Scuba Diving Glossary

This is a glossary of technical terms, jargon, diver slang and acronyms used in scuba diving. The definitions listed are in the context of scuba diving. There may be other meanings in other contexts.

Actual Bottom Time (ABT)
In repetitive diving, the total time actually spent under water (in minutes) from the beginning of descent until leaving the bottom for a direct continuous ascent to the surface or safety stop.
Adjusted No Decompression Limit
The time limit for a repetitive dive that accounts for residual nitrogen.
Air Compressor
A machine that compresses or pressurises air to fill your cylinders before going diving.
Air Pressure
The force per unit area exerted by the weight of air.
Alternate Air Source
Any device a diver can use in place of the primary regulator, in order to make an ascent while still breathing normally.
Anoxia
Absence of oxygen caused by inhaling a breathing gas that contains no oxygen or being unable to inhale any breathing gas.
Aqua-lung
Underwater breathing equipment consisting of a diving cylinder and diving regulator.
Archimedes Principle
Principle that states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid.
Argon
An inert gas that makes up less than one percent of air.
Ascent Rate
The proper speed for ascending, which is no faster than 18 metres/60 feet per minute. A rate slower is acceptable and appropriate.
Atmosphere Absolute (ATA)
1 ata is the atmospheric pressure at sea level.
Back Roll Entry
Leaving the dive boat by sitting on its rail/pontoon and rolling backwards into the water.
Bar
Short for barometric pressure. The method of measurement of air pressure used throughout most of the world and by the compressed gas industry - also a place where divers meet after a long hard day of diving.
Barotrauma
Injury caused by unequal pressure between a space inside the body and the ambient pressure, or between two spaces within the body - usually caused by a too rapid ascend.
Bends
See - decompression sickness
Body Suits
Garment which provides full-length, stylish abrasion protection when diving in conditions where a full wetsuit may not be needed. Body suits provide minimal insulation and may be worn under a traditional wetsuit for added insulation and ease of suiting up.
Boots
Neoprene boots worn with open heel fins.
Bottle
The commonly used name for a scuba cylinder in the United Kingdom.
Bottom Time
The time from the beginning of descent until the beginning of a direct, continuous ascent to the surface or safety stop.
Bounce Dive
In recreational diving, a bounce dive is a descent to maximum depth and then an ascent back to the surface with the least delay, in a dive profile resembling a spike. In commercial diving, bounce diving is the alternative to saturation diving.
Boyle's Law
Under constant temperature, the volume of a given mass of gas is inversely proportional to the absolute pressure of the gas.
Buddy
Your diving partner. Very valuable in times of trouble. A good buddy makes diving a lot more enjoyable.
Buddy Breathing
Two divers sharing the same demand valve, generally after an out-of-gas emergency.
Buddy System
Pair or sometimes threesome of divers that dive together as a team for safety.
Buoyancy
Upward force exerted by a fluid on any body immersed in it. Buoyant force can be explained in terms of Archimedes' principle. Scuba divers with good buoyancy control are able to stay neutrally buoyant at any required depth.
Buoyancy Compensator or BCD
Diving equipment worn by divers to provide life-saving emergency buoyancy and the ability to control ascent and descent rates.
Carbon Dioxide Poisoning
Is due to incomplete elimination of carbon dioxide.
Carbon Monoxide Poisoning
Usually due to compressor maintenance errors.
Certification Card
Proof of completed diver training and evidence of experience. If you forget it at home you might have problems getting on the dive boat or filling your cylinder. Don't despair, many dive shops can check your records on-line these days.
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.
Chemical Light Stick
The stick is a light source in which the two chemicals inside the stick mix to produce light. Divers attach these sticks to their tank valves or snorkels to increase the ability to be seen by other divers on night dives.
Chumming
Term used for attracting sharks with a mixture of blood and fish parts. Can be used to describe your buddy's behaviour when vomiting overboard from seasickness.
Closed Circuit Breathing Apparatus
aka, a rebreather, a device that reuses air by scrubbing or removing the carbon dioxide from it. There are usually no bubbles emitted.
Compressor
A machine used to increase the volume of usable air in a diving cylinder by raising its pressure.
Controlled Buoyant Lift
A diver rescue technique.
Cylinder
A cylinrical metal container used to safely store high pressure air. Most are made of steel or aluminum. Cylinders came in a variety of air capacities depending upon their pressure rating and size. In the metric system, the most common sizes are 8, 10, 12 and 15 litres of water capacity. In the USA, cylinders are commonly referred to as tanks. In the UK, cylinders are commonly referred to as bottles.
Cylinder Boots
The flat-bottomed, plastic, vinyl or rubber devices that fit over the rounded end of a scuba cylinder, allowing the cylinder to stand up.
D-Rings
D-shaped rings. Generally part of good BCDs and loved by technical divers.
Dalton's Law
The total pressure exerted by a mixture of gases is equal to the sum of the pressures of each of the different gases making up the mixture - each gas acting as if it alone were present and occupied the total volume.
Decompression Diving
Diving that requires planning stops during ascent to avoid decompression sickness. In recreational diving (no decompression diving), a decompression stop is considered an emergency proceedure only and is never an intentional part of the dive plan.
Decompression Sickness
A potentially lethal diving disorder caused by bubbles of inert gases, such as nitrogen or helium, coming out of solution and becoming trapped in the tissues, organs and blood vessels of the body causing symptoms ranging from rashes to death.
Decompression Stop
A pause during the ascent phase of the dive to allow safe release of inert gases from the tissues of the body and avoid decompression sickness.
Decompression Tables
Printed tables that provide divers with a way of avoiding Decompression Sickness by giving the maximum times that can be spent at depth, and by indicating the Decompression Stops and Surface Intervals needed for a particular depth and time profile to be carried out safely.
Deep Diving
For recreational divers a deep dive is one with a maximum depth of 40 metres / 130 feet.
Defogging Solution
"Fake spit" that prevents fog from building up inside the mask during diving.
Dehydration
Reduced water content in the body - caused by diving without drinking enough water, too much alcohol the night before the dive etc. Always drink lots of water before diving.
Depth Gauge
Device that indicates how deep you are.
Descent/Ascent Line
A line from a boat or buoy which can be used by divers to control their descent or ascent.
Diaphragm Regulator
A diaphragm first-stage design inherently prevents water from entering the first-stage mechanism itself. This helps to prevent internal corrosion and contamination buildup, which may lead to more consistent performance between service intervals.
Diluent
This is the gas used in a closed circuit rebreather to make up volume in the breathing loop as the diver proceeds to deeper depths and the gases in the breathing loop are compressed. Depending on the rebreather, and the type of diving, the gas used for diluent could be air, nitrox, trimix or even heliox.
DIN Valve
Screwable alternative to a yoke fitting for first stage - commonly used in Europe and has the advantage of reducing the chance of O-ring failure.
Dive Club
A group of people with an interest in SCUBA diving. Many dive clubs in Victoria are associated with commercial dive shops. However, there are also many non-commercial dive clubs, such as the Victorian Sub-Aqua Group (VSAG).
Dive Computer
Device that does all the had work for you. No more multi level diving calculations, no more PADI wheel - based on empicic data and theoretic models - not necessarily always right but a great help for repetitive diving. Beeps often.
Dive Flag
Flag typically used by a dive boat to indicate that it has 'divers down'. Can also be used by a diver using a buoy and line. Comes in two versions: the international marine (international code letter flag 'A', as used in Australia) and the red and white flag (red with white diagonal).
Dive Lights/Torches
Specially designed underwater lights used for night dives or dive in dark places such as wrecks or caves.
Dive Profile
A two dimensional representation of the two most important characteristics of the dive that a scuba diver must monitor to dive safely: depth and time. The profile is often used when describing a dive's likely decompression obligation.
Dive Shop
Supplier of scuba diving equipment or training, or organiser of dive expeditions or trips.
Dive Tables
See Decompression tables.
Divemaster
An appropriately experienced scuba diver who leads a group of less experienced divers underwater. Some of the various training agencies (e.g. PADI) have a training programme and certification level for a Divemaster.
Diver Propulsion Vehicles
Motorised vehicles used by divers to cover long distances to underwater without having to kick.
Divers Alert Network (DAN)
A non-profit organisation, exists to provide expert information and advice consistent with current literature for the benefit of the diving public. In Australia call 1800 088 200.
Diving Fins
These are used on the feet for faster swimming and to increase maneuverability. They are not called flippers... he is a dolphin.
Doing It Right (DIR)
Term invented by George Irvine III to describe the practices that he and others derived from Hogarthian principles, a rigorous diving discipline. Loved by technical divers.
Drift Diving
Scuba diving in a current.
Drowning
Death caused by inability to inhale anything but water.
Dry Suit
A diving suit designed to thermally insulate and provide protection to the skin of the diver.
Dual Regulator Systems
Dual regulator systems use two primary stage regulators on a single air supply to allow a diver access to continue to access his air supply in the event of a malfunction in the first primary stage regulator. Generally loved by technical divers.
Dual Tank Manifolds
Is used for double cylinder systems. It has two valves similar to single tank systems which are attached by a heavy-duty crosspiece with a valve outlet in the center.
Dual-Valve Manifold
Allows a single cylinder system to utilise two regulators on a single scuba tank.
Dysbarism
Medical conditions resulting from changes in ambient pressure.
Electro-galvanic Fuel Cell
An electrical device used to measure the concentration of oxygen gas in diving equipment.
Emergency Position-Indicating Radio Beacons (EPIRBs)
Tracking transmitters that operate as part of the Cospas-Sarsat satellite system. When activated, the beacons send out a distress signal that allows the beacon to be located by the satellite system and search and rescue aircraft to locate the divers or boats needing rescue.
Enriched Air Nitrox (EAN[x] percent of Oxygen)
This is one of the naming conventions for a non-normal mixture of Oxygen and Nitrogen. Air has approximately 21% Oxygen and 79% Nitrogen, this is the normal mixture of Oxygen and Nitrogen (also called normoxic). It you have more (or less) Oxygen in the mix, it is considered nitrox. If the mixture had 32% Oxygen with the balance as Nitrogen this would be labeled EAN32.
Equalisation
Equalisation is the act of forcing air into an open space to offset increasing hydrostatic pressure. This can be applied to such spaces as a dive mask or the diver's ears, and prevents what is known as a squeeze.
Equivalent Air Depth (EAD)
A way of expressing the narcotic effect of breathing gas mixtures that contain nitrogen, for example nitrox and trimix. EAD is the depth relative to the partial pressure of nitrogen in a normal air mixture (21%O2, 79%N2). When there is a lower than normal fraction of nitrogen in a gas mix, the partial pressures of nitrogen are lower at any given depth. This allows the diver to feel less narcotic effect from the nitrogen than when breathing air at the same depth.
Eustachian Tube
Hollow structure of bone and cartilage extending from the middle ear to the rear of the throat. By permitting air to leave or enter the middle ear, the tube equalises air pressure on either side of the eardrum - see equalisation.
Exposure Protection
Garments worn to prevent decreases in core body temperature and abrasion by the elements. Protection can range from lightweight body suits to heavily insulated dry suits.
Fathom
A linear unit of measurement (equal to 6 feet) for water depth - commonly used by fishermen.
Fins
Worn on the feet to increase maneuverability and allow for faster swimming. A great point of discussion between divers - who has the fastest and most efficient fins.
First Stage
The part of the regulator which attaches to the scuba tank valve and which is responsible for the first level of tank pressure reduction.
Frogman
A scuba diver, particularly a military diver on an undercover mission.
Full Foot Fins
These diving fins are predominantly used for snorkelling and warm water diving. They comprise of a rubber sock like unit to keep the fins on your feet.
Gas Blending
Filling diving cylinders with gas mixes such as nitrox or trimix.
Gas Embolism
A potentially lethal diving disorder caused by air or other gas bubbles entering the blood stream through wounds.
Gas Saturation
Gas saturation occurs when the level of dissolved gas in a particular body tissue has reached its maximum.
Guage
Any instrument used to measure or quantify, typically used to measure cylinder pressure, depth and direction in diving.
H-Valve
A tank valve with 2 outlets.
Haldanian
Related to Haldane's theory that nitrogen is taken up and given off in exponential fashion during a dive, and that there is some safe ratio of pressure change for ascent (originally, 2:1).
Halocline
Visible boundary between layers of water of different salinities. Appears as a barrier of mist.
Hang Tank
An extra tank, ideally of deco mix, staged at the decompression stop.
Hard Hat diving
Surface supplied diving, generally in professional diving, either wearing a modern diving helmet or the old-style standard diving dress and brass helmet.
Heliox
A breathing mixture of gases consisting entirely of Helium and Oxygen. This is used to eliminate Nitrogen narcosis and to control the affects of Oxygen toxicity, by eliminating the Nitrogen and reducing the Oxygen in the breathing mix. Another benenfit is reduced effort of breathing due to the lower density of helium. There are a few disadvantages, for working divers, helium distorts the voice and helium has less insulating value than an Oxygen/Nitrogen mix which results in divers becoming cold sooner.
Henry's Law
The amount of any gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coefficient of the gas in the liquid.
Hogarthian Principles
The Hogarthian configuration is named after Bill "Hogarth" Main. It is based on reducing equipment to a minimum streamlined configuration that nevertheless includes sufficient redundancy for extended decompression dives
Hookah
Surface-supplied compressed air apparatus, for use in shallow diving in calm waters. The air is delivered to one or more divers through a long hose.
Hose
Used to carry air in a SCUBA unit. Example of use, to connect the first stage to the second stage of a regulator.
HPNS
High Pressure Nervous Syndrome or Helium Tremors - caused by using breathing gases that contain helium at depth.
Hydrogen
Hydrogen is an inert gas and the lightest, most abundant gas in the universe. It is believed it could be used as a substitute for helium in deep commercial and military diving.
Hydrostatic Test - Hydro
Pressure test in which the cylinder is filled with water instead of air. Needs to be done yearly in Australia to be able to have your tanks filled.
Hyperbaric Chamber
Air-tight chamber that can simulate the ambient pressure at altitude or at depth - used for treating decompression illness.
Hypercapnia
Carbon dioxide poisoning generally caused by rebreathing your own exhaled carbon dioxide. A big problem at depth especially with rebreathers or high rates of exertion.
Hyperventilation
Either a deliberate and dangerous method intended to extend the duration of a free dive or the body's response to hypercapnia.
Hypothermia
A potentially lethal medical condition caused by cooling the body.
Hypoxia
Insufficient oxygen in the body - normally caused by inhaling a breathing gas that contains insufficient oxygen to support normal activities or consciousness.
Interstitial Emphysema
Gas trapped in the chest after lung barotrauma.
J-Valve
A J-Valve contains a spring-loaded mechanism which shuts off a diver's air supply when a certain tank pressure is reached - formerly used to trigger switching to reserve air supply and divers ascent.
K-Valve
Simple on/off valve.
Lift Bag
Like underwater balloons. Helps lifting heavy objects underwater using bags filled with air at depth.
Live Aboard
Dive boat with sleeping and eating facilities - a preffered way to dive for those who don't like to juggle their gear around a lot and for those whose life on the surface of this planet is just another surface interval.
Log Book
List of the dives a diver has recorded for proof of experience.
M-Value
An M-value is used in conjunction with a dive computer to express the tissue compartment’s maximum allowable pressure in feet of seawater absolute (fswa).
Manifold
Plumbing to connect 2 tanks so that one regulator can access gas in both tanks.
Mask Squeeze
Occurs during rapid descents if the diver neglects to equalize his mask. Blood shot eyes are the most common symptom.
Maximum Operating Depth
The depth at which the partial pressure of oxygen (ppO2) of a gas mix exceeds a safe limit.
Multilevel Diving
Planning profiles that credit you for slower nitrogen absorption when you ascend to a shallower depth. This provides more no-stop dive time.
Narced
Nitrogen narcosis - under the influence of nitrogen while diving.
Navy SEAL
A US Navy trained military diver.
Neoprene
Neoprene is the DuPont Performance Elastomers trade name for a family of synthetic rubbers based on polychloroprene (polymer form of Chloroprene). For diving and exposure protection applications, the air spaces in the neoprene are filled with nitrogen for its insulation value. Thick wet-suits made at the extreme end of their cold-water protection are usually made of 7mm thick neoprene. It should be noted that since neoprene contains many porous air-spaces, the material compresses the further underwater its exposed too. So a 7mm neoprene wet suit offers much less exposure protection under one hundred feet of water than at the surface. A new advance in neoprene for wet-suits has been the "super-flex" variety which combines spandex into the neoprene for a greater flexibility.
Night Diving
A dive at night - many underwater animals are nocturnal or behave differently at night.
Nitrogen Narcosis
Condition caused by breathing nitrogen at high pressure (at depth).
Nitrox
A breathing gas consisting of oxygen and nitrogen
No Decompression Limit (NDL)
The maximum time that can be spent at a depth before decompression stops are required. Also called "no-stop time".
No-Stop Dive
A dive made within no decompression limits because you don't have any required emergency decompression stops.
Normoxic
This is the term used to describe the normal mixture of gases found in the atmosphere. More specifically it refers to the percent of Oxygen in the mix. Since the atmosphere has 21% Oxygen, a mixture of gas with 21% Oxygen would be called normoxic.
Octopus - Occy
Extra second stage regulator alternate air source used by your buddy in an out of air emergency. Also referred to as an Occy. However, many divers, including many technical and DIR divers, in an emergency prefer to pass the second stage regulator from their own mouth to their budddy, and use the Octopus as their air source. The thinking is that your buddy will probably grab your second stage regulator from your mouth anyway, so you may as well know how to switch to your Octopus.
Offgassing
Reducing the load of nitrogen (and/or other inert gasses) on the surface or on the safety stop. Also known as outgassing.
Open Heel Fin
Type of fin used mainly with drysuits, and where boots are worn, open back to slot foot in, with strap to keep them on your feet.
Oxygen
Gas vital to all life on this planet which makes up 21% of air by volume.
Oxygen Toxicity
Is caused by using breathing gases that contain oxygen at high pressure (at depth). There are two primary types of Oxygen toxicity. One results from long exposures of elevated ppO2's and is called "The Loraine Smith Effect" or "Pulmonary Oxygen Toxicity" as the primary damage is to the lungs and airways. The other type of Oxygen toxicity results from short high ppO2 exposures and is called "The Paul Bert effect" or "CNS Toxicity" (Central Nervous System Toxicity) and is characterized by convulsions with little or no warning signs. CNS toxicity usually occurs with ppO2's above 1.6.
PADI
Professional Association of Diving Instructors. World's largest scuba training agency. Often reffered to as "Put Another Dollar In".
Partial Pressure of a Gas
The concentration of individual component gases of breathing gases. In simpler terms it may be thought of as the number of molecules per given volume of gas. More molecules per volume = higher partial pressures. In more specific terms it is the Fraction of the gas (F[x]) multiplied by the absolute atmospheres.
Pneumothorax
Collapsed lung.
Pony Bottle
Small scuba tanks strapped to your main dive gear - see D-ring. These tanks have their own first and second stages and can be used as an alternate air source. A requirement for solo diving.
Prescription Dive Mask
Special masks for divers needing refractive correction.
Pressure Group
A letter used on Dive Planners to designate the amount of theoretical residual nitrogen in your body.
Professional Diving
Diving for payment.
PSI
Pounds per square inch - a common measurement of air pressure.
Purge Valve
Purge valves allow masks and even regulators to be cleared easily without having to remove the mask from the diver's face or the regulator from the diver's mouth.
Rebreather
Breathing equipment that captures, cleanses and re-oxygenates exhaled breath so that it can be re-inhaled.
Recompression Chamber
A pressure vessel used to treat divers suffering from certain diving disorders such as decompression sickness.
Recreational Diving
A type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. Used to refer to diving to prescribed limits, including a depth no greater than 40 metres, using only compressed air and never requiring a decompression stop.
Regulator - Reg
Regulators reduce the highly compressed tank pressure to ambient pressure for easy breathing pleasure. A regulator set is typically sold as a first stage regulator, plus a second stage regulator. Most divers then add a reserve 2nd stage regulator, also called occy or octopus.
Repetitive Dive
A dive that follows another dive while there is still a significant amount of residual nitrogen in your body. For recreational diving this is considered to be within six hours of a previous dive.
Residual Nitrogen
The higher than normal amount of nitrogen remaining in a divers body after a dive.
Reverse Block
When the internal pressure of an air space is greater than the external pressure.
Reverse Squeeze
Pain or discomfort in an enclosed space (e.g., sinuses, middle ear, inside mask) on ascent from a dive.
Rigid Inflatable Boat (RIB)
Generally with solid keel and large inflatable tubes.
Rule of Thirds
An air management technique. Use a third of the tank to get to your destination, a third to get back, and the last third belongs to your buddy in case of an emergency.
Saftey Stop
A stop made within 5 and 6 metres / 15-20 feet for three or more minutes at the end of a dive for additional safety. The safety stop is recommended after all dives (air supply and other considerations allowing) and required on dives to 30 metres/ 100 feet or greater, and dives coming close to the no decompression limits. Dive tables based Dr. Bruce Wienke's Reduced Gradient Bubble Model (RGBM), suggest deeper initial safety stops. It is recommended that all recreational diving incorporate a one minute deep stop at a depth half that of the maximum, followed by a two-minute stop at 5-6 metres/15-20 feet.
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 ambient (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
Diver Propulsion Vehicle.
SCUBA
Self Contained Underwater Breathing Apparatus. The term SCUBA in common usage usually means open-circuit equipment in which gas (usually air) is breathed from a cylinder of compressed gas and then exhaled into the water, usually in the line of kit development started by Emile Gagnan and Jacques-Yves Cousteau. However, rebreathers (both semi-closed circuit and closed circuit) are also self-contained systems (as opposed to surface-supplied systems) and could be classed as SCUBA.
Second Stage
The part of the regulator you put into your mouth to breathe through. Most divers have a reserve 2nd stage regulator, also called an occy or octopus.
Semi-dry Suit
A wet suit with wrist and ankle seals to reduce entry and exit of water.
Shore Diving
Scuba diving that starts from the shore line.
Silt-out
A complete loss of visibility caused by silt being disturbed, typically by careless finning.
Sinus Squeeze
Sinus squeeze occurs during a rapid descent when a diver is unable to equalize the air space in the sinus cavity. A diver experiencing sinus squeeze will often experience pain and surface with blood in his mask due to the trauma caused by a squeeze.
Sinuses
Air spaces within the skull that are in contact with ambient pressure through openings in the back of the nasal passages.
Sinusitis
Inflammation or infection of the sinuses.
Skin
A lycra suit worn by a diver in warm water or under a wet suit.
Skin Diving
Another term for breath-hold or free diving - diving without the use of SCUBA equipment.
Skip Breathing
The practice of inhaling, holding the inhalation for a period of time and then exhaling in order to attempt to extend the time underwater by using less air. This practice can lead to a buildup of CO2 and symptoms of hypercapnia.
Sling/Stage cylinder
A reserve supply of breathing gas not carried with the diver generally used for decompression.
Snorkel
Breathing device consisting of a bent tube fitting into a swimmer's mouth and extending above the surface. Allows swimmer to breathe while face down in the water.
Solo Diving
The act of diving without a buddy. It is generally recommended to have at least an alternate air source (see - pony bottle, alternate air source) if you engage in such practice. Not a recommended dive practice and not allowed on most charter boats. However, loved by keen photographers and videographers.
Squeeze
Pain of discomfort in an enclosed space (sinuses, middle ears, inside a mask) caused by shrinkage of that space - occurs on descent. See reverse squeeze.
Stage Cylinder/Bottle
A reserve supply of breathing gas not carried with the diver. An example is a stage bottle at the safety decompression stop at 5 metres (15 feet) for divers who are low on air at that point.
Standard Diving Dress
Old-fashioned "hard hat" diving gear.
Subcutaneous Emphysema
Gas under the skin tissue.
Submersible Pressure Gauge - SPG
A gauge attached by a hose to a first stage and indicating remaining air pressure in a tank. Today, some dive computers support having a pressure transmitter attached to the first stage. Then the dive computer can receive the pressure transmitted and display it.
Surface Detection Aids
Equipment, such as flags, SMBs, flares, EPIRBs and whistles, carried by divers to maintain contact with dive boats or attract rescue when lost at sea.
Surface Interval
The time on the surface between dives. Divers need to track this time interval for planning decompression for the next dive.
Surface Marker Buoy (SMB)
A small inflated buoy that divers tow when underwater on drift dives to indicate their location to their boat.
Surface-Supplied Compressed Air Diving
Diving with the air continuously supplied by a compressor on the surface. See hookah.
Tank
The commonly used name for a scuba cylinder in the United States.
Technical Diving
A form of SCUBA diving that exceeds the scope of recreational diving allowing deeper and longer dives. Diver who likes lots of gadgets or even as little gadgets as possible - depending on the type of tekkie. Diving with rebreathers, different mixes of gas, lots of stage bottles, going very deep into caves and silty wrecks are part of the tekkie diver heaven. If you have a BCD with less than 6 D-rings, you are NOT a tekkie diver.
Thermocline
Sudden changes in water temperature with changing depth. They occur when warmer, lighter water forms a layer above a more dense, colder layer of water If strongly effected by currents it can be the other way round occasionally.
Time To Fly
Divers must wait approx. 24 hours after the last dive before flying to reduce the risk of decompression sickness.
Trimix
A breathing gas consisting of oxygen, helium and nitrogen. This proportions of each are changed according to the needs of the particular dive plan to help limit Oxygen toxicity and Nitrogen narcosis.
Undersuit
Used to keep a diver warm and comfortable when diving in a dry suit.
Vertigo
Dizziness brought on by the inequality of pressures in the inner ear. Vertigo (dizziness) is a sign of ear barotrauma and should not be ignored. Causes may range from minor ear squeeze to perforation of the eardrum to inner ear barotrauma.
Visibility - Viz
Visibility is the distance a diver can see underwater measured in metres or feet.
Wall Diving
Diving along the face of a vertical wall - requires good buoyancy control because there is no seabed to prevent the diver descending too deep.
Weight Belt
A heavy belt worn around the waist to help a diver descend, and stop them rising to the surface involuntarily. Usually contains a set of lead weights.
Wet Suit
A tightly fitting neoprene thermally-insulating diving suit that allows a limited volume of water inside the suit.
Wreck Diving
Diving on shipwrecks.
Y Valve
A cylinder valve with two outlets.

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