Glossary Of Scuba Diving Terms


A —


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.


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


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 pressurises 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 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.)


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 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.


American Nitrox Divers Incorporated.


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


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

Aqua Lung

The first self-contained-underwater-breathing-apparatus that used compressed air and a two-stage on demand regulator. Designed by Jacques Cousteau and Emile Gagnan.

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.


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


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

Arterial gas embolism

The condition characterised 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.


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


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


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.


Australian Underwater Federation


Aquatic World Awareness, Responsibility and Education. A PADI nonprofit environmental foundation that provides financial support for aquatic preservation endeavours, 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 vice-versa) through the scrubber.

B —


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.


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.


Device used to measure atmospheric pressure.

Barometric pressure

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


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.


See buoyancy compensator.


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


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.


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.


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.


British Sub-Aqua Club. The UK’s leading dive club and the sport’s National Governing Body, providing internationally-recognised diver training.


British Thermal Units or calories; measurement of heat.


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


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.


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.

C —


Cerebral Arterial Gas Embolism.

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 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)

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.

Cave Diving

Requiring much specialised 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 specialised 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.


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


Course Director. Level of instructor certification authorised to conduct instructor training.


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


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


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

Charles’ 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.

Circular Search Pattern

An underwater circular search is a procedure conducted by a diver swimming at a series of distances (radii) around a fixed reference point. The circular search is simple and requires little equipment. It is useful where the position of the object of the search is known with reasonable accuracy.


The Confédération Mondiale des Activités Subaquatiques (CMAS) is the World Underwater Federation. CMAS is the international umbrella organisation for diver training organisations. 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 organisations.


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.


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.


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.

Cubic Foot

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


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


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; non-tidal 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!

D —


Divers Alert Network. Nonprofit organisation 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.


See Decompression

Deco Mix

Gas mixture used during 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.


Dive Equipment & Marketing Association. Not-for-profit organisation of equipment manufacturers, training agencies, dive media, travel companies and dive retailers that seeks to promote scuba diving and snorkelling 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.


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.


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 (DPV)

Motorised vehicle used by divers to cove long distances underwater without having to kick.


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 aluminium alloy the tank is made of and the maximum fill pressure.

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.

E —


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.


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.


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, equalisation 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.

F —


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


A linear unit of measurement (equal to 6 feet) for water depth - commonly used by fishermen. Depth = V x T/2


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).

Foramen Oval

The opening between the left and right atria of the fetal heart which remains open in about 30% of people, allowing passage of bubbles into the arterial circulation which is believed to cause symptoms of arterial gas embolism.

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.


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).

G —


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 Analysis

A procedure whereby the fractions of gas in a breathing mixture are determined. Generally it is used to determine the fraction of oxygen to within 1 %, to control the effects of Oxygen toxicity. Most diving operations will analyse for oxygen only. In more complex Trimix’s, it is be desirable to analyse for the inert gases as well.

Gas Laws

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

Gas Saturation

Gas saturation occurs when the level of dissolved gas in a particular body tissue has reached its maximum.

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.


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.

Granny Line

Also known as a Gerry Line. A line which goes from the stern platform or hang bar to the anchor line.

H —


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 equilibrate 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.


A Halocline is a change in the salt content of the water, but is not often apparent to divers unless severe enough to cause buoyancy changes.

Hang Tank

An extra tank, ideally of deco mix, staged at the
decompression stop. Normally found at the rear of a dive boat in case of emergencies.


Restroom on a boat.


Mixture of helium and oxygen, usually reserved for very deep diving.


Second lightest gas; does 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.

High Altitude Diving

Is done in mountain lakes or other high altitude waters at or greater than 1,000 feet (300 meters) above sea level with increased risk of decompression sickness because of lower-than-sea-level atmospheric pressure at the surface; regular dive tables and some dive computers and depth gauges are inaccurate above sea level; special high altitude dive tables and recalibration of gauges and computer are required; specialty courses are available due to the complexity and added hazards of this activity.

High Pressure Nervous Syndrome (HPNS)

A condition which results from breathing Helium under high pressures. Early symptoms of HPNS are sometimes seen as shallow as 300FSW but more commonly over 600FSW. The severity also depends on the mix of breathing gases, Nitrogen can often moderate the affects of HPNS.

The early symptoms include muscle tremors, followed by changes in electroencephalogram (EEG) readings, impaired motor and problem solving skills. Other symptoms can include euphoria, nausea, vomiting, lack of appetite and drowsiness. Symptoms sometimes moderate or entirely disappear with continued exposure.

Hogarthian Principles

The Hogarthian configuration is named after Bill ‘Hogarth’ Main. It is based on reducing equipment to a minimum streamlined configuration that nevertheless includes sufficient redundancy for extended decompression dives.


Garment worn over the head to reduce thermal loss.


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.


The common term for the hydrostatic test required on scuba cylinders every five years to determine whether the tank walls are still strong enough for safe usage.


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.


A higher than normal P02level in the blood.

Hyperoxic And Hyperoxia

In general, these terms relate to a more than a normal amount of Oxygen. Hyperoxic refers to a mixture of gases with higher than normal Oxygen content (above 21%). Hyperoxia is the physiological condition associated with breathing too high of a partial pressure of Oxygen. The human body has a limit on both the partial pressure of Oxygen it can tolerate and the long term dosage of Oxygen. The partial pressure upper limit is generally considered to be approximately 1.6 ppO2 but most divers leave some margin for error and a more typical upper limit is 1.4 ppO2. When high partial pressures of Oxygen are inspired, convulsions may occur with little or no warning.


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


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


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


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


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

I —


International Association of Nitrox and Technical Divers. IANTD is the only EANx agency that offers training in all aspects of EANx through continuing education programs. This allows you to expand your knowledge and training with the top professionals in the field. IANTD instructors are well trained, highly experienced and extremely qualified. IANTD Standards and Procedures provide for the highest level of EANx education and training available. IANTD (IAND, Inc.) was founded by Dick Rutkowski, the former dive supervisor for the National Oceanic and Atmospheric Administration (NOAA). Mr. Rutkowski introduced the recreational diving community to the technology of EANx. This program was developed through NOAA during his tenure. Today the board of directors and the membership of IANTD are composed of many of the most experienced divers in the world pertaining to the use of breathing gases other than air.

Immersion Hypothermia

The lowering of body temperature when a diver is fully immersed in cold water without proper thermal protection.

Inflator Valve

A manually operated valve that puts compressed air into the buoyancy bladders of a BCD or into a dry suit.

J —

Jackstay Search Pattern

An underwater search conducted by divers who remain in contact with a rope marking the search line. Each time a diver reaches an end of the rope, that end is moved in a set direction. The divers may start together or at opposite ends of the rope.


Jellyfish are members of the cnidarians group of invertebrates. They are transparent and bell shaped with tentacles hanging down from their body. They are free-floating and seem serene, but in their tentacles there are stinging mechanisms that are used to catch prey and for defence.


A structure, usually made with rocks, extended into a sea, lake or river to influence the current or tide in order to protect a harbour.


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

K —

Kelp Surface Dive

A vertical, feet-first, method of descending into water of unknown depth or when obstructions or heavy plant (such as kelp) growth exists; performed by spreading arms and legs, then simultaneously bringing legs together while giving a strong downward stroke with arms thus propelling upper body out of water; body weight will then drive the diver downward; some also find this dive descent easier to equalise ears because there is less blood pressure in head than with pike (head first) dive.


Kilogram. Metric measure of weight. 1 kg = 2.21 pounds.


The velocity unit of 1 nautical mile (6080.20 ft.) per hour; equivalent to 1.689 ft. per second: to convert ft. per sec. into knots, multiply by 0.592.


A simple on and off valve.

L —

Lift Bag

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

Lift Capacity

The amount of buoyancy provided by a Buoyancy Compensator; varies according to size of the BC and according to the purpose of the BC, e.g., a BC intended for use in cold fresh water will provide greater lift capacity than one intended primarily for use in warm salt water.

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.


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

Low Volume Mask

A mask which has a smaller area between the glass and the diver’s face, usually with separate lenses for each eye; requires less air to purge if becomes flooded.

M —


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.


A skirted glass window constructed to provide air space between eyes and water and to permit both eyes to see in the same plane; a regular mask covers eyes and nose only; modern mask skirts are usually made of silicone rather than the older rubber ones.

Mask Squeeze

A painful condition when the air inside the mask is compressed by the external pressure creating suction on the face and eyes; can be alleviated by exhaling from the nose; can cause permanent eye damage if not equalised.

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 equalised through the Eustachian 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 only for mixes that contain a gas in addition to (or in place of) nitrogen (e.g., helium).


Maximum Operating Depth/Oxygen Depth Limit. The deepest that a diver can safely go using a particular gas mixture. For example, the MOD for EAN32 (32 per- cent oxygen) is 132 fsw (40 m).

Multilevel Diving

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

N —


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


National Association of Underwater Instructors. It is one of the oldest certifying agencies.

Nautical Mile

Also known as a “geographical mile” or “sea mile”; a unit of distance designed to equal approximately 1 minute of arc of latitude, 6080.20 ft.; approximately 1.15 times as long as the statute mile of 5280 ft.


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 problems under 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 80 fsw.


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


Abbreviation for the National Oceanic and Atmospheric Association.

O —

Octopus Regulator

An alternate second stage air source used by a diver’s buddy in an out-of-air situation, or reserve 2nd stage regulator.


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.

Open-Water Diving

The recreational diving done in an environment other than a swimming pool but with no overhead obstacles; examples include lakes, ponds, rivers, oceans, quarries.

Overhead Environment

A structure exists above which prevents the diver from making a direct vertical ascent to the surface.


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 metabolised by the tissues, so venous oxygen pressure is lower than arterial oxygen pressure.

P —


Professional Association of Diving Instructors. It is one of the largest certifying agencies.

Partial Pressure

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


Am abnormal collection of air outside the lining of the lung, between the lung and the chest wall, often a consequence of pressure injuries (barotrauma).

Pony Bottle

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


Any force exerted over an area.


Pounds per square inch, a common measurement of air pressure.

Purge Valve

Allow masks to be cleared without removal. Allows snorkels to be cleared easily.

R —


A closed-circuit system which filters exhaled air, then recirculates it for rebreathing by the diver; requires special training and maintenance.

Recreational Scuba Diving

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


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.

Rip Current

A strong current of limited area flowing outward from the shore, and may be visible as a band of agitated water with the regular wave pattern altered; current is caused by the rush of escaping water which is piled between shore and bar or reef by wave action through a gap in the bar or reef; such currents are dangerous to the uninitiated and are the cause of many drownings at ocean beaches; however, when located by divers they are often used to facilitate entry to areas beyond the bar or reef.

Rule Of Thirds

An air management technique. Use a third of the tank to get to your destination, a third to get back, and the last third belongs to your buddy in case of an emergency.

S —


Sub-Aqua Association. Diver training organisation in the UK.

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 mandatory for a safe ascent from a dive.


The amount of salt dissolved in a liquid, measured in parts per million.


Supplied Air Snorkelling for Youth / Supplied Air Snorkelling for Adults. BC-like PFD mounted with small compressed air cylinder and regulator that allows snorkeller to breathe comfortably on the surface but prohibits him from descending. SPG submersible pressure gauge. Required scuba gear that displays the amount of air pressure in the scuba cylinder; can be either analog or digital.


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.


Surface Consumption Rate. Measure used in calculations for determining air consumption rate at various depths.


Acronym meaning Self Contained Underwater Breathing Apparatus. Although now considered a word in its own right.

Scuba Resort

a place providing recreation and entertainment to vacationers with the main attraction as that of scuba diving. It may include training, Beach and/or boat diving and many specialties such as wall, drift, wreck, eco-diving and many others. It can be an All-Inclusive, a Live Aboard and or a land base operation. It will normally be supported by one or many of the training agencies.


Scuba Diving International. The recreational scuba training and certification arm of TDI.

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”.


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


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).


Scuba Lifesaving and Accident Management. YMCA diver rescue course.


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.

Solo Diving

Solo diving is the practice of scuba diving alone without a “dive buddy”. Solo diving, once discouraged, is now beginning to gain acceptance among experienced divers who have skills in self-sufficiency and redundant backup equipment.


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


The back end of a boat.


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

T —

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.


Technical Diving International. Maine-based certification agency for technical aspects of recreational scuba.


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


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


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

U —

Unbalanced Regulator

An unbalanced first stage regulator is affected by tank pressure. The tank pressure works to force the piston in the first stage open. As the gas in the tank diminishes, it is easier for the piston to be forced into a closed position.

This makes it increasingly harder to breath from the first stage as the dive progresses. This is an older style regulator and has pretty much vanished from today’s scuba gear market.

U-Pattern Search

A U shape search pattern that requires on line. This type of search pattern is suitable over many different bottom terrains and is a frequently used search pattern. A search team swims along the bottom in a long straight line, turns 90 degrees and swims for a short length, and then swims another long length.


Upwelling is vertical water movement. This occurs when surface water is moved away (offshore) due to winds blowing from north top south and acting as a force on the surface waters. Because water from somewhere has to fill the space left by the offshore movement of surface water, water fills in from below, creating a vertical current of water.

V —

Valsalva Manoeuvre

A method of manually equalising the inner ear. The pressure on the inside of the eardrum is increased by closing the nasal passage (pinching the nose shut) and blowing gently, forcing air into the inner ear via the Eustachian tube.


The constriction of the blood vessels in order to reduce heat loss from the blood through the skin.


Having a gland or glands for secreting venom; able to inflict a poisoned bite, sting or wound.


A sign of ear barotrauma and should not be ignored. Causes may range from minor ear squeeze to perforation of the eardrum to inner ear barotrauma.


Visual Inspection Program. Standardised visual tank inspection performed by a trained equipment technician, performed annually.


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

W —

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 33 feet 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.


A dive table used to plan muti-level dives. (Now obsolete.)


A point or side from which the wind blows; in the direction from which the wind blows; as opposed to, leeward.

Wreck Diving

Diving on natural or man-made shipwrecks; specialty courses are available.

Y —


To swing off course. This is normally caused by bad steering or the action of the waves.


Dual orifice valve.

Z —


In general terms, the zenith is the direction pointing
directly “above” a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of “above” is more specifically defined in astronomy, geophysics and related sciences (e.g., meteorology) as the vertical direction opposite to the net gravitational force at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir. The term zenithalso refers to the highest point reached by a celestial body during its apparent orbit around a given point of observation.

Zoom Lens

A lens that combines a range of focal lengths.