Glossary of Diving

May 11, 2020 | We Are. . .

The glossary contains terms, acronyms, expressions and other related nomenclature relative to scuba diving.  For each entry, you can learn more from the encyclopedia.


Absolute pressure

 The absolute pressure is compared against a perfect vacuum. It will read the actual surrounding pressure plus the pressure of the atmosphere. It is used in technical or altitude dive planning. The absolute pressure is not the pressure from the submersible pressure gauge as it represents the gauge pressure.

Argon is a noble gas used in diving for the sole purpose of drysuit inflation. Its higher density gives argon better insulating properties. It is not used as a breathing gas as it is about 2.3 times more narcotic than nitrogen.




Backmount is the expression used for regular scuba gear configuration where one or two cylinders are mounted and secured on the back of the diver.



The backplate is the base of a diving harness. It is what supports one or more back mounted tank and to which the straps for the shoulders and the waist are attached. They are mostly made of one piece of stainless steel or aluminium. In the last few years, lighter ones made of a soft nylon fabric with hard plastic plates has appeared.



A Bar is a metric unit of measure for pressure. It is used in most of the world as the standard pressure unit for measurement on SPG. In some countries where the imperial measurement is in use the unit is the PSI.


BCD – Buoyancy Control Device

A BCD or Buoyancy Control Device is an air cell that can be inflated or deflated. It is nowadays used by all divers to control buoyancy and help stay comfortably afloat at the surface. Most are in a jacket style of BCD but they can also be what is called wing BCD or even an hybrid of the two. In technical diving it is normally a wing attached to the backplate which can have a double internal bladder for added redundancy.



Bioluminescence is the ability of some aquatic organisms to generate light. It uses chemical reaction to achieve this. This ability has only been observed in aquatic organisms and only a few terrestrial insects such as the firefly.


Bolt Snap

Bolt snaps commonly called dog clips comes in many sizes and are used to attach various pieces of diving equipment. They can also be double ended with a clip on each end. They are said to be the best clip for securing equipment as they cannot be opened by pressing against the gate or without a deliberate action. They have a relatively simple safe and resistant mechanism. They are either made of stainless steel or brass to prevent corrosion.


Buoyancy, Buoyant

Buoyant or buoyancy is relative to the sinking or floating characteristics of an object in liquids. We can say objects are positively buoyant or have a positive buoyancy when they float. Inversely are negatively buoyant when they sink, and neutrally buoyant when they neither float or sink and remains at the same depth.


Burst Disc

A burst disc is a safety feature of cylinders valves to help prevent overpressurization. It will as the name implies burst with the excess pressure and drain the cylinder until it is at ambient pressure.



A current is the name for water movement which would make a floating object move from its starting point. This is how surface current speed are determined. Some of these currents are called rip current and could be dangerous for swimmers and divers as they can bring them far away from the shore.




A d-ring is used for clipping and securing equipment with clips. It is composed of a ring bent in a D shape and usually made of resistant material like stainless steel or very hard plastics. They are positioned mostly on the shoulder straps but can also be hanging from the side flaps of jacket or hybrid style BCDs or on the waist straps on harnesses to give other options then the shoulders.

Another use of D-rings is to help divers to grip and pull the straps on the various adjustable points of a jacket or harness BCD.


DCIEM – Defence and Civil Institute of Environmental Medicine

Its was in 1971 in Canada that the DCIEM or Defence and Civil Institute of Environmental Medicine was founded in a joint participation of military and civilian personnel. This lead to the development of dive tables that were more conservative than the US Navy tables.


DIN – Deutsches Institut für Normung

DIN or “Deutsches Institut für Normung”, meaning “German institute for standardisation” is a word used to design tank valve and regulator first stage fittings. The other connection used in scuba diving is the A-Clamp or Yoke.


Divers Alert Network – DAN

Divers Alert Network or DAN is a non-profit agency that collects and analyse information for divers safety. It has branches that sell different diving oriented services and products such as insurance or oxygen providing kits or First Aid and CPR courses.


DPV – Diver Propulsion Vehicle

DPV or Diver Propulsion Vehicle are also called underwater scooters. They are composed of a sealed electric engine with gears that rotates a propeller at various speeds.



A drysuit as its name implies is a thermal protection meant to keep divers dry. It has seals around the neck and wrist and is basically airtight. It requires its own supply of gas to be inflated to prevent the squeeze of it against the whole body. It is used in colder regions or when going deeper or longer even in tropical zones.


DSMB – Delayed Surface Marker Buoy

A DSMB or Delayed Surface Marker Buoy works the same as a SMB but it is inflated and deployed from underwater. It is composed of a surface marker buoy secured by a line on a spool or by a WDS. DSMB allow a diver to be located and spotted from the surface while still being at desired depth. It helps in preventing and avoiding accidents with boats and is considered mandatory in many places.


EAN – Enriched Air Nitrox

EAN or Enriched Air Nitrox is a mixture of gases for which content is the same as air but at different ratios. It has higher concentration of oxygen than the 20.8% found in regular air. EAN is used to reduce nitrogen exposition and saturation into the body. It gives longer bottom times at given depths than regular air.


END – Equivalent Narcotic Depth

END or Equivalent Narcotic Depth is used to express the relative narcotic effect of nitrogen at given depths. Also called rapture of the deep, it impairs divers when they get exposed to a certain pressure. It is different for every diver at different times. The END compares a mixed gas other than air such as nitrox or trimix to what the narcotic effect would if the dive was conducted on air.


Equalize, Equalization

To equalize or equalization is the act of applying the same pressure in an air space than the one of the surrounding or ambient pressure. Usually, in diving, it refers to the ears but can also apply to the mask or even to the lungs.



fsw – feet of seawater

fsw or feet of seawater is a measurement unit of the imperial system and represent the pressure in 1 foot of seawater or about 15 psi. 33 feet is also roughly 1 atmosphere.



Gauge pressure

The gauge pressure is the pressure shown by a gauge which was calibrated and zeroed using ambient surface pressure at sea level as a reference.




Heliox is a breathing gas mixture used in technical and commercial diving. Heliox is composed of Oxygen and Helium only. Having no Nitrogen it avoid nitrogen narcosis that happens in deeper dives.


HPNS – High Pressure Neurological Syndrome

HPNS or High Pressure Neurological Syndrome occurs on very deep dive usually past 100m and it affect the central nervous system. Syndromes include tremors, spasms, paralysie. Little is know on the physiological mechanics of HPNS.



Hypercapnia is defined as an increase of carbon dioxide in the body which leads to faster breathing rate and heartbeat as the body is trying to eliminate it.


Hyperoxic is a term used for breathing gases that has a higher percentage or fraction of oxygen than regular air. The term or word is not so commonly used because of the designation Nitrox or Enriched Air, which is used instead to cover hyperoxic gases. It is the opposite of hypoxic.



Hypothermia occurs when the human body cannot generate enough heat to maintain its core temperature. There is different stage in hypothermia. Symptoms and signs starts with shivering and can become as serious as delirium or loss of consciousness.


ICD – Isobaric Counterdiffusion

ICD or Isobaric Counterdiffusion is a term used to describe when an inert gas is diffusing in the tissues while another inert gas is diffusing out of the tissues. It is not a desirable effect as it leads to increased risk of gas bubbles formation in fast tissues.



J valve

A J valve is almost identical to a K valve but is equipped with a lever on the hand wheel allowing for a reserve pressure. It was used before submersible pressure gauges become easily available. Today J valves are not in use for recreational and technical diving anymore and limited to very specific circumstances by military, commercial or other specialized diving.



K valve

The K valve is the most common scuba cylinder valve. It has one knob on the side perpendicular to the post. Its clockwise rotation close the valve. K valves have either a DIN or a Yoke connector for the regulator.



lb – lbs

lb or lbs are the short unit designation for pounds. It is a measurement of weight coming from the imperial system of units. One pound is equal to exactly 0.453592 kilograms.


LDS – Local Dive Shop

A LDS is a store, shop or dive center that provides products and services related to diving. It can be diving logistics and diving trips, equipment maintenance, rental or sales or education and certification.


LPI – Low Pressure Inflator

The LPI or Low Pressure Inflator allows the BC or BCD to be inflated or deflated. The LPI is fed by its own hose connected to a low pressure port of a regulator first stage. There are only two buttons, one for inflation and the other for deflation.




Mangroves are coastal habitats composed of small trees and bushes that have adapted to the sea water and tides. The roots usually start from just above the surface line at high tide. This makes a vast maze of roots underwater and attracts a very wide biodiversity of marine life. Mangroves play an important role for coasts.

MOD – Maximum Operating Depth

MOD or Maximum Operating Depth counted in meters is the maximum depth at which a mixture of gas becomes unsuitable for breathing. The high partial pressure of oxygen might lead to seizures and convulsion. It is not used in regular air diving but taught in any enriched air courses or in mixed gases courses in technical diving.


MSW – Metre Sea Water

MSW or Metre Sea Water is the unit used in metric to measure the pressure divers are exposed to. For every meter of sea water the pressure is theoretically 0.1bar and is equal to 3.26336 fsw.




Narcosis also called nitrogen narcosis or inert gas narcosis is the name of the impairment that affect divers when the nitrogen in what we breath becomes narcotic under pressure. It has been given the name of rapture from the deep by Jacques Cousteau when divers started pushing the depth limits of diving beyond the 30 meters mark.


NDL – No Decompression Limit

The NDL or No Decompression Limit is a period of time that a diver can stay according to the depth before decompression is required. The NDL is used in recreational diving to calculate the maximum time a diver can stay and is used when planning dive. Dive computers will calculate this limit at set interval normally in just a few seconds to accommodate changes in depths through the dive.



Neoprene is a fabric made of a synthetic material with similar properties of rubber. This material is foamed to create a thick fabric with closed bubbles. A bit like a baked cake or bread. The bubbles are filled with nitrogen which creates a layer of gas that slow down eat diffusion. The insulation properties of neoprene are from the gas bubbles and not from the layer of water in the suit.



Nitrox is another name for enriched air which is a gas mix different than normal air. Other possible names are EAN or EANx where x is the percentage of oxygen eg: EAN32. It is named this way because just like most of the air we breath it is a mixture of nitrogen and oxygen. Nitrox has a different ratio of the two gases with higher oxygen percentage than regular air.



Normoxic is the name given to a breathing gas mix for which the percentage or fraction of oxygen is similar to the one of air, but contain a different mix of other gases. Along with hypoxic and hyperoxic, normoxic is one of the three kinds of breathing gas mixes for diving.



An O-ring is a ring made of synthetic rubber that allows two parts to be sealed together. The easiest application to see is the one between the tank and the regulator first stage. An O-ring allow airtight closure of different components working together. They make it possible for regulators and tanks to operate as they prevent air leaks or water infiltration.


OC – Open Circuit

Open circuit is the breathing system most commonly used for scuba diving. It is called open circuit because all the gas exhaled by the diver is expelled in the environment. A basic OC breathing apparatus is composed of a cylinder, a regulator first and second stage and a pressure gauge. The other systems are called CCR for closed circuit rebreather or SCR for semi-closed rebreather because they reuse in a loop all or part of the diver exhaled gas.



PSI – Pounds per Square Inch

The PSI or Pounds per Square Inch unit is from the imperial system of measurement and is used to express the value of pressure. Some countries such as the US still use it but most countries in the world use the bar which is the metric unit for pressure measurement.



Quick release

A quick release is a form of clip used for diving equipment. It is usually associated with a weight system but applies to many other purposes such as the releases of shoulder straps.




A rattle in diving is a piece of equipment designed to get attention by making a rattling sound. It is usually a hollowed tube with both ends plugged and containing metal balls. Shaking it create a sound from the shock of the balls.


RNT – Residual Nitrogen Time

The Residual Nitrogen Time or RNT is a way to convert the saturated nitrogen in the body in time to be easier to use with tables.




The name scuba comes from the equipment developed by Christian J. Lambertsen and is the acronym of Self Contained Underwater Breathing Apparatus. It was then used by the combat frogmen from the United States of America during the second world war. Nowadays it is used as a name for the gear which is composed of the tank, the regulator and the BCD.


SI – Surface Interval

The SI or surface interval is the amount of time between 2 dives which is used to calculate the limits in consecutive dives.


SMB – Surface Marker Buoy

A SMB or Surface Marker Buoy is a safety signalling device in the form of an inflatable tube of fabric brightly coloured. SMBs are used at the surface to be easily visible and spotted from far away. Some can also be deployed from underwater and are called DSMB.


SPG – Submersible Pressure Gauge

SPG or Submersible Pressure Gauge is a gauge that translate the pressure from the tank to a unit that has measurable properties. Similar to the fuel gauge of a car. They can either be in PSI for imperial system or bar for metric system. They are the same as their regular non-submersible counterpart but sealed to block water from getting into the mechanism preventing corrosion.


SS – Safety Stop

A Safety Stop or SS is a stop or pause that divers do at the end of a recreational dive before surfacing. They are not required in the case of an emergency but are very good practices to help prevent the onset of decompression sicknesses. They are done at 5 meters for a period of 3 minutes.



Table – Dive Tables

The dive tables were invented to deal with the effect of surrounding pressure that increase the nitrogen dissolved in our body. They were the first way to calculate times and depths to dive safely and prevent decompression sicknesses. Nowadays computers applies almost real time the calculations made to create the tables.



The trim is the position of a diver relative to the surface. Having a good trim means that the diver can float in a horizontal position parallel to the surface. It is important as it reduce drag and increase overall efficiency of movement underwater.



Trimix is a mixture of three breathable gases. Oxygen, Nitrogen and Helium. It is used in technical diving to reduce the effects of nitrogen narcosis. Because helium has lower density, Trimix is also used to reduce the effort of breathing caused by surrounding pressure at greater depths.




The process known as “upwelling” is caused by the winds blowing across the surface of the ocean and push water away. Colder water then rises up from deeper beneath the surface to take the place of the water that was pushed away. It is also the opposite of downwelling.




The volume is the term used to describe the three dimensional space that an object occupy. It can be counted in litres for liquids and gasses or cubic meters for solids. Volume in cubic meters is measured by multiplying the width by the height by the length of a squared object as an example.




A wing is an air cell used for buoyancy control. It is normally composed of a bladder protected by a nylon outer shell. A wing can have a horseshoe shape or a donut shape. It is attached to the plate of a diving harness. Some brands of scuba gear started to offer one piece wing style or hybrid jacket to the main market.


WOB – work of breathing

The work of breathing or WOB is in simple terms, the effort required by the lungs to breathe. But to be more accurate it refers to the energy expended to inhale and exhale a breathing gas. The WOB is usually expressed as work per unit volume.

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