Central Scotland Dive Club

Diving jargon buster

An Alphabetical Glossary of Diving Terminology

Possibly everything you ever wanted to know about diving jargon,
but were afraid to ask (again!)

Either click on a letter below to go straight to that group, or simply scroll down and browse through the lot – you never know what you might learn!
As you scroll down, an arrow will appear at the bottom right – click on it when you want to return to the top of the page.

Note: The information displayed here is for convenience and quick reference only, it is not a substitute for comprehensive research, should that be what you require.

If there's anything you want added to the list, let us know.


AAS (Alternative Air Source): Typically, this would be your 'octopus' – the spare second stage of your regulator. However, a safer AAS is to have a separate pony cylinder with its own self-contained regulator. Should you have a problem with your main air source, you then have a back-up.

ABT (Actual Bottom Time): The total elapsed time in minutes, from leaving the surface until ascent is initiated.

AED (Automated External Defibrillator): See Defibrillator below.

AGE (Arterial Gas Embolism): Caused when gas bubbles in the arterial bloodstream create a blockage. See CAGE.

ATA: Atmosphere Absolute; 1 ata is the atmospheric pressure at sea level; it is measured with a barometer.

ATM: Abbreviation for Atmosphere.

A-Clamp: The part of a regulator's First Stage that attaches it to the cylinder. Note: DIN-type First Stages attach differently.

A-Flag: (See Alpha Flag below:)

A-Frames: (Finnart Ocean Terminal) A popular shore-based dive training site on Loch Long. Location Map

Absolute Pressure: Atmospheric Pressure and Water Pressure added together.

Air: The gas that we normally breathe is a mixture of approximately 21% Oxygen, 78% Nitrogen and 1% other gases. (Precise breakdown here.)

Air Pressure: The force per unit area exerted by the weight of air; at sea level the air pressure is 1 bar. Air pressure decreases with altitude.

Air Spaces: In the human body, these are principally the lungs, ears and sinuses; which are important in diving because as the ambient pressure changes, the pressure in the air spaces is affected and needs to be equalised to avoid consequent discomfort, pain or injury. The air space in a diver's mask is also affected and it too requires to be equalised to avoid Mask Squeeze. Similarly, when diving in a drysuit, air needs to be injected to compensate for Suit Squeeze.

Algorithm: A set of equations incorporated into diving computers in order to compute nitrogen uptake and elimination from changes in depth and elapsed time.

Alternobaric Vertigo: Dizziness brought on by the inequality of pressures in the middle ear.

Altitude: The height above sea level, which is important to the diver in two ways:
Firstly, after a dive your body needs to safely release the extra nitrogen which it has absorbed. This 'off-gassing' is affected by ambient pressure, therefore, any increase in altitude after a dive may have consequences. Typically, flying after a diving holiday is the most likely scenario but, in some mountainous locations, even the drive home may involve a significant change in altitude. Observe your computer and dive table's 'no fly' times.
Secondly, although most diving is done at sea level, any inland lake you are planning to dive may be at altitude. If so, ensure your dive computer is adjusted accordingly, or that you are using appropriate dive tables for that altitude.

Alpha Flag: The international flag flown by dive boats to indicate they have divers below.

Alveolus (plural Alveoli): Air sac in the lung at the terminus of a bronchus where oxygen and carbon dioxide transfer occurs.

Ambient Pressure: The surrounding pressure. On land, it comes from the weight of the atmosphere (see Air Pressure); at any given depth, it comes from the weight of the water plus the weight of the atmosphere. It is 1 bar at the surface (1 atmosphere), and increases by 1 bar for every 10 metres depth.

Anaemia: Any reduction in the oxygen carrying capacity of the red blood cells.

Anoxia: The lack of oxygen in the bloodstream.

Anticoagulants: Medications that reduce the clotting ability of the blood; particularly dangerous to divers due to barotrauma of air-filled body cavities.

Aqualung: Breathing equipment for underwater diving. Invented by Jaques-Yves Cousteau and Emile Gagnan in 1943, it consists of a high-pressure cylinder containing breathing gas, together with a regulator to deliver the gas at ambient pressure to the diver.

Argon: An inert gas that makes up less than 1% of air.

Ascent Rate: The rate at which you ascend at the end of your dive. The SAA's recommended ascent rate is a maximum of 10 metres per minute.


BC: Buoyancy Compensator (see BCD below)

BCD: Buoyancy Compensation Device (aka BC or "Stab Jacket") is an inflatable vest worn by the diver that can be automatically or orally inflated to help control buoyancy. Note: Divers with drysuits may prefer to control their buoyancy underwater by adjusting the air in their suit. Divers with twin tanks may choose a different type of BCD known as a "wing".

BSAC: British Sub-Aqua Association

Backscatter: The water you dive in contains minute particles in suspension. Even in apparently good visibility the backscatter may spoil underwater photographs, as the light from your flash or strobe reflects right back into your camera's lens. Imagine driving at night in the snow then turning your headlamps full-on and you'll get the general idea. Try getting closer to your subject; turning the flash off; or (if you have a separate strobe) positioning it so as not to reflect directly into the lens.

Backwards Roll Entry: An easy method of entering the water from the side of a RIB, or some other boats. Close your drysuit valve; inflate your BCD; sit on the edge of the boat, with your back to the water; with one hand, hold your regulator and mask in place, and the rest of your loose kit with the other; then roll backwards. When you surface, check you're okay and signal accordingly to the boat and your buddy.

Bar: Measurement of pressure

Basic Equipment: Mask, fins and snorkel.

Barometric Pressure: The same as Atmospheric Pressure.

Barotrauma: Injury caused by pressure.

Beaufort Scale: A method for describing wind strength, created by Sir Francis Beaufort in 1805. Diving should only safely be undertaken in Force 4 or less. Read more here.

Bends: A form of decompression illness caused by dissolved nitrogen leaving the tissues too quickly on ascent. It is manifested by pain, usually in the limbs and joints; 'the bends' is sometimes used to signify any manifestation of decompression illness.

Boot: Most dive cylinders have a black plastic/rubber boot fitted which protects the base and allows the cylinder to stand upright when required. When unattended, for safety, cylinders should lie flat on the ground.

Bottle: Another name for cylinder/tank.

Bottom Time: The time between leaving the surface to the beginning of ascent. In multi-level diving, the time between descending below the surface and beginning the safety stop. (Other definitions may apply depending on the specific type of diving).

Bounce Dive: A descent directly to the maximum depth of the dive, followed by an ascent directly to the surface. (Observing ascent rates and safety stops.)

Boyle's Law: At a fixed temperature and for a fixed mass of gas, pressure x volume is a constant value; as pressure doubles, volume halves.

Bubble-check: An additiional check made with your buddy just below the surface at the start of a dive, to ensure there is no air leaking from your equipment. It's also an opportunity to check that no equipment has come loose upon entry – particularly from a boat.

Buddy: Your dive partner whom you have a mutual responsibility for during your dive.

Buddy-breathing: Technique used by two divers when sharing a single regulator.

Buddy Check: Undertaken before every dive to ensure that your buddy's (and your own) equipment is in place and working correctly.

Buddy Line: Piece of line used by divers to maintain contact with each other. May be useful in poor visibility, or on a drift dive.

Bühlmann: Professor Albert A Bühlmann. A Swiss physician who was principally responsible for a number of important contributions to decompression science at the Laboratory of Hyperbaric Physiology at the University Hospital in Zürich, Switzerland. Read more here.

Buoyancy: The tendency of an object to float or sink when placed in a liquid; objects that float are positively bouyant, those that sink are negatively buoyant and those that stay where placed are neutrally buoyant.

Buoyancy Compensator (BC/BCD): (See BCD above.)

Buoyancy Control: Essential for successful and safe diving, establishing neutral buoyancy at any depth is of fundamental importance.

Buoyant Ascent: Where a diver makes a faster than normal ascent to the surface as a result of over-buoyancy. If unintentional, it may be as a consequence of being underweighted at the end of the dive. For example: as a dive progresses the cylinder becomes lighter as the air it contained is consumed. It may also be as a result of inadequate buoyancy control, or an incorrectly adjusted drysuit dump valve. Any diver who has made a rapid ascent, intentional or otherwise, should be aware of, or monitored for, signs or symptoms of DCI.


CAGE (Cerebral Arterial Gas Embolism): The condition characterised by bubble(s) of air from a ruptured lung segment under pressure; the bubbles enter the pulmonary circulation and travel to the arterial circulation, where they may cause a stroke.

CESA (Controlled Emergency Swimming Ascent): A technique for ascending to the surface in an emergency, whilst continually exhaling to ensure the release of the expanding air in your lungs.

CMAS: Confédération Mondiale des Activités Subaquatiques. The World Underwater Federation.

CPR: Cardiopulmonary Resuscitation. An emergency procedure performed on a casualty suffering from cardiac arrest. It consists of chest compressions and artificial respiration, to maintain a flow of oxygenated blood to a casualty's brain and heart until full medical assistance can be administered. Everyone (that includes you!) should learn how to perform this procedure

Cam Band: The strap and locking mechanism which attaches the dive cylinder to the BCD. Some BCD's use a "Cinch Strap" with a lever-action metal clip.

Carabiner/Karabiner: A metal loop with a spring-loaded gate, commonly used by divers for connecting things, or attaching accessories to D-rings on their BCD.

Carbon Dioxide: CO2 is an odourless, tasteless gas that is a by-product of metabolism. It is a component of the air we exhale and is important for its role in the control of respiration. The level of CO2 in the human body is what triggers our impulse to breathe. Whilst diving (or snorkelling) it is essential to breathe properly and fully, to ensure there is no build-up of CO2 in your lungs to cause toxicity.

Carbon Dioxide Toxicity: Problems resulting from the build-up of CO2 in the blood; they may range from headache and shortness of breath to sudden blackout and possibly death in extreme circumstances.

Carbon Monoxide: CO is an odourless tasteless, highly poisonous gas given off by incomplete combustion of hydrocarbon fuels. It may be present in contaminated air fills from faulty compressors.

Carbon Monoxide Toxicity: Illness from inhaling excess CO; problems may range from headache to unconsciouness and death

Channel 16: On VHF radio this is the international marine distress frequency which is monitored continuously by the coastguard. If you are intending to dive from a boat at sea, make sure you have a functioning VHF radio – just in case.

Charles' Law: At constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature on the absolute temperature scale (i.e. the gas expands as the temperature increases).

Chokes: A form of decompression sickness caused by enough bubbles entering the lungs to interfere with gas exchange; manifested by shortness of breath. It can be fatal.

Clearing Techniques: Ways in which to equalise the pressure in the eustachian tubes with the ambient pressure while descending on a dive.

Closed Circuit Scuba: Apparatus designed to allow divers to re-breathe exhaled air after removal of CO2 and addition of supplemental O2. In contrast to the more common 'open circuit scuba', closed circuit is noiseless and produces no bubbles.

Club Diver: A level of diving accomplishment in the SAA. A Club Diver is a qualified open water diver and may dive with others certified as club divers or above. Maximum depth during training: 35 metres. Maximum depth after qualification: 50 metres.

Coastguard: HM Coastguard is a section of the Maritime and Coastguard Agency responsible for the initiation and co-ordination of all civilian maritime Search and Rescue (SAR) within the UK. Link

Compressor: A machine that compresses or pressurises air. For scuba purposes, air is compressed from the atmosperic level ( 1 Bar) to the working pressure of the cylinder, usually 200 to 300 Bar.

Compartment: A theoretical division of the body with an arbitrarily assigned half-time for nitrogen uptake and elimination. In designing decompression tables, the body is divided into a number of "compartments" for purposes of making calculations.

Compass: A device to aid navigation, with a magnetised needle which points to magnetic North. Some dive computers also incorporate a digital compass. Not always reliable, as the compass needle can be deflected by local conditions e.g., iron and steel wrecks, geological anomalies etc.

Compressed Neoprene: One of the materials which may be used in drysuit manufacture. The nature of standard neoprene is that the bubbles it contains compress at depth, thereby reducing its insulating properties as well as its buoyancy. Compressed (or crushed) neoprene goes some way to reducing this variability by it being pre-compressed in its manufacturing process.

Console: A device used to hold diving gauges. Some may contain a computer, compass and contents gauge.

Contents Gauge: A device which measures the pressure, and hence the contents, of a diving cylinder. Also known as a Submersible Pressure Gauge or SPG. Regular monitoring of these gauges enables you to be aware of your air consumption (and your buddy's) in relation to your dive plan, to ensure a safe return with around 50 bar as a safety margin at the end of your dive.

Co-ordinates: (See Grid Reference below.)

Crossover: (See Qualifications Crossover below.)

Cuff Dump: A type of valve which enables a diver to control the release of air within his drysuit. Cuff dumps have commonly been superseded by larger shoulder dump valves, but can be installed by those who prefer them.

Current: The movement of water can be caused by various factors and it is important for divers to be fully aware of local conditions. Some of the influencing factors include: tide, wind, temperature, rainfall etc.

Cylinder: A term used to define the container for the compressed air. May also be referred to as a bottle or tank.


DAN (Divers Alert Network): An organisation devoted to assisting divers in need. It conducts research into recreational diving safety and publishes the results to benefit divers worldwide. Link

DCI: (See Decompression Illness below.)

DCS (Decompression Sickness): (See Decompression Illness below.)

DIN Valve: A type of threaded fitting (as opposed to an A-Clamp) for connecting the first stage of a regulator to a cylinder. The two systems are themselves incompatible, but adaptors are available should they be required.

DSMB (Delayed Surface Marker Buoy): An inflatable tube (sometimes called a "sausage" or "blob") which a diver can inflate by various methods at the end of a dive, prior to ascending. This gives the boat crew a clear advance indicator of where the diver should surface. Attached to a reel, it also enables the diver in conducting a controlled ascent. However, the nature of the apparatus and its deployment also introduces an element of risk through potential entanglement and/or rapid ascent, therefore, continuous training in its use is advisable.

DV: (See Demand Valve below.)

Dalton's Law: The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume.

Deco: (See Decompression below.)

Decompression: Any change from one ambient pressure to a lower one always results in a reduction of gas pressures within the body.

Decompression Ceiling: On a dive computer, the decompression ceiling is indicated as the shallowest point to which you should ascend for safe decompression. The decompression floor is the deepest point at which your decompression stop time will not increase.

Decompression Dive: Actually, any dive where the diver is exposed to a higher pressure than when the dive began; the decompression occurs as the diver ascends. Typically, divers would refer to a Decompression Dive as one which extended beyond their computer's or dive table's Bottom Time, thereby requiring mandatory decompression stops upon ascending.

Decompression Floor: On a dive computer, the decompression floor is the deepest point at which your decompression stop time will not increase. The decompression ceiling is indicated as the shallowest point to which you should ascend for safe decompression.

Decompression Illness (DCI): A term to encompass all bubble-related problems arising from decompression, including both Decompression Sickness and Arterial Gas Embolism.

Decompression Sickness (DCS): (See Decompression Illness above.)

Decompression Stop: On ascent from a dive, a specified time spent at a specific depth, for the purposes of nitrogen off-gassing; when not mandatory, it is called a safety stop.

Decompression Tables: A set of guides used by divers to plan dive decompression requirements.

DeeP Stop System: The latest development in decompression tables introduced by the SAA.

Defibrillator (AED): A portable electronic device that automatically diagnoses the potentially life threatening cardiac arrhythmias of ventricular fibrillation and ventricular tachycardia in a casualty, and is able to treat them through defibrillation, the application of electrical therapy which stops the arrhythmia, allowing the heart to re-establish an effective rhythm.

Dehydration: A condition where the water content of the body is reduced. It is caused by immersion, alcohol, medications, excessive loss of fluids from vomiting and diarrhoea, or decreased intake of fluids.

Demand Valve (DV): The part of the regulator (Second Stage) that supplies the air to the diver's mouth. It delivers air at ambient pressure, regardless of depth.

Depth Gauge: A gauge used to indicate the depth of the diver at any given time.

Dive Computer (PDC): Personal Dive Computer carried by the diver that constantly measures water pressure (hence depth) and time based on a pre-programmed algorithm. The computer calculates tissue nitrogen and provides a continuous readout of the dive profile, including: depth, elapsed time of dive, and duration at current depth before obligatory decompression stops become necessary.

Dive Flag: See Alpha Flag above. May also be a collapsible flag carried by the diver for display on the surface at the end of a dive. A flag (or a DSMB) significantly increases a diver's visibility on the surface, making it easier for observers on the dive boat.

Dive Leader: A level of diving accomplishment in the SAA. A Dive Leader is someone who has had additional experience in both practical and theory and has proven their ability to lead divers underwater. A dive leader may dive with any certified grade of diver.

Dive Marshall: The individual tasked with various responsibilities on a given dive, including pre-dive risk assessment, logging divers details, tank contents, entry and exit times etc.

Dive Officer: An experienced and qualified SAA club official with the responsibility for ensuring the club's safe diving practices and training procedures.

Dive Profile: A graphical representation of a dive, showing depth and duration. Read more here.

Dive Slate: This, and a pencil, provides a simple means of being able to write notes before or during a dive, either for reference underwater, or for communicating with a buddy where dive signals would be insufficient. It's also possible to send a message to the surface via DSMB to communicate with those onboard the dive boat. It would be better for this likelihood to be pre-arranged for it not to go un-noticed.

Dive Supervisor: A level of diving accomplishment in the SAA. A Dive Supervisor is an experienced and responsible diver with a proven ability to organise and lead club expeditions.

Dive Tables: (See Decompression Tables above.)

Diving Signals: Since, in normal scuba diving, verbal communication is not possible, a system of clearly-defined and understood signals is important for effective communication underwater.

Dr ABC: The SAA System and mnemonic for divers to better understand the needs and objectives of effectively responding to emergency resuscitation scenarios.
D = Danger: Recognising the risks and minimising them before taking action.
R = Response: Assessing whether the casualty is conscious.
A = Airway: Ensure the casualty's airway is open.
B = Breathing: Check whether the casualty is breathing. Administer rescue breaths.
C = Chest Compressions: These should be done at a rate of 100 compressions per minute, but for every 30 chest compressions give two rescue breaths.
Note: Refer to the complete Dr ABC notes for full details.

Drift Dive: This kind of dive enables divers to deliberately take advantage of the prevailing current underwater and be transported from the entry point to the exit point. However, complicating factors increase the risk of diver separation and a drift dive should only be undertaken by experienced divers (and boat crew). An SMB (Surface Marker Buoy) may be deployed at the start of the dive to make it easy for the boat crew monitoring progress and location.

Drysuit: A diving suit designed to keep the diver dry (excluding head and hands). It is the preferred kind of suit for divers in the UK and enables diving to be undertaken all year round. Drysuits are generally of three different types of construction, neoprene, compressed neoprene and laminated material. Each has its advantages, and advocates.

Dump Valve: A device for releasing air under pressure from a drysuit or a BCD. Drysuit dump valves (typically on the left shoulder) allow for adjustment. Rotated clockwise they are fully closed; rotated fully anti-clockwise they are completely open, and click stops in between allow a diver to adjust the valve to dump automatically in order to maintain neutral buoyancy at any given depth. This, however, is a fine balance.


EAD (Equivalent Air Depth): The equivalent air depth, for a given nitrox mix and depth, is the depth of a dive when breathing air that would have the same partial pressure of nitrogen. So, for example, a gas mix containing 32% oxygen (EAN32) being used at 29 metres has an EAD of 24 metres.

EANx (Enriched Air Nitrox): (See Nitrox below.)

EFR (Emergency First Response): The C-Divers EFR Course provided individuals with an introduction to delivering Primary Care (CPR), Secondary Care (First Aid) and in the use of an Automated External Defibrillator (AED) – knowledge and skills everyone should aim to possess, not just divers. This course has now been superseded by a dedicated Sub Aqua Association course.

Ebb Tide: When the tide is going out.

Eddy: An eddy may be formed when the "normal" flow of a current is influenced by other factors, such as tidal changes and underwater contours, which at times may lead to the current in certain areas flowing in unexpected directions. In diving it's important to be aware of local conditions and how they might influence your dive.

Ear Clearing: The technique of equalising pressure inside the ears to that of the surrounding water pressure. This should be done every few metres when descending. Common methods include: pinching the nose and gently blowing internally until your ears "pop"; wriggling your jaw side to side; and swallowing – or a combination of this. Find which is most effective for you.

Elementary Diver: A level of diving accomplishment in the SAA. An Elementary Diver is fully pool trained but inexperienced in open water. When diving in open water, an elementary diver must be accompanied by a qualified Dive Leader or above. Maximum depth during training: 10 metres. Maximum depth after qualification: 10 metres.

Embolism: (See CAGE above.)

Entonox: A 50:50 mixture of nitrous oxide and oxygen, which is routinely carried by ambulance crews. It is potentially dangerous to administer to divers.

Equalisation: The process of adjusting the pressure within the air spaces to balance the ambient pressure. For example, clearing your ears; exhaling a little air through your nose into your mask to compensate for mask squeeze; and inflating your drysuit to offset suit squeeze. Your sinuses should be able to equalise automatically – so, do not dive when you have the cold, or are congested, as this automatic equalisation will be impared or impossible.

Eustachian Tubes: Short tubes which run from the throat to the middle ear which enable divers to equalise the pressure on either side of their ear drums.


Fin Ladder: A special kind of ladder on a boat to enable divers to climb out of the water more easily. A central shaft with rungs projecting horizontally from each side enables divers to climb without first having to remove their fins.

Fins: Equipment divers strap to their feet to improve propulsion in the water

Finning: Various techniques divers use in order to propel or orientate themselves in the water.

First Stage: The part of the regulator that attaches to the valve on the cylinder. It reduces the pressure of the gas in the cylinder for delivery at ambient pressure through the second stage. From other outlets, it delivers high pressure to the contents gauge and low pressure through the BCD and drysuit inflator hoses. A further hose delivers air to the diver's alternative second stage, the octopus.

Flood Tide: When the tide is coming in.

Freeflow: A situation where a regulator delivers a continuous flow of air, rather than on demand. Depending upon the extent and duration of the freeflow, if it can't be halted, the dive should be aborted and a controlled, safe ascent performed. Freeflows can happen for various reasons and If the cause of the freeflow is not obvious, or easily corrected, the regulator should be checked by an approved service centre before using it again.


GPS (Global Positioning System): Also known as Satellite Navigation or "Sat Nav", is a satellite-based system of navigation which allows for precise locations (or routes to them) to be ascertained. This may be useful in locating dive sites, in particular wrecks. It also has a safety benefit in that your own accurate location can be given if the need arises.

Gas Embolism: (See CAGE above.)

Gas Laws: Laws that predict how gases will behave with changes in pressure, temperature and volume. See Charles' Law, Dalton's Law; and Henry's Law.

Giant Step Entry: One of the methods of gaining entry to water at the start of a dive. After performing a buddy-check and ensuring your BCD is inflated (and your drysuit valve is closed), manoeuvre to the edge of the boat, or platform; ensure your entry point is clear, then, holding your mask and regulator in position with one hand and the rest of your gear with the other, take a large step forward towards the water and clear of the edge. Keeping your legs separated will ensure maximum water resistance, so you submerge as little as possible. Once you have established everything is fine, signal OK to the boat and your buddy.

Go for Gold: An SAA mnemonic to ensure divers using the SAA Dive Tables calculate exit and re-entry Residual Groups accurately.

Goody Bag: Generally a large mesh bag that divers can collect "treasure" in, whether it's scallops or something less edible, but more interesting.

Grid Reference: This is a series of number and/or letters used to define the location of a point on a map. Useful for locating dive sites for accuracy, or in the absence of other landmarks. More information here.


HP (High Pressure): A port (or maybe two) on your regulator's first stage may be marked HP. One of these would be used for the high pressure hose to your contents gauge. Some dive computers can also monitor your tank's contents, and if so, a little sensor/transmitter would be screwed into the other HP port to transmit a signal to your computer.

HPNS: (See High Pressure Nervous Syndrome below.)

Haldane: Dr John Scott Haldane, a British scientist who created the first diving tables, based on his research for the Royal Navy. He theorised that nitrogen is taken up and given off in exponentially during a dive, and that there is some safe ratio of pressure change for ascent – originally 2:1.

Half-time: Half the time it takes for a dissolved gas (such as nitrogen) in a tissue to equilibrate to a new pressure. Theoretical tissue half times are used in designing dive tables and alogrithms for dive computers.

Halocline: Water of differing salinity, in conjunction with temperature, tends to lie in layers as a result of the varying densities. Where the layers meet results in a noticeable blurry effect. (See also Thermocline.)

Heliox: A mixture of helium and oxygen which is used in very deep diving.

Helium: The second lightest gas does not cause the same problems of narcosis, as seen with nitrogen. Because of this, it is used for very deep diving.

Henry's Law: The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coefficient of the gas in the liquid.

High Pressure Nervous Syndrome HPNS: Convulsions or seizure-like activity arising from high gas pressure at depth, especially with helium.

Hydrostatic Test: In the UK, diving cylinders are required to be hydrostatically tested every five years (with a visual inspection every two and a half years).

Hyperbaric Chamber: An airtight chamber that can simulate the ambient pressure at altitude or depth and is used for the treatment of decompression illness. It is also referred to as a Recompression Chamber.

Hypercapnia: A higher than normal percentage of CO2 level in the blood. Also hypercarbia.

Hyperthermia: A body temperature higher than normal. It is less common than hypothermia, but may be as a result of overheating in a drysuit, or wetsuit.

Hyperventilation: A condition where an individual breathes too rapidly and has a lowered CO2 level in the bloodstream. It is commonly associated with snorkel divers, or divers who may panic.

Hypothermia: A body temperature colder than normal (37°C /95°F ). Severe problems start to manifest when body temperature drops to around 35°C/95°C.

Hypoxia: Lower than normal pO2 level in the blood; insufficient oxyen in the blood


IANTD: International Association of Nitrox & Technical Divers.

Incident Pit: A phrase used to describe escalating situations.

Inflatable: A small craft suitable for diving from. (See also RIB below.)

Inner Ear: That portion of the ear in the petrous bone that has to do with hearing organs and balance

Intermediate Hose: The hose between the first stage and the second stage.

Intermediate Pressure: The pressure of air between the first and second stages of a regulator; usually around 10–12 bar above the ambient pressure.


Jon-line: A short line with clips at either end which can be used to fasten a diver to something, perhaps to a shot line during decompression stops on ascent. (See also Buddy Line.)


KY Jelly: A water-soluble "personal" lubricant which many divers use to enable them to ease into the wrist and neck seals of their drysuit. By design, it is non-damaging to latex or neoprene. Other divers may have their own preferences, such as talc.

Karabiner: (See Carabiner above.)

Kelp: A type of seaweed commonly found in shallow waters around the coast.

Kevlar: A light, strong, abrasion resistant synthetic fibre that may be used to reinforce areas of dive gear which are subject to high wear; such as gloves, or drysuit knee pads.

Knot: A method for securing a line or rope. Some divers may prefer to fasten certain items of kit with knots, which can be cut in an emergency (such as entanglement), rather than with metal fastenings – which can't.
Also, a Knot is a boat's unit of speed: 1 knot = 1 Nautical Mile per hour. This is equal to 1.852 kilometres per hour, and approximately 1.151 miles per hour.


LP: Low Pressure. The LP ports on the first stage of a regulator are the ones to which the hoses for the second stage, octopus, BCD and drysuit inflator would be connected.

Latex: A natural rubber that is commonly used for the watertight wrist and neck seals on drysuits. Some divers prefer neoprene seals, which are warmer and more robust.

Latitude: The measurement in degrees of a point north or south of the equator (which is 0°). Used in conjunction with Longitude it enables the location of any point on the earth's surface to be described. This is particularly important at sea. Read more here.

Lead: The metal which is commonly used to provide divers with the weight to establish neutral buoyancy. It is generally available in shaped blocks which can be attached to a belt, or as shot bags. Since lead is poisonous, contact with the bare metal is best avoided.

Log Book: A book in which a diver can log the details of every dive which has been undertaken. This provides a dive history and is a record of the diver's experience.

Longitude: The measurement in degrees of a point east or west of the Greenwich Meridian (which is 0°). Used in conjunction with Latitude it enables the location of any point on the earth's surface to be described. This is particularly important at sea. Read more here.

Lift Bag: A bag designed to help lift heavy objects to the surface. It is inflated underwater by the diver and, if it provides sufficient lift, will carry the object to the surface.

Liveaboard: A dive boat which also provides accommodation for sleeping and eating. Apart from convenience, an advantage of liveaboards includes the fact that more distant (and less-dived) locations now become accessible.


MOD (Maximum Operating Depth): When diving with Enriched Air Nitrox, to reduce the risk of oxygen toxicity, each blend has its own maximum operating depth. Observing the MOD ensures that the oxygen partial pressure will not excede the maximum safe level of 1.4 bar. The SAA MOD's for standard nitrox mixes are as follows: EAN28: 40m; EAN32: 33m; EAN36: 28m; EAN40: 25m

msw: Metres of Seawater. This abbreviation is more likely to be encountered in scientific or technical documents.

Manifold: Divers using twin cylinders may use a manifold for connecting their valves (and contents). Other divers may prefer to have separate sets of regulators for each.

Mask: A diving mask enables you to see underwater. For safety, it should be fitted with tempered glass.The body of most masks these days is made from silicon, which should be light, flexible and comfortable. It should not leak, so when buying a mask try on lots to identify which ones fit best. If you require glasses, choosing a mask for which prescription lenses are available is a good idea. In a standard diving mask, the nose should be enclosed. This enables the pressure in the air space to be equalised via the nose, as the ambient pressure increases during descent.

Mask Clearing: This is an essential skill for all divers to learn and practise. During a dive it is possible, for various reasons, for your mask to leak or even flood. The solution is easy: tilt your head back, and whilst pressing the top of your mask against your forehead, exhale slowly through your nose into your mask. You will find that the air displaces any water that was present. Once you become adept at this skill, you will do it without thinking. You will also be comfortable with removing your mask completely, refitting it, then clearing it. Practising these skills in the pool, before progressing into open water is a real confidence builder for the beginner.

Mask Squeeze: This occurs when descending during a dive because of the increasing ambient pressure of the water surrounding you. It is easily eliminated by exhaling a little, through your nose into the mask, until the pressure is equalised.

Master Diver: A level of diving accomplishment in the SAA. A Master Diver is a diver who has, through nationally conducted examinations, proved to have gained a higher than average level of knowledge and ability, and is able to organise divers to achieve major tasks and projects underwater.

Mayday: This is the internationally recognised distress signal which a boat will broadcast over VHF radio Channel 16 in the event of "grave and imminent danger". Channel 16 is permanently monitored for such incidents. (See also Pan-Pan below.)

Micro-bubbles: These are formed in the venous bloodstream during most normal dives. They become filtered-out by the lungs, where their gas dissolves into the blood and gets exhaled in breathing.

Middle Ear: The air-containing space of the ear, which is affected by changes in ambient pressure. The pressure can be equalised via the Eustachian Tubes during ear clearing procedures.


NAUI (National Association of Underwater Instructors): A United States-based scuba diver training organisation.

NDL (No Decompression Limit): Or No Stop Time is the interval that a scuba diver may theoretically spend at a given depth, without having to perform decompression stops.

NOAA: In the United States, this is their National Oceanic and Atmospheric Association.

Nautical Mile: A standard marine measurement of distance. 1 Nautical Mile per hour = 1 knot – a boat's unit of speed. This is equal to 1.852 kilometres per hour, and approximately 1.151 miles per hour.

Navigation: The methodical process of getting from one specific location to another. A diver will likely use a compass for direction and may count finstrokes over time to estimate distance. Knowledge or awareness of currents, local conditions and underwater contours or features may also be used for reference.

Neap Tide: Tides are largely governed by the gravitational effects of the moon and the sun on the earth's oceans. Neap tides occur when the moon and sun are most out of alignment with the earth i.e., at the moon's first quarter and third quarter. This results in the lowest high tides and the highest low tides. (See also Spring Tide below.)

Neoprene: A type of synthetic rubber from which dive suits are commonly made.

Neutral Buoyancy: A state of buoyancy which divers strive to achieve throughout a dive.

Nitrogen: An inert gas that makes up 78% of air. Nitrogen is inert in that it does not enter into any chemical reaction in the body, but it can cause problems under pressure. See Decompression Illness above, and Nitrogen Narcosis below.

Nitrogen Narcosis: A depressed mental state, similar to alcohol intoxication, as a result of high nitrogen pressure. It usually does not become apparent when diving on compressed air until below 30 metres. Symptoms may range from confusion or drowsiness to coma. It has also been referred to as "raptures of the deep" and "the Martini effect". The condition disappears on ascending to shallower depths.

Nitrox: Essentially, any mixture of nitrogen and oxygen, however, in Enriched Air Nitrox for diving, the higher oxygen content would typically be around 28%, 32%, 36% or 40% depending upon the depth that was planned for any particular dive. Each cylinder should be analysed to determine its precise O2 content. Nitrox reduces the divers exposure to nitrogen, but the increased oxygen content introduces its own risks. (See Oxygen toxicity below.)

No-fly Time: As a safety precaution, dive tables and dive computers provide information on how long you must wait after your last dive, before it is considered safe for you to fly.


OWD: (See Open Water Diver below.)

O-Ring: A small but vital piece of equipment, the O-ring is designed to provide a high-pressure air-tight seal between your cylinder's valve and the first stage of your regulator. Always check that it is intact and in position. If it's not, you won't be diving – so make a point of carrying spares just in case. Remember, if it's missing or damaged, it's not just your dive you'd be spoiling, but your buddy's too.

Octopus: An extra demand valve used in an out-of-air situation; also referred to as an Alternative Air Source (AAS).

Off-gassing: Refers to the process of the absorbed nitrogen being released from the body's tissues after a dive.

Open Circuit Scuba: Standard suba apparatus is open circuit; where exhaled air is expelled into the water as bubbles and no part is rebreathed by the diver.

Open Water Dive: The type of dive where a diver can, if need be, return directly to the surface without hindrance – unlike cave diving, or wreck penetration.

Open Water Diver: A level of diving accomplishment in the SAA. An Open Water Diver has additional practical experience, but must be accompanied by a qualified Dive Leader or above. Maximum depth during training: 20 metres. Maximum depth after qualification: 20 metres.

Oxygen O2: The gas vital for all life on this planet makes up 21% of air by volume.

Oxygen Clean: All equipment coming into contact with high-pressure oxygen must be "O2 clean" and O2 compatible to reduce the risk of fire. The purpose of cleaning is to remove all traces of contaminates such as oil, grease, soot, fingerprints paint, lint, dust, rust, metal particles, cleaning solvents and detergents. Oxygen cleaning MUST be done by a qualified person and this means taking your equipment to an approved IANTD, TDI or ANDI service agent.

Oxygen Compatible: This means that a component, or the system, is compatible for use with pure oxygen Some regulator components e.g., silicon diaphragms, silicon or neoprene O-rings and teflon seats are incompatible with pure oxygen. These must be replaced if pure oxygen is to be used.

Oxygen Service: When an item or system is both oxygen clean and compatible, it is said to be in "Oxygen Service". Where pure oxygen is to be used, then all equipment MUST be in oxygen service.

Oxygen Toxicity: Damage or injury from inhaling too much oxygen. It can arise from either too high an oxygen concentration or oxygen pressure. The first sign of oxygen toxicity whilst diving can be seizures. O2 above a partial pressure of 1.6 bar may lead to toxicity. The SAA maximum is defined as 1.4 bar.


PADI (Professional Association of Diving Instructors): A popular system of learning to dive, often encountered whilst on holiday. However, learning to dive at home within a club environment, such as Central Scotland Dive Club, allows you to progress at your own pace and achieve a more comprehensive knowledge and skill-set, so when you do go on holiday, you can spend all your time diving, rather than learning.

PDC (Personal Dive Computer): An electronic device which may be wrist or console-mounted that monitors various aspects of your dive, including: depth; duration; temperature; ascent-rate etc. The information and dive profile can later be downloaded to a PC and saved or printed. (See also Dive Computer above).

PFO (Patent Foramen Ovale): A PFO is an opening in the heart between the right and left atria. For most people, it closes at birth – but in around 30% of the population it remains open and may allow the passage of bubbles into the arterial circulation, which could lead to an Arterial Gas Embolism (AGE).

PSI (Pounds per Square Inch): A measure of pressure that remains in standard usage in the United States. Within the UK diving community, pressure is measured in bar.

Pan-Pan: At sea, a call of "Pan-pan" on marine radio VHF Channel 16, is used to signify that there is an urgency on board a boat. As this channel is monitored continuously, the coastguard should respond promptly. Note the difference between this call and that of "Mayday".

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

Pee Valve: Drysuits were never designed for the convenience of users, and some enterprising manufacturers have attempted to make things a bit easier for needy divers – both male and female.

Plankton: This is found in the seas and oceans and consists of any drifting micro-organisms including: animals; plants; archaea or bacteria. It can play a large part in the quality of underwater visibility.

Pneumothorax: An abnormal collection of air outside the lining of the lung, between the lung and the chest wall; often the consequence of barotrauma. Remember to breathe normally through your regulator during ascent. Holding your breath whilst ascending increases the pressure in your lungs, as the ambient water pressure decreases.

Pony Cylinder: A small cylinder (typically 3 litres) used for emergencies. It acts as a self-contained Alternative Air Source (AAS) as it is equipped with its own independent regulator.

Positive Buoyancy: A state of buoyancy where the diver will have a tendency to float upwards.

Pot: (See Hyperbaric Chamber above.)

Pressure: Any force exerted over an area. (See Air Pressure and Ambient Pressure above.)

Profile: (See Dive Profile above.)

Pulmonary Barotrauma: The rupture of the lung surface from increased pressure of ascent from depth; usually due to a closed glottis, pulmonary blebs, or terminal airway disease. It causes Cerebral Arterial Gas Embolism (CAGE), pneumothorax and pneumomediastinum. Remember to breathe normally through your regulator during ascent. Holding your breath whilst ascending increases the pressure in your lungs, as the ambient water pressure decreases.

Purge Button: A button on the demand valve for manually activating the flow of air.


Qualifications Crossover: A comparison table which enables qualified divers of one agency to understand the comparable qualifications awarded by other agencies, as they may wish to "crossover" and continue their training.


RG (Repetitive Group): After any dive there is an excess of nitrogen in the diver's body. This excess is called residual nitrogen. The RG is a letter of the alphabet used to denote the amount of residual nitrogen in the diver's body. There are two types of RGs; the Exit RG and the Re-entry RG.

RNT (Residual Nitrogen Time): The time it would take to off-gas any extra nitrogen remaining after a dive.

Rebreather: Specialist scuba apparatus where the diver's exhaled air is recycled by removal of carbon dioxide (CO2) and augmented with oxygen. With this type of equipment, no exhaust bubbles are produced and sensitive sea creatures will be less likely to swim off.

Reciprocal Bearing: If you were following a compass bearing in one direction and wanted to return the way you came, you would go back following the reciprocal bearing i.e., at 180° from your original course. The rotating bezel on a dive compass makes this process easier.

Recompression: This may form part of the treatment of a diver suffering from Decompression Illness. Important Note: In-water recompression must NEVER be attempted, because of the increased risk from complicating factors.

Recompression Chamber: (See Hyperbaric Chamber above.)

Regulator: The apparatus used by a diver, consisting of a first stage, hose and second stage (demand valve) that enables the high-pressure contents of the cylinder to be reduced for delivery at ambient pressure.

Reef: A reef is generally a rocky (or coral) underwater feature, although sometimes an artificial reef is created by a wreck. Reefs can be a haven for sealife and are popular for divers to explore.

Reel: A diver may use a reel and line for various purposes: to ensure the return to the starting point; to measure distance swum; to assist in a search; and to attach to a DSMB at the end of a dive.

Repetitive Dive: Any dive done within a certain time frame after a previous dive.

Residual Nitrogen: Nitrogen that remains dissolved in a diver's tissues after surfacing from a dive.

Reverse Profile: This occurs when a repeat dive is deeper than the previous one. Such practice is discouraged by most dive organisations, including the SAA.

Reverse Squeeze: Discomfort or pain in an air space on ascending from a dive.

RIB (Rigid Inflatable Boat): Inflatable boat with a rigid hull which is popular with some divers.

Rip Tide (or Rip Current): A strong channel of water flowing away from the shoreline, typically through the surf line, and can occur on any shore that has breaking waves. Because of their inherent dangers, they should be avoided where possible, as swimming against one will be exhausting, so it may be preferable to swim parallel to the shore until you break out of the current, then attempt to swim shoreward at that point.

Rule of Thirds: For divers following the rule, one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve. When planning your dive, you should always consider your buddy's cylinder contents and breathing rate, not just your own.

Run-time: The dive table on the SAA Bühlmann DeeP Stop System conveniently shows cummulative run-times for the decompression/safety stops.


SAA (Sub-Aqua Association): An association of independent dive clubs

SAC (Surface Air Consumption): The rate at which a diver consumes air at the surface. Knowing this value is a useful indicator of how long the diver could safely dive for. Because of pressure, the air consumption will increase as the depth increases. For example: at 10 metres depth a diver will consume twice the volume they would have on the surface; at 30 metres depth, this will have increased to four times the surface volume. Actual air consumption on the dive will also be governed by other variables, including temperature, exertion and stress.

SCUBA (Self Contained Underwater Breathing Apparatus):

SMB (Surface Marker Buoy): An inflatable device to indicate that divers are below the surface. It may also be used on a drift dive for observers on the dive boat to monitor the progress of the divers.

SPG (Submersible Pressure Gauge): The diver's Contents Gauge, which displays the air remaining in the diver's cylinder.

SSAC: (See ScotSAC below.)

SSI (Scuba Schools International): An international diver training organisation.

Safety Stop: On ascent from a dive, a specified time spent at a specific depth for the purpose of nitrogen off-gassing. By definition, it is not mandatory for safe ascent from the dive. Compare with Decompression Stop.

Saturation Diving: Diving performed after the body is fully saturated with nitrogen. To become fully-saturated, the diver must stay underwater for a much longer period than is allowed in recreational scuba diving tables.

Saw-tooth Profile: In a saw tooth dive profile the diver ascends and re-descends a number of times during the dive. Each ascent and re-descent increases the risk of decompression illness.

ScotSAC (Scottish Sub-Aqua Club): A Scottish diver training organisation.

Scuba: Self-Contained Underwater Breathing Apparatus

Second Stage: Also known as the Demand Valve. The part of the regulator that delivers air at ambient pressure via a mouthpiece to the diver. The first stage has previously reduced the high-pressure air from the cylinder to an intermediate pressure which is supplied to the second stage via a hose.

Secondary Drowning: Water can damage the inside surface of the lung, collapse the alveoli and cause pulmonary oedema with a reduced ability to exchange air. This may cause death up to 72 hours after a near-drowning incident. This is called secondary drowning

Shallow Water Blackout: A sudden unconsciousness from hypoxia, that occurs amongst some breath-hold divers. It can also occur near the surface after a deeper dive, as a result of a reduction in the partial pressure of oxygen.

Shot Bag: A component of some divers' weight systems. An alternative to solid lead weights, they consist of bags containing small pellets of lead.

Shot Line: A line with a weight at one end and a buoy at the other. May be dropped onto a dive site from a dive boat. The benefits of having a shot line include: It's useful for divers in conditions of low visibility, strong tides, or on deep dives; divers can use the line to help buoyancy control; to ease long decompression stops; and to prevent drift when ascending in a current; the buoy also marks the dive site for observers on the dive boat.

Shoulder Dump: A valve typically on the left shoulder of a drysuit, which enables the diver to control the release of air in his suit. When rotated fully anti-clockwise it is open, and when fully clockwise it is closed. Click-stops in between allow for the degree of automatic dumping to be varied. Pressing on the valve, also opens it temporarily, without having to adjust the automatic setting.

Silt: The sediment of particles which settles at the bottom of a body of water. When disturbed, it can significantly reduce visibility and may remain suspended in the water for a considerable time.

Sinuses: Air spaces within the skull that should automatically equalise to ambient pressure, unless a diver has nasal congestion. One of the reasons for not diving with a cold.

Skin Diving: Another term for breath-hold diving. Diving without using scuba equipment.

Slack Water: Generally the preferred time to dive, between tidal changes either at high tide or low tide. Water movement, dependent upon local conditions, may reduce to zero.

Slate: (See Dive Slate above.)

Snorkel: A tube used by snorkellers and some divers for breathing on the surface.

Spring Tide: Tides are largely governed by the gravitational effects of the moon and the sun on the earth's oceans. Spring tides occur when the moon and sun are in alignment with the earth i.e., at a new moon and a full moon. This results in the highest high tides and the lowest low tides. (See also Neap Tide above.)

Square Course: Navigating a Square Course underwater consists of following an initial compass bearing for a certain distance (by counting finstrokes for example); then turning 90° (this can be right or left, but subsequent turns must all be the same), finning the same distance; repeating the turning and finning twice more should bring you back to where you started; although current may affect your success.

Stage-stop Diving: Diving for depths and durations that require decompression stops to be observed during ascent.

Strobe: A flashgun for underwater photography may be referred to as a strobe. Also, a small torch-like device which emits a regular flashing light, which may be a useful safety feature when used wisely, or an annoyance to other divers when used inconsiderately.

Suit Squeeze: Noticeable in a drysuit as a diver descends. As the ambient pressure increases, the more air is required to be injected into the suit to compensate for the squeeze.

Surface Interval: The time in between one dive and another. Longer surface intervals allow more time for off-gassing.

Squeeze: The effect of increased ambient pressure on air spaces.

Swell: The long-wavelength surface waves found in seas and oceans. More likely to be a feature of exposed coastlines, as the source of swell is likely to have been generated from storms and wind systems far out at sea.

Swim-through: A gap or short "tunnel" underwater, which divers (as the name suggests) can swim through.


Tables: (See Decompression Tables above.)

Talc: Talcum powder; used by some diver to make donning their drysuit easier. It also tends to cover everyone else and their kit which happens to be in the immediate vicinity. Other divers prefer lubricants such as KY Jelly, or even baby oil. Make sure that whatever you use, is not going to cause your neoprene or latex seals to perish.

Tank: Another word for a diving cylinder.

Technical Diving: Although some definitions vary, Technical (or Tech) Diving is generally that which goes beyond the normal bounds of recreational diving. Tech Divers are likely to dive deeper and longer, but doing so requires them to use special gas mixes, such as Trimix and Heliox, and often switch to higher oxygen content gases during decompression in order to keep the stop times manageable. Technical Diving must only be undertaken by very experienced, trained and disciplined divers. In the event of an emergency, coming directly to the surface is not a safe option.

Thermocline: The intersection between two layers of water that are of decidedly different temperatures; usually the colder layer is deeper.

Thinsulate: A synthetic fibre used in the insulating layers of some undersuits.

Tide: The regular rising and falling of the seas and oceans, as a result of the gravitational effect of the sun and moon.

Tide Tables: Charts used to predict the times and heights of high and low tides.

Tissue Compartment: A theoretical division of the body with an arbitrarily assigned half-time for nitrogen uptake and elimination. In designing decompression tables, the body is divided into a number of "compartments" for purposes of making calculations.

Trilaminate: A type of drysuit membrane construction. Other types of drysuit include neoprene and compressed neoprene.

Trimix: A mixture of helium, nitrogen and oxygen, which is often used in Technical Diving.

Trim Weight: When fine-tuning the weight and configuration of their kit and their swimming position, divers my attach trim weights of one sort or another. Some BCD's provide small pouches for this purpose.

Twinset: Scuba apparatus that has two dive cylinder clamped together. The can be configured as independent units, or combined via a manifold.


Undersuit: Worn by drysuit divers under their dive suit for warmth. Different types and makes are available, either one-piece or two-piece.


VHF: Very High Frequency radio used in marine (and other) communications.

Valsalva Manoeuvre: One method divers can use for equalising the pressure in their ears. Achieved by pinching the nose and exhaling to increase the pressure in the mouth/nose cavities and consequently via the eustachian tubes to the middle ear. It is named after Antonio Maria Valsalva, a 17th century Italian physician.

VENTID: A mnemonic acronym for helping to recall the signs and symptoms of oxygen toxicity.
V: Vision
E: Ears
N: Nausea
T: Twitching
I: Irritation
D: Dizziness

Visibility: The clarity of the water on a dive. Affected by various factors, including silt and plankton.

Viz: (See visibility above.)


Wall: An underwater feature, the equivalent of an underwater cliff-face.

Weasel: A popular brand of one-piece insulated undersuit.

Wetsuit: A type of protective clothing for those engaged in watersports. Diving wetsuits provide insufficient insulation for all year round diving in the UK. Choose a drysuit instead.


X-Bracket: A type of bracket for mounting a pony cylinder onto a main cylinder's cam band or strap. A better option is the quick-release type manufactured by Northern Diver.


Yo-yo Dive Profile: The type of diving where a diver repeatedly dives to the required depth and returns to the surface with little or no surface interval before repeating the process. Diving instructors may find themselves in this situation. For safety, each descent/ascent should be treated as a separate dive, so there should be no more that three in any twenty-four hour period.


Zooplankton: This heterotrophic type of plankton drifts in the currents of the ocean and seas. It contains a wide range of organism sizes, from small protozoans to large metazoans. Read more here.