# Glossary of underwater diving terminology: A–C

This is a glossary of technical terms, jargon, diver slang and acronyms used in underwater diving. The definitions listed are in the context of underwater diving. There may be other meanings in other contexts.
Underwater diving can be described as a human activity – intentional, purposive, conscious and subjectively meaningful sequence of actions. Underwater diving is practiced as part of an occupation, or for recreation, where the practitioner submerges below the surface of the water or other liquid for a period which may range between seconds to the order of a day at a time, either exposed to the ambient pressure or isolated by a pressure resistant suit, to interact with the underwater environment for pleasure, competitive sport, or as a means to reach a work site for profit, as a public service, or in the pursuit of knowledge, and may use no equipment at all, or a wide range of equipment which may include breathing apparatus, environmental protective clothing, aids to vision, communication, propulsion, maneuverability, buoyancy and safety equipment, and tools for the task at hand.
Many of the terms are in general use by English speaking divers from many parts of the world, both amateur and professional, and using any of the modes of diving. Others are more specialised, variable by location, mode, or professional environment. There are instances where a term may have more than one meaning depending on context, and others where several terms refer to the same concept, or there are variations in spelling. A few are loan-words from other languages.
There are five sub-glossaries, listed here. The tables of content should link between them automatically:
* Glossary of underwater diving terminology: A–C
* Glossary of underwater diving terminology: D–G
* Glossary of underwater diving terminology: H–O
* Glossary of underwater diving terminology: P–S
* Glossary of underwater diving terminology: T–Z

## A

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |

## B

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |
Subsection: Top, Ba, Be, Bi, Bo, Bu

## Ba
### back gas

    Breathing gas carried by a scuba diver in back mounted cylinders. Generally the primary breathing gas for the bottom or longest sector of a dive.

### backward kick

    See: Back kick (finning) A finning technique for moving backwards. Not an easy, powerful or elegant kick, but useful in many situations. The fins are angled outwards in opposite directions with the legs straight, then swept upwards and towards the diver by bending the knees in the power stroke. The knees may move downwards a bit at the same time by bending at the hips for stability. The return stroke feathers the fins by pointing them backwards in line with the body axis, to reduce forward thrust until the legs are straight again

### back mount

    The practice of carrying a scuba set on the back of the diver, supported by a harness, backplate or stabilisor jacket BCD. Compare with sidemount

### backpack

    A rigid or semi-rigid structure similar in function to a backplate, usually made of moulded plastic, but sometimes of metal, used either as a stiffener and reinforcement for a jacket style buoyancy compensator, or as the basis of a scuba harness independent of a buoyancy compensator. The backpack supports and stabilises the scuba cylinder on the diver's back.

### backplate

    Main article: Backplate and wing A plate, normally made from metal, which rests against the diver’s back, and to which the primary scuba cylinders are attached. Held to the body by harness straps over the shoulders and round the waist. Sometimes also crotch straps and chest straps. Usually used with a back inflation buoyancy compensator.

### backscatter

    See: Backscatter (photography) Light reflected back towards the diver or camera by suspended particles in the water.

### backup light

    Dive light carried as a spare to be used in case of failure of the primary light.

### backup regulator

    Also: "secondary regulator" A second complete scuba regulator connected to a cylinder or manifolded twin set. Compare with octopus regulator.

### back roll entry

    See: Scuba skills#Entries Water entry method in which the seated diver rolls backwards off the side of the boat, allowing the scuba cylinders to strike the water first.$^{:125}$

### bailout block

    A gas switching block specifically intended for connection of a bailout set to the main gas supply (which may be scuba or surface supply) which allows the diver to switch from main gas supply to emergency gas supply while continuously using the same mouthpiece, regulator second stage, full face mask or helmet. A bailout block is generally used on open circuit breathing apparatus, the equivalent function on a rebreather is provided by a bailout valve (BOV). The bailout block may be mounted on the side of a diving helmet or full-face mask, or may be mounted in a convenient place on the diver's harness, and includes a bailout valve, used to select the gas source, and one or more non-return valves to ensure that the emergency gas supply is directed only to the diver.

### bailout cylinder

    Main article: Bailout cylinder A scuba cylinder carried by an underwater diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure.

### bailout system

    Also: "EGS", "emergency gas supply" Main article: Bailout cylinder An independent breathing gas supply carried by a diver for use in case of failure of the main gas supply. Usually consists of a bailout cylinder with a first stage regulator, and either a second stage regulator or connected to a bailout block or bailout valve (q.v.) A submersible pressure gauge is also usually provided.

### bailout valve

    1. Also: "integrated open-circuit regulator" An open circuit demand valve built into a rebreather mouthpiece, or other part of the breathing loop, which can be isolated while the diver is using the rebreather to recycle breathing gas, and opened at the same time as isolating the breathing loop when the diver bails out to open circuit.
    2. A valve which opens the gas supply from the bailout cylinder of a surface supplied diver, used in case of surface gas failure, usually mounted on the side of a diving helmet or full-face mask, or on a manifold block on the diver's harness.

### baited remote underwater video

    Main article: Baited remote underwater video A system for assessing fish populations using video cameras to record fish attracted to a bait canister.

### balanced regulator

    See: Mechanism of diving regulators#Balancing Regulator designed to provide a consistent demand effort not affected by cylinder gas pressure or depth.

### bandmask

    See: Surface-supplied diving#Band mask A heavy duty full-face mask with many of the characteristics of a lightweight demand helmet. In structure it is the front section of a lightweight helmet from above the faceplate to below the demand valve and exhaust ports, including the bailout block and communications connections on the sides. This rigid frame is attached to a neoprene hood by a metal clamping band, hence the name.

### bands

    See: cylinder bands

### bar

    Main article: bar (unit) Metric unit of pressure commonly used in diving, equal to 100 kiloPascal, and nearly equal to standard atmospheric pressure.

### barodontalgia

    Main article: Barodontalgia Tooth pain caused by pressure change.

### barotrauma

    Main article: Barotrauma Injury caused by pressure difference.

### baseline

    See: Baseline (disambiguation)
    1. Also: "tank factor". Numeric value computed for a cylinder or manifolded set that relates volume and pressure in the imperial system of units. Computed by dividing nominal capacity (cubic feet) by cylinder working pressure (psi) to express cubic feet of volume per psi of fill pressure (sometimes multiplied by 100 to give cubic feet per 100 psi). Used to convert cylinder pressure to free gas volume.
    2. A line that is a base for measurement or for construction; see datum (calculations or comparisons)
    3. A data set which is a point of reference (engineering or science) for later data.

### basket

    See: diving stage

### BAT wing

    Buoyancy and trim wing. A back mounted buoyancy compensator cell used with sidemount harness. The buoyancy volume is mostly over the lower back.

### BCD

    See: buoyancy compensator

### BCD blowup

    Uncontrolled buoyant ascent caused by inability to release gas from the buoyancy compensator faster than it expands due to pressure reduction of ascent.
Subsection: Top, Ba, Be, Bi, Bo, Bu

## Be
### beach master

    See also: dive marshall A person on the beach who records when divers enter and exit the water. Typically used during recreational scuba training to keep track of the students, watch the gear, and provide assistance when required.

### bell

    1. See: diving bell
    2. Short tug on a lifeline, used in pairs, for signalling purposes. (see "pull" for comparison)

### bell-bounce dive

    See: Surface-supplied diving#Variations Surface oriented diving operation in which the divers are transported in and deployed from a closed bell, and are either decompressed in the bell at the surface or transferred under pressure to a deck decompression chamber for decompression.

### bell cursor

    See: Diving bell#Deployment of a modern diving bell Mechanism or structure for guiding and constraining the motion of a bell when in the close vicinity of the deployment platform to improve handling in bad weather.

### bell diving

    1. Any diving operation in which the divers travel in or work from a diving bell
    2. Diving operations in which divers are transported in and deployed from a closed bell, either as a surface oriented (bell bounce) or saturation dive.

### bell harness

    A safety harness made of strong webbing, which is fastened around a diver over the exposure suit, and allows the diver to be lifted without risk of falling out of the harness.

### bellman

    See: Diving bell#Deployment of a modern diving bell Standby diver deployed in the diving bell

### bellows ratio

    The swept volume ratio between the inner and outer bellows of the counterlung of a passive addition semi-closed rebreather.

### bell run

    The part of a bell dive operation from bell lock-off to bell lock-on (from and back to the life support system)

### bell stage

    A framework extending below a closed bell which keeps the base of the pressure vessel off the bottom sufficiently to provide clearance for the divers to use the bottom hatch when the bell is resting on the bottom or on the clump weight.

### bell umbilical

    See: Umbilical cable#Diver The combined supply and return hoses and cables for life-support, power and communications between a diving bell and the support platform

### belt slider

    Also: "triglide", "weight slider", "weight stop" Hardware item with two parallel slots which is fitted to harness or weightbelt webbing to prevent other components such as D-rings and weights from sliding along the webbing.

### benign conditions

    See: Benign water Sometimes also referred to as confined water. Environments of low risk, where it is extremely unlikely or impossible for the diver to get lost or entrapped, or be exposed to hazards other than the basic underwater environment.

### bent D-ring

    A D-ring which has been bent about 45° near the straight section on both sides, forcing it to project slightly from the harness when pushed to one side, allowing easier attachment of clips.

### bends

    Decompression sickness: Injury caused by bubble formation in the body tissues after hyperbaric exposure.
Subsection: Top, Ba, Be, Bi, Bo, Bu

## Bi
### BIBS

    See: built in breathing system

### bi fin

    Swimfin worn on one foot. One of a pair. The term is usually used in the context of freediving and finswimming to distinguish the style having a blade mounted on a single foot pocket from monofins, which have a single blade and two foot pockets. In scuba diving, surface-supplied diving and snorkelling they are usually referred to simply as fins or swimfins.

### Billy ring

    Three D-rings welded together along their straight sides so that one is perpendicular to the other two, which lie on the same plane. This uses the two flat rings to maintain the third in an upright position when mounted on harness webbing, allowing it to be more easily accessed to clip on stage cylinders. Named after Billy Deans.

### bioprene

    Diver and swimmer slang for subcutaneous fat, which provides some thermal insulation.

### BK safety hook

    A type of safety lifting hook which is held closed when under load by a lever system where the weight of the load holds the bill of the hook against the safety latch.

### black-water diving

    see also: blue-water diving Diving in mid-water at night where the bottom is out of diving range.

### blending stick

    See also: Gas blending for scuba diving#Mixing the gasesMixing tube in which gases are continuously mixed prior to intake by a compressor, usually at atmospheric pressure. Usually refers to manufacture of nitrox from air with added oxygen, but also used for trimix. Gas mixture is usually continuously analysed at the exit of the blending stick to monitor composition.

### blind traverse

    Passing through a cave from one entrance to a different exit which the diver has not used before.

### block adaptor

    Screw-in adaptor fitting which is fitted to a 200/240 bar DIN pillar valve to allow connection of a yoke regulator or filling whip.

### blowdown

    1. Procedure of pressurising a diving chamber or saturation habitat.
    2. Procedure of driving water out of the bottom of an open diving bell by displacing it with air or other breathing gas at ambient pressure.

### blowout

    Main article: Blowout (well drilling) Uncontrolled release of crude oil and/or natural gas from a well after pressure control systems have failed.

### blowout preventer

    Main article: Blowout preventer A large, specialized valve used to seal, control and monitor oil and gas wells. Generally operated remotely by hydraulic ram.

### blowout preventer stack

    An assembly of several blowout preventer rams which may be used to shut off a well.

### blue hole

    Main article: Blue hole A sink hole in a lake or the sea that is often the entrance to a cave. Blue holes in the sea are subject to tides so that their flows regularly reverse.

### blue-water diving

    See also: Underwater diving#Diving environment Underwater diving in mid-water where the bottom is not visible and is out of diving range.
Subsection: Top, Ba, Be, Bi, Bo, Bu

## Bo
### bolt snap

    Main article: bolt snap A metal connector comprising a hook with a spring-loaded axial sliding rod gate which must be manually retracted to allow the hook to be clipped onto something or removed. May be single- or double-ended, and if single-ended is usually fitted with a swivel ring opposite the jaws.

### bomb

    Commercial diver slang for high pressure gas storage cylinder of around 50 litres water capacity, also known as a "J".

### bommie

    Australian term for a coral mound or small, usually isolated reef, or coral head. From the Aboriginal word "bombara".

### bondage wing

    See: bungee wing Derogatory term for bungee wing based on fetishist terminology.

### bonnet

    The upper part of a copper helmet which encloses the diver's head and is sealed to the lower part or corselet (q.v.)

### booster pump

    Main article: Booster pump Machine used to increase pressure of a gas. Usually refers to the case where inlet pressure is above ambient pressure already.

### BOP

    See: blowout preventer

### BOP stack

    See: blowout preventer stack

### bottom gas

    The gas breathed by the diver at the deepest part of the dive. Compare with travel gas and decompression gas.

### bottom time

    Time used in calculating decompression obligation from decompression tables. For most tables this is defined as the elapsed time from starting the descent to starting the final ascent to the surface, excluding ascent and decompression time.

### bottom timer

    Device used to measure and record the total time spent underwater during a dive. They do not generally only record bottom time (q.v.).

### bottom out

    A rebreather counterlung becoming completely deflated during inhalation.

### bounce diving

    1. Also: "surface oriented dive" : In commercial diving, bounce diving is the alternative to saturation diving. Any dive where the diver is decompressed directly after the dive.
    2. In recreational diving, a bounce dive is a descent to maximum depth and then a direct ascent back to the surface with minimal bottom time, in a dive profile resembling a spike.

### BOV

    See: bailout valve

### bowline

    Main article: Bowline Knot used to form a secure, non-slipping loop at the end of a line.

### Boyle's law

    Main article: Boyle's law Relationship between pressure and volume at constant temperature in an ideal gas.

### breakdown room

    An area in a cave where a large amount of material has fallen from the overhead.

### breakout

    The point at which an object being lifted which is partly embedded in the bottom sediments overcomes the adhesion of the sediments and the force required to lift it drops rapidly to the apparent weight of the object.

### breakthrough

    The failure of a rebreather scrubber to remove all the carbon dioxide in the gas passing through the absorbent material.

### breastplate

    See: corselet

### breathing mixture

    Main article: Breathing gas Gas supplied to the diver to breathe, either directly to the diver or to the hyperbaric environment of the diving bell, dive chamber or saturation habitat. Colloquially just "gas" or "mix".

### breathing hose

    Large bore hose carrying the breathing gas in a rebreather breathing loop or a twin-hose demand valve.

### breathing loop

    The gas flow path in a rebreather comprising the diver's lungs, the mouthpiece, valves, hoses, counterlungs and scrubber through which gas is rebreathed.

### B-ring

    A small plate with a slot for a webbing belt and two side by side holes for clipping on equipment, generally used similarly to a D-ring in combination with a belt slider as an alternative to a butterfly slider

### Broco cutter

    A type of thermal lance initiated by an electric arc, in common use for underwater cutting work.

### BRUV

    See: Baited remote underwater video

### BTV

    See: Ear clearing#Methods Béance tubaire volontaire: Voluntary opening of the Eustachian tubes to equalise the middle ear.
Subsection: Top, Ba, Be, Bi, Bo, Bu

## Bu
### bubble models

    See: Decompression theory#Bubble models Decompression models based on the assumption that bubbles will form during non-symptomatic decompression.

### buccal pumping

    See: glossopharangeal insufflation

### buddy bottle

    Bailout cylinder carried by a scuba diver, particularly when diving solo.

### buddy breathing

    Main: Buddy breathing Sharing breathing gas from one demand valve by two or more divers, generally after an out-of-gas emergency.

### buddy check

    Main article: Buddy check A procedure carried out by scuba divers using the buddy system where each diver checks that the other's diving equipment is configured, fitted, and functioning correctly just before entering the water to dive.

### buddy diving

    Main article: Buddy diving A safety procedure where two or three divers monitor each other constantly during a dive and provide assistance or rescue when needed.

### buddy lights

    Warning lights visible to the diver's buddy that indicate rebreather system status.

### buddy line

    See: Buddy diving#Buddy lines A short line between two divers, used to maintain contact during a dive, generally in poor visibility, or other conditions where the divers might become separated and not be able to quickly locate each other.

### buddy system

    Main article: Buddy diving A procedure where two divers look out for the safety of each other, and give assistance if the other gets into difficulty.

### Bühlmann tables

    Main article: Bühlmann decompression algorithm Diving tables and decompression algorithm on which the tables are based, and some dive computers are programmed, based on the dissolved gas decompression model derived and tested by Dr Albert A. Bühlmann.

### built in breathing system

    Also: "BIBS" A demand breathing gas supply system with external exhaust used to provide chamber occupants with breathing gases other than the gas used to pressurise the chamber. Used for treatment gases and emergency breathing gas if the chamber is contaminated.

### bundle

    A set of gas cylinders fastened together for transportation and manifolded for use as a unit, also cylinder bundle.

### bungee (sidemount)

    See also: ring bungee Length of shock cord used to restrain the top end of side mount cylinders and keep them tucked in at the diver's shoulder while swimming. Usually clipped to a shoulder D-ring of the harness and looped around the cylinder valve. May be attached to the back of the harness between the shoulder blades, or run continuous from one shoulder D-ring, around the back under the arms to the other shoulder D-ring.

### bungee wing

    Back inflation buoyancy compensator with shock cord lacing or loops which exert a force on the bladder to oppose expansion during inflation and accelerate deflation.

### buoyancy

    Main article: Buoyancy
    1. Upward force on an object immersed in a fluid due to pressure exerted over the immersed surface.
    2. Resultant upward force of buoyancy and weight of an object immersed in a fluid.

### buoyancy check

    Procedure to test and adjust weights carried by an underwater diver. The diver wears all the personal equipment to be used for the planned dive, with the scuba tank(s) nearly empty, and the buoyancy compensator empty, in shallow water of the same density as expected on the dive, and adds or removes weights until neutrally buoyant. After the buoyancy check it is usual to distribute the weights for safety, trim and convenience.

### buoyancy compensator

    Also: "ABLJ", "BC", "BCD", "buoyancy compensator device", "horse collar", "stabilisor jacket", "stab jacket" or "wing" Main article: Buoyancy compensator (diving) An airtight bladder worn by a diver which can be filled with air and vented to adjust and control the buoyancy of the diver.

### buoyancy control

    The skill of maintaining the appropriate buoyancy at any time during a dive.

### burn tester

    Device for measuring the actual capacity of a battery relative to its nominal capacity, and the associated functional time for the device that the battery is powering.

### burn time

    The effective use time of a battery powered device. Mainly used in reference to dive lights and scooters.

### burst disk

    Main article: Rupture disk A non-reclosing pressure relief device used to protect a diving cylinder from overpressurization.

### butterfly clip

    A type of bolt snap with a tapered guide gate opening formed by a protrusion on both the piston and the fixed sides of the gate.

### butterfly D-ring

    A plate with two D-shaped cutouts on opposite sides of two to four parallel longitudinal slots for webbing. Used at the top back of the crotch strap in place of a butt-plate (q.v.) on minimalist sidemount harnesses as a clip-on point for equipment.

### butt-plate

    A rigid or fairly stiff flexible lower extension to a backplate or other scuba harness supporting butt-plate rails, used for clipping off the lower end of sidemount cylinders to the harness.

## C

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |
Subsection: Top, Ca, Ce, Ci, Co, Cu

## Ca
### cage diving

    See: Shark cage diving Diving in a cage designed to protect the diver from potentially aggressive large marine animals, usually sharks

### calibration gas

    Main: Calibration gas Gas of accurately known composition used to calibrate gas sensors.

### CALM

    See: catenary anchor leg mooring

### cam band

    A strap, usually of webbing, with a cam action tensioning buckle, generally used to secure a diving cylinder to a backplate, stabilisor jacket BCD or other form of diving harness.

### camel

    See: Lifting bag#Closed lift bags (camels) A closed lifting bag, for use at or near the surface, which retains air in rough conditions.

### canister light

    See: Dive light#Canister light Dive light comprising a light head connected to a battery canister by a cable.

### canoe diving

    Main article: Canoe diving Scuba diving from canoes, used when the dive site is beyond convenient swimming distances.

### carbon dioxide poisoning

    Main article: Carbon dioxide poisoning The toxic effects of carbon dioxide, due to incomplete elimination of carbon dioxide resulting from skip breathing, excessive work of breathing, scrubber failure in a rebreather system, or inadequate ventilation in a diving chamber or free flow helmet. Occasionally caused by contaminated gas supply.

### carbon monoxide poisoning

    Main article: Carbon monoxide poisoning The toxic effects of carbon monoxide, usually due to contaminated breathing gas supply.

### cardio-pulmonary resuscitation

    Main article:— Cardio-pulmonary resuscitation An emergency procedure which is performed in an effort to manually preserve intact brain function until further measures are taken to restore spontaneous blood circulation and breathing in a person in cardiac arrest

### CAS

    See: critical air supply

### cascade filling

    Main article: Cascade storage system Decanting from several storage cylinders in succession, generally using a procedure to maximise charge pressures. Often used in partial pressure gas blending.

### catenary anchor leg mooring

    Also: CALM Single point mooring type named for the catenary curve of the anchor cables that hold the buoy in position. Also referred to as single buoy mooring, monobuoy or loading buoy.

### caustic cocktail

    See: Rebreather diving#Scrubber failure A mixture of water and carbon dioxide absorbent caused by flooding the scrubber of a diving rebreather, and which may reach the diver's mouth through the breathing loop. The alkalinity depends on the absorbent used, and may injure the diver if aspirated.

### cave

    Main article: Cave A naturally occurring cavity in bedrock, or an underwater passage not illuminated by natural daylight, large enough to be entered by a human. Statute 810.13 of the Florida legislature defines a cave as: any void, cavity, recess, or system of interconnecting passages which naturally occurs beneath the surface of the earth or within a cliff or ledge, including natural subsurface water and drainage systems but not including any mine, tunnel, aqueduct, or other manmade excavation, and which is large enough to permit a person to enter. The word "cave" includes any cavern, natural pit, or sinkhole which is an extension of an entrance to a cave.

### cave arrow

    See: Line marker Directional line markers which point the way to an exit.

### cave fill

    Filling a scuba cylinder to a pressure significantly above the rated safe working pressure (charging pressure). Illegal in some jurisdictions, and increases risk of catastrophic failure.

### cave line

    1. Application: A distance line laid in a cave for navigation.
    2. Material: Small diameter braided synthetic cordage used for distance lines in cave diving .

### cave reel

    See: Distance line A reel specifically made for cave diving, used to lay and recover large lengths of cave line which is used as a temporary guide line to find the exit or a permanent guide line.

### cavern

    1. Two or more interconnected underground rooms or passages in bedrock, each large enough to be entered by a human.
    2. The initial space in an underwater cave system that is illuminated by natural daylight.
    3. A naturally occurring cavity in bedrock or an underwater passage, large enough to be entered by a human, which is illuminated by natural daylight, or in which it is possible from all points to see the exit by natural daylight.

### cavern dive

    Defined as a penetration dive under rock where visibility is greater than 40 feet, maximum penetration does not exceed 130 feet, maximum depth does not exceed 70 feet, the diver is always within the area illuminated by ambient sunlight, and does not pass through any restrictions.

### C-card

    See: certification card

### CCR

    See: closed circuit rebreather

### CCUBA

    See: closed circuit underwater breathing apparatus
Subsection: Top, Ca, Ce, Ci, Co, Cu

## Ce
### ceiling

    Solid overhead or decompression restriction to a safe direct vertical ascent to the surface

### cenote

    Main article: Cenote A sinkhole in Mexico. Generally with vertical or overhanging walls or shafts with water that open into a cave system.

### certification card

    A plastic card issued to a diver by a diver certification agency as evidence of having completed the diver training and experience required for the level of certification.

### CF

    See: clusterfuck

### CGA

    See: Compressed Gas Association

### CGA 850

    Compressed Gas Association valve standard CGA 850 is the standard for the scuba cylinder valve outlet for yoke connectors.

### chain chaser

    Ring or hook shaped components used for installing and recovering conventional mooring systems. The chaser is hooked around the chain and pulled in the direction of the anchor until it slides onto the anchor shank and is stopped by the crown. The chaser is then used to break the anchor out by pulling directly upwards.

### chamber dive

    Simulated dive in a hyperbaric chamber pressurised to equivalent pressure to the nominal depth of the dive.

### chamber operator

    Person competent to operate a diving chamber

### channelling

    Formation of void spaces in the granular absorbent in a scrubber which allow gas to bypass close contact with the absorbent material, allowing carbon dioxide to pass through the scrubber.

### charging pressure

    See also: working pressure Pressure stamped on a container for a permanent gas to indicate the maximum gauge pressure measured at, or corrected to the reference temperature (usually 15°C or 20°C) that may be applied at the time of filling.

### Charles's law

    Main article: Charles's law Relation of volume to temperature at constant pressure of an ideal gas.

### charlie foxtrot

    See: clusterfuck

### check-out dive

    A dive done to establish whether a diver's skills are current and responses appropriate and timely, equipment is configured and working correctly, or that team procedures are mutually understood and work as planned.

### chicken vest

    Sleeveless neoprene wetsuit vest with attached hood.

### chimney

    A section of a cave that is vertical or near vertical and like a shaft.

### Chinese lantern

    Connection between pipeline end manifold (PLEM) and single point mooring buoy (SPM) using two to four separate curved, flexible, underwater hose strings.

### choked flow

    See: constant mass flow

### choker (sidemount)

    See: Sidemount diving#Cylinders A strap around the neck of a sidemount cylinder used to hold the bolt snap closer to the neck so that the head of the cylinder stays closer to the diver's armpit. The choker can be a small webbing strap with a sliding buckle for adjustment, so it can be tightened to bring the clip closer to the neck or slacked off while in use.

### chokes

    See: Decompression sickness#Classification A symptom of decompression sickness manifested by shortness of breath, caused by a large number of venous gas bubbles in the lung capillaries which interfere with gas exchange.

### Christmas tree

    Also: "tree" Main article: Christmas tree (oil well) An assembly of valves, spools, and fittings installed on top of the wellhead and used primarily to control the flow, usually oil or gas, out of the well.

### Christmas tree ladder

    A boarding ladder which has a single central rail with rigid cantilevered rungs to each side, allowing use while wearing swimfins.

### Christo-lube

    Usually Christo-lube MCG111, an oxygen compatible lubricant suitable for use in breathing apparatus in oxygen service.
Subsection: Top, Ca, Ce, Ci, Co, Cu

## Ci
### circuit

    Cave dive route in which there is a one-way segment. The circuit can be simple or complex depending on the number of jumps involved.

### civils

    See: inshore diving

### closed bell

    Also: "dry bell" See Diving bell#Structure of a typical closed bell A closed or dry bell is a pressure vessel for human occupancy which is lowered into the sea to the workplace, equalised in pressure to the environment, and opened to allow the divers in and out. Divers may be decompressed in the bell or transferred under pressure to a hyperbaric chamber at the surface.

### closed-circuit rebreather

    See: Rebreather diving#Closed circuit rebreather Underwater breathing apparatus in which exhaled gas is scrubbed to remove carbon dioxide, and the oxygen is replenished to maintain a specific partial pressure, before returning it to the diver as breathing gas. See also ECCR and MCCR.

### closed circuit underwater breathing apparatus

    Military alternative term for closed circuit rebreather.

### clump weight

    A weight used to keep the guide wires of a diving bell aligned and tensioned.

### clusterfuck

    Also: "CF", "charlie foxtrot" Occasion when the situation diverges notably and usually uncontrollably from the plan, sometimes involving immediate hazard to life and limb, and often involving poor judgement.
Subsection: Top, Ca, Ce, Ci, Co, Cu

## Co
### cobra guard

    See: rams head

### code of practice

    A systematic set of professional standards or written guidelines and rules of procedures to be followed by members of a profession, trade, occupation or organization. A code of practice may be compiled and agreed on by members of a particular profession or written guidelines issued by an official body or a professional association to its members to help them comply with its ethical standards. A code of practice does not normally have the force of law, but is often required or compulsory practice for members of an organisation.

### coded welder

    Welder who is trained and qualified and assessed as competent for a specified type of welding under specified conditions.

### combat swimmer

    Main article: Combat swimmer Person trained in scuba diving or swimming underwater in a military capacity which can include combat

### command signal

    A signal from a diver in a team that requires a response from the other diver. There are three: "Are you OK", "Hold", and "Surface" (terminate the dive).

### commercial diving

    Main article: Commercial diving Working under pressure: Occupational activity where gas is breathed while immersed at pressure in excess of atmospheric pressure, usually underwater.

### completion

    Main article: Completion (oil and gas wells) The process of making a well, that has been drilled, ready for production.

### competent person

    Person who is able to perform a task or operation safely and according to required procedures. Recognition of competence may require formal assessment or accreditation, or registration with government body.

### compressed air

    Air at a pressure greater than ambient.

### Compressed Gas Association

    Main article: Compressed Gas Association An American trade association for the industrial and medical gas supply industries. The CGA publishes standards and practices that codify industry practices. In cases where government regulation is not specific, CGA documents are considered authoritative. CGA V-1 Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections covers diving cylinder valve outlets. Safety devices like burst disk overpressure protection are specified by the CGA Standard S1.1.

### compressed neoprene

    Foam neoprene that has been compressed to reduce volume. Used for dry suits and hot water suits which are less affected by depth induced buoyancy changes.

### compression

    The process of increasing the ambient pressure on a diver by descending into the water or pressurising the chamber.

### compression arthralgia

    Main article: Compression arthralgia Pain in the joints caused by exposure to high ambient pressure at a relatively high rate of compression

### compressor

    Main article: Compressor Machine which pressurises gas. Generally intake gas is at ambient pressure, outlet gas at higher pressure. High pressure breathing air compressor output pressure is usually 200 to 330 bar. Machines which compress gas at higher intake pressures are called boosters

### compressor log

    Book or file containing records of compressor operation, filling of cylinders and maintenance records.

### compressor operator

    Person who operates a compressor, either to fill cylinders, or to provide breathing air to surface supplied divers.

### condensate

    Main article: Condensation Liquid resulting from phase change from gas due to cooling, pressure increase, or both.

### conduit

    Tunnel or passage in a cave system.

### confined water

    Water that is enclosed and bounded sufficiently for safe training purposes. Generally implies that conditions are not affected by geographic or weather conditions, and that divers cannot get lost.

### conservative (decompression)

    See: Conservatism (diving) Decompression profile tending to minimise risk of decompression sickness at the cost of more decompression time for a given pre-ascent dive profile.

### constant pressure blackout

    Also: "isobaric blackout" A Freediving blackout which occurs while the diver maintains a near constant shallow depth, where reduction of oxygen partial pressure due to ascent is not a factor. Usually induced by pre-dive hyperventilation. Also referred to as shallow water blackout, which is an ambiguous term

### constant mass flow

    Also: "choked flow" Main article: Choked flow Sonic flow through an orifice – the maximum possible flow rate for the gas through the orifice for a given absolute upstream pressure.

### constant volume dry suit

    Dry suit with an automatic dump valve.

### constriction

    See: restriction

### containment grip

    See: whip sock

### contents gauge

    See: submersible pressure gauge

### continuous decompression

    See: Decompression practice#Continuous decompression Decompression without stops. Instead of a fairly rapid ascent rate to the first stop, followed by a period at static depth during the stop, the ascent is slower, but without officially stopping. Ascent rate may vary with depth, usually slowing as the depth reduces.

### contra-indications to diving

    Conditions (usually medical) that indicate that a person should not dive.

### control compartment

    The tissue compartment that dictates the ascent profile of a given dive because it is theoretically the highest risk compartment for DCS.

### controlled buoyant lift

    Main article: Controlled buoyant lift A rescue technique used by scuba divers to raise an incapacitated diver to the surface from depth.

### controlled emergency swimming ascent

    Also: "CESA" Main article: Controlled emergency swimming ascent An emergency technique for surfacing, usually when no breathable gas is available at depth. The diver fins upward while gently exhaling to keep expanding air in the lungs from causing lung expansion injuries.

### cookie

    Main article: Line marker Personal non-directional line markers that mark specific locations, or the direction of one's own exit at line intersections.

### copper helmet

    See: Standard diving dress A diving helmet of traditional design and construction, usually made from spun or beaten copper, with brass or bronze fittings. There are usually two main sub-assemblies; the bonnet is the roughly spherical part which covers the head and is provided with viewports, valves and various other fittings, and the corselet, which rests on the upper torso of the diver, and to which the bonnet is connected when in use, and which may be sealed to the suit and ballasted to compensate for the buoyancy of the airspace inside the helmet.

### corselet

    Also: "breastplate" See: Standard diving dress#General description Lower part of a standard copper helmet and some other heavy helmets, which clamps to the diving suit, rests on the diver's shoulders, and to which the helmet upper part, or bonnet, is clamped, screwed or bolted.

### counterlung

    See: Rebreather#Counterlungs Flexible bag or bellows in a rebreather which compensates for the changes in volume of gas in the loop during the breathing cycle.

### CPR

    See: cardio-pulmonary resuscitation

### cracking pressure

    Pressure required to open a spring-loaded valve. Often applied to the difference in pressure over the diaphragm of a demand valve required to open the valve to start flow. This may differ significantly from the pressure difference required to keep the valve open once flow has been initiated, and the pressure required to keep the valve open may vary with flow rate.

### critical air supply

    See: critical pressure The amount of breathing air required to safely exit a penetration dive. When the available air supply reaches the calculated critical pressure the dive has reached a planned turning point.

### critical difference hypothesis

    See: Decompression theory#Critical difference hypothesis Hypothesis that bubble formation during decompression will not occur if a critical pressure difference between tissue inert gas tension and ambient pressure is not exceeded.

### critical flicker-fusion frequency

    Also: CFFF The frequency at which a flickering light is perceived as continuous, a measurement used to evaluate visual temporal processing as an indicator of alertness and arousal in humans, including use as an experimental indicator of inert gas narcosis in divers, using a simple uncomplicated, non-invasive and objective methodology.

### critical pressure

    See: Scuba gas planning#Critical pressures Cylinder gas pressure which determines a safe limit to an underwater activity on scuba, such as start of ascent or turnaround during a penetration.

### critical ratio hypothesis

    See: Decompression theory#Critical ratio hypothesis Hypothesis that bubble formation during decompression will not occur if a critical ratio of tissue inert gas tension and ambient pressure is not exceeded.

### critical volume hypothesis

    See: Decompression theory#The critical volume approach Hypothesis that symptoms of decompression sickness will not be evident if a critical volume of tissue gas bubbles is not exceeded.

### cross-hauling

    Exerting a lateral force on the bell by using a separate cable from a winch. Used to prevent bell rotation on the lifting cable if a clump weight with guidelines is not used, to move the bell to a position other than directly below the launch and recovery system, and may be used to recover a bell in an emergency.

### crotch strap

    Harness strap that passes from the lower part of the harness at the back, through between the diver's legs, to the waistbelt in front of the harness, effectively securing the harness from sliding up along the torso. In commercial diving safety harness this is often in two parallel parts and allows the diver to be lifted by the harness without risk of falling out.

### crushed neoprene

    Proprietary material for dry suits manufactured by DUI in a process where the foam neoprene suit material is degassed by exposure to high hydrostatic pressure to reduce the volume after assembly. There is less buoyancy variation with depth as the material is less compressible after the treatment.
Subsection: Top, Ca, Ce, Ci, Co, Cu

## Cu
### cuff dump

    A spring loaded over-pressure dump valve mounted on a dry suit sleeve near the cuff, usually relatively small and non-adjustable, which dumps excess gas from the suit if it is raised sufficiently.

### cummerbund

    A wide flexible rubber waistband, which helps to maintain a watertight seal between the jacket and the pants of a two-piece dry suit.

### current limited

    See: Electro-galvanic oxygen sensor#Failure modes Common failure mode of an oxygen sensor cell in which an increase of oxygen partial pressure above the limiting level does not produce an increase in output current. Such a failure would prevent the control system from recognising an excessive partial pressure of oxygen.

### current shear

    Variation in flow velocity along an axis othogonal to the direction of flow. Current shear may be vertical, where shallow water flows at a different velocity to deeper water, horizontal, where flow velocity varies across the width of the current, or mixed. Wind induced current displays vertical shear and directional variation with depth in an Ekman spiral.

### cyalume

    See: Diphenyl oxalate Trademark name Cyalume is a solid ester whose oxidation products are responsible for the chemiluminescence in a glowstick.

### cyanosis

    Main article: Cyanosis The appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen.

### cylinder

    1. Diving cylinder: Refillable compressed gas container of water capacity between 0,5 L and 150 L used to supply gas for underwater diving.
    2. That part of a reciprocating compressor or booster in which the piston is moved to compress the gas. The internal space is cylindrical with a circular section. The external surface is usually finned for air cooling.

### cylinder boot

    See: Diving cylinder#Cylinder boot Rubber or plastic cover for the base of a scuba cylinder to protect it from abrasion, and in the case of domed end cylinders, to allow it to stand upright.

### cylinder bundle

    See: bundle

### cylinder neck

    See: Diving cylinder#Cylinder neck The part of the cylinder end which is shaped as a narrow concentric cylinder, and internally threaded to fit a cylinder valve.

### cylinder shoulder

    See: Diving cylinder#The pressure vessel The domed top of the cylinder between the parallel section and the neck with the cylinder valve.

### cylinder valve

    See: Diving cylinder#The cylinder valve Valve fitted to a compressed gas cylinder to control gas flow into and out of the cylinder. Also pillar valve.