Glossary

Glossary of Diving Terms

Actual Bottom Time (ABT)

ABT is the total time in minutes from leaving the surface until the ascent is started.

Air Embolism

A blocking of the bloodstream caused by air bubbles. Air embolisms can arise if air enters the blood system through lung damage such as ruptured alveoli. Air bubbles can interrupt or completely block the flow of blood to vital organs such as the heart, lungs or brain causing severe damage or even death.

Alpha Flag

An international flag used as a signal that divers are underwater nearby.

Alternate Air Source

A device used in the place of the primary air source (primary regulator) to enable normal breathing during ascent.

Archimedes Principle

A physical theory defined by Archimides stating that any body immersed in liquid is pushed upwards by a force that is equal to the weight of the liquid displaced by the body.

Ascent Bottle

Ascent bottles are spare air cylinders kept in reserve to enable decompression stops. Also called Bail Out Bottles.

Atmospheres

Units of pressure equal to the pressure of the atmosphere at sea level.

Back Roll Entry

A technique for entering the water while on board a boat. The diver sits with his/her back to the water and rolls backwards into it.

Bail Out Bottle

Bail out bottles are spare air cylinders kept in reserve to enable decompression stops. Also known as ascent Bottles.

Bar

A shortened form of barometric pressure. Bars are units of measurement for air pressure.

BCD

The Buoyancy Control Device (Buoyancy Control Jacket) holds your air tank and gives you control over your buoyancy beneath the water and helps you to float while on the surface.

Bends

Also known as Decompression Sickness. A bend happens as a consequence of nitrogen leaving the body too quickly when ambient pressure is lowered too fast, for example coming up in an emergency ascent.

Bottom Time

The time between going below the surface of the water to the start of ascent.

Bounce Dives

Bounce Dives are dives with surfaces within very short intervals. Bounce dives occur when securing an anchor or recovering an item that has been dropped in the water.

Boyle’s Law

Under a constant temperature, the volume of a given mass of a gas is inversely proportional to the absolute pressure of it.

BSAC

A diving body - The British Sub-Aqua Club

Buddy (Dive Buddy)

A person with whom you dive. You should never dive without a buddy as they can help in times of trouble and can generally make the experience of diving more enjoyable.

Buddy Breathing

Buddy breathing can occur if one of the buddies has problems with their breathing apparatus and share the air from one regulator.

Buoyancy

This is the force that is exerted upwards against a body by a fluid when a body is immersed in it, as defind by Archimedes’ principle.

C-Card

This is a diver’s certification card, which shows your level of ability. You need yo have your diver’s certification cards so that a dive center knows that it can safely allow you to dive with them. Having said this, many certification records can be checked online nowadays.

CAGE

Stands for Cerebral Arterial Gas Embolism.

Carbon Dioxide Headache

A type of headache occurring after diving usually caused by skipping breathing and a build-up of CO2.

CESA

Controlled Emergency Swimming Ascent

Charles’ Law

The degree of change in the volume or pressure of a volume of gas is directly proportional to the fluctuation in the absolute temperature.

Chumming

A technique used to attract sharks for shark dives. The technique involves priming the dive area with blood and fish parts. Chumming is a very contentious issue: some believe it causes no harm, others believe it will teach sharks to associate humans with food.

CMAS

A diving body - Confederation Mondiale des Activitias Subaquatiques

Cylinder

The tank that holds your air.

Dalton’s Law

Dalton’s Law states that the total pressure exerted by a mix of gases is equal to the total sum of pressure for each of the different gases making up the mix: each gas acts as if it were present by itself and occupies the total volume.

Deco Mix

A mixture of gas used during decompression.

Decompression

A change from a given ambient pressure to a lower own. This always results in decreased gas pressures in the body.

Decompression Dive

Also known as a deco dive, decompression dives are where the diver surpasses the recommended limits for safe recreational diving. As a result safety stops become mandatory to ensure a safe return to the surface.

Decompression Sickness

Also known as the bends. Decompression sickness arises as a result of nitrogen leaving the body too quickly when ambient pressure is lowered too fast, for example coming up in an emergency ascent.

Decompression Stop

The time spent at a given depth, for the purpose of nitrogen off-gassing. Decompression stops are also known as safety stops, when diving within recommended safe limits.

Dehydration

Dehydration relates to a lack of water in the body. Dehydration is caused by diving without having drunk enough water, or by having consumed too much alcohol the night before diving. It is important to drink lots of water before you dive to avoid dehydration.

Depth Gauge

Apparatus that tells you how deep you are.

Dive Computer

Dive computers help you to monitor your depth and time spent underwater. A dive computer will take much of the effort out of your depth/dive time calculations.

Dive Tables

Dive tables provide guides to recreational divers to help them stay within the safe recommended limits.

Dry Suit

These are suits designed to be water tight and therefore to keep water away from the skin (unlike wetsuits). Dry suits are generally used in cold conditions or deep dives as they keep the body warm: dry suits use the air space between the body and the suit as insultaion from cold water but you can also wear layers of clothing beneath them.

Dual Regulator Systems

A dual regulator system makes use of two regulators from one tank so that a diver can still breathe underwater in the event that the primary regulator fails.

Equalizing

Equalizing (also called equalization) is the act of driving air into a space to counteract an increase in hydrostatic pressure. Equalization can be applied to spaces such as the ears or a dive mask to prevent “squeezes”.

Eustachian Tubes

The eustachian tubes are channels in the head that extend from the ear to the throat. The eustachian tube allows air to pass both to and from the middle ear in order to equalize the air pressure on each side of the eardrum.

Fathom

A unit of measurement for used to define distance in terms of water depth.

First Stage

The first stage is a part of a diver’s regulator that is attached to the air cylinders. The first stage is the first point of tank pressure reduction.

Fins

Fins also called flippers by some. Fins are worn by divers on their feet and are used to asssist divers to move underwater: they help divers to swim faster and also aid maneuverability.

Gas Saturation

The saturation of gas in the body arises when the amount of dissolved gas within tissues has reached its limit.

Halocline

Halocline is a clearly visible divider seen in water which has layers that have different salinity. Halocine actually appears as an underwater layer of mist.

Hang Tank

A hang tank is a spare air tank that is normally hung at the decompression stop location.

Heliox

Heliox is used in deep diving environments where normal oxygen mixes are unsafe. Heliox is a mix of helium and oxygen

Helium

Helium gas mixes are used for deep diving. Helium absorption does not cause the same types of narcotizing problems as nitrogen at deeper depths.

Hookah

Hookahs are to supply air underwater from the surface used in calm water shallow dives. Air is supplied from the surface to the diver via a hose.

HP Hose

The high pressure hose extends from the first stage of the regulator to the air pressure gauge.

Hydrogen

Hydrogen is the lightest gas. It is also the most abundant gas known to man. Hydrogen may become a stadard gas mix used to replace helium mixes in the cases of deep, commercial and military dives.

Hyperbaric Chamber

The hyperbaric chamber is an air-tight space that simulates ambient pressure (both depth and altitude) to treat decompression sickness.

Hypothermia

Hypothermia arises as a result id the rapid and severe cooling of the body’s core to below 35 degrees. Hypothermia can have disatsrous effects on the body, causing potentially fatl arrhythmia (irregular heart beat).

J-Valve

J-valves contain a mechanism that closes the air supply from the tank at the point that a given tank pressure is met.

Lift Bag

A lift bag is used to lift heavy or large and awkward objects to the surface. Lift bags are filled with air to make the bag and its contents positively buoyant… ergo they float upwards.

Live Aboard

This is a boat which has facilities for living upon for a given time. if you take a diving trek offshore a live aboard means that you will not have to return to dry land before the next day’s diving.

Log Book

Divers use a log book to record the details of their dives. Log entries are stamped by the company you are diving with and provide a complete record of dive history and experiences. Some dive companies or dive boats may request to see your log book before they will alklow you to dive with them.

LP Hose

The LP satnds for low pressure. The low pressure hose extends from the regulator first stage to the second stage.

Manifold

This is pipework that connects two tanks so that you can access both with a single regulator.

Mask Squeeze

You may experience a mask squeeze if you do not equalize the air in your mask during a rapid descent. The most common result of a mask squeeze can be bloodshot eyes.

Mixed Gas

This is a mixture of non-air gases.

Multilevel Diving

Spending a period of time at several depths on a single dive - pretty much every dive you do.

NAUI

An acronym standing for the National Association of Underwater Instructors.

Nitrogen

Nitrogen is one of the gases that makes up air. Under pressure and at certain depths, the absorption of nitrogen in the body can cause nitrogen narcosis. If decompression rules are not adhered to nitrogen in the tissues can cause decompression sickness.

Nitrogen Narcosis

Nitrogen narcosis occurs as a result of increased nitrogen partial pressure at depths between 20 and 40 meters. Between these depths, nitrogen dissolves into the neurons impeding signal transmission giving the diver a narcotic sensation in the process. Decision making can be impaired and, combined with the stronger effects the deeper you go, can result in death.

Nitrox

Mixtures of oxygen and nitrogen that have less than the 78% of nitrogen that comprises the air that we breathe on land.

Octopus

The octopus is also known as an occy and is the reserve second stage regulator.

Offgassing

The transferring of dissolved gases out of the body.

Oxygen

Oxygen is the gas required for all life on our planet. Oxygen accounts for 21% of the gases that comprise to form the air we breathe on land.

Oxygen Toxicity

Oxygen toxicity occurs when we absorb too much oxygen. At around 66 meters, regular compressed air can lead to over-absorption of oxygen, which can in turn lead to some nasty effects, one of which can be seizures.

PADI

PADI is an acronym meaning the Professional Association of Diving Instructors.

Partial Pressure

Partial pressure is the pressure that is exerted by a single gas element within a mixture of gases.

Pneumothorax

Pneumothorax is caused by the presence of air in the pleural cavity around the lungs. This can result in a full collapse of a lung leading to severe breathing difficulties.

PSI

An acronym meaning pounds per square inch. PSI is a common measurement of air pressure.

Pulmonary Barotrauma

Pulmonary barotrauma is a rupturing of the surface of the lung caused by increased internal pressure. This will occur if air inside the lung is not exhaled during ascent: air that remains in the lungs expands during ascent.

Purge Valve

The purge valve enables a diver to clear a mask or regulator without actually removing the mask from the face or the regulator from the mouth.

Rebreather

Rebreathers are breathing apparatus that recycle the air a diver breathes, removing carbon dioxide and adding oxygen in its place.

Regulator

The regulator reduces the prssure of the air in the tank to an ambient pressure so that you can inhale it easily.

Residual Nitrogen

Residual nitrogen is the amount of nitrogen that ramains in your tissues after you have surfaced from a dive. Residual nitrogen always has to be factored into subsequent dives.

Residual Nitrogen Time

This is also known as RNT. RNT is the time it takes to for nitrogen to disperse after a dive.

Reverse Squeeze

A reverse squeeze causes pain or unease in enclosed spaces such as inside your mask, in your ear or in your sinuses as you ascend from a dive.

Safety Stop

Your safety stop is usually a 5 minute wait between 3 to 5 meters so that you can offgas as a precautionary measure.

Salinity

The amount of salt in a solution.

Saturation

The degree to which gases are dissolved into body tissues. A state of full saturation arises when the pressure of the gas that is dissolved into the body equals the ambient pressure of the gas itself.

Scooter

Scooters are Diver Propulsion Vehicle (DPVs). DPVs are machines that propel divers when they are underwater.

SCUBA

SCUBA stands for Self-Contained Underwater Breathing Apparatus.

Second Stage

The part of the regulator that you put into your mouth so that you can breathe.

Sinus Squeeze

Sinus sqeezes arise if the diver cannot equalize the air space in the sinuses during a rapid ascent. Sinus squeeze can be quite painful and cause nose bleeds.

Sinuses

Sinuses are air spaces in the head that are in touch with ambient pressure via openings in the nasal passage.

Sinusitis

An infection or inflammation of the sinuses.

Skin Diving

Another term meaning free diving i.e. diving without breathing equipment.

Snorkel

A small plastic tube that is used to breathe through whilst swimming face down on the surface.

Solo Diving

Diving without a buddy. Not a recommended practice.

Squeeze

Discomfort or actual pain caused by the shrinking of an enclosed air space (such as inside a mask, in the ears or in the sinuses). Squeezes can occur on descent and are the opposite of reverse squeezes, which occur on ascent.

Surface Interval

The time spent on the surface between consecutive dives.

TDT

An acronym standing for Total Dive Time.

Thermocline

Thermoclines are sudden variances in the temperature of water as you change depths. Thermoclines arise when warmer water creates a layer above colder water. Sometimes currents can force colder water upwards forming a layer above warmer water.

Trimix

A mixture of three gases (most commonly helium, oxygen and nitrogen). Trimix is usually reserved for deep diving.

Vertigo

A sensation of dizziness or imbalance caused by inequal pressure in the middle ear. Vertigo can be caused by an ear squeeze, ear drum perforation or more severely ear barotrauma.

Visibility

Visibility is used to describe the distance a diver can see underwater and is most commonly defined in terms of meters.

The Wheel

The Wheel is a dive table produced by PADI to plan multilevel dives.