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.

Oxygen Measurement for Divers

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LambBookSadly, much of our information about oxygen sensors has been born on Internet chat forums. A lot of that banter is simple anecdotal evidence as opposed to scientific fact. Thankfully, owner of Vandagraph Ltd. in the UK, John Lamb has released his 2nd Edition of “Oxygen Measurement for Divers.” It might not be your idea of a book to curl up with in front of a fire, but it is one of the best investments you can make as a rebreather diver or owner of an oxygen analyzer. Lamb carefully breaks down the measurement of oxygen into abundantly illustrated chapters covering everything from gas laws to blending methods. He dispels some of the misinformation about sensor life and current limitation behavior and offers best management practices for storage and use of common sensors. He reveals the Achilles Heels of sensors and describes future technologies that will improve the measurement of gases in the future. If you want to break through the mire of misinformation and be better informed about diving safety, this is a must-read book.

One of the most important safety practices that Lamb suggests is to replace sensors 12-18 months after manufacture (not after their first use). He describes the normal expected life of a sensor is about 9-10 months in pure oxygen at the surface. He notes that single failure rates are low and multiple failures are rare when these parameters are adhered to. He also advises that correct storage procedures must be adhered to. In his examination of sensors, he believes that temperature is the main cause of early sensor failure. For most divers this means that sensors should be replaced every dive season. For me, that means putting a simple alert in my iPhone Calendar. It is a simple act that can ensure that you are reading accurate PO2 in your rebreather.

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Counting on Collaboration

By | All Posts, Bahama Blue Holes, Cave Diving, Rebreather Diving, Sidemount Diving, Underwater Photo and Video, We Are Water, Women Underwater | No Comments

Brian Kakuk is a former U.S. Navy Diver, part owner of The Bahamas Underground Technical and Cave Diving facility in Marsh Harbor, Abaco Island as well as Founder/Director of the Bahamas Caves Research Foundation.  He is also a consultant to the Antiquities Monuments and Museums Corporation/National Museum of the Bahamas.

Farewell to the Team           

Most people who know me know of my military background. Navy diving was something I had aspired to since I was a child and being part of an elite team was something that I felt drawn to whole heartedly. I’ve always felt comfortable and most importantly, safe, when my peers and I functioned as a well-practiced crew, each person bringing their own expertise and talents to the table to efficiently reach our cumulative goal.

For last two weeks, a group of people diverse in talents, and varied in origins have conducted such efficient, expert and safety conscious efforts in association with our National Geographic Blue Holes Mapping and Educational Outreach program. Our assemblage of volunteers includes educators, explorers, technical experts, Bahamian NGO’s, government employees and of course, our students. We have all come together with a common goal of exposing the world the exquisite, rarely-seen part of our planet that exists right under our feet.

Although this December session was only two weeks long, the story behind the building of this team and the common goals for the protection of Abaco Caves go back more than 10 years. Working with the Bahamas National Trust, The Antiquities Monuments and Museums Corporation, Friends of the Environment, The Bahamas Caves Research Foundation, The University of Miami, The University of Florida, The Florida Natural History Museum and the Ministry of the Environment’s Forestry Department, the players in this team have worked hard to find common ground in conserving these irreplaceable, cultural treasures of the Bahamas.

In 2005, one of our team members, Dr. Kenny Broad, wrote our first exploration grant to The National Geographic Society. I was stunned when we received the great news that they felt our project was worthy of funding and promotion.

This was a major step in the team’s conservation efforts. With the support of The National Geographic Society, our diverse group has been validated. Such high-profile recognition of our efforts helped launch more grants, documentaries and magazine articles while countless images, posts and blogs within the cave diving world blanketed social media on the interwebs.

The Bahamian people are extremely proud of their heritage and culture. Blue Holes and underwater caves have long been steeped in mystery and Bahamian folklore. They have been culturally intertwined with the inhabitants of these islands both now and before Columbus made landfall.

The Crystal Caves of Abaco have been seen on television documentaries around the globe in multiple languages. We have demonstrated to governmental agencies that these sites are worth much more to the Bahamian people as they are now, than if they were exploited for other short term gain, commercial purposes. They truly represent the uniqueness, the timelessness and the unrivaled beauty of our small island nation.

Our basic goals were to map as much of the Crystal Caves of Abaco as possible, while getting our message out to as many students as possible, both locally and internationally. Mission accomplished!

But another important accomplishment was achieved. Two weeks ago, some of our team had never met. Others were long-time friends who had experienced some of the most exciting expeditions of our lives together. But from every part of this multi-faceted project, new friends were made, new professional associations were formed, new expeditions have been planned and most of all, we bonded as a team. The comradery created by our experiences during this effort will last a lifetime, from team leaders to students.

It is routine practice for conservation groups to paraphrase Baba Dioum, the Senegalese forestry engineer: “In the end we will conserve only what we love; we will love only what we understand; and we will understand only what we are taught.”

But from the beginning, this project has been the embodiment of his words. From the original exploration, the recognition of this exceptional ecosystem that affects biodiversity and human health, and the efforts to share images and insights about these places locally and beyond, we proudly say again: mission accomplished!

It is with the deepest gratitude that I thank all the team members, both old and new. Individuals, agencies, and ministries who have been tenacious, both then and now. You all helped to bring the Crystal Caves of Abaco out of the darkness and into the hearts of the people of the world.

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Island Biogeography

By | Bahama Blue Holes, Cave Diving, Rebreather Diving, Sidemount Diving, Underwater Photo and Video, We Are Water, Women Underwater | No Comments

Tom Morris is a biologist and diver who lives in Gainesville, Florida and turned 70 years old on this expedition. 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.

At its closest point, the Bahama Archipelago is a mere 50 miles from Florida, but it has virtually nothing in common with continental mainland animals, except for the ones that can fly (birds and bats). In fact, the only native land mammal found naturally in the Bahamas is the hutia, which is of South American origin. And there are only three species of snakes, all boas and probably descendants of a common ancestor, also of South American origin.   The same pattern holds true for frogs and lizards, and even insects.   So how is it that the archipelago is so biologically isolated from North America?

Some animal groups, notably the reptiles, with their waterproof skin and low metabolic rates, are able to survive relatively lengthy ocean crossings – think Galapagos and Seychelles Island tortoises and Komodo dragons. And I have personally seen diamondback rattlesnakes, which were at one time numerous on Florida’s barrier islands, floating miles out in both the Gulf of Mexico and the Atlantic Ocean, apparently none the worse for wear. But an animal floating or riding a log from Florida has to overcome two big obstacles to make it to the Bahamas; the trade winds and the Gulf Stream.

The trade winds are planetary winds, the largest and most consistent winds on earth, and the Bahamas lie directly in their path. The trades blow from an easterly direction over two-thirds of the time, pushing floating objects towards the mainland.

The wind blows from the west less than ten percent of the time. and is generally much weaker than the easterlies. But, even if favorable winds push a drifting animal towards the Bahamas, it will soon find itself in the Gulf Stream, and moving north at up to six miles per hour, toward the open Atlantic and almost certain death.

Animals and plants on islands have historical extinction rates far greater than their continental cousins. Everyone is familiar with the fate of many isolated island inhabitants, such as the flightless birds of New Zealand, who evolved in the absence of mammalian predators, and could not cope with human introduced rats, cats, pigs, and other animals. The Bahamian fauna face similar threats. The only animals I have seen dead on the Abaco roads have been cats and raccoons. Both invasive species are known to kill local animals, including the threatened Abaco parrot. And, perhaps more tragic, a scale insect from the mainland, brought in on Christmas trees from the mainland, are destroying the native Bahamian forests of Caicos Island.

But on a more positive note, the local newspaper, The Abaconian, reported today that a pregnant manatee from Florida, named Washburn, has been tracked crossing the Gulf Stream, and is now swimming in the waters of the northern Bahamas near Walker’s Cay (pronounced “key”). She arrived on Thanksgiving day. This is the same gal who was rescued from the cooling waters of Cape Cod, Massachusetts. Ain’t no trade winds or Gulf Stream gonna keep this girl from going where she wants to.

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Technical Cave Mapping on the Abaco Blue Holes Cave Diving Expedition – Episode 12 Expedition Files with National Geographic

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December 15, 2016  / Blue Holes Blog / Sebastien Kister

Sebastien Kister is a french cave diving instructor sharing his passion for the cenotes of the Riviera Maya in the Yucatan Penninsula with students from around the world. He applies his software engineer background in the development of software and measurement instruments aimed at making the underwater tasks of his fellow cave explorers and surveyors easier.

Planning the dive! Preparing the equipment, gearing up! Exploring underwater caves! Laying line! Diving, diving and diving again! Any cave diver is thrilled at the idea of any of those steps, the common parts of our underwater explorer life. But when it comes to surveying or mapping a cave, enthusiasm is not usually the first emotion that comes to our minds. Surveying Is a time consuming occupation that requires a high level of focus while it is done. It can only go wrong … between parallax errors while reading the compass, to errors in estimating or measuring the length of the line, to the too common errors done while recording the data on the slate. To make it worse, getting a quick visualization of the data requires mastering software that is far from user friendly for the average non-geeky cave diver.

These evils were the ones facing any good willing cave surveyor at the beginning of my professional cave instructor career in 2011. I decided to develop Ariane, a cave mapping solution, in an attempt to make surveying and mapping more accurate and user-friendly for cave divers. The software was initially tested in the field during the exploration of the Doggi cave system with the Q.D.T team in Mexico. This expedition collected nearly 20000 feet of data and allowed me to quickly tailor Ariane’s features to exactly what the cave explorers needed. After 5 years of work on Ariane, I have the satisfaction of seeing it used for mapping the caves in Abaco during this National Geographic project.

Having taken care of Charybdis, the software part of cave survey, only Scylla, the actual measurement of the line in the cave, was left. That’s where Mnemo comes in. The result of a year of development and testing, Mnemo is a small handheld device that records all the parameters necessary to survey a cave line: depth/Inclination, length of the line and azimuth. In order to not effect the safety and cave awareness of the diver, I designed Mnemo to require as little attention as possible from its user, the actual contrary of what traditional slate/compass survey requires. Mnemo slides along the cave line collecting this data, using only one cursor and easily visible colours to control and signal the survey events.

The second evil was thus taken care of: surveying a cave line is now (nearly) as easy as swimming along it! Some of the explorers were given the opportunity to test the unit here in Abaco. and the beginning section of Ralph’s cave was surveyed both by hand (by Brian Kakuk)  and with MNemo (operated by Sebastien Kister) yielding only a 1.8% difference. During the time the survey was done by hand it could have been done 5 times with MNemo

 

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Mapping the Labyrinth – Episode 11 from the Abaco Blue Holes Cave Diving Project with National Geographic

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Steve Bogaerts is a cave diving instructor and explorer originally from London, England who has been living in Mexico for the last 18 years. Steve first visited the Bahamian island of Abaco in 2003 and has been making regular trips since then to explore and map the incredible Crystal Caves. In 2015 Steve and Brian Kakuk were able to complete a project years in the making by connecting Dan’s and Ralph’s Caves—two of the most beautiful and important caves on the island, and the caves that are the subject of our current survey project.

Expedition Blog 10 / Dec. 12 / By Steve Bogaerts

Today was my last day working on the National Geographic Abaco Blue Holes project. It has been a very rewarding experience both to work with this talented multidisciplinary team and to dive the amazing caves Crystal Caves of Abaco. As one of the original explorers of these caves, I am continually awed by the surpassing beauty Mother Nature can create. Unfortunately very few people will have the chance to experience the beauty of these caves firsthand. To be able to share that beauty and wonder with other people is one of our mains aims in this project.

One of the best ways we can do this is through cartography. Bringing back a map of your exploration allows other explorers and scientists to follow your path, to study and learn more, and most importantly to raise awareness of the need to protect and preserve this unique and fragile environment. Over the years many people have contributed to the exploration of the caves of the Bahamas, but unfortunately much of the mapping data remains missing or of poor quality.

During this expedition, we are starting a complete resurvey of Dan’s and Ralph’s Caves, which Brian Kakuk and I finally managed to connect together after many years of effort in 2015. The resulting connected system is properly known now as Dan’s Cave and is one of the longest island cave systems in the world. The area surrounding Dan’s Cave has moreover recently been designated a protected conservation area by the government of the Bahamas. Producing a complete map of the caves will help in these continued efforts to protect and preserve this unique and fragile natural wonder.

Cave survey, however, is a long, detailed, and laborious process. In addition to all the complex equipment a cave diver requires to safely conduct dives normally, surveying specifically requires many additional tools and techniques. The survey process begins by taking a GPS location fix at the entrance of the cave, which is then connected to the beginning of the permanent guideline that runs throughout the cave underwater. The GPS fix allows us to plot the survey data and the location of the cave on surface topological maps.

From this initial fix, every point at which the permanent guideline in the cave changes direction or depth must be fixed in place with a locking line wrap. Each one of these tie-offs becomes a survey station. The surveyor records the depth at each of these stations using a digital depth gauge and shoots the azimuth to the next survey station along the guideline using an orienteering compass. They then measure the distance between the stations using a fiberglass tape or knotted line.

All this information together with any important features or comments is recorded on an underwater slate. This basic information allows the survey team to create a “stick map,” or skeleton outline of the permanent guidelines installed in the cave passageways. This basic map can be further fleshed out by measuring the distance to the walls, floor and ceiling at every station and creating a cross-sectional sketch. Photos and video may also be recorded along with further geo-referencing at selected sites using a ground penetrating radar location tool called a “pinger.” All of this information is then downloaded to a computer survey program that creates a 3-D rendering of the cave with embedded links to photo and video of significant areas of interest.

Surveying an underwater cave is inherently limited by the amount of time that can be spent underwater on any one dive. This is further complicated in these particular caves by the saw-tooth dive profiles of the cave passageways (zig-zagging up and down), the need to surface slowly to allow for decompression, and the fragility of the highly decorated passages. In addition these caves are extremely complex with maze-like passageways that create a complex three-dimensional jigsaw puzzle of intersecting permanent guidelines. The desire to survey the cave accurately has to be balanced very carefully against the need to protect both the cave and the survey diver from any harm.

Having said all of that, it is a very satisfying feeling to return to base camp with full survey slates and to watch the cave map grow as you input the data and gain greater insight into the hydrology and geomorphology of the area. As the map has grown, so too has my desire to discover more of the secrets of the Crystal Caves of Abaco. I hope to be back soon to continue this journey of exploration and survey.

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Documenting Inner Earth – Episode #10 from the Abaco Blue Holes National Geographic Cave Diving Project

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I have an odd business card. The title simply says, “Explorer.” In reality, I am the Explorer in Residence for the Royal Canadian Geographical Society, underwater photographer/cinematographer, writer, dive technology contractor, instructor and motivational speaker. In a nutshell, I do the creative things that help keep me underwater and most of the time, underground in water-filled caves. This hybrid career is an occupation that defies easy description and leaves me pinching myself every day with excitement over my rewarding work.

On this project I am reunited with friends with whom I have worked for over twenty years. It is a joy to be in the field with such a capable and effective team. We all fall into place and jump into roles that keep us extremely busy without too much direction. Whether you are washing the dishes, finding the next roll of toilet paper or blending life support gases, there is important work that keeps the expedition moving along. Everyone has to be a specialist of some sort but also a generalist who is motivated enough to see what needs to be done. Good teamwork means that everything runs smoothly and operations are safe and streamlined.

My specialty role involves capturing everything that happens with photos and video. That means I get to miss a few dishwashing sessions, but have a lot of tasks that need my constant attention. My day begins and ends with camera maintenance. Batteries need to be charged, dome ports polished, and O-rings, cleaned. Troubleshooting and making minor repairs are a constant issue. When you take cameras and lights underwater, things will go wrong and gear will get damaged regularly. I am running four separate cameras topside and three can be encased in waterproof, pressure-proof Aquatica housings. Each camera needs to record audio as well and without a sound guy on the team, I have to do my best to keep on top of that too.

When I hit the water I carry my life support equipment weighing approximately 150 pounds with an additional lighting kit of 45 pounds, camera strobes logging in at 22 pounds and two cameras that come to roughly 25 pounds. Each component is carefully weighted and trimmed so that it is relatively neutrally buoyant underwater. That means I have to push the mass through the water but not fight with the weight. Once I am submerged, my right brain fights with my left. I switch between video and still photography while monitoring life support swimming through an overhead environment. I also have to arrange a creative dance with my teammates and that is where the experience comes in. I am not able to talk to them, so we work on a combination of experience, telepathy and hand signals to orchestrate stunning pictures that tell the story of swimming through the veins of Mother Earth. On some television projects I have the luxury of underwater communications and a large support team, but this a raw, voluntary exploration. There is no budget beyond the reward of a job well done.

When I surface exhausted at the end of day, the job really begins. I rush back to our base camp, download footage and stills and start the editing process. Social media and news today is about relevance, so each evening we reach out to the world with a new expedition blog. We all take turns writing posts, but it is my role to create these short videos for you each night before I crawl into my hammock for a few hours sleep. I choose fun over polish in my edits and hope these simple nuggets of our work will bring you a little closer to understanding the life of an explorer. There isn’t a person on this team who would rather be anywhere else in the world right now. Whether we’re assembling activities for school kids, carrying equipment or surveying these stunning caves, we know we have the best job in the world.

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Bush Medicine and Blue Hole Botany on the National Geographic Abaco Cave Diving Exploration and Survey Project

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Maria Fadiman is a National Geographic Emerging Explorer and an associate professor in Geosciences at Florida Atlantic University. She is an ethnobotanist, focusing on the conservation of human ecological knowledge and the ecosystems in which we all live.

“I found it!” the student from the local school exclaimed as she stuck a crumpled leaf under her nose. Such shouting and foliage shoving couldn’t have made us happier. She was learning the bush.

So, what was an ethnobotanist doing as part of a team of expert divers with the Abaco Blue Holes Expedition? Although I hope to experience the caves one day (apparently brushing up on my snorkeling skills won’t quite cut it…whatever), the outreach includes not only the intricate world beneath the ground, but also what grows on top. We are working with local experts from the Dept. of Forestry and the Bahamas National Trust to re-connect students (and teachers) to their cultural knowledge about their own country’s plants. When people know how to use their plants, they value the forest.

Part of my learning about this world included the nights I spent camping at the entrance of the blue hole, all of us taking turns throughout the trip. I was lucky enough to be paired with Tom Morris, an expert diver and biologist who is also a dynamic storyteller (ask him about the rattle snake…or the cougar…or…right, you get it). As the sun set I lowered myself into the clear water at the mouth of the cave while ferns and limestone rock surrounded me. When the moon came out later and I craned my neck back I saw the tops of the pine trees floating up against the inky black sky with stars bursting with a glimmering intensity and I felt like I was in a Dr. Seuss world. I then looked down, brushed off my feet and got into the tent as the sand flies thought the night, and my skin, looked pretty good too.

“Chicken Toe” Marcus said to me the next morning. He is a local bush medicine expert who works with the Bahamian National Trust. I scribbled the name in my Rite in the Rain Notebook, and repeated “Chicken Toe”. Not only is it the local name of a plant (Tabebuia bahamensis), it was also fun to say. “Or Five Fingers,” he added. “You make a tea to strengthen the five senses.”

“Cool,” I thought. I could always use some help with any of my senses.

After a day of collecting, I marveled at what had looked to me like a mish mash of green in the morning, emerging through Marcus’ teaching as a medicine chest by evening. We then cut up the plants, stuffed them into pots and set them to boil. As I tasted each tea, and felt healthier by the gulp, Marcus let me know that the best tea he made was the “21 Gun Salute.” Apparently I would have to try that one next time, as it took 21 (surprise!) plants and extra time to prepare.

The next morning, Terrance Rodgers, who works for the Forestry Service, helped me brush up on my plant names, walking me around the forest above the blue holes. I quietly repeated each one to myself, practicing as I stood with the two experts, Marcus and Terrance, behind the table laden with plants and the containers of bush tea.

As the children came to the table we explained how to rub a soap leaf (Petitia dominguensis) on their arms. Some scrubbed right away, while others looked warily until their friends placed the leaf on their skin and exclaimed “It really does feel soft!” then all would give it a try. They crushed Sweet Margaret (Byrsonima sp.) under their noses and brushed their teeth with White Sage (Lantana camara). Each student then held out a cup to taste the teas. Amidst excitement, curiosity and some fear (which adding sugar usually assuaged), for many it was their first taste of their own country’s bush medicine.

The students then scrambled away from the table to identify plants, viscerally diving into the bush world. They reached out to touch palm fronds, laced fingers through bracken ferns, and with dogged determination smelled crumpled leaves looking for Sweet Margaret.

With some groups we would shout “Into the Bush!” and they would clamber further up a slope (only about five feet from the port-a-potty actually). At the end of the fourth day, the first to arrive at the top of the hill was a student who had shied away from the plants when he first came to the table. He now clutched a Soap Bush leaf in one hand and a piece of Chicken Toe in the other.

Balancing on a log he shouted, “I could live in the bush!”

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Beautiful Bones – Episode #7 of the Abaco Blue Holes Cave Diving Project – National Geographic

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Nancy Albury is the Abaco Manager and Curator of Paleontology for the National Museum of The Bahamas. She is responsible for management of the natural history collections as well as documentation and research of the blue holes, cave sites and their fossil assemblages.

December 10th, 2016

When I was a child, I was intrigued by a large cave on New Providence Island here in the Bahamas. After pestering my mother for years, we finally took a trip to see it. I remember standing in the cave for the first time, spellbound in the darkness as the bats fluttered past me like fairies in the dim light of my headlamp. It never entered my mind to be afraid of what might be in this dark place. Rather, I was hooked, captivated by the cave’s beauty and the mysterious animals that lived within.

It was later during college that I began cave diving. My diving buddies and I would sneak across cow pastures so we could dive a forbidden cave – a considerably greater level of excitement for me than the typical college merriment. And for the past 11 years I have worked for the National Museum of the Bahamas, a job that has given me the opportunity to explore, study and document the dry and flooded caves in the Bahamas, referred to as blue holes. These blue holes have remained my personal happy place over these many years, quiet secretive spaces that hide the mysteries of our natural world.

Blue holes are time capsules that contain some of the most intriguing collections of natural, geologic, and human history in the West Indies. Diving in the crystalline passages of a blue hole is the equivalent of time travel. Like reading chapters of a book, history unfolds in the wall rock’s layers of sand, coral and shells. Speleothems and bottom sediments hold a rich history of sea level, climate change, and the remains of plants and animals. All of these reflect the surface ecology of the time when they were deposited.

My real passion is the fossils, some dating over 4000 years old, of a variety of animals that are extinct and give us clues of how the ecosystems functioned before human occupation. Through the millennia, animals that came to drink became prey, or fell into the blue holes as they drank. Trapped and treading water, their ultimate fate was to drown and sink into the dark anoxic bottom sediments, mixing with wind-blown leaves and vegetation that grew around the blue hole during the same time. The growing list of species that we’ve recovered from these holes includes the remains of animals that indicate a reptilian ecosystem, dominated by giant tortoises and crocodiles, the apex terrestrial predator of its day, large rodents known as hutia, numerous species of birds (many now extinct), including some that were flightless. Evidence including seeds and are used to reconstruct the past vegetation and how the environment has changed through time.  Blue holes were dry caves during the Pleistocene Ice Ages and were home to bat coloinies and giant owls roosts who left left behind the skeletal remains of their meals. Later, during the higher sea levels of the Holocene, the remains of mammals, birds, reptiles, and fish also became entombed. Lucayans, the earliest humans in the Bahamas, who have no living descendants, were no less mystified than we are today and there is skeletal evidence of these paleoindians preserved in the anoxic salt water. They revered blue holes and caves as spiritual portals to the world beyond life, using them as places to bury their dead.

A friend of mine spent his life exploring the caves and finding prehistoric sites on Abaco. For years, he was hesitant to document the sites he’d found. Sadly, he recently passed away along with too much of his life’s work. I understood his reserve; the double-edged sword of exploration and discovery is the threat of its destruction and looting. However, the best conservation often starts with education about the value of these places that is not always self-evident. I was immediately supportive of this National Geographic project when proposed because it had the rare combination of innovative outreach, mapping, and image collecting that can help further The Museum of the Bahamas goals of making these hidden realms accessible to the public.

The incredible response from the children that we’ve taught this week is evidence of the team’s effective hands-on activities. Watching their faces light up with those special “ah-ha” moments was a flashback to the childhood adventures that inspired my own love of caves. I’m sure the children who have visited the site this week will remember these real life learning experiences for a lifetime and become tomorrow’s citizens who know and appreciate our natural history.

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Speaking Up for Safety

By | All Posts, Bahama Blue Holes, Cave Diving, Rebreather Diving, Sidemount Diving, We Are Water, Women Underwater | No Comments

Jill Heinerth, Dec. 10, 2016

Jill Heinerth serves as the documentarian for the Abaco Blue Holes Project. Her hybrid career includes teaching, photo-journalism, motivational speaking, consulting and pretty much anything that keeps her underwater. She is the Explorer in Residence for the Royal Canadian Geographical Society and a member of the inaugural class of the Women Divers Hall of Fame.

Kenny Broad found a hilarious video online about train safety. After a good laugh, we decided to try our hand at sending our own safety message out into the world. Our singing talents are rather weak, but our hearts are in the right place.

Cave and rebreather diving have been called the most dangerous “sports,” and unfortunately we have all lost our fair share of friends to lapses in judgment and poor choices before they got in the water. Years of accident analysis have shown that when divers break our “cardinal rules,” accidents happen.

In the 1980s, cave diving pioneer Sheck Exley determined that most fatalities were caused when at least one of these rules were broken:

  1. Get proper training from a qualified and active cave diving instructor before entering the overhead environment and then dive in conditions which are similar to or better than the environment you trained in.
  2. Run a continuous guideline from the open water so that in the event that visibility is lost, you will have a tactile reference to get you home safely.
  3. Always reserve at least two-thirds of your gas supply for the exit portion of your dive so that you can assist a teammate in the worst case scenario of a catastrophic gas loss.
  4. If diving deep, use an appropriate helium gas mix that leaves your head clear and your mind sharp.
  5. Dive with the appropriate redundant backup gear including at least three sources of light.

After more recent examination of cave and rebreather diving fatalities, we have added several more specific rules that mitigate the risks. These include:

  1. Personally analyze and properly mark the breathing gas in your cylinders.
  2. Complete a proper pre-dive check prior to diving a rebreather.
  3. Pre-breathe your rebreather for five minutes on the surface in a safe place and watch your electronic controls to determine proper function.
  4. Do not jump in the water if your rebreather fails a safety check.
  5. Change oxygen sensors frequently and pro-actively, and do not dive with fewer functioning sensors than the manufacturer’s standard for your unit.

Cave and rebreather training is available globally through training agencies and professional instructors that are good safety role models. If you choose to take on these activities, please seek out an active and experienced instructor who can take you through an industry recognized training program. The NSS-CDS, NACD, GUE, RAID, IANTD, TDI and other agencies can be found online.

So please forgive our lack of pitch and if you want to see a truly professional and humorous safety video, check out this one from our friends in railway safety.

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The Life Within Underwater Caves – Blue Holes Expedition with National Geographic

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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.

Expedition Blog 4 / Dec. 6 / By Dr. Keene Haywood

Driving south down the main high way on the island of 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 20 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 goal for our current project is to survey and map these complex and beautiful water filled passages, exploring the edges of what is known and unknown. But the project has other dimensions of exploration as well.

Exploration—the 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 takes elements of observation, documentation, and communication and combines them with this wonder to develop 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.

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REBREATHER GLOSSARY

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.