Scuba Diving Glossary A - G | Scuba Diving Glossary H - Z

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.

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

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 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.)

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 device a diver can use in place of the primary regulator, in order to make an ascent while still breathing normally.

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

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

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 atmospere is about 14.7 pounds of pressure per square inch.

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

ANDI
American Nitrox Divers Incorporated

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

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 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.

Ascent Bottle
An extra cylinder of air used on deep dives to allow decompression stops without fear of running out of air.

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.

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 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.

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.

BC or BCD
See buoyancy compensator.

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

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
Britsh Thermal Units or calories; measurment 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.

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 divers certification card for a specific level of achievement.

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

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 toother 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.


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 nontidal; 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.

D.A.N.
Divers Alert Network. Nonprofit organization that provides emergency and informational advice and assistance for diving injuries, promotes diving-related medical research and education, collects injury statistics, and offers dive safety services to its members and the diving community.

Dalton's Law
The total pressure exerted by a mixture of gases is equal to the sum of the pressures of each gas of the different gases making up the mixture. Each gas acting as if it were alone were present and occupied the total volume.

Decompression
Any change from one ambient pressure to a lower ambient pressure, always results in a reduction of gas pressure within the body.

Decompression dive
Any dive where the diver is exposed to a higher pressure than when the dive began, the decompression occurs as the diver ascends.

Decompression illness
DCI; a term to encompass all bubble-related problems arising from decompression, including both decompression sickness and arterial gas embolism.

Decompression sickness
DCS; a general term for all problems resulting from nitrogen leaving the body when ambient pressure is lowered. Can be divided into Type I (musculoskeletal and/or skin manifestations only) or the more serous Type II (neurologic, cardiac, and/or pulmonary manifestations).

Decompression stop
On ascent from a dive, a specified time spent at a specific depth, for the purpose of nitrogen off-gassing. When not mandatory it is called a safety stop.

Deep diving
For recreational divers a deep dive is a dive below 60 ft.

DEMA
Dive Equipment & Marketing Association. Not-for-profit organization of equipment manufacturers, training agencies, dive media, travel companies and dive retailers that seeks to promote scuba diving and snorkeling to the general public.

Depth gauge
A device that indicates how far a diver is below the surface.
Descent/Ascent Line
A line suspended from a boat, float or buoy used to permit divers to control their descents and ascents and to provide guidance to the bottom in poor visibility or strong currents; particularly useful on ascent to assist divers to make safety or emergency decompression stops between 10 and 15 feet.

Diaphragm
A dividing membrane or thin partition; the thin muscle separating the chest cavity from the abdominal cavity; the rubber (or other material) separating the demand chamber in a regulator from the surrounding water.

DIN
Deutsches Institut fur Normung. Design of tank valve popular in Europe in which the first-stage regulator screws into the tank valve. Recommended for high pressure tanks.

Dive computer
Device that constantly measures depth and time, based on a pre-programmed algorithm, the computer calculates tissue nitrogen uptake and elimination in several theoretical compartments and provides a continuous readout of the dive profile, including: depth, elapsed time of the dive, duration at current depth before decompression becomes mandatory, and a warning if the rate of ascent is too fast.

Dive Flag
May be either a red rectangle with a white diagonal stripe or a blue and white double tailed pennant. Flags are used to warn watercraft to stay away because there are divers below.

Dive lights
Specially designed underwater lights used for night, cave or wreck diving.

Dive Tables
A printed collection of dive times for specific depths, by which the divers can avoid contacting DCS. Most tables are based on Haldanian theory for nitrogen up-take and elimination.

Diver propulsion vehicle
Motorized vehicle used by divers to cove long distances underwater without having to kick.

DOT
Department of Transportation. U.S. government agency that regulates the manufacture, testing and transport of compressed gas containers, including scuba cylinders. DOT stamp appears on scuba tanks, followed by the alphabetic designation for the steel or aluminum alloy the tank is made of and the maximum fill pressure.

DPV
Diver propulsion vehicle, underwater scooter that allows a dive to cover an increased distance underwater. Popular at some resorts.

Dry Suit
A water-tight garment that keeps the diver's body warm by providing insulation with a layer of gas, such as air, for diving in waters that are too cold for comfortable wetsuit protection, usually below 65'F.

EAN
Enriched Air Nitrox. A N2/O2 (nitrogen/oxygen) breathing gas containing more oxygen (typically 32 or 36 percent) and less nitrogen than plain air. Used by recreational divers to increase either bottom time or safety margin by decreasing the amount of nitrogen absorbed. Requires predive testing of gas mixture and adherence to strict depth restrictions.

Ebb Current
A movement of tidal current away from shore or down a tidal stream; tide that is flowing out or causing a lower water level.

Eddy
A circular movement of water, in a comparatively limited area, formed on the side of a main current; may be created at a point where the mainstream passes a projection or meets an opposite current.

Equalization
The act of forcing air into an open space to offset increasing water pressure.

Eustachian tube
A short tube connecting the back of the nose to the middle ear. If clogged, by mucus, equalization is next to impossible.

Exposure protection
Garment worn to prevent decreases in core body temperature and abrasions. Protection can range from thin body suits to heavy dry suit

Fahrenheit
The temperature scale used in the United States. F=(C/.556) +32

ffw
feet of fresh water. Measurement of depth in fresh water.

First Stage
Regulator attached to the scuba tank that lowers the tank pressure to ambient pressure plus a pre-determined pressure (e.g., ambient + 140 psi).

Forward Roll Entry
A method of entering the water while in full SCUBA gear from a boat transom or a pier by bending at the knees and, while firmly securing the mask, rolling forward into the water landing on one's shoulders and tank.

Free Diving
Diving without any scuba or other equipment and synonymous with breath-hold diving.

fsw
feet of sea water. Measurement of depth in sea water.

FSW
Feet of sea water; used to indicate either an actual depth, or just a pressure equal to that depth (e.g., in an hyperbaric chamber


Galley
Kitchen on a boat.

Gas absorption and elimination
Dissolved gases such as nitrogen are absorbed into the blood and tissues during the course of the dive. The level of saturation depends on the depth of the dive. The elimination of these gases is very important in preventing decompression sickness. The length of time required for elimination depends on the duration and depth of the dive.

Gas Laws
Laws that predict how gases will behave with changes in pressure, temperature and volume.

Gauge pressure
Pressure exclusive of atmospheric pressure, when diving, gauge pressure is due to the water pressure.

Giant Stride Entry
The most common method of entering water from a boat transom, pier, etc., where the standing diver takes a large step into the water while securely holding mask, tucking chin and bringing fins quickly together to keep himself at the surface for a controlled descent.

GPS
global positioning system. A worldwide system of navigation based on a ring of stationary satellites. Small, even handheld, GPS devices can be used to accurately determine speed and direction of travel, and pinpoint dive site locations.