Back gas

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

Back roll entry or backward roll entry

Water entry method in which the seated diver rolls backwards off the side of the boat, allowing the scuba cylinders to strike the water first.

Backscatter

Light from a flash or strobe reflecting back from particles in the lens’ field of view causing specks of light to appear in the photo. Backscatter can be a common problem in underwater photography because particulate matter can be very dense and include plankton which would otherwise be near transparent. Backscatter can be reduced and in many cases removed altogether using various photographic techniques.

Backmount

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

Backward kick

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

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.

Backpack

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

Backup light

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

Backup regulator

A second regulator connected to a cylinder or manifolded twin set.

Bail-out cylinder or Bail-out tank

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.

Balanced regulator

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

Bar

Metric unit of pressure commonly used in diving, equal to 100 kiloPascal, and nearly equal to standard atmospheric pressure.

Barotrauma

Injury caused by pressure difference.

BC or Buoyancy Compensator

An airtight bladder worn by a diver which can be filled with air and vented to adjust and control the buoyancy of the diver.

BCD or Buoyancy Control Device

See BC or Buoyancy Compensator.

Bends

Decompression sickness: Injury caused by bubble formation in the body tissues after hyperbaric exposure.

Bladder

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

Blending stick

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

Blind traverse

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

Block adaptor

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

Blue hole

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

Boatmaster

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

Bolt snap

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

Booster

Machine used to increase pressure of a gas. Usually refers to the case where inlet pressure is above ambient pressure already.

Bottom time

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

Bottom timer

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

Bottom gas

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

BOV or Bail-Out Valve

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

Bow

The front end of the boat.

Bowline

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

Boyle’s law

Relationship between pressure and volume at constant temperature in an ideal gas. PV = k, where P is pressure, V is volume and k is a constant. Means that if pressure decreases (such as during an ascent) then the volume of gas increases (assuming temperature is constant). And vice versa. Why you need to add air to your BCD when you descend, and release air from your BCD when you ascend.

Breathing gas

Gas supplied to the diver to breathe, either directly to the diver or to the hyperbaric environment of the diving bell, dive chamber or saturation habitat.

Breathing loop

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

BSAC or British Sub Aqua Club

The British Sub-Aqua Club has been recognized since 1954 by the Sports Council as the national governing body of recreational diving in the United Kingdom. The club was founded in 1953 and at its peak in the mid-1990s had over 50,000 members declining to over 30,000 in 2009. It is a diver training organization that operates through its associated network of around 1,100 local, independent diving clubs and around 400 diving schools worldwide. BSAC is unusual for a diver training agency in that most BSAC instructors are volunteers, giving up their spare time to train others, unlike many other agencies, in which instructors are paid employees, or self-employed.

Bubble models

Decompression models based on the assumption that bubbles will form during non-symptomatic decompression.

Buddy

Your nominal partner when you practice the “Same Day, Same Ocean” diving principle

Buddy breathing

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

Buddy check

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

Buddy system

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

Bühlmann algorithm

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

Bungee

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

Buoyancy

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

Buoyancy check

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

Buoyancy control

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

Burst disk

A non-reclosing pressure relief device used to protect a diving cylinder from over-pressurization.

Butt-plate

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

BWRAF

Acronym for pre-dive safety check.

CAGE or Cerebral Arterial Gas Embolism

Cage diving

Diving in a cage designed to protect the diver from potentially aggressive large marine animals, usually sharks.

Capillary depth gauge

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

Canister light

Dive light comprising a light head connected to a battery canister by a cable.

Carbon dioxide or CO2

An odorless, tasteless gas that is a byproduct of metabolism; is excreted by the lungs in exhaled air.

Carbon dioxide poisoning

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

Carbon monoxide or CO

An odorless, tasteless, highly poisonous gas given off by incomplete combustion of hydrocarbon fuels.

Carbon monoxide poisoning

The toxic effects of carbon monoxide, usually due to contaminated breathing gas supply.

Cave

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

Cave arrow

Directional line markers which point the way to an exit.

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.

Cave reel

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

Cavern

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

Cavern diving

Visibility greater than 12 meters0 Maximum penetration of 40 meters, maximum depth of 21 meters, and always within the ambient sunlight area. No passing through restrictions.

CCR or Closed Circuit Rebreather

Ceiling

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

Cenote

A cenote is a sinkhole with exposed rocky edges containing groundwater. It is typically found in the Yucatan Peninsula and some nearby Caribbean islands. The term derives from a word used by the low-land Yucatec Maya to mean any location with accessible groundwater. Commonly dove by cavern and cave divers. Generally found in Tulum, Akumal, and Playa del Carmen.

CESA or Controlled Emergency Swimming Ascent

Emergency procedure where the diver makes an ascent from depth after a breathing gas supply failure.

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.

Chimney

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

Chokes

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

Circuit

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

CMAS or Conféderation Mondiale des Activitiés Subaquatiques

Confédération Mondiale des Activités Subaquatiques is an international federation that represents underwater activities in underwater sport and underwater sciences, and oversees an international system of recreational snorkel and scuba diver training and recognition. Its foundation in Monaco during January 1959 makes it one of the world’s oldest underwater diving organizations. A founding member and key proponent of CMAS was the French underwater explorer and diving pioneer Jacques-Yves Cousteau who was chosen to be the inaugural President.

CNS or Central Nervous System

CNS oxygen toxicity

Formerly known as Paul Bert effect, Central nervous system oxygen toxicity manifests as symptoms such as visual changes (especially tunnel vision), ringing in the ears (tinnitus), nausea, twitching (especially of the face), irritability (personality changes, anxiety, confusion, etc.), and dizziness. This may be followed by a tonic–clonic seizure consisting of two phases: intense muscle contraction occurs for several seconds (tonic); followed by rapid spasms of alternate muscle relaxation and contraction producing convulsive jerking (clonic). The seizure ends with a period of unconsciousness. The onset of seizure depends upon the partial pressure of oxygen in the breathing gas and exposure duration. However, exposure time before onset is unpredictable, as tests have shown a wide variation, both amongst individuals, and in the same individual from day to day. In addition, many external factors, such as underwater immersion, exposure to cold, and exercise will decrease the time to onset of central nervous system symptoms. Decrease of tolerance is closely linked to retention of carbon dioxide. Other factors, such as darkness and caffeine, increase tolerance in test animals, but these effects have not been proven in humans.

Exposures, from minutes to a few hours, to partial pressures of oxygen above 1.6 bars (160 kPa)—about eight times the standard atmospheric partial pressure—are usually associated with central nervous system oxygen toxicity and are most likely to occur among patients undergoing hyperbaric oxygen therapy and divers. Since sea level atmospheric pressure is about 1 bar (100 kPa), central nervous system toxicity can only occur under hyperbaric conditions, where ambient pressure is above normal. Divers breathing air at depths greater than 60 meters face an increasing risk of an oxygen toxicity “hit” (seizure). Divers breathing a gas mixture enriched with oxygen, such as enriched air (nitrox), can similarly suffer a seizure at shallower depths, should they descend below the maximum operating depth allowed for the mixture.

Code of practice

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

Commercial diving

Working under pressure : Occupational activity where gas is breathed at pressure in excess of atmospheric pressure, usually underwater.

Compressed Air

Air at a pressure greater than ambient.

Compressor

Machine which pressurizes gas. Generally intake gas is at ambient pressure, outlet gas at higher pressure. High pressure breathing air compressor output pressure is usually 200 to 330 bar.

Confined Water dives

Confined water is a general term that refers to either a swimming pool or confined open water. Confined open water is an open water site that offers swimming-pool-like conditions with respect to clarity, calmness and depth. It has both shallow water and water sufficiently deep to allow student divers to meet all skill performance requirements. Water that is enclosed and bounded sufficiently for safe training purposes. Generally implies that conditions are not affected by geographic or weather conditions, and that divers can not get lost.

Controlled buoyant lift

A rescue technique used by scuba divers to raise an incapacitated diver to the surface from depth.

Cookie

Personal non-directional line markers that mark specific locations, or the direction of one’s own exit at line intersections.

Counterlungs

Flexible bag or bellows in a rebreather which compensates for the change in volume in the loop during breathing.

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 !

Cyanosis

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

Cylinder valve

Valve fitted to a compressed gas cylinder to control gas flow into and out of the cylinder.

Dalton’s law

Gas law describing the relation of component pressures of gases in a mixture to the total pressure.

DAN or Divers Alert Network

A non-profit organization for assisting divers in need and medical research on recreational scuba diving safety.

DCI or Decompression Illness

Illness caused by decompression. Includes decompression sickness and arterial gas embolism due to lungs over-expansion barotrauma. Include DCS and AGE.

DCS or Decompression sickness

A condition arising from dissolved inert gases coming out of solution during decompression as bubbles in the tissues, organs and blood vessels of the body causing symptoms ranging from rashes to death. Can be divided into Type I (musculoskeletal and/or skin manifestations only) or the more serous Type II (neurologic, cardiac, and/or pulmonary manifestations).

Dead air space or dead air volume

1. The volume of a breathing apparatus which holds exhaled air, which is subsequently inhaled directly. without passing through a scrubber to remove carbon dioxide and without oxygen addition.
2. The volume of inhaled air, which does not take part in gas exchange either because it remains in the conducting airways or in alveoli that are poorly perfused.

Decant

Transfer gas between cylinders by differential pressure. No energy is input, flow will stop when pressures equalized.

Decompression

Reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure, and the process of allowing dissolved inert gases to be eliminated from the body tissues during this reduction in pressure.

Decompression algorithms

Specified step-by step procedures used to calculate the decompression stops needed for a given dive profile. The algorithm can be used to generate decompression schedules for a particular dive profile, decompression tables for more general use, or be implemented in dive computer software.

Decompression bar or Decompression trapeze

A horizontal bar or bars suspended at the depth of intended decompression stops by buoys used to make decompression stops more comfortable and more secure and provide the divers’ surface cover with a visual reference for the divers’ position.

Decompression chamber

Hyperbaric chamber used for decompressing divers and emergency therapeutic recompression.

Decompression gas or Decompression mix

Gas breathed during decompression with a composition designed to accelerate decompression, usually by an increased oxygen content.

Decompression schedule

A specific ascent rate and series of increasingly shallower decompression stops that a diver uses to allow inert gases to be eliminated from the body tissues during ascent after a specific hyperbaric exposure, to reduce the risk of decompression sickness.

Decompression stop

A pause during the ascent phase of a dive that a diver spends at a constant relatively shallow depth to allow safe release of inert gases from the body tissues to avoid decompression sickness.

Deep dive

A deep dive is conducted at a depth over 18 meters.

See Deep diving courses

Deep stops

Decompression stops which are deeper than the deepest stops required by decompression algorithms using dissolved phase models.

Deep water blackout

Loss of consciousness caused by cerebral hypoxia on ascending from a deep breath-hold dive, when the swimmer does not necessarily experience an urgent need to breathe.

Dehydration

A condition where the water content of the body is reduced.

Demand valve

Mechanism for providing the user with breathing gas flow only when required.

Depth gauge

A pressure gauge calibrated to measure depth as a function of ambient pressure.

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.

Diluent

Gas mixture used to dilute the oxygen in the loop of a closed circuit rebreather to a partial pressure suited to the depth.

DIN or Deutsches Institut für Normung

Usually refers to G5/8″ x 14 tpi parallel thread fittings used to connect a cylinder valve to a filling connection or regulator first stage. Available in 200 bar and 300 bar versions which should only be inter-connectable in safe combinations. Design of tank valve popular in Europe in which the first-stage regulator screws into the tank valve. Recommended for high pressure tanks.

DIN plug

Screw in adaptor which can be used with many recent 200/240 bar DIN pillar valves to allow connection of Yoke regulators or filling whips.

Distance line

A line used by scuba divers as a means of returning to a safe starting point in conditions of low visibility, water currents or where pilotage is difficult.

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

Flag used to indicate that there are divers in the water. There are two versions: the international code letter flag Alpha, and the red flag with white diagonal bar.

Dive Leader

EN 14153-3 / ISO 24801-3 standard competence for recreational scuba diver. A level 3 “Dive Leader” has sufficient knowledge, skill and experience to plan, organize and conduct their dives and lead other recreational scuba divers in open water, to conduct any specialized recreational scuba diving activities for which they have received appropriate training, plan and execute emergency procedures appropriate for the diving environment and activities. If diving and environmental conditions are significantly different from those previously experienced, they require an appropriate orientation with regard to local environmental conditions, and must have appropriate specialized training and experience to lead on dives which have more demanding operational parameters.

Dive profile

The variation of depth with elapsed time during a dive, often depicted as a graph.

Dive tables or Decompression tables

Printed cards or booklets that allow divers to determine a decompression schedule for a particular dive profile and breathing gas.

Dive time

The total elapsed time spent underwater during a dive.

Divemaster

A Divemaster is a diving qualification used throughout most of the world in recreational scuba diving for a diver who has overall responsibility for a group of divers. As well as being a generic term, Divemaster also refers to the lowest professional rating of many training agencies, such as PADI, SSI or SDI. The certification is a prerequisite for becoming an instructor in recreational diving with most agencies.

Diving stage

A platform on which a diver stands which is hoisted into the water, lowered to the workplace at the bottom, and then hoisted up again to return the diver to the surface and lift him out of the water.

Doppler ultrasonic bubble detection

Ultrasonic signals reflected from bubble surfaces to identify and quantify gas bubbles present in venous blood.

Downstream valve

Valve in which the closure is downstream of the orifice. Pressure in the line tends to assist in opening the valve. When spring-loaded a downstream valve may open automatically if the pressure difference is excessive, thus functioning as a pressure relief valve.

DPV or Diver Propulsion Vehicle

DPVs are underwater scooters that carry divers through the water using a fin based propulsion system. These underwater marvels range from rather lethargic units to hot rod speeders capable of ripping your mask off. Primarily used in cave and military diving, DPVs are becoming more popular in recreational diving.

D-ring

A ring shaped like a capital D, usually of stainless steel, stitched or buckled to a diver’s harness and used as an attachment point for lifeline, cylinders or other equipment.

Drift diving

Any dive where the diver is transported significantly by drifting with currents during the dive.

Drop weight

Weight used during descent and ascent, but left on the bottom at the guideline during the deep part of the dive when it is not needed due to suit compression.

Drowning

The process of experiencing respiratory impairment from submersion/immersion in liquid.

Dry suit

A watertight suit worn to keep the diver dry and to provide protection from the environment. Thermal insulation may be provided by the suit or garments worn under the suit.

DS or Direct System

See LPI or Low Pressure Inflator

DSMB or Delayed Surface Marker Buoy

An inflatable marker buoy deployed from underwater to indicate the position of a diver and to control ascent rate. Can also be used to mark a position or signal an emergency.

Dump valve

Valve used to release excess air from a Dry suit or buoyancy compensator.

EAD or Equivalent Air Depth

A way of approximating the decompression requirements of nitrox mixtures at depth by comparison with the depth at which air would require the same decompression.

EAN or Enriched Air Nitrox

Mixture of nitrogen and oxygen for use as breathing gas. Usually with oxygen percentage higher than air.

Ear clearing methods

See Equalization

ECCR or Electronic Closed Circuit Rebreather

ECCCR or Electronically Controlled Closed Circuit Rebreather

eLearning

PADI eLearning is an online learning tool available for some PADI courses and programs. Divers who complete a PADI eLearning program have met all or most of the knowledge development requirements for the program, and may be required to take a quick review with an instructor. eRecords verify requirements met and expire within one year of the last knowledge development section completed.

END or Equivalent Nitrogen Depth or Equivalent Narcotic Depth

The depth at which the narcotic effects of nitrogen in a given Trimix mixture at a given depth are equivalent to the effects of air. Used to choose nitrogen content of a Trimix breathing gas for a planned dive profile.

Equalization

Equalization is any of various maneuvers to equalize the pressure in the middle ear with the outside pressure, by letting air enter along the Eustachian tubes, as this does not always happen automatically when the pressure in the middle ear is lower than the outside pressure.

ESA or Emergency Swimming Ascent

See CESA

Eustachian tube

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

Exceptional exposure

A dive in which the risk of decompression sickness, oxygen toxicity, and/or exposure to the elements is substantially greater than on a normal working dive.

Exposure suit protection

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

Feather breathing

Technique for emergency breathing from a free-flowing demand valve where the diver manually controls air flow by opening and closing the cylinder valve.

Fenstop

See Dump valve

FFM or Full Face Mask

Diving mask covering the eyes, nose and mouth, and provides the diver with breathing gas.

Filtration

Process for removing impurities from a fluid. Particulates are commonly removed by passing the fluid through porous material with pore size small enough to trap the particles (e.g. micron filters). Liquids and gases are commonly absorbed or adsorbed by the surface of the filter medium (Activated carbon, Molecular sieve, Silica gel), or may be chemically combined with the medium (Sodalime) or catalytically converted (Hopcalite) into a less objectionable substance.

Fin keepers

Elastic rubber straps used to help prevent fins from falling off the diver’s feet.

First stage

See Regulator first stage

Flutter kick

Finning style where the fins are alternately muved up and down by movements of the full, usually fairly straight leg. Thrust is developed on both up and down strokes.

Fogging

Condensation of water vapor on the inside surface of a mask or helmet faceplate, reducing visibility.

Forward roll entry

Water entry technique used by scuba divers from a boat or platform too high or unsuitable for backward roll entry. The diver bends forward at the hips and waist and falls forward into the water, making a partial somersault and breaking the water with the cylinder, back and shoulders. Not suitable for heights more than about 2 meters.

Free air or Free gas

Gas at normal atmospheric pressure. Usually refers to the volume of some amount of compressed gas when allowed to expand to atmospheric pressure at constant temperature.

Free flow

1. Constant flow rate air supply
2. Malfunction of a demand regulator where the valve sticks in the open position, allowing a constant rate of flow.

Freediving

Underwater diving that does not involve the use of external breathing apparatus, but relies on a diver’s ability to hold his or her breath until resurfacing. also Breath-hold diving and apnea.

Frenzel maneuver

Technique for equalizing the middle ear by pinching the nose closed and moving the back of the tongue upwards.

Frog kick

Finning technique where thrust is developed by sweeping the fins horizontally toward each other with the fins twisted into a nearly vertical plane, with the soles facing each other, followed by a recovery stroke which develops negligible thrust where the fin blades are feathered. The legs are fairly straight during the power stroke.

FSW or Feet of Sea Water

Used to indicate either an actual depth, or just a pressure equal to that depth.

Gap

The space between two cave guidelines. Usually between a main guideline and a branch line.

Gas blending

Mixing breathing gases for diving, filling diving cylinders with gas mixes such as nitrox or trimix.

Gas embolism

Blockage of blood vessel by a bubble of gas.

Gauge mode

Operating mode for a personal dive computer where the decompression calculation is disabled, and the unit operated only as a timer and depth gauge. Typically used when diving with gas mixtures not supported by the algorithm, in which case decompression tables are used to monitor and control the decompression schedule.

Gauge pressure

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure.

Gay-Lussac’s law

Relation between temperature and pressure in an ideal gas for a constant volume.

General gas law

1. Relation between the variables pressure, volume and temperature for a given mass of a given mixture of an ideal gas.
2. Thermodynamic equation of state for gases for the variables volume, pressure, temperature and number and atomic weight of molecules.

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.

Golden rule

The convention in cave diving that anyone can turn the dive at any time for any reason.

Gold line

The permanent main guideline in a cave system, that usually starts well inside the cave. Often yellow or gold in color.

Gradient factors

A way of modifying the M-value of a decompression algorithm to a more conservative value. Often used to bias the algorithm towards deeper stops.

Guide line

See Distance line

H-valve

Cylinder valve body with two outlets and two valve mechanisms which can be independently controlled so that two regulator first stages can be fitted. Similar to Y-valve but in configuration where the second valve is parallel to the primary, though the secondary valve can sometimes be swiveled.

Haldanian or Haldanean

Decompression models based on the principles described by John Scott Haldane.

Half mask

Diver’s mask which covers the eyes and nose but not the mouth.

Half times

See Tissue half times

Harness

Straps and webbing with associated buckles, D-rings and other accessories used to support the breathing apparatus and secure it to the diver. The harness often has other functions such as supporting weighting and buoyancy control systems and for recovery of the diver from the water.

Heliair

Trimix blends made by topping up helium with air.

Helicopter turn

Manoeuver in which a (usually horizontally trimmed) diver uses small fin movements to rotate on the spot on a vertical axis.

Heliox

Mixtures of helium and oxygen for use as a breathing gas.

Helium

An inert gas which is used as a component of breathing gas mixtures for deep diving.

Helium analyzer

An instrument used to identify the presence and concentration of helium in a mixture of gases.

Henry’s law

Description of the relation between solubility of a given gas in a given liquid as pressure varies.

HIRA or Hazard Identification and Risk Analysis

A risk management procedure for identifying hazards and assessing the risk associated with them and ways to reduce the risk to an acceptable level.

Hogarthian configuration

A scuba combination of backplate, wing, one-piece harness with crotch-strap, regulator arrangement including long-hose primary with a necklaced secondary demand valve, and, if used with twin cylinders, an isolation manifold. Named after Willian Hogarth Main, a developer and proponent of the system.

Hog looped

A scuba configuration where the primary demand valve has a long hose which is routed under the righr arm, usually tucked under a light battery canister on the waist belt of the harness, and around behind the neck to reach the mouth from the right hand side. Part of the Hogarthian configuration.

Hood

Close-fitting thermal head protection, usually neoprene foam, but also latex on some dry suits.

HP or High pressure

Generally gas pressures in excess of 30 bar. In diving context gas working pressures do not frequently exceed 300bar, but pressures in hydraulic systems may be considerably higher.

HP hose

High Pressure Hose. Goes from 1st stage to air pressure gauge

HPNS or High-Pressure Nervous Syndrome

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 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.

Hydreliox

Deep diving breathing gas mixture of hydrogen, helium and oxygen.

Hydrogen

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

Hydrostatic pressure

Pressure due to the weight of the water column above a point at depth.

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.

Hydrox

Deep diving breathing gas mixture of hydrogen and oxygen.

Hyperbaric chamber

See Decompression chamber

Hypercapnia

Excessive level of carbon dioxide in the body.

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.

Hyperthermia

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

Hyperventilation

1. A deliberate deep breathing to reduce blood carbon dioxide level to extend the duration of a free dive.
2. Rapid breathing as the body’s response to hypercapnia.
3. Rapid, often shallow breathing, associated with panic.

Hypothermia

A lowering of deep body temperature due to heat loss.

Hypoventilation

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

Hypoxia

Abnormally low tissue oxygen concentration: Insufficient oxygen in the body to support normal activities or consciousness.

IANTD or 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.

ICD or Isobaric Counter Diffusion

The diffusion of gases in opposite directions caused by a change in the composition of the external ambient gas or breathing gas without change in the ambient pressure.

Inert gas

A gas which is not metabolically active, used to dilute the breathing gas.

Inflator

See LPI or Low Pressure Inflator

Ingassing

Inert gas uptake in body tissues during a dive or other hyperbaric exposure.

Instructor

A diving instructor is a person who trains underwater divers. This includes free-divers, recreational divers including the subcategory technical divers, and professional divers which includes military, commercial, public safety and scientific divers. Depending on the jurisdiction, there will generally be specific published codes of practice and guidelines for training, competence and registration of diving instructors, as they have a duty of care to their clients, and operate in an environment with intrinsic hazards which may be unfamiliar to the lay person. Recreational diving instructors are usually registered members of one or more recreational diver certification agencies.

See PADI Open Water Scuba Instructor

Internal condition of cylinder

The state of the internal surface of a cylinder regarding corrosion, contamination and cracking.

IP or Intermediate Pressure or Interstage Pressure

The reduced pressure between the first and second stages of a diving regulator.

Isolation manifold

Connection between two scuba cylinders which when open allows free flow of gas in both directions between the cylinders, but has an isolation valve to block this flow.

Isolation valve

Valve in an isolation manifold used to close the gas passage through the manifold and isolate the contents of the two cylinders. Used to prevent a leak on one cylinder from causing the other cylinder to also lose gas.

J-valve

Scuba cylinder valve with lever operated reserve mechanism.

John Scott Haldane

John Scott Haldane (2 May 1860 – 14/15 March 1936) was a Scottish physiologist famous for intrepid self-experimentation which led to many important discoveries about the human body and the nature of gases. He also experimented on his son, the equally famous J. B. S. Haldane, both for extending his father’s interest in diving and as a key figure in population genetics and the development of the modern evolutionary synthesis even when he was quite young. Haldane locked himself in sealed chambers breathing potentially lethal cocktails of gases while recording their effect on his mind and body. In 1907 Haldane made a decompression chamber to help make deep-sea divers safer and produced the first decompression tables after extensive experiments with animals. Most decompression models are based on Haldanian models.

Jonline

A short line used to connect to a shotline or anchor line, allowing the diver to move a short horizontal distance away to decompress. The line helps compensate for vertical movement in the anchor line or shot line due to waves.

Jump

A path from a main guideline to another which is not in contact.

Jump line

A short cave line used to connect between two permanent lines that are not in contact. May also be used to search for the other end of a break in a cave line and repair the break.

Junior diver certifications

Divers who are under age 15 may earn a Junior diver certification as indicated in a course instructor guide. All course requirements, except dive depths, apply to junior divers unless noted specifically in the course instructor guide. Certification for juniors, as for adults, is based on mastery learning. Junior Diver certification cards state the restrictions based on age. After certification, 10-11 year olds are restricted to diving with a parent, guardian or PADI Professional to 12 meters maximum depth. 12-14 year olds must dive with an adult certified diver, the maximum depth is limited to 21 meters.

K-valve

Scuba cylinder valve without reserve mechanism.

Knot

The knot is a velocity unit equal to one nautical mile (1.852 km) per hour.

Lanyard

A piece of cordage used to secure or lower things, usually it is used where there is a risk of losing the object.

LED or Light Emitting Diode

Commonly used in dive lights.

Lifeline

A line connected securely to the diver at one end and anchored at the other end at the diving control point, which is handled by a line tender, and is used to communicate with the diver and provide a means of finding the diver for a surface standby diver, and for assisting the diver to the surface and back to the control point if necessary.

Lift bag

A robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag’s buoyancy.

Light head

The part of a canister light which emits light, and is held in the hand or mounted on the helmet.

Line marker

Line arrows, cookies and sometimes clothes pegs. Used to indicate direction to exit, midway point between exits, jumps and personal markers to identify divers on a guide line.

Liveaboard

A large boat which provides transport, accommodation and services for vacationing divers.

Log book

Record of dives kept as proof of experience. Optional for recreational divers, but legally required for professional divers in many jurisdictions.

Long hose

5 feet to 7 feet interstage hose used on one of the regulators used by cave and other technical divers, which allows gas sharing through narrow spaces.

Loop

See Breathing loop

Loop volume

Volume of the breathing loop of a rebreather.

Lorrain-Smith Effect

See Pulmonary oxygen toxicity

Lost buddy drill

Standardized procedure followed when a diver realizes that their buddy is not where they should be. Procedures may vary depending on the circumstances and training organizations.

Lost line drill

Standardized procedure to be followed when the guideline to the surface is lost in a penetration dive, often in conditions of low visibility and darkness.

Lowry Technique

A combination of Valsalva and Toynbee maneuvers : pinching the nose to close the nostrils, and blow and swallow at the same time.

LP or Low Pressure

LP Hose

Low Pressure hose. Goes from 1st stage to 2nd stage or to inflator.

LPI or Low Pressure Inflator

A low pressure hose extends from the first stage of the regulator that provides accessory air to inflate the buoyancy compensator. A Low Pressure Inflator could also be connected to a dry suit valve to inflate / input air into a dry suit. Attachments have been made to adapt to Low Pressure Inflators including a Tire Inflator, O2 Analyzer Flow Restrictor, and Air Gun.

M-values

At a given ambient pressure, the M-value is the maximum theoretical value of absolute inert gas pressure that a tissue compartment can take without presenting symptoms of decompression sickness.

Macro Photography

A method of getting close-up pictures of a subject by using macro accessories attached to the camera’s lens. Where the size of the image projected in the film or lens is equal or smaller than the real object.

Manifold

See Isolation manifold

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 equalized.

Maze cave

Cave structure characterized by multiple branches and changes in direction.

MCCR or Manual Closed Circuit Rebreather

A closed circuit rebreather which relies on the diver to control the gas mixture in the loop.

Mediastinal emphysema

Air from an over expanding lung escapes into the center 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 eardrum, which is exposed to any change in ambient pressure. Air pressure in the middle ear space can only be equalized through the Eustachian tube, which controls the middle ear to the back of the nose.

Mixed gas

Breathing gas for diving other than air, but usually implies a helium based mixture.

MOD or Maximum Operating Depth

Limiting depth for safety based on partial pressure of oxygen of a breathing gas mixture.

Modified flutter kick

Version of the flutter kick finning style which reduces risk of silting by directing thrust more directly backwards. Two techniques exist: One version has the legs bent at the knees so that the fins are placed relatively high and on average are aligned more horizontally. The other version has one fin stationary below the moving fin to deflect downwash. Leg movement is restrained, and ankle movement used for precision maneuvering.

Modified frog kick

Version of the frog kick finning style which reduces risk of silting by directing thrust more directly backwards. Performed with bent knees and fins raised above the line of the torso.

Monkey diving

The use of sidemount configuration and procedures with a single cylinder.

Monofin

A type of swim fin typically used in fin swimming and free-diving. It consists of a single surface attached to foot pockets for both of the diver’s feet.

MSD or Master Scuba Diver

Master Scuba Diver is a scuba diving certification or recognition level offered by several North American diver training agencies, such as the National Association of Underwater Instructors (NAUI), the Professional Association of Diving Instructors (PADI), Scuba Diving International (SDI), and Scuba Schools International (SSI). Other agencies offer similar programs under other names, such as “Elite Diver”. Each of these agencies touts their program at this level as the highest, non-leadership program.

See the PADI Master Scuba Diver program

MSDT or Master Scuba Diver Trainer

See the PADI Master Scuba Diver Trainer program

MSW or Meters Sea Water

Unit of pressure equal to 1/10 bar. Not a linear measure of depth. The pressure exerted by seawater varies slightly with temperature and salinity, but for practical purposes the convention is that each meter imposes a pressure of 0.1 bar. Sometimes the convention is that each meter is equivalent to 0.1 atmosphere (0.1013 bar).

Multilevel dive

A dive profile in which the diver remains in more than one distinct depth ranges for a significant period before beginning final ascent to the surface (excluding decompression stops).

NACD or National Association for Cave Diving

Narcosis

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

NAUI or National Association of Underwater Instructor

The National Association of Underwater Instructors (NAUI Worldwide) is a non-profit 501 association of scuba instructors. It is a recreational dive certification and membership organization established to provide international diver standards and education programs. The agency was founded in 1960 by Albert Tillman and Neal Hess.

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, so 1,852 km.

NDL or No Decompression Limit

The maximum time which a diver can remain at a specified depth without incurring a stage decompression obligation in terms of the specified decompression tables or algorithm.

neo-Haldanian or neo-Haldanean

Decompression models based on later modifications of the principles described by John Scott Haldane.

Neoprene

Synthetic elastomer used in the form of foamed sheets as the material for most wetsuits and some dry suits.

Net cutter

A handle with a hooked blade used to cut netting or cordage to free the diver from entanglement.

Night Dive

A night dive is generally conducted any time between sunset and sunrise.

Nitrile rubber

A synthetic elastomer used for most standard O-ring seals.

Nitrogen

The major component gas of air and many breathing gas mixtures used in diving. Important in diving as an active agent in nitrogen narcosis and decompression sickness.

Nitrogen narcosis

Also known as narcs, inert gas narcosis, raptures of the deep, Martini effect: A reversible alteration in consciousness that occurs while breathing gases containing nitrogen under elevated partial pressure similar to alcohol intoxication or nitrous oxide inhalation, and can occur during shallow dives, but usually does not become noticeable until greater depths, beyond 30 meters.

Nitrox

See EAN

Nitrox stick

A mixing tube used to blend oxygen with air before compressing to make nitrox breathing gas.

No-mount diving

A specialized overhead-environment strategy for dealing with particularly tight restrictions which may involve divers wearing a very basic harness or simply hand-carrying cylinders.

NOAA or National Oceanographic and Atmospheric Administration

The National Oceanic and Atmospheric Administration is an American scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere. NOAA warns of dangerous weather, charts seas and skies, guides the use and protection of ocean and coastal resources, and conducts research to improve understanding and stewardship of the environment.

Normoxic

1. A breathing gas mixture with oxygen content approximating atmospheric air.
2. A breathing gas for diving which contains sufficient oxygen to minimize risk of hypoxia at atmospheric pressure.

O-ring

A mechanical gasket in the shape of a torus; a loop of elastomer with a circular cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the contact surfaces.

Octopus regulator

A secondary demand valve fitted to a first stage diving regulator for use as an alternative air source for another diver in case of an emergency.

OEA or Oxygen Enriched Air

See EAN

Off-gassing

See Outgassing

On-gassing

See Ingassing

OOA or Out of air

An emergency situation where the supply of breathing gas to the diver has stopped.

Open Circuit

Breathing apparatus which discharges exhaled gas into the environment, without any further use.

Open Water

Open water is a body of water significantly larger than a swimming pool offering conditions typical of a natural body of water encountered by divers.

1.  Water where there is no physical obstruction to a direct ascent to the surface.
2. Water which is open to influences by weather and climatic conditions.

OPV or Over-Pressure Valve

A pressure relief valve which automatically opens at a set pressure to allow excess gas to escape.

Outgassing

Diffusion of gas out of the tissue into the blood, and transport to the lungs where it diffuses into the lung gas and is eliminated by exhalation.

Overhead

A physical or procedural obstruction to a direct ascent to the surface. Physical overheads include cave, cavern or culvery ceilings, fishing nets, ship hulls, and wreckage. Procedural overheads are generally a decompression obligation.

Overweighting

Carrying more weight than is necessary to achieve neutral buoyancy at all times in a dive.

OW or Open Water

OWSI or Open Water Scuba Instructor

See Instructor

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 analyzer

Instrument for measuring the partial pressure of oxygen in a gas mixture.

Oxygen clean

Cleaned for oxygen service by appropriate methods and materials and tested for contaminants.

Oxygen clock

In some diver training courses for these types of diving, divers are taught to plan and monitor what is called the oxygen clock of their dives. This is a notional alarm clock, which ticks more quickly at increased oxygen pressure and is set to activate at the maximum single exposure limit recommended in the National Oceanic and Atmospheric Administration Diving Manual. For the following partial pressures of oxygen the limits are: 45 minutes at 1.6 bar (160 kPa), 120 minutes at 1.5 bar (150 kPa), 150 minutes at 1.4 bar (140 kPa), 180 minutes at 1.3 bar (130 kPa) and 210 minutes at 1.2 bar (120 kPa), but it is impossible to predict with any reliability whether or when toxicity symptoms will occur. Many nitrox-capable dive computers calculate an oxygen loading and can track it across multiple dives. The aim is to avoid activating the alarm by reducing the partial pressure of oxygen in the breathing gas or by reducing the time spent breathing gas of greater oxygen partial pressure. As the partial pressure of oxygen increases with the fraction of oxygen in the breathing gas and the depth of the dive, the diver obtains more time on the oxygen clock by diving at a shallower depth, by breathing a less oxygen-rich gas, or by shortening the duration of exposure to oxygen-rich gases.

Diving below 56 m on air would expose a diver to increasing danger of oxygen toxicity as the partial pressure of oxygen exceeds 1.4 bar (140 kPa), so a gas mixture must be used which contains less than 21% oxygen (a hypoxic mixture). Increasing the proportion of nitrogen is not viable, since it would produce a strongly narcotic mixture. However, helium is not narcotic, and a usable mixture may be blended either by completely replacing nitrogen with helium (the resulting mix is called heliox), or by replacing part of the nitrogen with helium, producing a trimix.

Oxygen compatible

Made from materials which are suitable for oxygen service.

Oxygen compatible air

Air which has been filtered to reduce contaminants to a level suitable for blending with high pressure oxygen.

Oxygen service

Suitable for operating with significantly higher levels of oxygen than normal atmospheric air. Often implies special cleaning procedures, use of oxygen compatible materials, and design to reduce ignition risk.

Oxygen toxicity

Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at increased partial pressures. It is also known as oxygen toxicity syndrome, oxygen intoxication, and oxygen poisoning. Historically, the central nervous system condition was called the Paul Bert effect, and the pulmonary condition the Lorrain Smith effect, after the researchers who pioneered its discovery and description in the late 19th century. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lungs and eyes. Oxygen toxicity is a concern for underwater divers, those on high concentrations of supplemental oxygen (particularly premature babies), and those undergoing hyperbaric oxygen therapy.

See CNS oxygen toxicity and Pulmonary toxicity

PADI or Professional Association of Diving Instructors

The Professional Association of Diving Instructors (PADI) is the world’s largest recreational diving membership and diver training organization founded in 1966 by John Cronin and Ralph Erickson. Cronin was originally a NAUI instructor who decided to form his own organization with Erickson, and to break diver training down into several modular courses instead of the single universal course then prevalent. PADI courses range from minimal entry level to relatively advanced recreational diver certification, several specialized diving skills courses, usually connected with specific equipment or conditions, some diving related informational courses and a range of recreational diving instructor certifications. Under the PADI TecRec brand, they also offer various technical diving courses. PADI’s sister company Emergency First Response Corp provides a range of first aid and CPR programs for lay people,

Panic

A sudden sensation of fear which is so strong as to dominate or prevent reason and logical thinking, replacing it with overwhelming feelings of anxiety and frantic agitation consistent with an animalistic fight-or-flight reaction.

Partial Pressure

The pressure that a component gas of a gas mixture would exert if it alone was present in the volume occupied by the gas mixture.

Paul Bert effect

See CNS oxygen toxicity

Penetration

Entering a region with no direct vertical access to the surface, such as a cave or the interior of a wreck.

Perfusion limited

The assumption in a decompression model that perfusion has the dominant influence on gas uptake and release.

PFO or Patent Foramen Ovale

A shunt between the right and left side of the heart that allows some blood to circulate back through the body without going to the lungs first. This means that micro-bubbles don’t get removed from the blood stream by the lungs efficiently, making DCS more likely.

Pneumofathometer

Instrument to measure depth of a diver using a gauge calibrated in MSW, FSW, or often both, to measure the ambient pressure at the diver by measuring the pressure in a hose filled with air with an open end at the diver.

Pneumothorax

Air or other breathing gas in the chest cavity, often resulting in a collapsed lung.

Pony cylinder

Relatively small scuba set carried as an independent alternative breathing gas source by a scuba diver.

PpO2 or Partial Pressure of Oxygen

PpN2 or Partial Pressure of Nitrogen

Pre-dive safety check

See Buddy check

Primary light

The main light to be used on a dive. Usually the most powerful.

Primary regulator

The regulator which the diver intends to breathe from for most of the dive. Particularly when diving with back-mounted manifolded twin cylinders.

Primary tie-off

First tie off of the guideline in a penetration dive. This is usually made in a place with free vertical access to the surface.

Professional diving

Diving which is done as part of the diver’s employment or professional occupation.

Progressive penetration

An incremental approach to cave and wreck exploration. Each dive goes a bit further so that the divers develop a familiarity with the environment.

Project AWARE or 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.

PRV or Pressure Relief Valve

PSI or Pounds per Square Inch

Unit of pressure in the Imperial system.

Public Safety Diving

The underwater work conducted by law enforcement, fire department rescue, and search & rescue/recovery dive teams.

Pulmonary oxygen toxicity

Pulmonary oxygen toxicity, also known as Lorrain-Smith effect, symptoms result from an inflammation that starts in the airways leading to the lungs and then spreads into the lungs (tracheobronchial tree). The symptoms appear in the upper chest region (substernal and carinal regions). This begins as a mild tickle on inhalation and progresses to frequent coughing. If breathing increased partial pressures of oxygen is not discontinued, patients experience a mild burning on inhalation along with uncontrollable coughing and occasional shortness of breath (dyspnoea) Physical findings related to pulmonary toxicity have included bubbling sounds heard through a stethoscope (bubbling rales), fever, and increased blood flow to the lining of the nose (hyperaemia of the nasal mucosa). The radiological finding from the lungs shows inflammation and swelling (pulmonary oedema). Pulmonary function measurements are reduced, as noted by a reduction in the amount of air that the lungs can hold (vital capacity) and changes in expiratory function and lung elasticity. Tests in animals have indicated a variation in tolerance similar to that found in central nervous system toxicity, as well as significant variations between species. When the exposure to oxygen above 0.5 bar (50 kPa) is intermittent, it permits the lungs to recover and delays the onset of toxicity.

The lungs, as well as the remainder of the respiratory tract, are exposed to the highest concentration of oxygen in the human body and are therefore the first organs to show toxicity. Pulmonary toxicity occurs with exposure to partial pressures of oxygen greater than 0.5 bar (50 kPa), corresponding to an oxygen fraction of 50% at normal atmospheric pressure. Signs of pulmonary oxygen toxicity begins with evidence of tracheobronchitis, or inflammation of the upper airways, after an asymptomatic period between 4 and 22 hours at greater than 95% oxygen, with some studies suggesting symptoms usually begin after approximately 14 hours at this level of oxygen.

At partial pressures of oxygen of 2 to 3 bar (200 to 300 kPa)—100% oxygen at 2 to 3 times atmospheric pressure—these symptoms may begin as early as 3 hours after exposure to oxygen.[34] Experiments on rats breathing oxygen at pressures between 1 and 3 bars (100 and 300 kPa) suggest that pulmonary manifestations of oxygen toxicity may not be the same for normobaric conditions as they are for hyperbaric conditions. Evidence of decline in lung function as measured by pulmonary function testing can occur as quickly as 24 hours of continuous exposure to 100% oxygen, with evidence of diffuse alveolar damage and the onset of acute respiratory distress syndrome usually occurring after 48 hours on 100% oxygen.[34] Breathing 100% oxygen also eventually leads to collapse of the alveoli (atelectasis), while—at the same partial pressure of oxygen—the presence of significant partial pressures of inert gases, typically nitrogen, will prevent this effect.

Pulmonary oxygen toxicity is an entirely avoidable event while diving. The limited duration and naturally intermittent nature of most diving makes this a relatively rare (and even then, reversible) complication for divers. Guidelines have been established that allow divers to calculate when they are at risk of pulmonary toxicity.

Purge

To press the purge button on a demand valve to induce a gas flow which is intended to clear the demand valve interior of water or other substances.

Purge button

Button or flexible area on the front or side of a demand valve which allows the user to manually open the second stage valve to provide gas flow without inhalation.

Purge valve

Valve in snorkel or mask which allows water to drain either under gravity or as a result of exhalation into the air space.

Pyle stops

Named after Richard Pyle, an early advocate of deep stops. An additional brief deep decompression stop, typically 2 minutes long and half way between the maximum depth and the first conventional decompression stop.

Quick link

Oval connector shaped like a chain link with a screw gate on one side.

Rapture of the deep

See Nitrogen narcosis

Ratio decompression

A technique for calculating decompression schedules for scuba divers engaged in deep diving without using dive tables, decompression software or a dive computer.

RDP or Recreational Dive Planner

A no-stop decompression table developed by DSAT.

Rebreather

A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user’s exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content of each breath. Oxygen is added to replenish the amount metabolized by the user. This differs from an open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. Rebreather technology may be used where breathing gas supply is limited, such as underwater or in space, where the environment is toxic or hypoxic, as in firefighting, mine rescue and high-altitude operations, or where the breathing gas is specially enriched or contains expensive components, such as helium diluent or anesthetic gases.

Recompression

Subjecting a diver to pressure after an ascent from a dive as treatment for decompression illness or to prevent decompression sickness. Preferably done in a recompression chamber, but occasionally done as in-water recompression.

Recompression chamber

See Decompression chamber

Recreational diving

Any scuba diving activity from the surface to 40 meters. No decompression stop. Recreational diving or sport diving is a type of diving that uses scuba equipment for the purpose of leisure and enjoyment. Recreational diving or sport diving is a type of diving that uses scuba equipment for the purpose of leisure and enjoyment. In some diving circles, the term “recreational diving” is used in contradistinction to “technical diving”, a more demanding aspect of the sport which requires greater levels of training, experience and equipment. In other circles, technical diving is considered a subset of recreational diving as opposed to professional diving, which is done as part of the diver’s work.

Reel

Mechanism used to store, deploy and recover long lengths of line.

Regulator

1. A mechanism for controlling the output pressure of a high pressure gas supply.
2. As 1, with a demand valve incorporated which provides the diver with breathing gas at ambient pressure.

Regulator first stage

Diving regulator component which reduces gas pressure from storage pressure in the cylinder to interstage pressure for supply to the second stage and for suit and BC inflation.

Regulator freezing

Locking of the regulator mechanism caused by freezing of the water due to expansion cooling of the regulated air. Often causes the mechanism to lock open, causing free flow and further cooling.

Regulator second stage

The part of a diving regulator which provides pressure reduction from intermediate pressure to ambient pressure on demand.

Repetitive dives

Any dive which is done while the tissues retain residual inert gas from a previous dive.

Reserve gas or Reserve pressure

Gas which is not intended to be used during the dive, and is reserved for contingencies.

Reserve valve

Cylinder valve with a lever operated bypass valve to release the gas below reserve pressure.

Residual gas

Gas remaining in a cylinder from the previous fill.

Residual nitrogen

Nitrogen in excess of normal atmospheric saturation remaining in the diver’s tissues after a dive.

Restriction

Section of a cave which is difficult to pass through due to lack of space. A minor restriction is too small for two divers to swim through together, a major restriction requires the diver to remove equipment to fit through.

Reverse profile

1. Repetitive dive which is deeper than the previous dive.
2. Multilevel dive in which a later level is deeper than an earlier level.

Reverse squeeze

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

RGBM or Reduced Gradient Bubble Model

A bubble model decompression algorithm developed by Bruce Wienke.

RIB or Rigid Inflatable Boat

Small and fast boat with solid keel and large inflated tubes.

Ring bungee

Length of shock cord with metal rings and a clip used to control the position of the top end of a sidemounted cylinder. Differs from a regular bungee in having the rings. A popular configuration is a bolt snap connected to a ring by a quick link, with a length of bungee from the ring to another quick link which is used to connect the assembly to a D-ring on the back of the harness. The bolt snap is clipped to the shoulder D-ring and the cylinder neck bolt snap is clipped to the ring.

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.

RMV or Respiratory Minute Volume

The volume of gas inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person’s lungs in one minute.

RNT or Residual Nitrogen Time

Time penalty in a repetitive dive equivalent to time at depth which would produce the residual nitrogen in the diver at the start of the dive.

ROV or Remotely Operated Underwater Vehicle

Rule of Thirds

Cave and wreck penetration breathing gas management convention where no more than one third of the gas in a cylinder may be used on the inward part of the dive, and the other two thirds is kept for exit: One third for the planned exit, and one third in case of an emergency.

Run time

Time elapsed since the start of a dive.

Run time schedule

Decompression schedule and dive plan based on elapsed time from the start of the dive, All waypoints and events are specified in terms of elapsed time with start of descent at zero.

S-Drill or Safety drill

An air sharing exercise based on deploying the long hose primary regulator.

SAC rate or Surface Air Consumption rate

A measure of air consumption in units of pressure over time, usually psi/minute, adjusted to surface pressure, used to estimate air endurance of a cylinder of specific size. Useful for those who work in imperial units. SAC has a constant value for a given diver and represents gas used on the surface at rest.

SAFE or Slowly Ascend From Every dive

Safe air

Term used for Nitrox by ANDI

Safety reel

Reel for use in an emergency, usually for searches to find lost buddy or lost guideline or to jump a line break. Relatively short line.

Safety stop

A voluntary (not required by the decompression schedule) additional decompression stop intended to reduce risk of decompression sickness.

Samba

Contractions experienced by breath-hold divers when approaching hypoxic blackout.

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.

Scooter

See DPV

SCR or Semi-Closed Circuit rebreather

A rebreather which either dumps part of each breath to the environment or continuously adds gas and dumps the excess.

Scrubber

Canister containing material (sorb) which chemically combines with carbon dioxide to remove it from the gas passed through the canister.

SCUBA or Self Contained Underwater Breathing Apparatus

May be open or closed circuit.

SDI or Scuba Diving International

SCUBA Diving International, launched in 1999, is the sister organization of Technical Diving International. Thus, SDI was created by dive professionals from the technical diving field. This gives the organization a perspective that other recreational diving certification agencies do not have, which is teaching recreational diving through the lens of experienced technical diving. SDI’s philosophy is to take recreational scuba diving to new levels, enhancing older – maybe even outdated – diving practices by incorporating new diving technology and emphasizing safety. The curriculum is set up to take divers from the beginner level to instructor level and structures its courses around a logged dive and specialty course approach. Divers who progress through SDI’s recreational diving courses are then in a position to advance to technical diving with the courses offered by Technical Diving International.

Search pattern

Systematic procedure for covering the search area sufficiently to be reasonably sure of finding a given target. Several patterns are in general use for underwater searches, depending on the target, the terrain, and available facilities.

Second stage

See Regulator second stage

Semi-dry suit

A wet suit with wrist and ankle seals, and usually a more watertight zipper than usual, to reduce flushing of water through the suit.

Shallow water blackout

A loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than 5 meters, when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it.

Shivering

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

Shore diving

Diving from a shore entry.

Sidemount

A scuba diving equipment configuration which has diving cylinders mounted alongside the diver, below the shoulders and along the hips, instead of on the back of the diver.

See Sidemount courses

Silt-out

A situation when underwater visibility is rapidly reduced to zero, usually when a diver disturbs silt deposits.

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).

Slate

Rigid plastic tablet used for writing messages or notes.

SMB or Surface Marker Buoy

See DSMB

Snorkel

Tube with a bend and mouthpiece used for breathing air from above the water surface when the wearer’s mouth and nose are submerged.

Snorkeling

Swimming at the surface of the water while breathing through a snorkel. the snorkeler is almost always equipped with a diving mask or swim goggles, and usually swim fins.

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.

Sorb

Carbon dioxide absorbent material used in rebreather or life support system scrubber to remove carbon dioxide from the breathing gas so it may be recycled.

SPG or Submersible Pressure Gauge

Gauge attached to the first stage regulator and used to monitor pressure remaining in the diving cylinder.

Spool

Circular device for storing line. No moving parts, small, compact, economical and reliable alternative to a reel for relatively short lines.

Squeeze

Injury or discomfort caused by increase or decrease in volume of gas space in the diver’s body or equipment due to a change in ambient pressure.

SSDE or Surface Supply Diving Equipment

SSI or Scuba Schools International

In 1999 SSI merged with the National Association of Scuba Diving Schools.

Stage cylinder

1. A cylinder used for a stage of a long penetration dive, also known as drop cylinder, which is placed on the distance line to be collected on the return.
2. Also generically used to refer to decompression gas cylinders carried as sling cylinders.

Staged decompression

The practice of making decompression stops.

Stern

Back of the boat.

Subcutaneous emphysema

Gas under the skin resulting from lung overpressure injury.

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.

Supervised Diver

EN 14153-1 / ISO 24801-1 standard competence for recreational scuba diver. The level 1 “Supervised Diver” has sufficient knowledge, skill and experience to dive, in open water, to a recommended maximum depth of 12 m, which do not require in-water decompression stops, under the direct supervision of a dive leader, in groups of up to four level 1 scuba divers per dive leader provided the dive leader is capable of establishing physical contact with all level 1 scuba divers at any point during the dive, only when appropriate support is available at the surface, and under conditions that are equal or better than the conditions where they were trained.

Surf

The mass or line of broken water formed by waves breaking on a shore or reef.

Surface decompression

A procedure in which some or all of the staged decompression obligation is done in a decompression chamber immediately after surfacing instead of in the water.

Surface Interval

The time spent by a diver at surface pressure after a dive during which inert gas which was still present at the end of the dive is further eliminated from the tissues.

Surge

Reciprocating water movement parallel to the bottom surface caused by the passing of a wave overhead.

Swell

A series of surface gravity waves that is not generated by the local wind.

Swim thru

Usually a short underwater tunnel. Light can usually be seen from both ends. NOT a cave dive. Adds a bit of excitement to your dive.

Taravana

A form of decompression sickness found among Polynesian island natives who habitually do multiple repetitive deep breath-hold dives.

Task loading

A multiplicity of responsibilities leading to an increased risk of failure on the part of the diver to undertake some key basic function which would normally be routine for safety.

TDI or Technical Diving International

TDI was founded in 1994 by Bret Gilliam, John Comly, John Crea, David Sipperly and Mitch Skaggs after a split away from International Association of Nitrox and Technical Divers (IANTD) in 1993. The agency aimed to provide training materials and education for specialized diving situations. Some courses offered by TDI include open circuit courses such as diving with Nitrox as well as Rebreather courses. They also provide training for overhead environments like caves and wrecks, mixed gas training and were one of the first diving agencies to create a comprehensive insurance plan for technical diving instructors.

Technical Diving

An extension of the scope of recreational scuba diving to applications with greater technical complexity and higher inherent risk. Definitions vary, but diving with 2 or more tanks, multiple breathing gases, helium based gases, closed circuit rebreathers, or under extensive overhead environment are generally considered as technical diving.

Test pressure

Pressure at which the cylinder will be hydro-statically tested for revalidation. Usually 1.5 or 1.67 x working pressure.

Thalmann algorithm

The Exponential/linear decompression algorithm used in the 2008 US Navy decompression tables.

Therapeutic recompression

A procedure for treating decompression sickness by recompressing the diver, thus reducing bubble size, and allowing the gas bubbles to re-dissolve, then decompressing slowly enough to avoid further formation or growth of bubbles, or eliminating the inert gases by breathing oxygen under pressure.

Thermocline

A thin but distinct layer in a large body of fluid, in which temperature changes more rapidly with depth than it does in the layers above or below.

Thermodynamic decompression model

Hypothesis that bubble formation during decompression will not occur provided absolute ambient pressure exceeds the total of the partial gas tensions in the tissue for each gas.

Tissue

A part of the body characterized 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.

Tissue compartments

Imaginary tissues which are designated as fast and slow to describe the rate of saturation.

Tissue half times

The time it takes for the tissue to take up or release 50% of the difference in dissolved gas capacity at a changed partial pressure.

TNT or Total Nitrogen Time

Equivalent time of hyperbaric exposure for a repetitive dive used with some decompression tables.

Toynbee maneuver

Method of equalizing the middle ears by pinching the nose and swallowing.

Travel Gas

Gas mixture used for descent and ascent when the bottom gas is not suitable for breathing at shallower depths.

Traverse

Pass through a cave by entering at one point and exiting at another.

Triangular profile

A triangular dive profile is one in which, after a descent at constant rate, and a short bottom time at maximum depth, the diver maintains a constant, slow ascent to the surface or first decompression stop. A plot of depth against elapsed time takes a triangular shape.

Trimix

Mixture of three gases for breathing. Oxygen, nitrogen and helium are the gases used. The gas fractions will usually be specified.

Turbidity

The cloudiness or haziness of a fluid caused by individual particles (suspended solids) that may be invisible to the naked eye, similar to smoke in air.

Turn the dive

Start the return on a dive which has reached the planned turning point in terms of depth, time, gas supply or distance.

Twilight zone

Deeper than 60m in the sea, or the part of a cave or cavern that has dim but discernible ambient light.

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.

UBA or Underwater Breathing Apparatus

Equipment used to supply breathing gas to an underwater diver. Usually refers to the part of the system carried underwater by the diver.

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.

Undertow

A subsurface flow of water returning seaward from shore as result of wave action.

Upstream valve

Valve, (usually regulator first stage or demand valve), where the valve mechanism moves against the flow when opening, and the pressure difference over the valve tends to close it.

Upwelling

An oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water

Valsalva maneuver

A method of manually equalizing 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.

Valve cage

Structure or frame fitted to scuba cylinder to protect the cylinder valve or manifold and regulator first stage from impact damage and roll-off.

Valve drill

Safety exercise in which the diver shuts down, tests regulators and re-opens the manifold valves on a twin set in a specific order.

Valve guard

Protective structure or frame fitted to the top of a bulk storage cylinder to protect the cylinder valve from mechanical damage.

Van der Waals equation

Thermodynamic equation of state for a real (non-ideal) gas.

Vasoconstriction

The narrowing of blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. This makes the skin turn paler because less blood reaches the surface, reducing the radiation of heat. On a larger level, vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure.

Vasodilation

The widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles. When blood vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly the arterioles) decreases blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). In addition, the response may be localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire systemic circulation).

Vertigo

A type of dizziness, where there is a feeling of motion when one is stationary.

VGE or Venous Gas Embolism

VIP or Visual Inspection Program

Annual visual internal inspection of a scuba cylinder.

Visual gap

Gap between guidelines which is small enough that each line can be seen from the other.

Visual jump

The procedure of crossing a visual gap without the use of a jump line.

Visual Inspection

Internal and external inspection of a pressure vessel as part of revalidation procedure.

VPM or Varying Permeability Model

A decompression model and associated algorithms based on bubble dynamics.

VVAL18

See Thalmann algorithm

Wall Diving

Diving along the face of a near vertical cliff wall, particularly if the bottom is below the range of the diver’s equipment and certification. This requires good buoyancy control.

Water Pressure

Force per unit area exerted by the weight of water, each 10 meters of sea water exerts a pressure equivalent to one atmosphere (1 bar).

Weighting system

Weights, generally made of lead, to counteract the buoyancy of other diving equipment, and the belts, pockets or harnesses used to support them.

Wet notes

A small notebook of waterproof paper carried by some divers.

Wet Suit

A close fitting, thermally-insulating, foam neoprene diving suit that allows a limited volume and movement of water inside the suit.

Wing

Back inflation buoyancy compensator cell.

Working pressure

Maximum filling pressure rating for the cylinder at standard temperature.

Wreck Diving

Recreational or technical diving on and inside of shipwrecks.

Wrist slate

A small plastic writing surface attached to the diver’s wrist

Yoke adaptor

A fitting used to connect a regulator or filling whip with a DIN thread connection to an international connection cylinder valve.

Yoke valve

A fitting or valve used to connect a regulator or filling whip to a diving cylinder using the “international” connection.

Y-valve

Cylinder valve body with two outlets and two valve mechanisms which can be independently controlled so that two regulator first stages can be fitted. Similar to H-valve but in Y configuration. Also known as Slingshot valve.

Z-knife

Line cutting tool with a replaceable blade in a slot.

ZHL-16

Bühlmann decompression algorithms.

Zodiac

See RIB