Scuba Glossary: A - B - C


Abdominal hernia  –  A weakness of the abdominal wall through which the intestines or other intraabdominal contents protrude. Dangerous to a diver if a loop of air-containing intestine is trapped outside the abdomen.

Absolute  –  Pressure calculated by using a vacuum as the zero point and including the gauge and atmospheric pressure in the calculation.

Actual bottom time (ABT)  –  Total elapsed time in minutes from leaving the surface until ascent is initiated.

Adjustable buoyancy life jacket  –  Also known as ABLJ or horse collar buoyancy compensator. A combination of buoyancy compensator and inflatable life jacket worn on the chest and round the neck.

Adrenaline  –  A hormone secreted by the adrenal gland into the circulatory system which stimulates the heart, blood vessels and respiratory system.

ADV  –  Automatic Diluent Valve: A demand valve set into the breathing loop of a re-breather to inject diluent gas into the loop when the loop volume falls and there is not enough gas for inhalation.

AGE  –  Abbreviation for arterial gas embolism.

Air  –  A gas mixture containing 21% oxygen, 78% nitrogen, and 1% other gasses (mainly argon); compressed air is used for recreational scuba diving.

Air compressor  –  A machine that compresses or pressurizes air; for scuba purposes, air is compressed from the atmospheric level (14.7 psi at sea level) to the capacity of the tank, usually between 2500-3000 psi.

Air Consumption  –  The depletion of breathing gas by a diver during the course of a dive.

Air Dome  –  A section of cave which traps air or other gas at the top. This gas is not directly connected to the surface.

Air embolism  –  A condition that occurs when air enters the bloodstream through ruptured alveoli into the pulmonary capillaries. The air in the bloodstream then forms bubbles, which can block blood flow to the body’s tissues.

Air pressure  –  The force per unit area exerted by the weight of air; at sea level the air pressure is 14.7 psi. (air pressure decreases with altitude.)

Algal bloom  –  A rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system. Some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells. Visibility can be severely impaired over a period of hours to days. Sometimes referred as a Red Tide.

Algorithm  –  A set of equations incorporated into diving computers in order to compute nitrogen uptake and elimination from changes in depth and elapsed time.

Alpha flag  –  An International maritime signal flag, meaning, ‘Diver down, keep clear’.

Alternate air source  –  A redundant air supply. A device a diver can use in place of the primary regulator, in order to make an safe ascent while still breathing normally. This can be either an octopus second stage or a separate tank and regulator, sometimes called a pony tank.

Alternoberic Vertigo  –  Un-even release of pressure from the inner ear. Causing vertigo, dizziness and spins.

Altitude diving  –  Diving at a location where the water surface is at an altitude which requires modification of decompression schedules, (more than about 300 m (980 ft) above sea level.

Altitude sickness  –  An illness brought on by the sudden reduction in pressure of ascent to altitude.

Alveolus  –  Air sac at the terminus of a bronchus where oxygen and carbon dioxide transfer occurs.

Ambient Light  –  It is the available sunlight underwater used as a source of illumination.

Ambient pressure  –  The surrounding pressure; on land, comes from the weight of the atmosphere (see air pressure), at depth, comes from the weight of the water plus the weight of the atmosphere. One atmosphere is about 14.7 pounds of pressure per square inch.

Analog instrument  –  Device that uses a needle moving around a dial to provide information.

Anchor Line  –  A rope, cable or chain that attaches a boat to its anchor.

ANDI  –  American Nitrox Divers Incorporated

Anemia  –  Any reduction in the oxygen carrying capacity of the red blood cells.

Anoxia  –  A medical condition caused by a severe lack of oxygen in the body.

Anticoagulants  –  Medications that reduce the clotting ability of the blood. Particularly dangerous to divers due to barotrauma of air-filled body cavities.

Aphasia  –  An impairment of language ability which may range from having difficulty remembering words to being completely unable to speak, read, or write.

Aqua Lung  –  The first self-contained-underwater-breathing-apparatus that used compressed air and a two-stage on demand regulator. The aqua lung was designed by Jacques Cousteau and Emile Gagnan.

AR vest  –  A waistcoat (vest) style harness of heavy cloth with strong adjustable webbing straps so that the diver can not slide out under any predictable circumstance.

Archimedes principle  –  Any object wholly or partly immersed in fluid will be buoyed up by a force equal to the weight of the fluid displaced by the object.

Argon  –  An inert gas that makes up less than one percent of air (sometimes used as a dry suit gas).

Arrhythmias  –  Irregularities in the rhythm and rate of the heart, particularly dangerous to divers due to the underwater environment.

Arterial bubble model  –  Decompression model in which the filtering capacity of the lung is assumed to have a threshold radius of the size of a red blood cell and sufficiently small decompression bubbles can pass to the arterial side, especially during the initial phase of ascent.

Arterial gas embolism  –  The condition characterized by bubble(s) of air from a ruptured lung segment under pressure; the bubbles enter the pulmonary circulation and travel to the arterial circulation, where they may cause a stroke.

Artificial Respiration  –  Any means by which an alternating increase and decrease in chest volume is artificially created while maintaining an open airway in mouth and nose passages; mouth to mouth, mouth to nose and mouth to snorkel resuscitation are examples.

Artificial Spit  –  Small bottles of “special” liquid used by divers to keep their masks defogged. There are many different manufactures that offer a gel, paste or spray.

Ascent Bottle  –  An extra cylinder of air used on deep dives to allow decompression stops without fear of running out of air. They come in many different sizes from 6 c.f. to a full 80 c.f. tank.

Ascent/Decent line  –  Line suspended from a boat or a buoy for a diver to use to control their rate of ascent or descent.

Asthma  –  A common condition manifested by narrowing of air passages within the lungs. One reason for the narrowing is excess mucous in the airway.

ATA  –  Atmosphere absolute; 1 ATA is the atmospheric pressure at sea level; is measured with a barometer.

Atmosphere   –  The blanket of air surrounding the earth, from sea level to outer space. Also, a unit of pressure; “one atmospheres is pressure of the atmosphere at sea level, I.e., 760 mm Hg. Two atmosphere is twice this pressure, 1520 mm Hg, etc. Abbreviated atm.

Atmosphere Absolute  –  The ambient pressure including the air column over the water. The air column = 1 ATM. at sea level. In sea water, another atmosphere is added each 33 FSW (Feet of Sea Water) . There is an increase in pressure per foot of sea water equivalent to 1/33 or .03030303 . So ATA may be calculated by multiplying the depth (FSW) by .0303030 and then adding 1 for the air above the water. i.e. the ATA at 46 FSW = (46 * .0303030) + 1 = 2.3939 ATA. to convert ATA to FSW. ATA – 1 * 33 = FSW.

Atmospheric pressure  –  Pressure of the atmosphere at a given altitude or location.

AUF  –  Australian Underwater Federation

AWARE  –  Aquatic World Awareness, Responsibility and Education. A PADI nonprofit environmental foundation that provides financial support for aquatic preservation endeavors, develops conservation-oriented educational materials and initiates public awareness campaigns.

Axial flow scrubber  –  An axial scrubber is a scrubber design in which the breathing gases move from top to bottom (or vise-versa) through the scrubber.



Backpack  –  A rigid or semi-rigid structure similar in function to a back plate, usually made of molded 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 stabilizes the scuba cylinder on the diver’s back.

Backplate  –  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  –  When suspended particles in the water, such as sand, are illuminated by light from a flash, they reflect the light back to the lens. The particles appear as specks or snow in the photo.

Backward kick  –  A fining 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.

Backward Roll Entry  –  Means of entering the water in SCUBA gear from a sitting position such as from the gunnel of a boat whereby the diver, while securely holding his mask, leans backward and rolls into the water onto his tank and shoulders. Checking for an all clear is recommended.

Bailout  –  training technique used in some SCUBA classes wherein the student jumps into the pool while holding all equipment in hand and then dons the equipment on the bottom of the pool; or, pertaining to or consisting of a means for relieving an emergency situation.

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 valve  –  

  1. An open circuit demand valve built into a re-breather mouthpiece, or other part of the breathing loop, which can be isolated while the diver is using the re-breather 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.

Balanced regulator  –  Regulator designed to provide a consistent demand effort not affected by cylinder gas pressure or depth.

Barometric pressure  –  Same as atmospheric pressure with the exception that it varies with the weather.

Barotraumas  –  Any disease or injury due to unequal pressures between a space inside the body and the ambient pressure, or between two spaces within the body; examples include arterial gas embolism and pneumothorax.

BAT wing  –  Buoyancy And Trim wing. A back mounted buoyancy compensator cell used with side mount harness. The buoyancy volume is mostly over the lower back.

BC or BCD  –  See buoyancy compensator.

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.

Bell man  –  Standby diver in the diving bell.

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

Bell umbilical  –  The combined supply and return hoses and cables for life-support, power and communications between a diving bell and the support platform.

Bends  –  A form of decompression sickness caused by dissolved nitrogen leaving the tissues too quickly on ascent; is manifestation of decompression sickness.

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.

Bladder  –  A pouch within a Buoyancy Compensator which holds the amount of air the diver desires to provide proper buoyancy.

Body suit  –  Garment that provides full length abrasion protection.

Bootie  –  A piece of foot protection, usually made of neoprene, worn inside an open-heeled fin; serves to protect the diver’s feet while walking to and from the dive site and prevents blisters from the fins while swimming; also provides warmth, depending on thickness. May come in a varying sole thickness.

Bottom time  –  The time between descending below the surface to the beginning of ascent.

Bow  –  The front end of a boat.

Boyle’s law  –  At a fixed temperature for a fixed mass of gas, pressure times volume is a constant value.

Breath-hold diving  –  Diving without life support apparatus, while holding one’s breath.

BTU  –  British Thermal Units or calories; measurement of heat.

Bubble  –  A collection of air or gas surrounded by a permeable membrane through which gases can enter or exit.

Buddy  –  Diving partner.

Buddy Breathing  –  Sharing of the same air supply by two or more divers; an emergency technique used when one person’s air supply is exhausted or unavailable due to equipment malfunction.

Buoyancy  –  The upward force exerted on an object in liquid, whether the object sinks or floats. Objects that float are positively buoyant, those that sink are negatively buoyant and those that stay where placed are neutrally buoyant.

Buoyancy compensator  –  An inflatable vest worn by the diver that can be automatically or orally inflated to help control buoyancy; abbreviated BC or BCD (Buoyancy Control Device).

Burst disk  –  Thin copper disk held in place with a vented plug. Designed to rupture if tank pressure is greatly exceeded.



CAGE  –  Cerebral arterial gas embolism.

Capillary depth gauge  –  Made up of a small tube. Uses Boyle’s law to determine depth.

Carbon dioxide  –  CO2; an odorless, tasteless gas that is a byproduct of metabolism; is excreted by the lungs in exhaled air.

Carbon dioxide toxicity  –    Problems resulting from buildup of CO2 in the blood; they may range from headache and shortness of breath, all the way to sudden blackout.

Carbon monoxide  –  CO; odorless, tasteless, highly poisonous gas given off by incomplete combustion of hydrocarbon fuels.

Carbon monoxide poisoning  –  CO bonds with hemoglobin and prevents blood cells from carrying oxygen. This causes oxygen deprivation in the tissues and can even cause death.

Carbon monoxide toxicity  –  Illness from inhaling excess CO; problems may range from headache to unconsciousness and death.

Cave Diving  –  Requiring much specialized training and equipment, this involves diving into an overhead environment including caves, abandoned mines or quarries and natural springs or sinkholes where the exit is not always visible. “Overhead environment” means a structure exists which prevents the diver from making a direct vertical ascent to the surface.

Cavern Diving  –  Requiring specialized training, this involves diving into an overhead environment including caves, abandoned mines or quarries and natural springs or sinkholes; differs from Cave Diving in that the exit should always be visible. “Overhead environment” means a structure exists which prevents the diver from making a direct vertical ascent to the surface.

C-Card  –  Refers to a diver’s certification card for a specific level of achievement.

Central nervous system oxygen toxicity  –  CNS   –  High oxygen levels which affects the central nervous system. The condition can occur during deep dives with fatal consequences.

CD  –  Course Director. Level of instructor certification authorized to conduct instructor training.

CDAA –  Cave Diving Association of Australia. The CDAA was formed in September 1973.

CDC  –  Centers for Disease Control and Prevention. An U.S. government agency within the Department of Health and Human Services which, among other functions, maintains the Traveler Hotline with information on geographic distribution of diseases and inoculations required/recommended for travel to other countries.

Celsius  –  Metric unit for temperature. C=(F-32) x .556

cf  –  cubic foot. A measure of volume. Scuba cylinders are manufactured in standard sizes, such as 30, 50, 72 and 80 cf.

Charles’s Law  –  The amount of change in either volume or pressure of a given volume of gas is directly proportional to the change in the absolute temperature.

Closed circuit scuba  –  Apparatus designed to allow divers to re-breath exhaled air after removal of CO2 and addition of supplemental 02. In contrast to “Open Circuit”, closed circuit scuba is noiseless and produces no bubbles.

CMAS  –  The Confédération Mondiale des Activités Subaquatiques (CMAS) is the World Underwater Federation. CMAS is the international umbrella organization for diver training organizations. It was founded in Brussels on September 28, 1958. One of the founding members was the French underwater pioneer Philippe Tailliez. As such, it is one of the world’s oldest scuba diving organizations.

Compartment  –  A theoretical division of the body with an arbitrarily assigned half time for nitrogen uptake and elimination. In designing decompression tables the body is divided into finite number of compartments for purposes of making calculations.

Computer  –  A device that monitors nitrogen in the body during a dive though mathematical algorithms. The device allows divers to multilevel dive and extend bottom time beyond what a dive table allows.

Coral  –  Invertebrates that secrete an internal, hard skeleton structure composed of calcium carbonate, which is absorbed from the surrounding water.

Core temperature  –  The internal temperature of the body, 98.6F is the normal temperature of the human body. Deviation from this temperature even a few degrees could be life threatening.

Cummerbund  –  An overlapping waistband with Velcro used to secure a Buoyancy Compensator snugly around the diver’s waist.

Current  –  A horizontal movement of water; currents can be classified as tidal and non-tidal; tidal currents are caused by forces of the sun and moon and are manifested in the general rise and fall occurring at regular intervals and accompanied by movement in bodies of water; nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from weather conditions. For general rules: 1 to 2 kt current is Light, 2 to 3kt current is Mild, 3 to 4kt is Strong and 5kt is Very Strong for the average diver!


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