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.

TEDYouth 2015

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The following links take you to Session Three of the day where Jill spoke around :50 into the feed.

More information and feeds are available at TEDYouth.

TEDYouth 2015

TEDYouth is a day-long event for middle and high school students, with live speakers, hands-on activities and great conversations. Scientists, designers, technologists, explorers, artists, performers (and more!) share short talks on what they do best, serving both as a source of knowledge and inspiration for youth around the globe. The show was stream worldwide to over on hundred events in English, Spanish and Arabic. I was able to speak about underwater exploration and the importance of understanding our human connections to our water resources.

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Free Seminars at DEMA Trade Show

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For those attending the DEMA Trade Show in Orlando this week, please join me at one of my free seminars!

Thursday, November 5 10 – 11 am in Room S310F Size Matters: Lean and Mean Pro Video

Professional explorer Jill Heinerth talks about the new paradigm of videography for expeditions and professional shoots where a small footprint and lightweight gear yields huge results. She’ll talk about how small cameras paired with Light and Motion’s bulletproof lights can make magic for television, store marketing and great vacation memories.

Friday, November 6 1-1:45 pm at the Image Resource Center on the Show Floor – Booth #357 Quality in Carryon

Professional explorer and filmmaker, Jill Heinerth will feature exciting images and video clips from caves and wrecks around the world, sharing tips on how to shoot 4K video and magazine ready still images from a camera package that easily fits in the overhead luggage bin.

2-3 pm in Room S330B Exploration

Suunto Ambassador, Explorer, Cave Diver and Film Maker Jill Heinerth

Jill Heinerth presents vignettes from her expeditions using interesting technology in underwater caves and deep ocean environments.

Saturday, November 7  11-11:45 at the Technical Diving Resource Center Show Floor booth #1757 The Science of Cave Diving

Jill Heinerth shares stories of scientific expeditions around the world and how cave divers have supported research and exploration.

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Sexism in the Scuba Industry Article Available

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DIVER_V40_I7sexism-1lDIVER Magazine has once again demonstrated great leadership by posting my cover feature on sexism online. It is available here as a PDF download or on the DIVER Magazine website.

PDF File: DIVER Magazine Article

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Devil Ray Dance

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During my recent work on #ProjectoAzores, I had the opportunity to swim with the majestic mobilas or Devil Rays in Santa Maria, Azores. It was one of the most beautiful dives of my life. As I hung out in the open ocean current the rays came closer and closer in an arcing line until they literally hovered over my head pausing there until I exhaled. They seemed to be waiting for a tickle of bubbles before they moved around and circled in again.

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TEDYouth November 14, 2015

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I’m proud to be speaking at TEDYouth in New York.

TEDYouth is a day-long event for middle and high school students, with live speakers, hands-on activities and great conversations. Scientists, designers, technologists, explorers, artists, performers (and more!) share short talks on what they do best, serving both as a source of knowledge and inspiration for youth around the globe. All speakers volunteer their time and attendees are offered free admission based on an application essay that highlights their desire to participate.

TEDYouth 2015: Made in the Future

TEDYouth 2015 is taking place at the Brooklyn Museum on Saturday, November 14, from 11am to 6pm.

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Sensors in Chaos!

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Q. If you saw this (three sensors out of agreement) what would you do?HandsetIMG_3462l

A. When three sensors are out of agreement on the Shearwater Petrel controller, the solenoid will not fire. You will see a message “Voting Logic Failed.” You should bail to open circuit. A diluent flush may confirm which (if any) of the sensors is working properly in the event that you have a long transit out of an overhead environment and are concerned about the volume of open circuit gas available to you. In that case you could theoretically run the unit manually on one working sensor. However, if two of the rogue sensors get close to each other you should be aware that the voting logic may reactivate the solenoid based on two similar bad sensors. Lowering the set point is a good idea, but the best solution is to use open circuit bailout or SCR as a gas extender. Your sensors can’t be trusted.

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Shearwater Petrel Calibration Tip

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Calibration Errors on a Shearwater Petrel

Q. I’m trying to calibrate my unit. The HUD shows 1.0 with green and yellow rows of lights. I have fully flushed with oxygen. The handset reads .97, .98 and 1.00 already. The millivolts on the first screen look right and are in range. What is it failing calibration? Looking at the failure screen the millivolts are off the charts, each reading over 200? What’s going on?

A. Check your System Settings. Ensure the Calibration O2 is the correct value. After a battery failure, numerous parameters in your computer such as “units” reset. Your cal gas might be registered at 0, rather than .98. Check that value. Once you reset it, the calibration will likely pass.

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More Tips for Traveling with Rebreathers

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Mask, fins, rebreather and you are ready to go, right? There are many small items that you might not think about that can ruin a trip in their absence.


Inquire about the local power supply. As an example, Mexico has the same current as the US, but many wall outlets do not have grounding plugs. You will need to purchase “cheaters” that convert your three-prong plugs to two-prong. Power strips are worth their weight in gold since wall outlets may be few and far between. In Europe, recessed wall plugs may not fit the fancy converter you bought at the airport. Those converters also often lack a hole for the grounding plug, making them impossible to use without the “cheaters” mentioned above. Many chargers are already rated for 120 and 240v power supplies. Do not waste converters on these devices as they are not needed.


Disinfectant may be challenging to take to your destination. Steramine tablets are very convenient for travel since they are dry and will not spill in luggage or take up precious weight allowance. If you have forgotten to pack Steramine, Betadine is relatively easy to find in foreign pharmacies (though a few people are allergic to Betadine). Dilute it in a tub to soak your breathing loop. If you are camping on an expedition, use one of your shipping cases as a bath. If you cannot find Betadine, Listerine will get your through a trip with reasonable cleanliness, but it does not kill all bacteria. Use proper disinfectants as soon as available.


You can never give enough credit to duct tape. It can repair an awful lot of damage in a difficult situation. I always carry some on trips, but rather than taking a heavy roll, I just wind a few yards around a business card for a small supply. A product called “Rescue Tape” is also fantastic. It is made of clear silicone self-binding tape. It does not have a sticky backing, but instead, bonds to itself in mere moments. It is extremely useful.


Clear plastic Ziplock bags allow you to neatly separate and pack gear so that it does not get lost during inspections. If you simply roll your DSV in your pajamas, the screener is likely to pick up the garment and send the sensitive device crashing to the floor in pieces. Slide delicate items into dive boots and fin pockets. Tape them in place or put them in a Ziplock bags if necessary.


Always make a photocopy of your critical personal identification like your passport and carry it separately in case of loss. Consulates can easily assist if you have back-up documents. Also hand-carry documentation about your rebreather and MSDS sheets for sorb or any other questionable items. Be prepared to share this information with screeners.


Place a friendly letter in your rebreather case that describes the device. Indicate on your letter that all parts are “safe for airline transport on passenger aircraft.” Invite the TSA screeners to contact you via cell phone and even offer up your seat assignment if they need it. Leave a photo of yourself wearing your CCR with a big smile in the case. It helps them to quickly understand what they are looking and let’s them know you are trying to be safe and compliant.

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Travel by Boat of Car with Your CCR

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When using a car and boat, your dive destination may be a lot closer to home, but there are still a few special tips to consider.

  • Bring extra bungee cords for boat travel. Many boats are specifically constructed for single tank divers. The benches may be awkward for a rebreather diver. Bring your own method of securing the rebreather to the bench or the floor.
  • Consider the orientation of your rebreather scrubber. If your rebreather is lying on its back, vibrating on a moving boat or in a car for hours, will your particular canister design be subjected to drastic settling? Some canister designs are equipped with springs that help to resolve settling issues, but if the orientation of the rebreather in relation to the settling forces is incorrect, you could get some channeling of material.
  • Fully assemble and check your rebreather before leaving the dock. You don’t want to be packing sorb on a bouncing boat and also don’t want to capture diesel fumes within the breathing loop.
  • Beware of extreme heat. If you gear is sitting sealed in a hot car for a long period of time, you can damage the oxygen sensors.
  • Beware of extreme cold. If your packed rebreather sits in a freezing car overnight, the moisture in the sorb can freeze causing damage to the canister and dusting in the material itself.
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Aquiferious Art Exhibit Coming to Gainesville

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Reflected 1020237lThe Thomas Center Galleries announce the fall opening of AQUIFERious, an expansive exhibition to be presented in the Historic Thomas Center’s Main and Mezzanine galleries from Thursday, Oct. 1, 2015 to Wednesday, Jan. 6, 2016. The opening reception for the exhibition is Thursday, Oct. 1 from 5:30-7 p.m.
AQUIFERious is curated by Margaret Ross Tolbert, and is based on her award-winning book of the same title. A multi-media art experience in the truest sense, the exhibition will juxtapose Tolbert’s dramatic, large-scale springs paintings and eloquent poetry with contributing artists’ and scientists’ photographs, videos, cartography and writings. Featured will be the underwater photography by Jill Heinerth, Mark Long and Tom Morris, and cave maps by Eric Hutcheson.
Among the most remarkable natural wonders of the Sunshine State is the seldom-seen, underground Floridan aquifer system, a 100,000 square mile life force that reaches into neighboring states and feeds more than 1,000 freshwater springs throughout Florida. AQUIFERious is intended to give the audience a greater understanding and appreciation of Florida’s subterranean water system, while celebrating its extraordinary beauty and that of the fragile springs that arise from the system’s depths.
11947801_10206566042301648_4309935710810697204_oOver the course of the exhibition, the Thomas Center Galleries will present a number of programs and opportunities for discussion about the springs, and how they serve as a rich platform to connect art and science.
The opening reception of AQUIFERious will be followed by a music concert inspired by the land and waters of Florida and beyond, performed by Patchwork, an award-winning, five-piece female folk/bluegrass that is a favorite of Florida festivals. The concert will begin at 7 p.m. The reception and concert are free and open to the public.
Striking images from the exhibition can be viewed at
The Thomas Center Galleries are located at 302 NE Sixth Ave., Gainesville, FL 32601. Gallery hours are Monday through Friday from 8 a.m.-5 p.m. and Saturday from 1-4 p.m. The galleries are closed for observed holidays. For additional gallery information, please contact Russell Etling, cultural affairs manager, at 352-393-8532 or visit

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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.