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.

Bailout Bottle Trim for Rebreathers

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A Lesson from Sidemount Divers


Sidemount diving has taken the diving community by storm. The popularity of this style of carrying tanks has grown tenfold in the last few years. Now rebreather divers are taking a lesson from sidemount divers to help them streamline their bailout bottles. Standard stage rigging generally hangs bailout tanks nearly perpendicular to the diver’ s body. This is neither streamlined nor conservation-minded. It is far more effective to mount your tanks in a way that aligns them with your body. There are numerous ways to achieve this, but here are two simple solutions:

1. Use your SMS 75 or SMS100 sidemount harness on your rebreather.

SideBungeeKnot9054l2. Make your own simple sidemount bungees: – Take a length of heavy bungee cord (approximately 18 inches) and place a knot in one end. – Thread the bungee cord through a hole in the backplate of your rebreather. Use a hole on the edge of the plate, halfway down or roughly below your scapula. The knot should be on the back side of the plate so it does not press into your shoulder or back.

3. Loop the free end of the bungee through the ring of a small bolt snap or other type of clip and secure it to the clip with two zip ties.

4. Clip to the lowest shoulder D-ring on your harness.

5. Place a butterfly D-ring on your crotch strap or use a butt plate (as shown in the photo) for the lower tank attachment point.

6. Mount the lower bailout bottle clip on the tank roughly 180° opposite from the tank valve hand wheel.SideBungee9055l

7. Use the bungee to snug the tank close to the body by looping it over the tank valve so the tank tucks into your armpit region.

8. Move the knot to shorten or lengthen the bungee and adjust for size.

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Interview with Nick Hollis

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JillRPOVJoe Cocozza, DIVER Magazine and PodDiver Radio Host interviews Nick Hollis about the Explorer rebreather.

Click here for full interview.


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What’s Your Adventure?

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I’m very proud to be featured in Suunto’s new adventure video!

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Get Shot in North Florida

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

There are few things I love more than shooting people… underwater! When I am home in North Florida by the crystalline underwater springs, I love to arrange private photo experiences for interested divers or teams. For some people, it is a chance to learn more about underwater photography. For others, it gives them a beautiful portfolio of shots to share with friends. I also arrange to shoot professional video content for teams and individuals who want some HD footage they will treasure.CasPeanut*0225l

I can accommodate teams of up to three divers in the cave and more if we are shooting in open water. On the first dive, I’ll get you acquainted with the photographic process while concentrating on portrait type shots. On the second dive we get a little more adventurous and travel further with you not only acting as the model, but also as a lighting assistant. It’s a fun day of interesting diving and I guarantee your satisfaction with the final results. After a couple of days of editing on my part, I’ll upload a portfolio of high resolution images to DropBox so that you can download all the original files. I can offer advice on where you can get great quality prints for your wall or other merchandise with your photo’s printed on the surface. You’ll be able to share the work online, make prints, and use the images on you website. Essentially, you can do anything you like with the exception of selling the shots (that can be arranged through commercial shoot). If you are interested, I will also use the shots in magazine articles and books with your permission.P1tunnel0096l

Drop me note if you are interested!


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Rosemary Lunn Takes on Rebreather Diving

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Women Underwater reached out to dive professional Rosemary Lunn of The Underwater Marketing Company to ask about challenges in learning how to dive a new rebreather. She offers some personal experiences and candid advice for others who want to take the plunge.

Rosemary Lunn:

My greatest challenge with equipment has got to be with rebreathers. I seem to be fortunate to be in the right place at the right time on many occasions when it came to rebreathers. I have a definate love / hate relationship with this piece of kit.

My first rebreather dive was in Stoney Cove in 1996 on a Drager Dolphin. It may seem hard to imagine now, but at that time rebreathers were rarely, if ever seen by recreational divers. Technical divers were all diving doubles / twins. Depending on which training agency you listened to, nitrox was considered a technical or voodoo / devil gas; and the first mass produced rebreather, the Inspiration, was still on the drawing board. The only people with access to rebreathers were the military for covert opreations, therfore you never saw them. A lot of heads turned as I walked down the car park to the water, as everyone oggled the Drager Dolphin.

Rosemary Lunn at InnerSpace in Grand Cayman. Photo: Courtesy of Peter Den Haan

Rosemary Lunn at InnerSpace in Grand Cayman. Photo: Courtesy of Peter Den Haan

I think it is fair to say that Drager created a game changer when they launched the Dolphin. A few years later they followed it up with the ill-fated Drager Ray. This was a 22 metre rated semi-closed recreational rebreather and was to pay a part in my rebreather education. It was one of those ‘right place, right time’ moments again. I was offered the chance of working for Drager Dive and given 24 hours notice to leave the UK and join the team. My boyfriend at the time gave me the choice of him or Drager. I cried all the way down to Dover. How could I turn up an opportunity like this? I spent the summer of 1999 as part of an international team taking divers diving on rebreathers all over Europe. It was an amazing job. I remember ringing my Mum with the news I had the job and saying “it was better than winning the lottery!”. Money could not buy this job.

When we were not travelling to incredible destinations in Holland, France, Spain, Italy, etc, we were diving. We stayed in everything from grand luxury hotels to wooden bunks in basic huts. We lived on chocolate, diet coke and listened to Cher’s ‘Believe’ album on repeat for three months. We spent our days building and checking units; going diving; and then cleaning and packing up units. I was in 7th heaven. Until the one day I was task loaded. The one day I made a mistake. It was my own stupid fault. I built and carefully check 12 Rays and then ran out of time to get my unit ready. I slung my Dolphin together. I thought I’d tightened down the scrubber canister properly. I hadn’t.

I took a caustic cocktail at 20 metres. It felt as though someone had poured liquid concrete down my throat and it had instantly set and changed into a steel scaffolding bar. My esophagus was rigid from throat to stomach. I was very lucky I’d ingested it, not breathed it in. At the time I didn’t know this. I must admit I was quite scared and not happy wondering just how badly I had damaged my lungs. (I hadn’t). I was escorting a pair of diving professionals whose idea of buddy diving was ‘same ocean, same day’. Trying to get them to dive in roughly the same area, let alone surface because I’d taken a cocktail, was hard work. The cocktail hospitalised me and destroyed my confidence in this amazing technology in one stroke. It took years to rebuilt my confidence.

Rebreathers are an amazing tool. They have opened up exploration and enabled certain dives to take place that would have been impossible on open circuit. Not everyone should be diving them however. You need a certain mindset and attitude and you need to dive them regularly. They require the three “C’s”. Checks, care and concentration.

Time passed and I did some training on a couple of other units, but I never trusted them. I couldn’t relax. And I have never really had a head or natural feel for maths. So when I checked my handsets I wasn’t always confident it was safe or about to kill me. I have lost too many friends and colleagues diving on rebreathers, so my first thought was, “so when are you going to kill me then”?

Photo of Rosemary Lunn by Karl Shreeves

Although I have not got hundreds of hours of diving time on rebreathers, I have a fair knowledge of them. I found this invaluable when I was asked to organise the logistics on an international safety conference called ‘Rebreather Forum 3′. This three day event took 3 years of development and organisation, and two years of editing the proceedings. The proceedings and a number of lectures are available online, for free, for anyone who wants to access them. If you have any interest in rebreathers then check out

It wasn’t until 2013 when I was working the renowned “Inner Space” week hosted and organised by Divetech in Grand Cayman, that I finally began to get my mojo back for breathers. I was fortunate enough to be trained by Matthew Addison on the Hollis Explorer. If truth be told I had no idea why Hollis had gone down the line of developing and building a recreational unit. There was so much choice in the market place, why on earth would you want to dive an Explorer? My head turned 180 degrees during my training. This unit was basic. This unit was simple. Above all this unit was fun! Even I could understand it. And now I see its place. If you want to get into rebreathers, this is a useful unit to learn how to dive one, and how to become disciplined in your checks, care and concentration.

Twelve months later I wanted to expand my rebreather education and trained on the Poseidon Mark VI with Steve Newman. My courage and my mojo returned in spades. I fell in love with the unit. This too was fun. This too was simple. But what this had over the Explorer was it had legs. Whereas the Explorer is a recreational breather and so therefore limited, the Poseidon is a marmite unit. “The growing up spread you never grow out of”. As your diving develops, so does this unit’s capability. I like the fact that one day I will be able to take it trimix diving and to technical depths. Am I cautious when I dive it? Yes. Do I respect it? Yes. Am I happy and relaxed diving this? Yes. It has taken me 15 years to find the rebreather I want to have a long term relationship with. Today I am proud to say I am an ambassador for Poseidon Rebreathers.

The moral of the tail. If you are interested in getting into rebreather diving, and you find that one unit is not to your liking, there is choice out there. You may well find that another unit suits your diving, and you too will fall in love with this extraordinary piece of technology.

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Death of the Canister Light

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My first primary light for cave diving was so heavy I had to mount it on two large D-rings and hang it from the bottom of my tanks. The bulky rectangular housing concealed two large sealed lead acid batteries. A hefty cord lead to a metal light head that could double as a sledgehammer. One day it switched on in the van and burned a hole in the carpet before I could get it shut off. I used to wait to the very last moment possible before turning the light on, well into the cavern zone, trying to save every last minute of burn time for the dive at hand. SolaTech6000249

In the last two decades of technical diving, lights have changed more than anything else. They’ve shrunk. The battery technology has improved and burn times are extended. Even my backup lights are now far brighter and reliable than my earliest primary light. We’ve switched from incandescent bulbs to HID, HMI, and LED. We moved through lead acid to nickel metal hydride and now lithium batteries. Yet, for some reason, divers still have a hard time letting go of that antiquated canister and cord.

I’m not one to point fingers, because it took until early this year when I finally dropped the canister in favor of a powerful handheld – the Light & Motion Sola Tech600. I had tried other handhelds, but they were still large, negatively buoyant lights equipped with clunky high profile handles. They lacked the elegance, comfort and streamlining necessary to convince me. After researching lighting technology for an article I wrote for DIVER magazine, I stepped into some Light & Motion gear and wondered why I had waited so long. Free from a canister and cord, my rig is more streamlined, my hands more nimble and luggage weight is cut substantially. The beam (check out their beam comparisons) offers the perfect balance between a great signaling light and excellent coverage. The Tech600 fulfills a solid day of diving and yet I can easily recharge it in 75 minutes over lunch if I want to. With three power settings that are arguably brilliant, I can conserve power when needed. It also gives me peace of mind seeing small LED fuel gauges that confirm I have plenty of power left to finish the day of diving.

When it comes to travel, it is a no brainer. I can’t break it and neither can the airlines (I don’t know which of us is worse). The light can be switched into an airline safe lockout. The package with the charger is minuscule and light. More importantly, it is not intimidating to the Transportation Safety Authority (TSA) guards who squirm at the x-ray machine and set off lights and buzzers when viewing canister lights in your bags.

P1tunnel0096lI’m a convert. After watching others use these elegant lights, I reached out to Light & Motion and now I own most of the products they make.

So if you are still tethered to a canister, it is time to break the umbilical and move into the next generation. Try a Sola Tech600 and I guarantee you’ll never go back.

Jill Heinerth, Explorer

PS – Ladies and small hands: Light & Motion makes two sizes of hand mount and also offers myriad mounting options from D-ring to T-handle to a new Goodman style handle adapter.

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$1500 Training Credit for PRISM2 Buyers

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Dive Rite In Scuba is offering an exciting promotion this month. They are offering a $1500 training credit for buyers of a PRISM2 rebreather and $1000 for training when buying an Explorer. If you want to jump in on this really terrific deal call the shop at 815-267-8400 or email




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Boom Baby Boom – Volume One

By | All Posts, Cave Diving, Rebreather Diving, Robert McClellan, Uncategorized, Underwater Photo and Video | No Comments

I’m so proud to announce that my husband Robert McClellan has released his first book. It has been a real journey of discovery for him. I recall the day almost eight years ago, when he expressed the desire to share his story. The revelations he shares in Boom Baby Boom represent a lifetime of challenge, discovery and hard knocks.

BoomCoverFRONTWe’ve been married for seven years and he has been an incredible asset to Heinerth Productions as a writer, producer and talented audio engineer. We’re both explorers in life, but his unique social perspective was formed on the working class streets of Philadelphia, where sometimes, the most effective way to enforce the social contract was to give a guy a punch in the nose.

As a young man, Robert immersed himself in the vibrant Philly music scene. This led to several great opportunities to work as a concert production manager with some of the most acclaimed musicians of the time. The list of artists includes such well-known acts as U2, Ray Charles, James Brown, Cindy Lauper, Jimmy Cliff, Culture Club, The Four Tops, and dozens more. But, the rock and roll lifestyle, and all its excesses, eventually caught up to him, and Robert fell off the tour bus.

Fortunately, the bus stop happened to be at a Navy Recruiter’s office.

Robert became a U.S. Navy Combat Photographer, including duty with the famed SeaBees. His award winning photography and journalism gained attention, and he was assigned as a instructor at the U.S. Navy Schools of Photography, at NAS, Pensacola, Florida. He collected numerous service awards include the Naval Commendation and Navy Achievement medals. Brief missions in Operation Desert Storm left lasting scars that forever changed his outlook on life. After leaving active duty, and completing nursing school, Robert continued his service to the country in the Army National Guard. He was a Medical Platoon Sergeant with the 1/156th Armor Battalion. An injury ended his service, and today Robert is among the many fortunate veterans who receive excellent care through the VA health system.

Before we met, he continued life’s explorations as a long haul truck driver, concert promoter, music artist manager, afternoon talk radio host, and travel nurse. Those careers were intermingled with several attempts at drug and alcohol rehab and fortunately, Robert found himself sober. And broke.

His only possession, and old motor home, ran out of gas at Navarre Beach, with an Irish Setter and $26 in his pocket. He couldn’t have been happier. He got active in the local recovery community and landed a job as a detox nurse at a prestigious treatment facility. Continuing to grow in sobriety and trying to remain of some service to others, he began working part-time in the medical departments at some of the most notorious prisons in Florida.

robhappyJEH4288Again, growth and life changes came about, and he decided that the Black Hills of South Dakota would be a good place to explore. He became a nurse at the VA medical center in Hot Springs, and also produced music festivals and concerts in Rapid City.  After a few Dakota winters, Robert admitted the error of his ways and transferred to a VA hospital in north Florida.

We met online while I was working on a documentary in the Florida Everglades. Months passed and the email romance blossomed until we finally had a first date at a beautiful North Florida Spring. While he continued nursing at the local VA, we met for bicycle dates and dinner in town, but there was no turning back for either of us.

As Creative Director of Heinerth Productions, Inc., Robert gets to explore his boundaries every day. Whether it is a new documentary film idea or a book project, he embraces the opportunity to create something worthwhile, something that he can be proud of and that will resonate with people around the world. He’s also the customer service and shipping department, and rides his bike 14 miles round trip to our local post office to get our shipments in the mail.

The rest of the story plays out every day as a heart warming romantic comedy, where the planets have aligned and the best is yet to come.

Boom Baby Boom – Volume One is a remarkably candid and brutally honest collection of essays and short stories, told in the authentic voice of an American Baby Boomer. From the rough and tumble streets of North Philadelphia, to the glamorous world of rock concerts and endless lines of cocaine, Robert will carry you along on a journey that winds through his extraordinary life. With first person experience as a prison nurse, a combat photographer and recovering alcoholic, this book is full of interesting anecdotes, with plot twists that will leave you smiling, sometimes through your tears. It still chokes me up to learn of the things he has endured and overcome, but today I am incredibly proud of my husband and best friend!

Available on Amazon now!

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Pre-Breathing and CO2

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Dr. Simon Mitchell of New Zealand, recently conducted a study that will no doubt rock diver’s understanding of their pre-breathe sequence. The study has not yet been published, but Mitchell felt it was important enough to present a preview at Eurotek last weekend in Birmingham, England. He also posted to CCR Explorers today. He says:

I presented two studies at Eurotek.

The first aimed to evaluate the efficacy of the 5 minute prebreathe in evaluation of CO2 scrubber function prior to diving a rebreather. This has been a controversial subject and we sought to gather some definitive data. Since this study is being written up for publication in a medical journal I am limited to the amount of detail I can provide here because “pre-publication” on the internet could compromise acceptance by a journal. However, I am happy to provide some basic details and the essential findings of the study… since having presented it at a conference the results will probably be discussed on line anyway.

We randomised diver subjects (the majority of whom were rebreather divers) to undertake 5 minute prebreathes on an Evo Plus rebreather. They were instructed to conduct the prebreathe in the same way that is recommended in training. Thus, they sat at rest breathing on the unit with the nose blocked. They were instructed to terminate the prebreathe if they considered they had any symptoms of CO2 toxicity. 60 prebreathes were conducted, comprised of 20 in each of 3 conditions: normal scrubber, completely absent scrubber, partial failure of the scrubber. The “partial failure” was achieved by leaving the O ring and spacer out of the inspo scrubber assembly. The subjects were blinded to the condition, and we masked any changes in resistance to breathing associated with the different scrubber conditions. We measured a variety of physiological parameters during each prebreathe including pulse rate, breathing rate, tidal volume (the volume of each breath), minute volume (the volume of gas moved in and out of the lungs each minute), the inspired PCO2, and the end tidal PCO2.

The primary outcome was comparison of the proportion of subjects who terminated the prebreathe in each condition.

No subjects terminated when a normal scrubber was in place (as you would expect).

25% of subjects did not terminate the prebreathe when there was no scrubber present despite dramatic changes in the physiological parameters.

90% of subjects did not terminate the prebreathe in the partial failure condition despite significant CO2 break through and some changes in the physiological parameters.

The changes in physiological parameters were fascinating and helped considerably with interpretation of the above results, but I will not discuss those at this stage because this will form much of the discussion in the paper. Once the study is published we will be able to exploit the significant educational potential of those results.

The obvious conclusion is that even in a study where there would have been a high expectation of scrubber problems among subjects, the 5 minute prebreathe had only mediocre sensitivity for detecting complete absence of a scrubber, and extremely poor sensitivity for detection of a significant partial failure. We therefore believe that it is not a valid intervention. I hasten to add that this is NOT to say that there should be no prebreathe. The prebreathe also gives the user the opportunity to ensure that other systems (like the oxygen controller) are working correctly. But it does not need to be 5 minutes long in the belief that this allows detection of problems with the carbon dioxide scrubber.

I will write on the second study in a separate post.

Hope this makes sense.

Simon Mitchell

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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.


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:

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

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

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

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

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

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

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

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

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

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

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

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

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

Barotrauma – A pressure related injury.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

ffw – Water depth as measured in feet of freshwater.

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

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

FHe – The fraction of helium in a gas mixture.

FN2 – The fraction of nitrogen in a gas mixture.

FO2 – The fraction of oxygen in a gas mixture.

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

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

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

fsw – Water depth as measured in feet of seawater.

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

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

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

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

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

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

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

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

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

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

Hydrostatic imbalance – See static lung load.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

mfw − Water depth as measures in meters of freshwater.

msw − Water depth as measured in meters of seawater.

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

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

msw – Water depth as measured in meters of seawater.

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

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

Nitrox – See enriched air nitrox.

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

Normoxic – A mixture of gas containing 0.21 ata oxygen.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Scrubber – See CO2 absorbent.

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

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

Setpoint – See PO2 setpoint.

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

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

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

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

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

Stack – Slang terminology referring to the CO2 absorbent canister.

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

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

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

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

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

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

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

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

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

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

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

Whole-body oxygen toxicity – See pulmonary oxygen toxicity.

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

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