RebreatherPro

Beyond SCUBA with Jill Heinerth

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.

AP Diving Offers Exciting New Features

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AP Diving is showing off some new bling at the NEC Diving Show in Birmingham. They’ve aggressively jumped into the market with a bright new heads up display at a very reasonable price.

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The unit has been fully vetted with CE Approval setting a high bar for other manufacturers to follow. The unit can be retrofitted to existing rebreathers as an upgrade or ordered on new ones.  The following features are posted in their press release today:

  • Vivid OLED colour display – with excellent readability even in very poor viz
  • Conditional colouring indicates status changes – Green = good, Yellow = info alert, Red = warning
  • New Ascent Rate and Ceiling Height graphical displays
  • Live information in-line-of-sight throughout the dive
  • Super adjustable articulated mount allows positional preference
  • Upgrade path for all AP rebreathers with Vision electronics
  • Ideal for photographers, film-makers or anyone preferring hands-free monitoring
  • CE Approved

For further details on this development and other new features, see:

http://us1.campaign-archive2.com/?u=fa7b024f6b9aabf24bdd1c393&id=e2e91b8b07&e=f36ba02db9

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Dive Safety – An Interview of Jill Heinerth

By | All Posts, Cave Diving, Rebreather Diving, Sidemount Diving, Underwater Photo and Video, Women Underwater | No Comments

ALERT DIVER, the prestigious and excellent publication of Divers Alert Network recently reached out to me with some questions about how I view dive safety. Establishing a culture of dive safety is of great importance to dive leaders and is central to Divers Alert Network’s mission. They’ll be sharing these thoughts and those of other experts in coming issues of their magazine.

ALERT DIVER: Recreational diving culture; what does it mean to you?

JILL HEINERTH: Sport diving is a community made up of many different subcultures. These small groups of divers are knitted together by their shop, club, charter operator or perhaps agency affiliation. Some of these tribes are known for their technical expertise, their great trips or safe operations. Others are tagged for aggression, cockiness or exclusivity. It’s like any other participation sport. People tend to congregate in smaller groups and roam with pack like behavior. If you’ve been in diving long enough, you’ll find that people drift in and out, switch sides and change their behaviors. Sometimes change is brought on by the wisdom of experience, sometimes through the example of great leadership and other times influenced by the shocking impact of witnessing an incident or tragedy.

ALERT DIVER: What are characteristics of a safety-aware diver?

JILL HEINERTH: In my opinion, a safety aware diver is one who is fully engaged in their participation in diving. He/she understands and has accepted risk and takes full personal responsibility for outcomes. A safety-aware diver is one who looks on a given dive and asks him/herself, “Am I fully capable of self rescue in this scenario and am I fully capable and willing to execute a buddy rescue if needed?” A safe diver, would only enter the water if the answer was an unequivocal “yes” to both questions.

ALERT DIVER: What is the role of training agencies in shaping and disseminating a culture of safety?

JILL HEINERTH: Training agencies have the opportunity to set the ground rules right from the beginning and guide divers to recognize that the general safety rules have been developed from practical experiences. I understand that training agencies have a responsibility to their stakeholders to sell classes and materials, but ultimately the sport benefits when a safe culture is rooted during entry level training and is carried through consistently in continuing education. When shortcuts are allowed and tolerated, then an attrition of knowledge and decay of safe practices results. One instructor that slips through the cracks without following standards can affect hundreds of future divers that can also move on to affect another generation of divers. Maintaining high standards and ensuring strict quality assurance is critical to nurturing a consistent climate of safe diving practices.

ALERT DIVER: How can dive operators contribute to the culture of dive safety?

JILL HEINERTH: I suppose I have become more conservative as I have gained the wisdom of experience. At the risk of sounding old, I sometimes look back on my early years as a Divemaster and realize that some of my colleagues bowed to the constant pressure to take clients on the “most exciting dive of their lives.” For some that lead to cutting corners and stretching standards in the hopes it would create return customers and big tips. These days, operators are under increased competition to offer the best adrenaline-laced experience they can possibly summon.

But I learned early that enthusiasm is infectious. If you love what you are doing, then your clients will love their experiences with you. There is wonder and satisfaction just being underwater. When people get away from their office or cold climate and arrive in a tropical destination, what they are really looking for is positivity, a communal participation in remarkable experiences and fun. It’s great if you get blessed with a stunning manta ray, but it can be just as exciting to see a jaw fish with a mouth full of eggs. A Divemaster is a skilled professional, but also a motivational speaker. Their knowledge and engagement in their passion is what will ultimately be remembered and that doesn’t require great depths or unnecessary risks.

ALERT DIVER: What kind of social support should divers expect when diving?

JILL HEINERTH: I believe that divers should seek a nurturing environment. (I cringe when I hear instructors or Divemasters yelling at a client). A learning environment or a diving tribe should be supportive, free of harassment or peer pressure and inclusive of all genders and experience levels. Diving should be mutually respectful. Each diver should be given the opportunity and be encouraged to take full responsibility for his/herself. Anything less than that is disrespectful to the individual and team and is patently unsafe.

ALERT DIVER: How can the culture of dive safety be promoted?

JILL HEINERTH: My Great Uncle Jock used to tell me that “a friend knows the song in your heart and sings it back to you when you have forgotten the words.” That lesson speaks volumes to me in terms of diving. Human beings are so inclined to find camaraderie that they are often prone to peer pressure. Keeping that in mind, it is important that individuals, instructors, operators and shop owners all work together to promote safe diving practices and pledge to point out issues that evolve over time. In doing so, they should recognize that positive role modeling will go a lot farther than negative reinforcement.

As a young diver in Tobermory, Canada, I was taking a class from a great role model, Dale McKnight. He was a master at mentoring academic and physical diving skills but also the psychological factors in diving. We had worked hard for days, practiced skills and made plans to complete the deepest and first decompression dive of our lives. We were on the boat heading to the site when Dale told us that we had done such a great job that he would reward us with an extra ten feet of depth and five more minutes of bottom time. We could use the contingency plans we had constructed the night before. My colleagues hooted and hollered in excitement while I felt a deepening angst growing in the pit of my stomach. With my head bowed down, I quietly muttered that I did not feel ready for that dive… that I would sit on the boat. I was disappointed and embarrassed. Dale tried to reel me back into the dive, but I was dejected and not ready.

After allowing a few minutes of chest beating and gratification, Dale admonished the other divers for permitting him to shift a safe, organized plan into a “trust-me” dive. At first, I did not understand what was happening, but soon recognized that he was patting me on the back. I had passed his test. By aborting my dive, I was being rewarded. I’m so glad to know that Dale is still teaching today, because he taught me an important lesson that may have even saved my life. A true survivor needs to know when to get within a hair’s breadth of complete success and then be willing to turn back and call it a day.

I have a special note for my rebreather diving colleagues. A safe culture of rebreather diving includes three simple actions.
  1. Use a checklist (automated or on paper) every time your prepare your unit for a dive.
  2. Complete a five-minute pre-breathe of your unit in a safe seated position with your nose blocked and paying attention to your displays.
  3. Do not enter the water if anything has failed your test and abort your dive immediately in the safest manner possible if a failure occurs underwater.

Adherence to these three rules must be uncompromising for yourself and everyone on your team.

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On Motorcycles and Rebreathers

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A few years ago, I completed a Motorcycle Safety Class and was struck by the similarities to training rebreather divers. It was really good to be a student instead of an instructor for a change. It’s one of those experiences that reminds you what performance pressure feels like and reinforces the qualities of good instruction.

I have to say that I am accustomed to thriving and performing well in new learning situations. However, I had absolutely no background in motorcycles and started off at the bottom of the heap amongst my fellow students. Everything was new.

I was pleased to be given procedures and checklists that kept me on track. A pre-ride check let me assess the readiness of my bike. An ignition-check mantra helped me recall the steps for safely starting the motorcycle. Rules of the road engaged the entire class in safe operation on the driving range.

But when all was said and done, there were safe operators, risky operators, people who didn’t have a clue and those of us who were trying hard to learn and never make the same mistake twice. Discovery learning always works best for me. There is nothing like almost dropping the bike to highlight the lesson of not using the front brakes in a curve!

I was also struck by the importance of giving-in a little and letting the bike become an extension of my body. When you fight technology, things don’t work very well. When you look up and move with the bike, things get much easier. Looking far down the road and anticipating the possibilities ahead, keep you safe.

Finally, I was very aware, that the calm demeanor of my instructor was a critical aspect that contributed to my best performance. When a second coach raised his voice, class performance fell apart and people started to make mistakes.

So what can you learn from this as a new rebreather diver?
  1. Find a patient instructor who will allow you to make some mistakes, so you can learn important lessons through discovery.
  2. Don’t be afraid to make mistakes. If you weren’t going to make any, you wouldn’t need to be in a class.
  3. Work towards becoming a physiological extension of your rebreather. Don’t keeping fighting it for buoyancy or over-thinking it for counterlung volume.
  4. Use checklists and verbal keys for important safety steps.
  5. Keep your head in the game and don’t let yourself be distracted by the performance of others
  6. Look ahead and anticipate problems and issues. Rehearse those scenarios until you become fluent in the new technology and motor skills.
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Rebreather Bailout Gas Management

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In My Experience

Nearing twenty years of rebreather cave diving, I have witnessed many dives that resulted in open circuit bailout. Here is a partial incident list:

Cockroach ate edges of non-return valve leading to carbon dioxide build-up
Diver sucked non-return valve through plastic web during negative loop check
Rubber hose tore open where it attached to rebreather head (dry-rot, age)
Rebreather laid horizontally in car and settled scrubber contents leading to almost immediate breakthrough
Diver failed to put scrubber canister in unit
DSV lever unscrewed and came off in diver’s hand leaving hole into loop
Mouthpiece torn and unusable
Wet sensors completely unreliable
Mysterious electronics failure leads to unreliable sensors
Low battery voltage causing rebreather to reset and inject oxygen
Lithium battery explodes out of back of rebreather
HMI light causes unshielded displays and HUD to turn off
Diver over-breathes counterlungs in high flow cave
Scrubber used beyond manufacturer’s specifications

Catastrophic failures on open circuit scuba are usually manifested in events like high-pressure seat failure in a first stage, hose rupture or manifold damage, burst disk and valve breakage. Technical divers spend ample time rehearsing valve drills and abort scenarios, since gas loss equates to time pressure. They manage the emergency and get home quickly!

On a rebreather, failures more commonly develop slowly. In some ways, a hose rupture or first stage failure is one of the easiest issues to deal with. In many cases, the diver simply reaches back, turns off the valve and feathers it on and off as needed or simply bails to the off board bailout tank.

In the early days of rebreather training, we used to put a considerable emphasis on keeping the diver on the loop. These days, we teach students the myriad options available to them in emergency scenarios, but encourage divers to bailout to open circuit if they have any doubt about the safety of what they are breathing or if the task load is too large. If in doubt, bail out.

Examining the type of failures that could lead to the necessity of an open circuit bailout will help the diver choose how much gas they wish to carry.

Confusing Data

When the face of an oxygen sensor gets wet, it will tend to read low and slow. Sensors that get wet on the wiring side may read high. If a diver is in doubt of their sensor readings, a vigorous flush with diluent gas will help them determine if any of the sensors are reliable and accurate. After determining which, if any sensors, are accurate, the diver may allow the system’s voting logic to get them out of the cave or run the unit manually with the single accurate sensor during their aborted dive. If the diver is at all uncertain about the accuracy of their sensors, from flooding, poor calibration, current limitation in old sensors or other electronic failures, then an open circuit bailout is not just warranted but is the only safe option.

Catastrophic Loop Failure

Mechanical problems may also cause catastrophic loop failures that demand open circuit bailout. Ripping or tearing a breathing hose in a tight passage, counterlung tears, dry-rotted rubber hoses, lost or torn mouthpieces and breakage of the DSV lever itself are all examples of failures that leave the loop unrecoverable.

Carbon Dioxide Breakthrough

Carbon dioxide issues are the most insidious problems leading to unrecoverable loop failure. Partial flooding may lead to channeling of scrubber material just as easily as improper packing. Using carbon dioxide material beyond its specification may also lead to rapid breakthrough. Damaged non-return valves could also lead to carbon dioxide build-up.

Given the scenarios above, I have personally chosen to carry ample open circuit to get myself out of the cave for almost all of my dives. In a few cases of extreme exploration, my team has opted to share bailout beyond a certain point of penetration. I find that carrying two, 80 cubic foot tanks in a sidemount style is very easy and comfortable and will get me out of most dives. I use smaller tanks for shorter penetrations. Beyond that, staged gas is preferred. Understanding that carbon dioxide issues will significantly elevate the surface air consumption rate of an exiting diver, I am very conservative with gas planning.

Both IANTD and the NSS-CDS have adopted a standard for bailout gas, requiring that a dive team carry 1.5 times enough gas to get a single diver out of the cave. Arguably, this results in greater team conservatism than is offered to open circuit divers in a pinch. However, it also leaves a team of three with the need to stay together and swap tanks throughout the exit so that a diver is never left without open circuit gas. Other divers have taken drastically different approaches. Some advocate a one-hour rule that gets every diver topside with one hour of any consumable in reserve, whether it is batteries, gas, scrubber or bailout.

My best advice to rebreather divers is to make a careful risk assessment prior to their dive and visualize the worst-case possible scenario. Only then, can you make the right decision for you and your team about the amount of gas to carry in reserve.

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Advice to CCR Photographers and Videographers

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Rebreathers are ideal for capturing incredible animal action. When you slide through the water column without making bubbles, suddenly you are a part of the environment, not simply an invader making noise and a curtain of gas. That being said, we use a different part of our mind to process creative thought than the part of the brain necessary for intellectual processing of rebreather skills. Videography and photography create an enormous task load and one that robs you of a sense of time. It takes a lot of skill and patience to force your mind to depart from the creative aspects of image-making and stay on top of monitoring your rebreather. I often shoot video and photos using my rebreather, but I am keenly aware that I have to stay on top of it and I advise my models to be extra vigilant with watching our time too.

Additionally, our buoyancy skills are hampered slightly with a rebreather. If you are a gas-miser, then your buoyancy skills will not be as fine-tuned as they are on open circuit scuba. You will need to monitor your onboard diluent and oxygen closely and allow for use of greater volumes. Conservation of the environment is critical and therefore, you should use a camera and viewfinder combination that allows you to look over the unit rather then through a small window, stuck against your mask.

Some features that help an underwater image-maker are:

1. HUD – A heads up display is critical. More importantly, it should be one that is capable of showing “exact PO2″ and not just whether the diver is within range of his/her selected PO2. In my opinion, those types of HUDs do not protect the diver from making an inappropriate choice and not realizing their mistake. They also do not catch the diver who has forgotten to raise their set point at depth.

2. NERD – A new form of HUD, the NERD was created by Shearwater Research to show all your handset information in a small view find placed against your mask.

3. Some “Auto Set Point” features on some rigs will help ratchet up the set point as the diver descends, preventing you from slipping back to a lower partial pressure as oxygen is metabolized, but these are only appropriate for direct descents and not necessarily for slow or saw-toothed descents.

4. Vibrating Alarms – The Hammerhead-style rebreathers and some others such as the Sentinel, include a mouthpiece that vibrates in a life threatening situation. A vibrating alarm is nearly impossible to ignore and is far superior to simple audio alarms which may or may not be heard through a hood. Having also owned rebreather with simple audio alarms, I can tell you there is a big difference. Hoods impair hearing and older divers lose their ability to hear high frequencies, especially after a lifetime of listening to bubbles roar past their ears. Easily fifty percent of my diving colleagues cannot hear the beeps on their wrist computers.

So, despite the opportunities for animal interaction that rebreathers may present, ensure that you are making the right choice to use one for a particular dive mission. It all comes down to risk assessment. If you need every bit of your mind to run your camera well, then leave the breather behind. Perhaps that is why I still love to keep my free-diving skills up to snuff!

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Overcome Fear

By | All Posts, Cave Diving, Rebreather Diving, Sidemount Diving, Women Underwater | No Comments

This message is a universal one. Every diver should know how to embrace fear to survive. In this short nine minute video I describe life lessons that have helped me face the worst and come home safe. It has a special focus for rebreather divers about basic preparation that will help you prevent most common rebreather diving accidents.

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Algorithm Comparisons

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Matching Your Backup Computer to Your Rebreather

If you are like me, you might have a backup CCR computer that is loaded with a different algorithm that your rebreather. Owning more than one rebreather, I want to know that the algorithm is a reasonably close match for each unit. Ryan Crawford from the UK has made some comparison profiles on different algorithms to help CCR divers see how different computer algorithms match up on different rebreathers. The comparisons are made to a depth of 45m so that air is used as the diluent. This keeps the comparisons within the scope of recreational rebreather diving. He compared the following algorithms: Suunto Fused RGM – found in the DX watch-sized CCR computer Gradient Factors – AP Projection VPM-B – V-Planner DCAP – Poseidon WeDive VGM – VR Proplanner Comparisons were made using some commonly used settings as described below: Suunto Fused RGM at five available settings Gradient Factors as recommend from the AP Inspiration Vision manual 90/95, 50/90 and 15/85 VPM-B settings of 0 and +2 DCAP in Poseidon WeDive app replicates the Poseidon MKVI actual dive data VGM carried out with zero bubble control As far as possible all other settings were kept the same, such as descent and ascent rates etc. The following data is intended as a guide only and should be individually verified. The setpoint for 15m and shallower has been left at 0.7 bar and switches to 1.3 bar at 18m as might be expected for actual dives.

RBalgorithmcomparisonCHART

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Suunto Gives Direction to Team Sedna

By | All Posts, Cave Diving, Rebreather Diving, Sedna Expedition, Sidemount Diving, Underwater Photo and Video, Women Underwater | No Comments

The Ambit 2S Sport Watch

suunto-ambit2-s-white-hr-l-1Suunto has provided Team Sedna with Ambit 2S GPS fitness watches to track our journey through the Arctic. These advanced devices are used by athletes around the world to track their fitness, journeys and adventures. This particular version of the Ambit is specially designed to fit women’s narrower wrists yet provides full features of the watch. The Ambit has become an open source craze among computer savvy athletes. The data can be repurpose using community shared apps. Clever programmers have created apps such as a cupcake counter, letting a runner know how many cupcakes they have burned off. Other more serious apps help swimmers, triathletes, cyclists plan their training for events and life goals.

Sedna swimmers will gather heart rate data, GPS location, speed and duration in the water. We’ll be using the movescount.com website to log date and shout it out to the world.

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Kevin Gurr May Save Your Life

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White Paper on Oxygen Sensors May Save Your Life

O2 cells Tempstik 7735lThis article, painstakingly written by Kevin Gurr and aggressively peer reviewed by colleagues, may be the most important thing you will read about your rebreather. Your oxygen cells are the key to your life support and yet many people abuse them and use them beyond their working life. Kev’s article spells out everything you need to know about cells and why we are all so adamant about proactively changing them out. Read this and read it again. The information may save your life.

This is a very long article, but well worth your time. You can read it or download it here: Oxygen-Sensors-for-use-in-rebreathers-release-V1

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Thermal Protection Under the Ice

By | Rebreather Diving, Sedna Expedition, Sidemount Diving, Underwater Photo and Video, Women Underwater | No Comments
The Santi Ladies First Suit

 

lf_drysuitReliable drysuits are critical for survival in cold water. Without proper exposure protection, one could expect to survive for less than 45 minutes in water near freezing. In less than 15 minutes, unconsciousness would be likely.

 

Santi Diving has outfitted the Sedna team with the first drysuits ever designed specifically for women. These “Ladies First” suits have many special features. The crotch has been recut and reinforcement for better leg motion and comfort. But beyond custom tailoring the suit offers many unique features. It has been redesigned from their unisex line with a right shoulder to left hip zipper. This permits better placement of the left shoulder dump, especially in small women. The zipper is a TIZIP Masterseal design that is flexible and therefore comfortable. The ripstop nylon fabric also allows for stretch and easy movement. As with all Santi suits, they have not cut any of the features made for serious divers. Larger gusseted leg pockets are easy to reach and the right pocket has an extra zipped small pocket for quick access items.

 

The Santi Ladies First drysuit is produced from lightweight Ripstop nylon fabric especially designed for Santi to achieve the best possible stretch. It is light and soft but at the same time very durable and flexible, which is exactly what any diver needs. The cold water hood is made with supple stretchy neoprene. Recognizing that women’s head to neck ratio is great than a man’s, they added an expansion zipper in the back of the hood. Now it can be easily donned without tearing hair and then zipped for perfect fit.

 

lf_undersuitOur exposure protection is a full package. Each woman will wear a custom engineered and fitted undersuit made of Thinsulate. The garment also has a net layer, which makes the undersuit even more durable and practically indestructible. The lining is made of soft 190 gram micropolar insulation and the outside layer is constructed of polyamide additionally reinforced with polyester fabric in the most vulnerable areas. A layer of Merino wool next to the skin will add to comfort and warmth.

In between the Merino and the undersuit, swimmers will wear an electrically heated vest which covers the torso and is connected to electrically heated dry gloves. Cinematographers who are likely to experience longer exposure times will wear a heated full undersuit. Dry gloves dock to rings on the wrist to keep the heated gloves from getting wet and a double layer ice hood will keep heads warm. The only exposed flesh will be around the swimmers mouth.

 

HeatingPackageThe wiring for the heated undergarments is ported through the dry suit inflation valve, which has been replaced with a dual-purpose thermal valve. An E/O connector hanging from the chest plugs into a lithium ion battery pack provided by Halcyon for the project. These specially designed battery packs are intended to provide up to two hours warmth. Electric undergarments demand high-amperage and therefore high capacity lithium batteries are the best choice. The pack will be mounted on the diver’s harness.

 

The entire package should allow each woman to remain submerged in water that would be categorized as deathly cold. Close to icebergs and floes, we expect temperatures as low as 28°F/ -1.8°C. One tenth of a degree colder and the entire ocean would be solid!

 

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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.