Scuba Diving Definitions

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

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

Air Agas 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 (1 ATA at sea level) to the capacity of the tank, usually between 200-300 Bar.

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 1 atmosphere absolute. (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.

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.

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.

Analogue Instrument Device that uses a needle moving around a dial to provide information.

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).

Arterial Gas Embolism (AGE) The condition characterised by bubbles of air from a ruptured lung segmentunder pressure; the bubbles enter the pulmonary circulation and travel to the arterial circulation, where they may cause a stroke.

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.

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


BAR Approximately equal to 1 atmosphere. A common measurement of air pressure.

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. Body Suit Garment that provides full length abrasion protection.

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


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.

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

Buoyancy  The upward force exerted on an object in liquid, whether the object sinksor 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 odourless, tasteless gas that is a by-product of metabolism; is excreted by the lungs in exhaled air.

Carbon Dioxide Toxicity Problems resulting from build-up of CO2 in the blood; they may range from headache and shortness of breath, all the way to sudden blackout.

Carbon Monoxide CO; odourless, tasteless, highly poisonous gas given off by incomplete combustion of hydrocarbon fuels.

Carbon Monoxide Poisoning CO bonds with haemoglobin 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.

C-Card Refers to a divers certification card for a specific level of achievement.

Charles’s Law The amount of change in either volume or pressure of a given volume of gasis 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 CO2and 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 tablesthe 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.


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 or 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 18m.

Depth Gauge A device that indicates how far a diver is below the surface.

Dive Computer Devicet hat 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 Maybe either white with a blue dovetail (Australia and Europe) or a red rectangle with a white diagonal stripe (USA). May be displayed on afloat or from a dive boat. 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 Haldaniantheory for nitrogen up-take and elimination.

Diver In relation to water, we refer to a person doing SCUBA as a “Diver”. Diver Propulsion Vehicle (DPV) Motorized vehicle used by divers to cove long distances underwater without having to kick.

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 12 deg C.


EAN Enriched air nitrox.

Equalisation 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 suits.


First Stage Regulator Regulator attached to the scuba tank that lowers the tank pressure to ambient pressure plus a pre-determined pressure (e.g., ambient + 10 Bar).

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


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.


Haldanian Related to Haldane’s theory that nitrogen is absorbed up and released in an exponential manner during a dive, and that there is some safe ratio of pressure change for ascent.

Half Time Half the time it takes for a dissolved gas in a tissue (such as nitrogen) to equililbrate to a new pressure, or to reach full saturation at a new pressure. Theoretical tissue half times are used in designing dive tables and algorithms for dive computers.

Heliox Mixture of helium and oxygen, usually reserved for very deep diving. Helium Second lightest gas; dies not cause problems of narcosis to the same extent as seen with nitrogen, and is therefore used for very deep diving.

Henry’s Law The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coefficient of the gas in the liquid.

Hoods Garment worn over the head to reduce thermal loss.

Hookah A surface-supplied compressed air apparatus, for use in shallow diving in calm waters. The air is delivered to one or more divers through a long hose. See Surface Supplied Compressed Air Diving.

Hydrogen An inert gas, and lightest of all the elements, has been used in experimental diving situations.

Hydrostatic Test Pressure test in which the tank is filled with water instead of air and raised to five thirds the maximum working pressure, causing the water to expand and be displaced.

Hyperbaric Chamber Air-tight chamber that can simulate the ambient pressure at altitude or at depth, is used for treating decompression illness.

Hypercapnia A higher than normal P02level in the blood.

Hyperthermia A body temperature warmer than normal, less common in diving than Hypothermia, but can occur from overheating in a wet suit.

Hyperventilation Over breathing to the extent that the blood carbon dioxide level is lowered, may lead to tingling in fingers and dizziness.

Hypoventilation Under breathing to the extent that the blood carbon dioxide level is elevated, may be manifested by carbon dioxide narcosis.

Hypoxia Lower than normal PO2 level in the blood, insufficient oxygen in the blood.

Hypothermia A body temperature colder than normal (98.6F), severe problems start to manifest when body temperature reaches about 95’F.


J-Valve Contains a spring loaded valve that shuts off a divers air supply at approx. 300psi.


K-Valve A simple on and off valve.


Lift Bag After being tied to an object to be lifted, the bag is inflated and will start to rise.

Live Aboard A dive boat with sleeping and eating accommodations. Commercial live aboard boats are usually between 50 and 130 feet long, and can carry from 10 to 30 divers for up to a week or more.

Logbook A diary of a divers dive history. Provides evidence of the depth and breadth of a divers experience.


Manifold Used on double cylinder systems. Has 2 valves similar to single tank systems attached by a heavy duty crosspiece with a valve in the centre.

Macro Photography A method of getting close-up pictures of a subject by using Marco accessories attached to the camera’s lens.

Mask Squeeze Occurs in rapid descents where the diver neglects to equalize his/her mask.The increase pressure causes tissues around the eyes to swell.

Mediastinal Emphysema Air from an over expanding lung escapes into the centre of the chest. This puts pressure on the heart and major blood vessels, interfering with circulation. Symptoms are shortness of breath and feeling faint.

Middle Ear Air containing space of the ear bordered on one side by the tympanic membrane, which is exposed to any change in ambient pressure. Air pressure in the middle ear space can only be equalized through theEustachian tube, which controls the middle ear to the back of the nose.

Mixed Gas Any non-air mixture (e.g., nitrox), although some authors use the term onlyf or mixes that contain a gas in addition to (or in place of) nitrogen(e.g., helium).

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

Multilevel Diving Spending a period of time at several different depth on a single dive.


Narcosis Depressed mental state, anywhere from confusion or drowsiness to coma.

Nitrogen Inert gas that makes up 79% of air. Nitrogen is inert in that it does not enter into any chemical reaction in the body, but it can cause problemsunder pressure (see nitrogen narcosis, decompression sickness).

Nitrogen Narcosis Depressed mental state from high nitrogen pressure; usually does not begin to manifest on compressed air until below 25m.

Nitrox Any mixture of nitrogen and oxygen that contains less than the 79% nitrogen as found in ordinary air.

NOAA Abbreviation for the National Oceanic and Atmospheric Association.


OEA Oxygen enriched air – synonym for nitrox.

Open circuit scuba Apparatus used in recreational diving. Exhaled air is expelled into the water as bubbles, no part is re-breathed by the diver.

Oxygen Often seen as using the chemistry abbreviation 02, gas vital for all life on this planet; makes up about 21% of the air by volume.

Oxygen Therapy Administration of any gas, for medical purposes, that contains more than 21% oxygen.

Oxygen Toxicity Damage or injury from inhaling too much oxygen; can arise from either too high an oxygen concentration or oxygen pressure. One of the most dramatic manifestations of oxygen toxicity while diving can be seizure.

Oxygen Window Difference between total gas pressures in arterial and venous blood; exist because oxygen is partly metabolized by the tissues, so venous oxygen pressure is lower than arterial oxygen pressure.


Partial Pressure Pressure exerted by a single component of a gas within a gas mixture, or dissolved in a liquid.

Pneumothorax A mabnormal collection of air outside the lining of the lung, between the lung and the chest wall, often a consequence of pressure injuries(barotraumas).

Pony Bottle A small scuba cylinder strapped to a divers main tank for emergency use.

Pressure Any force exerted over an area.

PSI Pounds per square inch, a common US measurement of air pressure. Purge Valve Allow masks to be cleared without removal. Allows snorkels to be cleared easily.


Recreational Scuba Diving Diving to prescribed limits, including a depth no greater than 130 ft, using only compressed air, and never requiring a decompression stop.

Regulator In scuba, any device that changes air pressure from one level to a lower level.

Repetitive Dive Any dive whose profile is affected by a previous dive is considered repetitive.

Residual Nitrogen Time The time it would take to off-gas any extra nitrogen remaining after a dive. Residual Nitrogen Time is always taken into consideration when determining the safe duration for any repetitive dive.

Reverse Squeeze Pain or discomfort in enclosed space (e.g., sinuses, middle ear, inside mask) on ascent from a dive.


Safety Stop On ascent from a dive, a specified time spent at a specified depth, for purposes of nitrogen off gassing. By definition it is not a mandatoryf or a safe ascent from a dive.

Salinity  The amount of salt dissolved in a liquid, measured in parts per million. Saturation The degree to which a gas is dissolved in the blood or tissues, full saturation occurs when the pressure of gas dissolved in the blood or tissues is the same as the surrounding pressure of that gas.

Saturation Diving  Diving performed after the body is fully saturated with nitrogen. To become fully saturated the diver must stay under water for a much longer period than is allowed in recreational scuba diving tables.

SCUBA Acronym meaning Self Contained Underwater Breathing Apparatus. Sea Level The altitude of the world’s oceans; all oceans are at sea level.

Second Stage Regulator The regulator that follows, in line, the first stage regulator, and delivers compressed air to the diver. Usually associated with the mouthpiece.

Shallow Water Blackout A sudden unconsciousness, from hypoxia, that occurs among some breath hold dives. Often occurs near the surface after a deeper dive, hence”Shallow Water”.

Shivering The body’s attempt to create heat through muscular activity.

Sinuses Air spaces within the skull that are in contact with ambient pressure through openings into the back of the nasal passages.

Skin Diving Another name for breath-hold diving; diving without the use of a breathing equipment (may include a snorkel).

Squeeze Pain or discomfort in an enclosed (sinuses, Middle ear, inside a mask), caused by shrinkage of that space, occurs on decent.

Submarine A heavy walled vessel that can withstand pressures under water and allow occupants to breath air at sea level pressures and travels under itsown power.

Submersible Pressure Gauge Gauge to monitor air supply during the course of a dive.

Super Saturation An unstable situation where the pressure of a gas dissolved in the blood or tissues is higher than the ambient pressure of that gas.

Surface Interval Length of time on the surface, usually out of the water, between two consecutive dives.

Surface Supplied Compressed Air Diving Diving with the air continuously supplied by a compressor on the surface can be used for both sport and professional diving. See Hookah.


Tank Boot Flat-bottomed, plastic, vinyl or rubber devices that fit over the rounded end of a scuba tank, allowing the tank to stand up.

Thermocline Intersection between two layers of water of that are of distinctly different temperatures, usually the colder layer is deeper.

Tissue Apart of the body characterized by specific characteristics, such as muscle, bone, or cartilage. The term is also used to refer to any partof the body with a specific half time for loading and unloading nitrogen or even a theoretical compartment.

Trimix Mixture of helium, nitrogen and oxygen, used for very deep diving.


Vasoconstriction The constriction of the blood vessels in order to reduce heat loss from the blood through the skin.
Vertigo A sign of ear barotraumas and should not be ignored. Causes may range from minor ear squeeze to perforation of the eardrum to inner ear barotraumas.

Visibility The distance a diver can see underwater measured in feet. Normally vertically not up and down.


Wall Diving Occurs on rock and reefs that run vertically, usually run from shallow to deep.
Water Pressure Force per unit area exerted by the weight of water, each 10m of sea water exerts a pressure equivalent to one atmosphere, or 14.7psa.

Wet Suit Any suit that provides thermal protection underwater by trapping a layer of water betweens the diver’s skin and the suit.
Wheel A dive table used to plan multi-level dives.


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