreath on the Bell Island shipwrecks to recognize the sacrifice of the men who perished after their four vessels were struck by U-Boat torpedoes. The deceased and the people who were rescued will be in our hearts this week. They and the countless other departed soldiers and veterans will be treasured for their sacrifice. We shall not forget.

AI Robotic Cave Diver Mission a Success

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In 1995, deep in a canyon in the Sierra Mazateca Mountains I sat around a campfire with Dr. Bill Stone. We were wrapping up a project in the Huautla Resurgence; a cave of mammoth proportions that we hoped would become the deepest cave on earth. With spring rains arriving early, we had experienced flooding in the canyon that was destroying the visibility of our cave. But Bill had an idea; a way to see in the dark. Two years later he had built the world’s first 3D mapping device that could accurately define the cave in three dimensions and even link it to surface topography with ultra low frequency radio beacons. The United States Deep Caving Team’s Wakulla2 Project was born.

I had the honor of driving the Wakulla mapping as a part of a team of 150 volunteers who worked together to create the first accurate 3D map of a subterranean system. It was cave diving history but also a leap forward in engineering and technology that was destined for far greater motives. Since that time Bill has further developed his devices, working with NASA on plans to send an autonomous mapper to Jupiter’s moon Europa where it can explore the ocean trapped beneath the frozen surface. He’s been hard at work in Alaska and Antarctica, diving in deep caves in Mexico and tinkering in the lab.

This week, a small but extremely capable version of his autonomous mapper made history at Wes Skiles Peacock Springs State Park in Florida. First hooked to a tether, engineers learned about the mapper’s behaviors, watched through its camera and tweaked software. Watching the robot navigate through real cave passages was remarkable. Finally, it was ready for the test. We unplugged the tether and watched the first robot cave diver explore and accurately map a submerged cave.

What’s next? There is still much to do to extend range and capability before Sunfish can be sent into the unknown to go where no person has been before, but the teamwork, professionalism and skill of Stone Aerospace engineers and volunteer cave divers has made a mark in the history of cave diving and artificial intelligence.

Jill Heinerth piloting the 3D mapper during the United States Deep Caving Team Wakulla2 project in 1998.

Bill Stone’s DepthX (DEep Phreatic THermal eXplorer) AUV.

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Protect Your Noggin’ – Diving Helmets

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I have never figured out why so few North American technical divers wear helmets. Perhaps helmets never reached the Pantheon of hip here. Divers in other parts of the world would never consider exposing their scalps to the ceiling of a cave or wreck without protection. Diving in an overhead environment, there is a high likelihood that you will bump your head. It might not be a major incident, but if you use a Diver Propulsion Device (DPV), contact with a rock or hull of a ship could knock you out. A helmet will not only protect you, it offers an opportunity to attach lights and even a GoPro in a handy location.

Not all helmets are ready to dive “off the shelf” and there are several important points to bear in mind for your DIY helmet project.

The Shell

Start with a basic helmet with adjustable, interior suspension straps. Foam filled helmets are very common for skateboarding, cycling and other sports, but they are unsuitable for submersion. The foam inside those helmets is extremely buoyant and will make it almost impossible to sink. I have seen some people painstakingly dig foam out of their helmets or add weights to the interior, but the end result usually destroys the safety structure of the helmet and even makes it un-diveable. Kayaking, construction and some styles of rock climbing helmets offer the feature of interior suspension straps that can be quickly adjusted with a small fly wheel using one hand. The adjustment wheel is a small round dial or ratcheting wheel that adjusts the circumference of the interior headband. This feature will be very valuable if you switch between differing neoprene hood thicknesses in various water temperatures.

Chin and Side Straps

Adjust the chin strap so it is comfortable and secure without feeling like it is strangling you. If the helmet strap is too short, you may have to replace it with a bungee strap. Any modification such as this may destroy its safety rating out of the water, but unless you have overhead risks such as those encountered by sump divers, this may be okay.


The air vents in the side of the helmet offer thermal comfort out of the water and bubble outlets underwater. These holes can also serve as mounting points for lights. It is far better to use these engineered outlets for mounting rather than cutting or drilling new holes in a way that might damage the structure of the helmet.

The Lights

I use Light and Motion GoBe lights. They are activated with a simple push button switch. Lights that require turning a bezel may not be suitable for this application. Bezels can be tough to operate with one hand while the helmet is on your head. You might find the entire light spinning in place instead of triggering the switch. I use a GoBe SPOT on the left side of the helmet and point it slightly downward. I want the left light to illuminate a notebook or wrist slate when I am writing with my right hand. My preference for the right side is the GoBe SEARCH. It is a little brighter and has a wider beam. I point that light so it illuminates the cave in front of me when I am in the horizontal swimming position. It acts as the best possible backup light and is actually bright enough to serve as a primary. The GoBe lights have one more important feature. They never need to be opened for charging. The charging cable snaps on the light in place on the helmet. It is quick and easy to charge without breeching any seals and is therefore unlikely to flood.


Decide whether it is important to be able to remove your lights from your helmet. You can permanently mount the lights for best security or you can use some very snug bungee cord to hold them in place. In this case, I recommend a small bolt snap fastened to the back of the light. Snap the clip onto the bungee cord as a secondary point of security in case the light slides out of its snug sleeve of cording. If you ever have to deploy the light, you can clip it off as needed.

When choosing the attachment site be careful to consider your swimming position and ergonomics. Lights that are aimed effectively may not look symmetrical or level when the helmet is placed on a table. What is important is that they are well aimed for your swimming position. Before you commit to a permanent location for the lights, take the helmet on a few dives and try using the lights. Ensure your stream of bubbles does not cascade across the face of the light. Bubbles can create a distracting flicker that also appears as an emergency light signal to a dive buddy. I prefer Hollis SE500 side exhaust regulators since they are breathable in either a right handed or left handed exhaust position.

Primary Light

Some divers choose to mount their primary light on their helmet. That can make activities such as surveying easier. If your light is a canister style design, you can purchase a releasable or permanent saddle that can be affixed to the helmet, such as models manufactured by Light Monkey. Generally divers choose the releasable version for ease of gearing up. I prefer keeping my primary light in my hand for easy aim. The hand-mounted Sola Tech600 by Light and Motion is my preferred primary light.

GoPro Cameras

There are many great helmet mounts available for GoPro cameras. The stock mount accepts the male camera clip into a female receiver which is affixed to the helmet with included double sided tape. The male side of the mount pivots to enable you to point the camera in the proper direction. Once you are geared up in your new helmet, do a short test run of the camera and double check the field of view, otherwise you might end up with an entire file looking down the front of your chest rather than out into the blue.

Remember that if you use your head-mounted lights and/or camera, you need to think about being very stable with your head movements. If you are constantly looking around you may be creating unusable footage on your camera and if you blind your fellow diver with glaring lights, you may soon be looking for another buddy!

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Explorer in Residence

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Sofnolime versus Spherasorb

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Finally some real data exists comparing the relative durations of Sofnolime 797 versus Spherasorb. Rosemary Lunn of The Underwater Marketing Company posted this to X-Ray Magazine describing recent scientific studies on sorb.

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Don’t Mix Metric!

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What’s wrong with this valve? If you think it looks a little beat up, you should see the other guy. This metric valve was installed in an Imperial tank by a diver. The fill operator hooked it up for a fill without knowing about the “attempted service” by the unknowing diver. Fortunately, the fill operator had stepped out of the compressor area when the incorrect threads gave out and the valve launched out of the cylinder, stripping the threads along the way. Nearby, while eating my lunch, I heard the loud bang and hiss and was glad to note that it was not followed by pandemonium or screaming. Nobody was hurt. The valve took the fill whip with it and smashed out a light fixture creating a hole in the ceiling. This could have been a deadly accident. Consider this a reminder that you should never mix threads between Imperial and Metric tanks and valves. Better yet, leave your tank maintenance to a pro.

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Absolute pressure – The total pressure imposed by the depth of water plus the atmospheric pressure at the surface.

Absorbent pads – Absorbent material placed in a breathing loop; used to soak up moisture caused by condensation and metabolism.

Accumulator – A small chamber that provides a collection vessel to ensure proper gas flow of oxygen to a solenoid valve.

Active-addition – A rebreather gas-addition system that actively injects gas into the breathing loop (such as a constant-mass flow valve in certain kinds of semiclosed rebreathers).

Atmospheres absolute (ata) – The absolute pressure as measured in atmospheres.

Atmosphere (atm) – A unit of pressure equivalent to the mean pressure exerted by the Earth’s atmosphere at sea level, or by 33 fsw, or by 10 msw (equal to 1.0 bar or 14.7 psi).

Automatic diluent valve (ADV) – A mechanically-activated valve that adds diluent gas when increasing pressure associated with descent or lowered volume triggers the device.

Axial scrubber – A type of CO2 absorbent canister design. In this design, the gas flows through the canister in a linear fashion from one end of the canister to the other.

Backplate – A plate made of stainless steel, aluminum or acrylonitrile butadiene styrene (ABS) plastic which attaches to a rebreather and allows for the use of a webbed or soft harness system.

Bailout – A failure requiring a dive to be terminated, usually using open-circuit gas.

Bailout gas – Tanks carried by the diver to allow for escape from a serious situation, often conducted with open-circuit technique.

Bailout valve (BOV) – An open-circuit regulator built into the mouthpiece assembly that allows a diver to switch from closed-circuit mode to open-circuit without removing the mouthpiece from their mouth. When the loop is closed, the BOV activates, supplying open-circuit gas directly from the onboard diluent tank (in a closed-circuit rebreather) or supply gas cylinder (in a semiclosed-circuit rebreather).

Bar – A unit measure of pressure, roughly equivalent to 1 atm.

Barotrauma – A pressure related injury.

Bottom-out (counterlung) – A term used to refer to the situation when a rebreather counterlung becomes completely collapsed after a full inhalation.

Boom scenario – An explosion or implosion of a hose or other component usually resulting in rapid gas loss or catastrophic loop failure.

Boyle’s Law – The volume occupied by a given number of gas molecules is inversely proportional the pressure of the gas.

Breakthrough − The point at which a scrubber allows CO2 to bypass the scrubbing process to be re-inspired. The fraction of inspired CO2 normally rises extremely quickly once breakthrough is reached.

Breathing hose – Large bore hoses in a rebreather breathing loop, through which the breathing gas travels.

Breathing loop – The portion of a rebreather through which gas circulates, usually consisting of a mouthpiece, breathing hose(s), counterlungs, non-return valves and a CO2 absorbent canister.

Buddy lights – Warning lights that indicates system status including life-threatening oxygen levels; usually monitored by the buddy diver.

Buoyancy control device (BCD) – An inflatable bladder which allows a diver to precisely adjust buoyancy.

Calibration gas – A gas of a known composition used to calibrate gas sensors, particularly PO2 and PCO2 sensors.

Carbon dioxide (CO2) – Waste gas generated by the process of metabolism and exhaled by the diver into the breathing loop.

Carbon dioxide retention − Condition in which arterial CO2 is seen to increase in divers due to insufficient ventilation, excessive dead space in the breathing loop, or ineffective CO2 scrubber filtration.

Catastrophic loop failure – A complete failure of the breathing loop of a rebreather such that it cannot be recovered in closed-circuit mode; usually occurring from a ripping or tearing and subsequent flooding of a unit or a carbon dioxide emergency.

Central nervous system (CNS) – The human brain, spinal cord, and associated major neurological pathways that are critical for basic life-support processes, muscular and sensory systems.

Central nervous system oxygen toxicity − A serious form of oxygen toxicity, usually caused by exposure to breathing mixtures with an oxygen partial pressure in excess of 1.6 ata. Symptoms may include visual disturbances, hearing anomalies, nausea, twitching, dizziness and severe convulsions.

Chain of custody − Refers to the chronological documentation that captures the seizure, custody, control, transfer, analysis, and disposition of physical or electronic evidence, typically for legal purposes.

Channeling (of scrubber canister) − Condition in which improper packing or excessive settling forms channels that allow some CO2 to pass through the scrubber without being absorbed.

Check valve – A one-way, non-return valve that directs gas to move in only one direction through the breathing loop.

Closed-circuit rebreather (CCR) – A type of rebreather that usually includes some form of oxygen control system and generally only vents gas upon ascent.

CO2 absorbent – A material that chemically binds with CO2 molecules (Sodasorb, Drägersorb®, lithium hydroxide, Sofnolime®, Micropore ExtendAir, etc.).

CO2 absorbent canister – A canister in the breathing loop containing CO2 absorbent.

Condensation – Water that forms when water vapor cools and forms liquid droplets. In a rebreather, heat conduction through the breathing hoses and other components of the breathing loop lead to condensation. This process may be exacerbated by materials with greater heat conductivity and lessened with insulation of the breathing loop components.

Conduction (thermal) – Heat flow between objects in physical contact; the inverse of insulation.

Constant mass flow valve – A type of valve that allows a constant mass of gas molecules to flow at a fixed rate.

Constant volume flow − A type of valve that delivers a constant volume, independent of ambient pressure, thus a flexible number of gas molecules.

Convection (thermal) – Heat flow through circulating currents in liquid or gas environment.

Counterlung – A collapsible bag connected to a rebreather breathing loop, which expands as a diver exhales and collapses as a diver inhales.

Cubic feet (ft3) – A unit measure of volume, defined as the space occupied by a cube one foot on each side; 1 ft3 = 28.3 L.

Current limited (oxygen sensor) − A condition in which a change in the load applied to a sensor is not met with a change in the current supplied by the sensor.

Dalton’s law (of partial pressures) − States that the total pressure exerted by the mixture of gases is equal to the sum of the partial pressures of individual gases.

dcCCR – Diver-controlled closed-circuit rebreather. A manually operated rebreather which requires the diver to monitor oxygen levels and manually inject oxygen as needed to maintain an appropriate setpoint. Also known as a manual CCR (mCCR).

Decompression dive – Any dive that requires staged stops during ascent (determined by the decompression algorithm used).

Decompression model/algorithm − Mathematical algorithm used to compute decompression procedures. A variety of computational models and derivatives are available in tabular or dive computer form.

Decompression illness (DCI) – Injury that includes arterial gas embolism (AGE) and decompression sickness (DCS).

Decompression sickness (DCS) – Injury seen especially in divers, caused by the formation of inert gas bubbles in the blood and tissues following a sudden drop in the surrounding pressure, as when ascending rapidly from a dive, and characterized by severe pains in the joints, skin irritation, paralysis, and other symptoms.

Demand regulator – A valve that delivers gas from a pressurized source at or near ambient atmospheric pressure when the diver inhales.

Diffusion – The process in which molecules move from a region of high concentration to a region of low concentration.

Diluent – A cylinder in a closed-circuit rebreather that contains a supply of gas which is used to make up the substantial volume within the breathing loop; a mixture capable of diluting pure oxygen.

Diluent purge valve/diluent addition valve – A manual valve used to add diluent gas to a breathing loop, usually through the counterlung or a gas block assembly.

Display integrated vibrating alarm (DIVA) – A light-emitting diode (LED) heads-up display module mounted close to the diver’s mask, offering information about various states of the rebreather such as PO2; this style includes a vibrating warning alarm when oxygen levels are unsafe.

Downstream – A relative direction with respect to the flow of gas through the breathing loop of a rebreather; the direction of travel of the diver’s exhaled gas.

Downstream check-valve – A one-way, non-return valve that directs exhaled gas to flow in one direction only, for a rebreather. This would typically be the mushroom-type valves that prevent subsequent re-inhalation of used gas and directs exhaled gas towards the CO2 scrubber canister.

Dynamic setpoint – Also referred to as a floating setpoint, it is a setpoint that changes to optimize gas use, no stop time and other consumables and dive variables. The floating setpoint can be determined by an electronic system or modified manually by a diver using a mCCR.

Equivalent air depth (EAD) – A formula used to help approximate the decompression requirements of nitrox. The depth is expressed relative to the partial pressure of nitrogen in a normal breathing air.

eCCR – An electronically controlled closed-circuit rebreather in which an electronics package is used to monitor oxygen levels, add oxygen as needed and warn the diver of developing problems through a series of audible, visual and/or tactile alarm systems.

Elastic load – A load on the respiratory muscles originating from the rebreather and/or diving suit. Materials in the suit and rebreathing bag may restrict breathing. As the diver breathes, the volume of rebreathing bag(s) changes making the depth of the bag(s) change. This depth change means a change in pressure. Since the pressure change varies with bag volume it is, by definition, an elastic load.

Electronically-monitored mSCR – A mechanical SCR with electronic monitoring. Electronics are used to inform the diver of PO2 as well as provide warnings and status updates, however the gas control is manually controlled by the diver.

Endurance (of scrubber) − The time for which a CO2 scrubber operates effectively. The duration varies with individual size, work rate, scrubbing material, depth, and ambient temperature.

Equivalent narcotic depth (END) – A formula used as a way of estimating the narcotic effect of a breathing mixture such as heliox or trimix.

eSCR – An electronic semiclosed-circuit rebreather where an electronics package monitors the PO2 and adds gas to maintain a floating setpoint that optimizes gas use and compensates for changing levels of diver exertion.

Enriched air nitrox (EAN) – A gas mixture consisting of nitrogen and oxygen; with more than 21% oxygen.

Evaporation (thermal) – The heat energy expended to convert liquid water to gaseous state. Evaporative heat loss results from humidifying inspired gases and the evaporation of sweat on the skin.


Exhalation counterlung – The counterlung downstream of the diver’s mouthpiece.

Failure mode, effect, and criticality analysis (FMECA) − Summarizes the study of all components that could fail, and identifies the type of failure, the probability, and severity as well as possible causes of the failure and mitigation and emergency procedures.

ffw – Water depth as measured in feet of freshwater.

Floating setpoint (dynamic setpoint) − A setpoint that changes to optimize gas use, no stop time and other consumables and dive variables. The floating setpoint can be determined by an electronic system or modified manually by a diver using a mCCR.

Flush (as in flushing the loop) – Replacing the gas within the breathing loop by injecting gas and venting bubbles around the edge of the mouthpiece or through a vent valve.

FHe – The fraction of helium in a gas mixture.

FN2 – The fraction of nitrogen in a gas mixture.

FO2 – The fraction of oxygen in a gas mixture.

Fraction of gas – The percent of a particular gas in a gas mix.

Fraction of inspired gas – The fraction of gas actually inspired by the diver.

Fraction of inspired oxygen (FIO2) – The fraction of oxygen inspired by the diver. In SCR operation, this figure is calculated using a formula that takes into account the diver’s workload.

fsw – Water depth as measured in feet of seawater.

Full face mask − Mask system that encompasses the entire face, in contrast with a typical regulator held in the mouth alone.

Galvanic fuel cell sensor − An electrochemical transducer which generates a current signal output that is both proportional and linear to the partial pressure of oxygen in the sample gas. Oxygen diffuses through a sensing membrane and reaches the cathode where it is reduced by electrons furnished by simultaneous oxidation of the anode.

Gas narcosis – A form of mental incapacity experienced by people while breathing an elevated partial pressure of a gas.

Harness – The straps and/or soft pack that secures the rebreather to the diver.

Heads-up display (HUD) – A light-emitting diode (LED) display module mounted close to the diver’s mask offering information about various conditions within rebreathers, such as PO2.

Heat exchange − Divers experience four primary avenues of heat exchange important in the diving environment – radiation, conduction, evaporation and convection.

Heliox – A binary gas mixture consisting of helium and oxygen.

Helium (He) – An inert gas used as a component of breathing gas mixtures for deep dives because of its very low density and lack of narcotic potency.

Henry’s law – The amount of gas that will dissolve in a liquid is proportional to the partial pressure of the gas over the liquid.

Hydrophobic membrane – A special membrane that allows gas to flow through it, but serves as a barrier to water.

Hydrostatic imbalance – See static lung load.

Hyperbaric chamber – A rigid pressure vessel used in hyperbaric medicine. Such chambers can be run at absolute pressures up to six atmospheres (more for some research chambers) and may be used to treat divers suffering from decompression illness.

Hyperbaric medicine – Also known as hyperbaric oxygen therapy, is the medical use of oxygen at a higher than atmospheric pressure.

Hypercapnia/Hypercarbia – Elevated levels of CO2 in the body due to inadequate breathing, generally induced by elevated respiratory loads and/or inspired CO2. The level of CO2 maintained varies from person to person (e.g., CO2 retainers maintain relatively high levels). Effects of hypercapnia may include shortness of breath, headaches, migraines, confusion, impaired judgment, augmented narcosis, panic attacks, and loss of consciousness. Dangerous levels can be reached while the diver remains unaware. Recovery may take many minutes under optimal conditions.

Hyperoxia – A concentration of oxygen in the breathing mixture that is not tolerated by the human body, generally occurring when the inspired PO2 rises above about 1.6 ata. Symptoms include visual and auditory disturbances, nausea, irritability, twitching, and dizziness; hyperoxia may result in convulsions and drowning without warning.

Hyperoxic linearity – The condition that a PO2 sensor is linear at partial pressures of oxygen above the highest calibration point.

Hypothermia – Condition of low body temperature, defined by a core temperature falling below 35ºC (95ºF), substantially below the normal core temperature range of 36.5-37.5°C (97.7-99.5°F). Reaching a state of frank hypothermia is very unlikely in normal operational diving.

Hypoxia – A concentration of oxygen in the breathing mixture that is insufficient to support human life, generally occurring when the inspired PO2 drops below about 0.16 ata.

Inhalation counterlung – The counterlung upstream from the diver’s mouthpiece block.

Insulation (thermal) – The resistance in heat flow between objects in physical contact; the inverse of conduction. The standard unit of insulation is the ‘clo,’ with 1.0 clo (1 clo = 0.18°C·m2·h·kcal-1 = 0.155°C·m2·W-1 = 5.55 kcal·m2·h-1).

Integrated open-circuit regulator – A second-stage, open-circuit regulator which is built-in to a mouthpiece block; also known as a bailout valve (BOV).

Layering (thermal protection) – Base layer (hydrophobic) to wick water away from the skin and reduce conductive heat flow; mid-layer with high insulation value to reduce conductive heat flow; shell layer barrier to reduce convective heat flow.

Liquid crystal display (LCD) − An energy efficient display that relies on the light modulating properties of liquid crystals.

Light-emitting diode (LED) − A small, low power light source used for warning lights on rebreathers.

Lithium hydroxide (LiOH) – A type of CO2 absorbent material.

Loop vent valve – The adjustable overpressure-relief valve attached to the bottom of the exhalation counterlung, which allows excess gas and accumulated water in the breathing loop to be vented. Also known as an OPV.

Manual bypass valve – A valve on a rebreather that allows the diver to manually inject gas into the breathing loop.

Manual diluent addition valve – The valve on a rebreather that allows diluent gas to be manually injected into the breathing loop.

Manual oxygen addition valve – The valve on a rebreather that allows oxygen to be manually injected into the breathing loop.

Maximum operating depth (MOD) – The maximum operating depth of a breathing gas before reaching a predetermined PO2, usually 1.4 ata or higher. This depth is determined to safeguard the diver from oxygen toxicity.

mCCR – A manually operated closed-circuit rebreather which requires the diver to monitor oxygen levels and manually inject oxygen as needed to maintain an appropriate setpoint. Also known as dcCCR or diver-controlled CCR.

Metabolism – The physiological process where nutrients are broken down to provide energy. This process involves the consumption of oxygen and the production of CO2.

mfw − Water depth as measures in meters of freshwater.

msw − Water depth as measured in meters of seawater.

Mixed-gas rebreather – A rebreather that contains a gas mixture other than pure oxygen in the breathing loop.

Mouthpiece (of CCR) – The portion of a rebreather breathing loop through which the diver breathes. This usually includes a way to prevent water from entering the breathing loop and sometimes includes an integrated open-circuit regulator (BOV).

msw – Water depth as measured in meters of seawater.

Narcosis – A form of mental incapacity experienced by people while breathing an elevated partial pressure of a gas such as nitrogen or CO2.

Near eye rebreather display (NERD) – A heads-up display that duplicates the wrist unit or primary controller.

Nitrox – See enriched air nitrox.

No-decompression dive – Any dive that allows a diver to ascend directly to the surface, without the need for staged decompression stops. Also referred to as a no-stop dive.

Normoxic – A mixture of gas containing 0.21 ata oxygen.

Notified body − Agent that acts as the certifying authority and verifies that equipment testing was conducted properly in compliance with all applicable requirements.

Offboard diluent – A diluent gas tank that is clipped externally to a rebreather.

Offboard oxygen – An oxygen tank that is clipped externally to a rebreather.

Organic light-emitting diode (OLED) – A display type that does not use a backlight and is able to display rich blacks that offer greater contrast in low light applications such as diving.

Onboard diluent – A diluent tank that is integrally mounted on a rebreather.

Onboard diluent regulator – A first-stage regulator which attaches to the onboard diluent tank of a rebreather.

Onboard oxygen – An oxygen tank that is integrally mounted on a rebreather.

Onboard oxygen regulator – A first-stage regulator which attaches to the onboard oxygen tank.

Overpressure relief valve (OPV) – the adjustable valve attached to the bottom of the exhalation counterlung, which allows excess gas and accumulated water in the breathing loop to be vented; also known as a loop vent valve.

Open-circuit scuba (OC) – Self-contained underwater breathing apparatus where the inhaled breathing gas is supplied from a high-pressure cylinder to the diver via a two-stage pressure reduction demand regulator, and the exhaled gas is vented into the surrounding water and discarded in the form of bubbles.

Optode − An optical sensor device that measures a specific substance usually with the aid of a chemical transducer.

Oxygen consumption (VO2) − A measure of the work intensity. Resting VO2 is usually assumed to be 3.5 mL·kg-1·min-1 (1 metabolic equivalent [MET]). Aerobic capacity (VO2 max) can be described as multiples of 1.0 MET. Recommendations for minimum VO2 max to be maintained by divers range from a low of >6.0 MET to >13 MET.

Oxygen (O2) control system – The components of a rebreather which manage the concentration of oxygen in the breathing loop. The system usually includes sensors, electronics and a solenoid valve that injects oxygen.

Oxygen rebreather – A type of closed-circuit rebreather that incorporates only oxygen as a gas supply. The earliest form of closed-circuit rebreather, designed for covert military operations, submarine escape and mine rescue operations.

Oxygen (O2) sensor – Any sensor that produces a signal related to O2 pressure or concentration. In diving, the most common type is a galvanic cell that generates an electrical voltage that is proportional in strength to the partial pressure of oxygen exposed to the sensor.

Oxygen toxicity – Symptoms experienced by individuals suffering exposures to oxygen that are above normal ranges tolerated by human physiology. See pulmonary oxygen toxicity and central nervous system oxygen toxicity.

Partial pressure – The portion of the total gas pressure exerted by a single constituent of a gas mixture calculated by multiplying the fraction of the gas by the absolute pressure of the gas.

Passive addition – Gas addition systems utilized by some semiclosed-circuit rebreathers to passively inject gas into the breathing loop; usually achieved by a mechanical valve that opens in response to a collapsed bellow or drop in breathing loop gas pressure.

PN2 – The partial pressure of nitrogen in a gas mixture, usually referring specifically to the breathing gas mixture inhaled by a diver.

PCO2 – The partial pressure of carbon dioxide in a gas mixture, usually referring specifically to the breathing gas mixture inhaled by a diver.

PO2 – The partial pressure of oxygen in a gas mixture, usually referring specifically to the breathing gas mixture inhaled by a diver.

PO2 setpoint – The PO2 set by the diver, used to determine when a solenoid valve injects oxygen into the breathing loop.

psi − Unit of pressure measured in pound per square inch (1 psi = 55 mm Hg = 6.9 kPa).

Pulmonary oxygen toxicity – Pulmonary irritation typically caused by prolonged exposure to breathing mixtures with oxygen partial pressures in excess of 0.5 ata. This form of oxygen toxicity primarily affects the lungs and causes pain on deep inhalation as well as other symptoms.

Quality assurance (QA) − Methods to prevent mistakes or defects in manufactured products. QA can be applied to physical products in pre-production and post-production to verify that specifications are met.

Radial CO2 absorbent canister (radial scrubber) – A cylindrical CO2 absorbent canister design wherein the gas flows laterally from the outside to the inside of a hollow tube (or vice-versa), like a donut.

Radiation (thermal) – The flow of electromagnetic energy from any object to any cooler object separated by space (air or vacuum).

Rebreather – Any form of life-support system where the user’s exhaled breath is partially or entirely re-circulated for subsequent inhalation.

Redundancy − The duplication of critical components or functions in a system with the intention of increasing reliability, usually in the form of a backup in case of primary system failure.

Respiratory load – Any load or breathing impediment that makes it harder to breathe. Respiratory loads include breathing resistance, elastic loads and static lung load (hydrostatic imbalance). Elevated inspired CO2 will make a person breathe more which increases the effects of other respiratory loads.

Respiratory minute volume (RMV) – The volume of gas inhaled and exhaled during one minute of breathing.

Safety stops – Stops carried out during ascent not required by the decompression model being followed for the dive.

Scrubber – See CO2 absorbent.

Semiclosed-circuit rebreather (SCR) – A type of rebreather that injects a mixture of nitrox or mixed gas into a breathing loop to replace that which is used by the diver for metabolism; excess gas is periodically vented into the surrounding water in the form of bubbles.

Sensor validation − Methods to confirm the appropriate function of sensors, typically oxygen sensors.

Setpoint – See PO2 setpoint.

Shoulder port – The plastic shoulder connectors in a breathing loop which connect the breathing hoses to the counterlungs, sometimes serving as water traps to divert condensation and leaked water into the counterlungs and down to the overpressure relief valve (OPV).

Skip breathing – The practice of inhaling, holding the breath and then exhaling slowly in order to attempt to extend the time underwater by using less air. This practice can lead to buildup of CO2 (hypercapnia).

Sodalime – A general term referring to a chemical agent which reacts and bonds with CO2 and is commonly used in the scrubbers of rebreathers.

Solenoid valve – A valve that opens when electricity is applied to an electromagnetic solenoid coil; usually the type of valve used to inject oxygen into the breathing loop of a closed-circuit rebreather.

Solid state sensor − A sensor with no mobile parts that detects or measures a physical property.

Stack – Slang terminology referring to the CO2 absorbent canister.

Stack time – A term used to describe the predicted time that a canister of CO2 absorbent will last before it needs to be replaced.

Static lung load (SLL; hydrostatic imbalance) − The pressure gradient between the outside and inside of the chest imposed by underwater breathing apparatus. Comfort and performance can be adversely affected, especially during exertion. The lungs can be thought of as having a center (lung centroid) located approximately 17 cm below and 7 cm behind the suprasternal notch on the chest. SLL represents the difference between the pressure delivered by the breathing apparatus (at the start of an inspiration) and the pressure at the lung centroid. If gas is delivered to the diver at a pressure equal to the depth of the lung centroid then no SLL is imposed. A person immersed to the neck has pressure inside the chest at atmospheric and outside the chest at the elevated water pressure. This represents negative SLL and can be measured as the depth of the lung centroid. A negative SLL will make a person breathe at smaller lung volumes, while a positive SLL makes a person breathe at larger lung volumes. For scuba diving, the placement of the regulator determines the SLL. A regulator in the mouth of an upright diver imposes a negative SLL. If the vertical diver is head down then the SLL would be positive. A prone diver may have a slightly positive SLL. A diver swimming shoulder down will not have an SLL imposed. With rebreathers, the placement of rebreathing bags and the amount of gas therein determines SLL. Since gas collects at the top of the bags, the orientation of the diver also matters. The pressure delivered by the breathing apparatus is determined by the depth of the bottom of the gas bubble. The SLL is then equal to the difference between this pressure and the pressure at the lung centroid. A back-mounted bag will impose a negative SLL. A chest-mounted bag will impose a positive SLL. Over-the-shoulder bags with the right amount of gas in them may have a neutral SLL, but the actual SLL varies with gas volume and can be positive or negative. If a diver with an over-the-shoulder bag rebreather swims with a shoulder down then the SLL may be negative since the gas will collect in the upper bag; should the gas volume be large enough that all breathing is in the lower bag then the SLL will be positive. Should the gas volume in the upper bag be such that an exhalation forces some gas into the lower bag, then a sudden large pressure increase is required by the respiratory muscles.

Statistical dependence − A condition in which two variables are not independent.

Technical diving − A form of scuba diving that exceeds conventional limits, generally including dives that are deeper than 130 ft (40 m), using mixed gas, requiring multiple cylinders or decompression, or taking place within overhead environments.

Temperature stick − An array of thermal sensors aligned in the scrubber canister to monitor the thermal activity of the scrubber (measuring the advance of the thermal front) to provide information on scrubber depletion. Also known to as a Temstick or Thermal profile monitor (TPM).

Trimix – A gas mixture containing three constituents; usually oxygen, nitrogen, and helium.

Upstream – A relative direction with respect to the flow of gas through the breathing loop of a rebreather; the opposite of downstream.

Upstream check-valve – A one-way valve system that permits inhaled gas to flow from the inhalation breathing hose to the mouthpiece, but prevents exhaled gas from flowing backwards. This valve is part of the breathing loop system that enables circular flow of gas.

Venting breath – A type of breathing pattern used to purge gas from a breathing loop; accomplished by inhaling through the mouth and exhaling through the nose into the mask or around the edge of the mouthpiece, thus creating bubbles.

Volume-averaged pressure (aka resistive effort) − Terminology used by US Navy Experimental Diving Unit (NEDU) to describe work of breathing (WOB) in correct physical units and physiological terms. It is equivalent to the difference between inhalation and exhalation pressures averaged across a diver’s breath, and is sensitive to flow resistance.

Voting algorithm/logic − The procedure in which rebreather electronics rely upon output from multiple sensors to determine when oxygen needs to be added and when sensors are faulty and signals need to be ignored. This approach assumes statistical independence of sensors, which may not be valid since the sensors are exposed to the same conditions for part of their history, possibly all of it if they are from the same manufacturing lot, and they are monitored by the same measurement system.

Whole-body oxygen toxicity – See pulmonary oxygen toxicity.

Work of breathing (WOB) – The effort required to complete an inspiration and expiration cycle of breathing. For a breathing apparatus, the work of breathing can be affected by breathing hose diameters, check valve design, scrubber design, depth, absorbent material, and other factors. The placement of counterlungs does not affect the WOB, but is a respiratory load by itself.

Workload – A representation of the level of physical exertion; often measured through oxygen consumption in a laboratory setting.