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Scuba Terms                             

ABLJ - Adjustable buoyancy lifejacket.

Absolute Pressure - The total pressure at a certain depth, including atmospheric pressure. The sum of atmospheric pressure and hydrostatic pressure making the "Absolute Pressure".

Absolute Zero - The lowest possible temperature possible, as measured in Kelvin or Rankine scales. The temperature where individual molecules stop moving because of zero Kinetic Energy.

Actual Bottom Time (ABT) - A term used in common dive tables to represent the amount of time a diver spent underwater. This time begins upon descent and ends upon the beginning of the ascent.

Adjusted No-Decompression Limit - The maximum bottom time for a repetitive dive. Equal to the no decompression limit plus residual nitrogen time.

Air - The mixture of gases present at standard temperature and pressure in the Earth's atmosphere. Used in human respiration.

Air Embolism - The rupture of a body tissue as a result of increased pressure of gas within a body tissue. Characteristic injury resulting from divers holding their breath while ascending underwater.

Altitude diving - Diving at a greater height then sea level, for example in a lake on a mountain. Significant in recreational diving because the decompression stops used for dives at altitude are different from those used for the same dive profile at sea level.

Alveoli - Microscopic air sacs in the lungs where gas exchange occurs with the circulatory system.

ANDL - see Adjusted No-Decompression Limit

Anoxia - A medical term used to describe severe hypoxia- tissue damage due to lack of oxygen.

Aorta - The largest vessel of the systemic arterial system, from which the main arteries carrying oxygenated blood branch and subdivide into smaller and smaller vessels.

Aqua-lung - The original name for the first open-circuit SCUBA diving equipment, developed by Emile Gagnan and Jacques Cousteau in 1942. It consists of a high pressure diving cylinder and a diving regulator that supplies the diver with breathing gas at ambient pressure, via a demand valve. Before that, there were a few attempts at constant-flow compressed-air breathing sets. Aqualung and Aqua Lung are registered trademarks for diving equipment.

Archimedes' Principle - A physical principle which states that a floating object displaces an amount of water that is equal in weight to the floating object. Used by divers to explain and calculate the buoyancy of an object.

Argon - A gas known for its high atomic weight and properties as an insulator. Potential applications in diving include its use as an insulator in drysuit diving.

Arterial Gas Embolism - Also referred to as AGE, a condition in which gas bubbles enter the arterial system and cause damage by blocking blood flow to vital organs, most commonly the brain. This is generally caused by air passing through the walls of the alveoli into the bloodstream.

Atmospheric Pressure - The amount of pressure as a result of the weight of the Earth's atmosphere. Atmospheric pressure is 14.7 pounds per square inch , or one atmosphere, at sea level.

Atrium - Chamber of the heart which provides access to another chamber called the ventricle.

Avogadro's Number - A constant number used to describe the number of atoms or molecules in one mole of a given substance. Equal to 6.022 x 10 to the power of 23.

Bar - A unit of pressure. Used by divers to describe the force acting on a given area (usually per centimeter). Equal to the force of one atmosphere.

Barotrauma - Injury to the body as a result of pressure.

BC - see buoyancy compensator. Also known as BCD

Bends, The - A common term for decompression sickness.

Benthic - Bottom dwelling: living on the floor of the sea or lake.

BIDP - Bali International Diving Professionals

Bladder - A pouch or other flexible enclosure with waterproof or gasproof walls.

Bottom Time - The duration of a dive starting when a diver begins a descent underwater and ending at the beginning of the diver's ascent.

Bottom Timer - A small, usually hand-held or wrist-mounted device that calculates bottom time.

Bourdon Tube - A c-shaped or spiral tube used in pressure (depth) gauges. Increased pressure acts on the tube by changing its shape. This movement is used to change the position of a gauge needle.

Boyle's Law - A physical law stating that when temperature is a constant, the volume of a given is inversely proportional to its pressure. Expressed as pV=constant.

Bronchi - Plural of bronchus, which is a division of the trachea.

Bronchiole - Small branch of the bronchus that carries air to and from the alveoli.

Bronchospasm - Bronchoconstriction, or the sudden narrowing of the smaller airways, of a spasmodic nature.

BSAC - British Sub-Aqua Club. The BSAC is the governing body for the sport of scuba diving in the UK. It is a club-based organization that offers training for members through a branch, or intensive courses through a BSAC school. The club was founded in 1953 and has around 40,000 members. It is a diver training organization that operates through its associated network of around one thousand local, independent diving clubs and around thirty diving schools. BSAC is unusual for a diver training agency in that most BSAC instructors are amateurs operating in diving clubs as opposed to professionals operating in diving schools.

Buoyancy - Defined in Archimedes' Principle as the amount of upward force acting on a submerged object. The force is equal to the weight of the water displaced by the object.

Buoyancy Compensator - Acronym for buoyancy control device. Usually a jacket or backpack containing an air bladder that can be inflated and deflated to control a diver's buoyancy underwater.

Buoyancy Control - The ability to maintain neutral buoyancy. Common causes of buoyancy problems include a current pushing a diver either up or down, being either over- or under-weighted, overinflation of the buoyancy compensator, or lack of the actual skill.

Burst Disk - A small fixture located on a scuba tank valve that is designed to release pressure from the tank once a certain internal pressure is reached. This is a safety feature that minimizes the risk of tank over pressurization.

CC Closed circuit Rebreather ( see Rebreather below)

Capillary - Microscopic blood vessels where the gas exchange takes place between the bloodstream and the tissues or the air in the lungs.

Carbon Dioxide - A waste gas produced by the metabolism of oxygen in the body. Carbon Dioxide acts as a respiratory stimulance and can cause depression of the Central Nervous System (CNS).

Carbon Monoxide - A highly poisonous, odorless, tasteless and colorless gas formed when carbon material burns with restricted access to oxygen. It is toxic by inhalation since it competes with oxygen in binding with the hemoglobin, thereby resulting in diminished availability of oxygen in tissues.

C-Card - An acronym for a certification card. Distributed to divers by certifying agencies as evidence that an individual has completed a certain level of diver training.

Celsius - A unit of measurement for temperature where zero degrees is the freezing point of water.

Cerata - Finger-like projections on the back of a nudibranch (sea slug).

Charles' Law - A law of physics stating a relationship between pressure, volume and temperature of a gas; if pressure is constant, the volume of the gas is proportional to its thermodynamic temperature. Also, if pressure is constant, the volume of a gas expands by a constant fraction as temperature increases.

Chart Datum (CD) - The level of the lowest tide there has ever been at a particular place. All depths on a chart and heights of tide refer to it.

Cilia - Long, slender microscopic hairlike processes extending from cells and capable of rhythmic motion.

CITES - The Convention on International Trade in Endangered Species.

Closed-Circuit - A scuba system in which exhaled gasses are not released as exhaust, rather, they are filtered and recycled into the gas supply.

CMAS - Confédération Mondiale des Activités Subaquatiques. (World Underwater Federation of Underwater Activities). Established 1959.

CPR - Cardiopulmonary resuscitation.

Dalton's Law - A physical law stating that the total pressure of a gas mixture is equal to the sum of the partial pressure of its components.

DCS - see decompression sickness

Decompression - Used to describe the equalization of gas in body tissues once they have been saturated with nitrogen.

Decompression Diving - Decompression diving involves diving beyond the standard, No-Decompression time/depth limits and requires one or more mandatory decompression stops during ascent. Failure to make the required in-water stops is a violation of the decompression tables and may result in Decompression Sickness. The rationale for using decompression diving is to extend divers' bottom time. Decompression diving is a diving procedure and is not limited to a specific type of equipment.

Decompression Illness - Also referred to as DCI, decompression illness is a term to describe dysbaric injuries related to scuba diving. This diagnosis stems from the uncertainties in many cases about the mechanistic causation of neurological symptoms. Moreover, it is sometimes impossible to differentiate clinically between neurological DCS and AGE. An alternative approach has been suggested in which the clinical manifestation of a patient's decompression syndrome is described without attempting to determine the pathophysiology. The term "decompression illness" (DCI) is suggested to encompass all manifestations of diseases following a reduction in ambient pressure, such as ascending from a dive.

Decompression Sickness - Also referred to as DCS, a syndrome caused by bubbles of inert gas forming in the tissues and bloodstream during or after ascent from a dive. DCS is manifested in two major forms, DCS I and DCS II.

  • DCS I - Decompression sickness involving only muscle and joint pain, fatigue and/or skin symptoms (itching, rash).
  • DCS II - Decompression sickness that includes symptoms involving the central nervous system, respiratory system or circulatory system.

Decompression Stop - A period of time a diver must spend at a constant depth in shallow water at the end of a dive in order safely to eliminate inert gases from the diver's body to avoid decompression sickness.

Dehydration - An abnormal depletion of water and other body fluids.

Demersal - Living on the sea-floor.

Deutsches Institut fuer Normung (DIN) - The agency that sets standards for compressed gas systems in Europe. The DIN fitting that connects the first stage of a regulator to the tank on a high pressure scuba systems is named after this agency.

Deviation - The angle between compass north and true north.

Diameter Indexing Safety System (DISS) - Intermediate pressure port where a hose attaches, leading to demand valve or other apparatus.

DIN - see Deutsches Institut fuer Normung

DIR - Doing It Right

Dive Computer - An electronic device used by a scuba diver to measure the dive profile and to display information needed for a safe dive, avoiding decompression sickness. Dive computers address the same problem as decompression tables, but perform a continuous calculation of the partial pressure of gases in the body based on the actual dive profile. As the dive computer automatically measures depth and time, it reduces the need for the diver to carry a separate watch and depth gauge and is able to warn of excessive ascent rates and missed decompression stops. Many dive computers also provide additional information to the diver, for example, the water temperature, or the pressure of the remaining breathing gas in the diving cylinder.

Dive Tables, Decompression Tables or Tables - printed cards or booklets that allow divers to determine for a particular dive profile and breathing gas, the Decompression stops required for that dive in order to avoid decompression sickness. With tables, it is assumed that the dive profile is a square dive, meaning that the diver descends to full depth immediately and stays at the same depth until resurfacing (approximating a rectangular line when drawn in a coordinate system where one axis is depth and the other is duration).

Diver Propulsion Vehicle (DPV) - An underwater propulsion vehicle used to increase the speed of movement of divers and equipment.

Doppler Ultrasound Bubble Detector - A device used in hyperbaric medicine to detect the presence of bubbles in the blood stream that may lead to decompression sickness.

Dorsal Fin - The fin running across the back of the fish.

Drysuit - A garment worn by divers for thermal protection. Usually a body suit constructed of various water tight materials. Functions by preventing water from coming into contact with a diver's body. Usually accompanied by thermal undergarments.

Emarginate Tail -

EMS - Emergency Medical Services

Enriched-Air Nitrox (EAN) - A nitrogen/oxygen mixture containing more than 21 percent oxygen, usually made by mixing air and oxygen.

Epiglottis - Thin structure behind the tongue that shields the entrance of the larynx during swallowing, preventing the aspiration of debris into the trachea and lungs.

Erythroprotein - A protein which is synthesized mainly in the kidneys and stimulates red blood cell formation.

Esophagus - Portion of the digestive tract that lies between the back of the throat and stomach.

Exertion - Exercise above that required for a relaxed swim in calm water. The main causes of exertion during a dive are current or extra equipment (such as for photography or specialty diving).

Exposure Suit - A garment worn by divers for thermal protection. Usually a wetsuit or a drysuit.

Fatigue - Complaints of being tired, experiencing a lack of sleep or a generalized tiredness.

Filling Station - A facility that provides the service of filling scuba cylinders (tanks) with compressed gas.

First Stage - The first of two stages that make upp a scuba regulator assembly. Responsible for reducing the high pressure from a scuba cylinder to an intermediate pressure that is delivered to the second stage.

Fossa Ovalis - Oval depression in the wall of the heart remaining when the foramen ovale closes at birth (See patent foramen ovale).

Free Diving - Breath-hold diving, without the use of scuba.

fsw - feet of sea water.

Fahrenheit ffw

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

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

First Stage

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

 Forward Roll Entry

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

Free Diving

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


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).

Global Positioning System - Acronym for Global Positioning System. Triangulates various satellite signals to determine position on the Earth. Often used by divers and boaters to navigate the position above an object underwater.

GPS - see Global Positioning System

Gradient - The difference in pressure, oxygen tension, or other variable as a function of distance, time or other continuously changing influence.

Half-Time - The time required for a body tissue to absorb or release half of total gas required to reach equilibrium with the surrounding gas pressure.

Hectocotylus - The modified arm of a male octopus or squid, used to transfer sperm to the female during reproduction.

Heliair - Is a breathing gas consisting of mixture of oxygen, nitrogen and helium and is often used during the deep phase of dives carried out using Technical diving techniques. It is easily blended from helium and air and so always has a 21:79 ratio of oxygen and nitrogen with the balance consisting of helium.

Heliox - is a diving breathing gas, consisting of oxygen and helium, that is often used during the deep phase of dives carried out using technical diving techniques. It is a more expensive gas than its close rival Trimix and is more often used in commercial diving. As sound travels faster in heliox than in air, voice formants are raised, making divers' speech incomprehensible to people not used to it.

Helium - is a colorless, odorless, tasteless chemical element, one of the noble gases of the periodic table of elements. Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas. The second most abundant element in the universe, significant amounts are found on Earth only in natural gas. It is used in cryogenics, in deep-sea breathing systems, for inflating balloons, and as a protective gas for many purposes. Helium is not toxic and has no biological effect.

Hermaphrodite - Having both male and female reproductive organs.

HWL - High water level.

Hydrostatic Pressure - The amount of pressure as a result of the weight of water above a diver.

Hypocapnia - An increased level of carbon dioxide in the blood. A known result of shallow skip breathing or excessive hyperventilation in free diving as well as Deep Diving on compressed air. Hypocapnia let nitrogen narcosis increase as well making it more likely that Oxygen Toxicity will occur. Heat loss can be increased, Heart rate and rhythm altered. Decompression Illness will be more likely to occur.

Hypothermia - A medical term used to describe a drop in the body's core temperature as a result of exposure to cold.

Hypoxemia - Inadequate oxygen supply in the arterial blood.

Hypoxia - Inadequate oxygen supply to the body tissues.

IAHD - International Association of Handicapped Divers

IANTD - International Association of Nitrox and Technical Divers

Inert - Having little or no tendency to react chemically.

Intercostals' Muscles - The muscles between the ribs which contract during inspiration to increase the volume of the chest cavity.

Ischemia - Inadequate blood flow to a part or organ. 


Kelvin - A unit of measurement for temperature. The magnitude of one degree Kelvin is equal to that of one degree Celsius.

Larynx - The organ of voice production, also known as the voice box; the opening from the back of the throat into the trachea (windpipe).

Laryngospasm - Severe constriction of the larynx in response to the introduction to water, allergies, or noxious stimuli.

Lift Bag - A specialized bag that is used to assist a diver in bringing a submerged object to the surface. After attaching the bag to a submerged object, the diver adds air to the bag to increase its buoyancy.

Logbook - A recorded history of an individual's diving experience. Usually contains, at least, the dive number, location, date, maximum depth and bottom time.

LPM - Liters per minute. A measurement of a flow rate of gas or liquid.

LWL - Low water level.

Mediastinum - The space within the chest located between the lungs, that contains the heart, major blood vessels, trachea and esophagus.

Metabolism - The conversion of food into energy and waste products.

MHW - Mean high water.

Mixed Gas - Any breathing medium that was mixed using oxygen and other gases, most commonly, helium, nitrogen, or air. Mixed gas could have only a single inert gas (e.g., heliox, nitrox) or multiple inert gases (e.g., trimix -- nitrogen, helium and oxygen mixes: see next page).

MLW - Mean low water.

msw. - Metres of seawater: a unit of pressure. 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).

Multiday - When more than one day of diving was performed in this particular dive series with a surface interval of < 24 hours between consecutive dives. "Multiday" and "single-day" are mutually exclusive.

Multilevel Dive - A dive in which the diver remains at several depths for a period of time before beginning final ascent to the surface: this contrasts with square dive, which involves a singe depth (see next page). Many different levels can be visited in one dive before finally ascending -- for example, a diver descends to 60 feet / 18 meters and stays for 10 minutes then descends to 80 feet / 24 meters and stays for five minutes, ascends to 50 feet / 15 meters for 10 minutes and then to 20 feet / 6 meters for five minutes before surfacing.

NASDS - National Association of Scuba Diving Schools. In 1999 NASDS merged with Scuba Schools International.

NAUI - National Association of Underwater Instructors.

NDL - see No Decompression Limit

Nematocysts - Stinging cells such as those possessed by sea anemones.

Nitrogen - A gaseous element known for its colorless, inert properties and its crucial role as a component of living organisms. Used by divers in varying concentration as part of the breathing gas mixture. Occurs in air at approximately 78% by volume. Known to affect divers in several applications, see nitrogen narcosis, decompression sickness.

Nitrogen Narcosis - A condition where divers behave as if intoxicated as a result of exposure to high partial pressures of nitrogen.

Nitrox - A nitrogen-oxygen mixed gas that contains an oxygen percentage other than 21 percent. "Oxygen-enriched air" or "enriched-air nitrox" refer to nitrox mixtures with oxygen levels greater than 21 percent. The most common nitrox mixtures are NOAA Nitrox I and NOAA Nitrox II, 32 percent oxygen and 36 percent oxygen, respectively. Nitrox is used to extend the NDL.

No-Decompression - A dive not requiring a staged stop during ascent to the surface. This type of dive can be made with either tables or computers.

No Decompression Limit - A theoretical mathematical representation of the amount of nitrogen, in minutes, that body tissues can absorb without substantial risk of decompression sickness.

NOAA - National Oceanic and Atmospheric Administration. Part of the U.S. Department of Commerce. Responsible for describing and predicting changes to the Earth's environment as well as conservation and management of resources in the marine and coastal environment to ensure sustainable economic opportunities.

Nystagmus - Spontaneous, rapid, rhythmic movement of the eyes occurring on fixation or on ocular movement.

Oblique - An indirect or evasive angle.

Occlude - To close off or stop up; obstruct.

Open Water - The name given by various certification agencies to designate the primary level of scuba training.

OC- open Circuit scuba using tanks with regulators that exhaust directly into the water

O-Ring - A small ring made of rubber or similar material. Used to seal watertight fittings.

Oxygen - A gas known for its colorless, odorless properties and its crucial role in the respiratory process of living organisms. Used by divers in varying concentrations as part of the breathing gas mixture. Occurs in air at approximately 21% by volume. Known to affect divers in several applications, see oxygen toxicity.

Oxygen Toxicity - A medical term used to describe the over-exposure of body tissues to increased concentrations of oxygen. A characteristic risk of breathing mixed gases under pressure. Can result in unconsciousness and death.

PADI - Professional Association of Diving Instructors. The world's largest recreational diving membership organization and diver training organization. PADI professionals make underwater exploration and adventure accessible to the public while maintaining a high industry standard for dive training, safety and customer service.

Partial Pressure - Describes the pressure exerted by any component gas in a mixture of gases. For example the pressure exerted by oxygen in breathing air.

Patent Foramen Ovale - A hole in the septum (wall) between the right and left atria of the heart.

Pectoral Fins - The pair of fins at the front of the fish, behind its head.                 

Pericardium - A double-layered membranous sac surrounding the heart and major blood vessels connected to it.

Pharynx - Portion of the airway at the back of the throat, connecting mouth, nasal cavity and larynx.

Platelet - A round or oval disk found in the blood of vertebrate animals that are involved with blood clotting.

Pleura - Membranes surrounding the outer surface of the lungs and the inner surface of the chest wall and the diaphragm.

Pony Bottle - A common term for a secondary gas cylinder carried by divers as a redundant air source.

Prescription - A written order for dispensing drugs signed by a physician.

Primary Assessment - Assessment of the Airway, Breathing and Circulation (pulse) in an ill or injured person; also known as the ABCs.

PSI - An acronym for pounds per square inch. Used to describe the force of pressure acting on a given area. For technical accuracy, psi must be expressed as psig (pounds-force per square inch gauge) or psia (pounds-force per square inch absolute; that is, gauge pressure plus sea level atmospheric pressure, or psig plus approximately 14.7 psi). psig can be further subdivided into psivg (psi vented gauge) and psisg (psi sealed gauge), the difference being that psisg measures the difference in psi between a chamber of air sealed at atmospheric pressure and the pressure at the measuring point, while psivg measures the difference between the measuring point and the local pressure. Sometimes psid (psi difference) is seen; this should always refer to a measurement of the difference between two pressures.

1 psi approximately equals 6.895 kPa

PSIA - An acronym for pounds per square inch absolute. Used to describe the force of pressure acting on a given area.

Pressure - (symbol: p) is a measure of force per unit area. The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·s-2·m-1). Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (PSI) and the bar. In the United States air pressure is still measured in inches of mercury (as in the mercury barometer). Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, because it gives the same numbers as the older millibar (mbar).  P = F / A  where p equals pressure, F equals force, and A equals area. Often F, is taken to be the of the magnitude of the mean vector force normal to the surface of area A upon which it exerts; the "surface" not necessarily being a that of a body, but for example the cross sectional area of a conduit.

Rapid Ascent - The currently recognized recommended ascent rate is no faster than 60 feet / 18 meters per minute. A rapid ascent occurs when a diver ascends faster than the recommended rate. Rapid ascents are often uncontrolled and can be caused by overinflation, poor buoyancy control, being underweighted or panic.

Rapture Of The Deep - A common term for decompression sickness.

RBT - see Residual Bottom Time

Rebreather - RB is a type of breathing equipment that provides an oxygen-based breathing gas and recycles exhaled gases. This recycling reduces the volume of breathing gas used making a Rebreather a lightweight and compact machine for supplying breathing gas for long durations in environments where humans cannot safely breathe from the atmosphere. There are some different types of Rebreather:

  • Oxygen Rebreather - This is the oldest type of Rebreather and was commonly used by navies from the early twentieth century. The only gas that it supplies is oxygen. As pure oxygen is toxic when inhaled at pressure, oxygen rebreathers are limited to a depth of 6 meters (20 feet); some say 9 meters (30 feet). Oxygen Rebreather are also sometimes used when decompressing from a deep open-circuit dive, as breathing pure oxygen makes the nitrogen diffuse out of the blood quicker. In some rebreathers, e.g. the Siebe Gorman Salvus, the oxygen cylinder has two first stages in parallel. One is constant flow; the other is a plain on-off valve called a bypass; both feed into the same exit pipe which feeds the breathing bag. In the Salvus there is no second stage and the gas is turned on and off at the cylinder. Some simple oxygen rebreathers had no constant-flow valve, but only the bypass, and the diver had to operate the valve at intervals to refill the breathing bag as he used the oxygen.
  • Semi-Closed Circuit Rebreather (SCR) - Military and recreational divers use these because they provide good underwater duration with fairly simple and cheap equipment. Semi-closed circuit equipment generally supplies one breathing gas such as air, Nitrox or Trimix. The gas is injected at a constant rate. Excess gas is constantly vented from the loop in small volumes. The diver must fill the cylinders with gas mix that has a maximum operating depth that is safe for the depth of the dive being planned. As the amount of oxygen required by the diver increases with work rate, the oxygen injection rate must be carefully chosen and controlled to prevent either oxygen toxicity or unconsciousness in the diver due to hypoxia.
  • Fully closed circuit Rebreather (CCR) - (Military, photographic and recreational divers use these because they allow long dives and produce no bubbles. Closed circuit rebreathers generally supply two breathing gases to the loop: one is pure oxygen and the other is a diluent or diluting gas such as air, nitrox or trimix. The major task of the fully closed circuit rebreather is to control the oxygen concentration, known as the oxygen partial pressure, in the loop and to warn the diver if it is becoming dangerously low or high. The concentration of oxygen in the loop depends on two factors: depth and the proportion of oxygen in the mix. Too low a concentration of oxygen results in hypoxia leading to sudden unconsciousness and ultimately death when the oxygen is exhausted. Too high a concentration of oxygen results in oxygen toxicity, a condition causing convulsions, which when they occur underwater can lead to drowning. In fully automatic closed-circuit systems, a mechanism injects oxygen into the loop when it detects that the partial pressure of oxygen in the loop has fallen below the required level. Often this mechanism is electrical and relies on oxygen sensitive electro-galvanic fuel cells called ppO2 meters to measure the concentration of oxygen in the loop. The diver may be able to manually control the mixture by adding diluent gas or oxygen. Adding diluent can prevent the loop's gas mixture becoming too oxygen rich. Manually adding oxygen is risky as additional small volumes of oxygen in the loop can easily raise the partial pressure of oxygen to dangerous levels.

Recompression - A treatment for decompression sickness or air embolism where an individual is re-introduced to a controlled high pressure environment and gradually returned to normal pressure. Usually occurs in a pressurized chamber, commonly known as a decompression chamber.

Redundancy - Or being Redundant is to prevent an entire system of failure by using duplicates. In Scuba diving this is why you have for example a double inflator, and a backup light while night diving.

Regulator - A device used to regulate the pressure inside a high pressure air cylinder and deliver gas to a diver at breathable ambient pressure (ambient pressure is the water pressure at depth i.e. 10m : 1 bar).

Repeat Dive / Repetitive Dive - More than one dive was made on the day of interest, with some period of time spent at the surface between dives (surface interval). "Single dive" and "repeat dive" are mutually exclusive.

Residual Nitrogen Time (RNT) - A theoretical mathematical representation of the amount of nitrogen absorbed in body tissues after a dive. Expressed on dive tables in minutes which are added to the no-decompression limit for a repetitive dive.

Respiration - The exchange of gases between a living organism and its environment; the act of breathing.

Respiratory Arrest - Cessation of breathing.

RIB - Rigid-hull inflatable boat.  

RNT - see Residual Nitrogen Time

SAA - Sub-Aqua Association.

Safety Stop - A safety measure that divers take to decrease the risk of decompression sickness. The diver halts his ascent at a fairly shallow depth (usually 15 feet) toward the end of a dive to offgas nitrogen absorbed under pressure.

Salinity - A term used to describe the concentration of salt in water.

Sclerite - see spicule.  

Scooter - see Diver Propulsion Vehicle

SCUBA - Self Containing Underwater Breathing Apparatus

SDI - Scuba Diving International. Qualification agency created in 1999. It grew out of sister company TDI, which specializes in the more advanced disciplines of technical dive training.  

Second Stage - The part of a scuba regulator responsible for reducing the increased pressure delivered by the first stage to an ambient pressure that is breathable by a diver.

Single-Day - Only one day of diving was done in this particular dive series. "Single-day" does not denote the number of dives, but rather a single day of diving (for example: four dives could be made in a single day: or a single day of diving could include one dive only).

SMB - Surface Marker Buoy

Snorkel - A bent tube fitting into a swimmer's mouth and extending above the surface; allows swimmer to breathe while face down in the water.

Snorkeling - Swimming at the surface of a body of water while equipped with a diving mask, a shaped tube called a snorkel, and usually swimfins. Using this equipment provide the possibility to swim, look underwater and breath through the snorkel at the same time.

Snuba - Is a popular form of "introductory diving" (diving done in the presence of a professional, insured guide, and following a very short lesson, but not requiring a SCUBA certification).

SPG - see Submersible Pressure Gauge

Spicule - Hard, needle-shaped, calcium carbonate skeletal structure contained within soft corals. Also known as a sclerite.

Square Dive - A dive in which a diver descends to a single depth and remains until beginning the final ascent to the surface. This contrasts with multilevel dive,  for example, a diver descends to 60 feet / 18 meters and stays at 60 feet for 30 minutes before ascending. Square dives and multilevel dives are mutually exclusive.

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

Stage Bottles - Secondary scuba cylinders that are used to carry different gas mixtures for consumption during various stages of decompression dives. Often used by technical divers.

Strobe - A common term used to describe an underwater flash unit used in underwater photography.

Submersible Pressure Gauge - An acronym for submersible pressure gauge. A device used to by divers to determine the gas pressure inside a scuba cylinder.

Supine - Lying flat on back, with face upward.

Surfactant - A substance produced in the lungs to reduce surface tension in alveoli and small airways.

Swell - Caused by waves formed by past wind, or wind at a distance.

Symptom - Any non-observable condition described by the patient.

Tables - see Dive decompression Tables

TBT - see Total Bottom Time 

Technical Dive - A Technical Dive is defined as one in which one of the following conditions existed and not limited too:

  • Diving deeper than 40 meters / 130 feet;
  • Using a breathing mixture other than compressed air;
  • Decompression or overhead diving (diving in shipwrecks or caves);
  • Special training and equipment were used.

Thermocline - A distinct area underwater which is the border between layers of water that differ in temperature and salinity.

Thorax - The upper part of the trunk (main part of the body) between the neck and the abdomen which contains the heart, lungs, trachea and bronchi.

Tidal Range - The difference in height between high and low water.

Total Bottom Time (TBT) - A term used in common dive tables to represent the amount of time a diver spent underwater, including residual nitrogen time. Total bottom time is the sum of actual bottom time and residual nitrogen time.

Trachea - The air passage that begins at the larynx and ends as the beginning of the principal right and left bronchi.

Trimix - Gas mixture, Nitrogen, helium and oxygen mixes.

Truncate Tail - Shaped as in the diagram to the right.

TTL - An acronym for "Through The Lens" which describes a camera feature set where the power of a flash or strobe is controlled by camera according to the amount of light the camera senses through the lens.


 Valsalva Maneuver - The forced inflation of the middle ear by exhaling with the mouth closed and the nostrils pinched.

Venous Gas Emboli - Inert gas bubbles in venous blood (which return to the heart and lungs).

Ventricle - Thick-walled, muscular chamber in the heart which receives blood from the atrium, pumping it through to the pulmonary or systemic circulation.

Venules - Small veins.

Visibility - "VIS" A measurement of how far a diver can see while underwater. Usually dependent on several factors including bottom composition, turbidity, suspended matter and water temperature.

Visual Reversal - A perceptual phenomenon where the distance to an object appears farther away than the actual distance.

Weight Belt - buoyancy control system, A belt worn around a diver's waist which holds weights used to make the diver less buoyant from suites or other buoyant gear worn.

Wetsuit - A garment worn by divers for thermal protection. Usually a body suit constructed of neoprene. Functions by providing insulation from the cold and by allowing a thin layer of water to enter the suit which is retained within the suit and warmed by the divers body.

Within Limits - Represents divers who were diving within their computer or table limits.










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