# Glossary of underwater diving terminology: H–O

This is a glossary of technical terms, jargon, diver slang and acronyms used in underwater diving. The definitions listed are in the context of underwater diving. There may be other meanings in other contexts.
Underwater diving can be described as a human activity – intentional, purposive, conscious and subjectively meaningful sequence of actions. Underwater diving is practiced as part of an occupation, or for recreation, where the practitioner submerges below the surface of the water or other liquid for a period which may range between seconds to the order of a day at a time, either exposed to the ambient pressure or isolated by a pressure resistant suit, to interact with the underwater environment for pleasure, competitive sport, or as a means to reach a work site for profit, as a public service, or in the pursuit of knowledge, and may use no equipment at all, or a wide range of equipment which may include breathing apparatus, environmental protective clothing, aids to vision, communication, propulsion, maneuverability, buoyancy and safety equipment, and tools for the task at hand.
Many of the terms are in general use by English speaking divers from many parts of the world, both amateur and professional, and using any of the modes of diving. Others are more specialised, variable by location, mode, or professional environment. There are instances where a term may have more than one meaning depending on context, and others where several terms refer to the same concept, or there are variations in spelling. A few are loan-words from other languages.
There are five sub-glossaries, listed here. The tables of content should link between them automatically:
* Glossary of underwater diving terminology: A–C
* Glossary of underwater diving terminology: D–G
* Glossary of underwater diving terminology: H–O
* Glossary of underwater diving terminology: P–S
* Glossary of underwater diving terminology: T–Z

## H

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |
Subsection: Top, Ha, He, Hi, Ho, Hu

## Ha
### habitat

    Main article: Underwater habitat An underwater structure inside which divers can carry out dry welding or which is fitted out with life support facilities.

### Haldanian

    See: Decompression theory#Critical ratio hypothesis Decompression models based on the principles described by John Scott Haldane.

### half+15

    See: Half + 15 bar An alternative scuba reserve gas management strategy to the rule of thirds for breathing stage cylinders. The cylinder is breathed down to half of its starting pressure plus 200 psi (or 15 bar), so that a stage cylinder filled to 3000 psi would be breathed down to 1700 psi before being dropped. Primary cylinder gas (usually back gas) is retained for use in contingencies.

### half duplex

    See: Half duplex Voice communications system where users take turns to send and receive.

### half mask

    Main article: Diving mask Diver's mask which covers the eyes and nose but not the mouth.

### half times

    See: tissue half times

### halocline

    Main article: Halocline A strong variation in salinity over a small depth range within a body of water. Often visible as a blurred or shimmering region due to uneven refractive index.

### hand-off cylinder

    A diving cylinder, complete with regulator, which can be handed off (manually transferred) to another diver in an emergency, so that the two divers are not obliged to remain in close proximity during the exit and ascent. Transfer of a hand-off cylinder should not compromise either diver's buoyancy to the extent that they cannot make a normal, controlled ascent at neutral buoyancy.

### hang

    To remain stationary at a specific depth and location, particularly when decompressing.

### hang off

    See: Dive profile#Hang-off profile Commercial diving procedure where the diver ascends to a shallower depth to minimise in-gassing during periods of waiting for surface support. The diver generally ascends to 30 ft waiting depth, and bottom time at that depth is not counted towards decompression obligation. When the support is ready, the diver returns to working depth and time elapsed from leaving hang-off depth accumulates towards decompression obligation. The profile appearance approximates a square wave form, or yo-yo profile.

### hard hat diving

    Surface supplied diving, generally in professional diving, either wearing a modern diving helmet or the old-style standard diving dress with copper helmet.

### hardwire communications

    Voice communications using a cable for transmission.

### harness

    Straps and webbing with associated buckles, D-rings and other accessories used to support the breathing apparatus and secure it to the diver. The harness often has other functions such as supporting weighting and buoyancy control systems and for recovery of the diver from the water. In professional diving the harness is used as a strong point to attach the lifeline or umbilical to the diver.

### hat

    Commercial diver term for diving helmet.

### HAZID

    Hazard identification study: A systematic qualitative assessment of potential hazards and threats to health, safety, equipment, property, environment, production, or reputation. May be followed by a risk assessment.

### hazmat diving

    Main article: Hazmat diving Diving in a known hazardous materials environment. The environment may be contaminated by hazardous materials, the diving medium may be inherently a hazardous material, or the environment in which the diving medium is situated may include hazardous materials with a significant risk of exposure to these materials to members of the diving team. Special precautions, equipment and procedures are associated with hazmat diving.
Subsection: Top, Ha, He, Hi, Ho, Hu

## He
### head-up display

    Also: "HUD" See: Head-up display A visual display mounted where it is normally in the diver's field of vision.

### Health and Safety Executive

    Also: "HSE" Main article: Health and Safety Executive UK government department responsible for occupational diver safety in UK.

### heavy gear

    See: standard diving dress

### heliair

    Main article Trimix (breathing gas) Trimix blends made by topping up helium with air.

### helicopter turn

    Manoeuver in which a horizontally trimmed diver uses small fin movements to rotate on the spot on a vertical axis.

### heliox

    Main article: Heliox Mixtures of helium and oxygen for use as a breathing gas.

### helium

    Main article: Helium An inert gas which is used as a component of breathing gas mixtures for deep diving.

### helium analyzer

    Main article: Helium analyzer An instrument used to identify the presence and concentration of helium in a mixture of gases

### helium speech unscrambler

    See: speech unscrambler

### helmet diving

    See: hard hat diving

### Henry's law

    Main: Henry's Law Description of the changes in solubility of a given gas in a given liquid as pressure varies.
Subsection: Top, Ha, He, Hi, Ho, Hu

## Hi
### HID light

    High intensity discharge light: Used in cave diving light heads (q.v.).

### HIRA

    Hazard Identification and Risk Analysis: A risk management procedure for identifying hazards and assessing the risk associated with them and ways to reduce the risk to an acceptable level.
Subsection: Top, Ha, He, Hi, Ho, Hu

## Ho
### Hogarthian configuration

    A scuba combination of backplate, wing, one-piece harness with crotch-strap, regulator arrangement including long-hose primary with a necklaced secondary demand valve, and, if used with twin cylinders, an isolation manifold. Named after William Hogarth Main, a developer and proponent of the system.

### Hogarthian loop

    See: Doing It Right (scuba diving), and Human factors in diving equipment design#Hose routing A scuba configuration where the primary demand valve has a long hose which is routed under the right arm, usually tucked under a light battery canister on the waist belt of the harness, and around behind the neck to reach the mouth from the right hand side. Part of the Hogarthian configuration (q.v.).

### Hoke valve

    See: Standard diving dress#Mixed gas systems Valve on the US Navy MK V helium helmet. The gas supply at the diver was controlled by two valves. The "Hoke valve" controlled flow through the injector to the "aspirator" which circulated gas from the helmet through the scrubber, and the main control valve used for bailout to open circuit, flushing the helmet, and for extra gas when working hard or descending.

### hold-back rigging

    Rigging provided to restrain excessive buoyancy of a lifting bag when it is attached to the load and inflated.

### homebrew

    Amateur blended mixed gas.

### hood

    Close-fitting thermal head protection, usually neoprene foam, but also latex on some dry suits.

### hook breathing

    See: recovery breathing

### hookah

    See: airline diving

### hopcalite

    Main article: Hopcalite Catalyst sometimes used in breathing air compressor filters to oxidise carbon monoxide to carbon dioxide. Hopcalite is a mixture of manganese oxide, copper oxide and a small amount of silver oxide.

### horse collar

    See: adjustable buoyancy life jacket

### hose dash size

    Number indicating the bore of a hose in multiples of 1/16" preceded by a dash. for example a –04 hose, typically used for a pneumofathometer hose would have a nominal bore of 1⁄4 inch (6.4 mm).

### hose organiser

    Also: hose tidy Equipment to position and stabilise a hose, particularly in the context of sidemount or sling mounted scuba cylinders, where the regulator air hoses are stored against the cylinder when not in use.

### hot mix

    A breathing gas mixture with a high percentage of oxygen for the depth at which it is used. Using a gas mixture with a high partial pressure of oxygen.

### hot stab coupling

    See: Hot stab Quick connection coupling that can be connected by an ROV underwater, often to provide hydraulic power. The hot stab is the connector unit supplying the utilities, and it is usually inserted into a port after pulling out the plug stab, which is used to protect the port when it is not in use. Typically the alignment is not critical and the port will guide the stab in as it has concentric flow channels and o-ring seals. It may lock into place when fully inserted, or may require an external force to hold it in place while in use.

### hot water suit

    See: Diving suit#Hot water suits A loose fitting wetsuit supplied with heated water through a hose in the diver's umbilical, which links the diver to the surface support. The diver controls the flow rate of the water, allowing them to vary the warmth of the suit.

### HP

    High pressure, generally gas pressures in excess of 30 bar. In most of the world a high pressure diving cylinder is a 300 bar cylinder, but in the US it may refer to anything over 3000 psi working pressure. In a diving context gas working pressures do not frequently exceed 300 bar, but pressures in hydraulic systems and high pressure water jetting equipment may be considerably higher.

### high-pressure nervous syndrome

    Main article: High-pressure nervous syndrome A neurological and physiological diving disorder that occurs when a diver descends below about 500 feet (150 m) while breathing a helium–oxygen mixture.

### HRC

    1. See: hyperbaric rescue capsule
    2. See: hyperbaric rescue chamber

### HRU

    See: hyperbaric rescue unit

### HSE

    See: Health and Safety Executive
Subsection: Top, Ha, He, Hi, Ho, Hu

## Hu
### HUD

    See: Head-up display

### H-valve

    See: Scuba cylinder valve#Dual outlet valves Cylinder valve body with two outlets and two valve mechanisms which can be independently controlled so that two regulator first stages can be fitted. Similar to Y-valve but in configuration where the second valve is parallel to the primary, though the secondary valve can sometimes be swivelled.

### hydro

    Slang/abbreviation for hydrostatic test (q.v.)

### hydrophobic membrane

    A membrane that is freely permeable to gas but does not allow the passage of water at low pressure differentials. Used in some rebreather scrubbers to keep water from the absorbent material

### hydrostatic pressure

    Main article: Hydrostatic pressure Pressure due to the weight of the water column above a point at depth.

### hydrostatic test

    Main article: Hydrostatic test Non-destructive test to revalidate pressure vessels which uses water as a test medium. The vessel is pressurised to the test pressure (q.v.) and measured for permanent set.

### hydreliox

    See: Hydreliox Deep diving breathing gas mixture of hydrogen, helium and oxygen.

### hydrox

    Main article: Hydrox (breathing gas) Deep diving breathing gas mixture of hydrogen and oxygen.

### hyperbaric evacuation

    See: Hyperbaric evacuation and rescue Evacuation of divers under pressure from a saturation chamber to a hyperbaric rescue craft via a pressurised transfer system.

### hyperbaric evacuation system

    See: Hyperbaric evacuation system System for evacuating divers under pressure from a saturation system in an emergency. The whole system set up to provide hyperbaric evacuation, including planning, procedures, equipment used for evacuation, reception facility, contingency plans, possible safe havens and anything else requited for a successful hyperbaric evacuation.

### hyperbaric lifeboat

    Also: Self-Propelled Hyperbaric Lifeboat (SPHL) See: Hyperbaric lifeboat A lifeboat with a hyperbaric chamber and life support system built into it for evacuating saturation divers in an emergency.

### hyperbaric rescue capsule

    A self contained buoyant hyperbaric chamber intended for emergency evacuation of saturation divers under pressure from a platform which has become so dangerous that it is considered safer to put the divers into the sea in the HRC to be picked up by a rescue vessel for transfer to another hyperbaric system for decompression.

### hyperbaric rescue chamber

    Normally a pressure vessel adapted to function as a means of hyperbaric evacuation with buoyancy chambers and lifting points, but not fitted in a lifeboat hull.

### hyperbaric reception facility

    See: Hyperbaric reception facility A unit which allows for connection of hyperbaric lifeboats and hyperbaric rescue capsules. Usually modular and portable so they can be moved to a suitable venue for standby with the hyperbaric rescue vessel.

### hyperbaric evacuation unit

    Also: "HEU", "HRU" See: Hyperbaric evacuation unit Generic term for the unit provided to evacuate divers from a saturation system. It may be an HRC or a SPHL or some other suitable pressure vessel.

### hyperbaric rescue vessel

    See: Hyperbaric rescue vessel A ship provided with equipment to handle the recovery of hyperbaric lifeboats and hyperbaric rescue capsules.

### hypercapnia

    Main article: Hypercapnia Excessive level of carbon dioxide in the body, to a partial pressure of above 45 mmHg.

### hyperfilter

    See: Gas blending for scuba diving#Blending nitrox Additional filter to produce air of oxygen compatible quality, usually for partial pressure gas blending. This may be built into the filling system or temporarily connected when required.

### hyperoxic linearity

    See: Rebreather diving# Property of an oxygen sensor to produce linear output at partial pressures above the highest calibration point.

### hyperventilation

    1. A deliberate deep breathing to reduce blood carbon dioxide level to extend the duration of a free dive.
    2. Rapid breathing as the body's response to hypercapnia.
    3. Rapid, often shallow breathing, associated with panic.

### hyperventilation-induced blackout

    See: underwater blackout syndrome

### hypocapnia

    Main article: Hypocapnia Abnormally low tissue and blood carbon dioxide concentration.

### hypothermia

    Main article: Hypothermia A lowering of core body temperature, usually due to heat loss.

### hypoxia

    Main article: Hypoxia (medical) Abnormally low tissue oxygen concentration. Insufficient oxygen in the body to support normal activities or consciousness.

### hypoxic training

    prolonged underwater distance swimming or extended breath-hold intervals.

## I

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### IAOGP

    See: International Association of Oil and Gas Producers

### ICD

    See: isobaric counterdiffusion

### IDRF

    International Diving Regulators and Certifiers Forum, previously International Diving Regulators Forum. A voluntary group of diving regulators and certifying agencies formed to work together toward mutual recognition and to identify and implement best practices in diver training with the object of harmonizing cross border diver training standards outside Europe. Members include Australia, Canada, France, Norway, South Africa and United Kingdom.

### IDSA

    International Diving Schools Association: Formed with the primary purpose of developing common international diving standards for all occupational divers, Offshore, Inshore and Inland, and specialist related non-diving qualifications e.g. Supervisor, DMT and LST.

### IMCA

    See: International Marine Contractors Association. International trade association for the marine contracting industry.

### inert gas

    See: Decompression theory#Inert gas uptake (Ingassing) A gas which is not metabolically active, used to dilute the breathing gas.

### ingassing

    See: Decompression theory#Inert gas uptake (Ingassing) Inert gas uptake in body tissues during a dive or other hyperbaric exposure.

### inherent unsaturation

    See: Decompression theory#Inherent unsaturation Metabolic reduction of total gas pressure in the tissues .

### inshore

    1. Legal: Within the territory of a country in which national legislation applies. (e.g. for occupational health and safety). Generally within the 12 nautical mile limit. (as opposed to "offshore")
    2. Oceanographic: (also 'nearshore) the region of the shoreface, where waves are affected by the seabed.

### inshore diver

    Colloquial term for a diver who works on inland dive sites or coastal waters not associated with the oil and gas industry. Also referred to as "civils" as much of this work is connected with civil engineering works.

### integrated open-circuit regulator

    See: bailout valve

### integrated weight system

    See: Diving weighting system#BCD integrated weights Any system for carrying dive weights on the buoyancy compensator or diving safety harness, avoiding the use of a separate weight harness or weightbelt.

### internal condition of cylinder

    The state of the internal surface of a cylinder regarding corrosion, contamination and cracking.

### International Association of Oil and Gas Producers

    See: International Association of Oil and Gas Producers The international forum of the oil and gas producing industry.

### interstitial emphysema

    Gas trapped in the spaces between organs after lung barotrauma.

### inverter line

    See: tripping line

### in-water recompression

    See: In-water recompression, Hyperbaric treatment schedules#In-water recompression schedules Recompression of a diver by returning to a specified depth in the water, followed by decompression on a specified gas, commonly oxygen, as treatment for decompression sickness or as prophylaxis for incomplete or missed decompression after a dive.

### IP

    Intermediate Pressure, or Interstage Pressure. The reduced pressure between the first and second stages of a diving regulator. Also referred to as LP (Low Pressure) in this context.

### isobaric blackout

    See: constant depth blackout

### isobaric counterdiffusion

    See: Isobaric counterdiffusion The diffusion of gases in opposite directions caused by a change in the composition of the external ambient gas or breathing gas without change in the ambient pressure.

### isolation manifold

    See: isolation manifold Connection between two scuba cylinders which when open allows free flow of gas in both directions between the cylinders, but has an isolation valve which can be used to block this flow.

### isolation valve

    See: Scuba manifold#Isolation valve Valve in an isolation manifold (q.v.) used to close the gas passage through the manifold and isolate the contents of the two cylinders. Used to prevent a leak on one cylinder from causing the other cylinder to also lose gas.

## J

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |
Subsection: Top, Ja, Je, Ji, Jo, Ju

## Ja
### jackstay

    See: Jackstay#Diving jackstay A line secured at both ends to serve as a support or guide.

### jackstay search

    See:Underwater searches#Jackstay search Underwater search techniques using one or more jackstays to guide the searcher.

### jack-up rig

    Main article: Jackup rig A type of mobile platform that consists of a buoyant hull fitted with three or more movable legs, capable of raising its hull over the surface of sea. The buoyant hull enables transportation of the unit and all attached machinery to a desired location. Once on location the hull is raised to the required elevation above the sea surface on its legs supported by the sea bed.

### J-cylinder

    Bulk gas storage cylinder with internal volume of about 50 litres.
Subsection: Top, Ja, Je, Ji, Jo, Ju

## Je
### Jersey upline

    See: upline
Subsection: Top, Ja, Je, Ji, Jo, Ju

## Ji
### JIC fitting

    Main article: JIC fitting A type of pipe and hose fitting with a 37-degree flare seating surface. Commonly used in umbilical hose connections.

### JIM suit

    Main article: JIM suit An atmospheric diving suit manufactured by Underwater Marine Equipment Limited.
Subsection: Top, Ja, Je, Ji, Jo, Ju

## Jo
### jocking harness

    webbing strap system used with diving helmets to hold the helmet assembly down on the diver to prevent buoyancy lifting it when underwater.

### Jonline

    Main article: Jonline A short line used to connect to a shotline or anchor line, allowing the diver to move a short horizontal distance away to decompress. The line helps compensate for vertical movement in the anchor line or shot line due to waves.

### JSA

    Main article: Job safety analysis A procedure to integrate health and safety principles and practices into a particular task or job. Each basic step of the job is analysed to identify potential hazards and controls for each hazard.

### J-search

    See: Underwater searches#Variations on the jackstay search A variation on the movable jackstay search suitable for a single diver.
Subsection: Top, Ja, Je, Ji, Jo, Ju

## Ju
### jump

    See Cave diving#Skills A path from a main guideline to another which is not in contact

### jump camera

    A camera mounted on a frame, which when lowered to the bottom of a body of water, takes a photograph, usually of the bottom under the camera. The frame constrains the camera to a fixed camera to subject distance, resulting in photographs of uniformly sized areas of bottom, equivalent to quadrats.

### Jump jacket

    A harness with integral buoyancy jacket specifically designed for commercial diving work with helmets and bells.

### jump line

    See: Cave diving#Skills A short cave line, usually stored on a spool, used to connect between two permanent lines that are not in contact. May also be used to search for the other end of a break in a cave line and repair the break

### jump spool

    A reel or spool with a relatively short line intended to be used as a jump line (q.v.).

### J-valve

    See: Scuba cylinder valve#Reserve valves Scuba cylinder valve with lever operated reserve mechanism.
Subsection: Top, Ja, Je, Ji, Jo, Ju

## K

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |

### kayak diving

    Main article: Kayak diving Diving from a special purpose kayak used to get to the site where the distance from a suitable entry and exit point is inconvenient for shore diving.

### K-cylinder

    Bulk high pressure gas storage cylinder size designation (approximately 50 litres internal volume)

### Kelly tube

    See: tube

### K-valve

    See: Scuba cylinder valve#Plain valves Scuba cylinder valve without reserve mechanism.

## L

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |
Subsection: Top, La, Le, Li, Lo, Lu

## La
### lanyard

    Main article: Lanyard A piece of cordage used to secure or lower things; usually it is used where there is a risk of losing the object.

### LARS

    See: Launch and Recovery System

### laryngospasm

    Main article: Laryngospasm An uncontrolled or involuntary muscular contraction (spasm) of the laryngeal cords which causes a partial blocking of breathing in, while breathing out remains easier.

### latent hypoxia

    While freediving, an arterial oxygen partial pressure which is sufficient to sustain consciousness at depth, but when ascending drops to hyoxic levels due to the reduction of ambient pressure, associated with ascent blackout.

### launch and recovery system

    See also: Launch and recovery system (diving) Mechanised system for lowering a diving bell, diving stage, hyperbaric evacuation unit, submersible or ROV from a vessel, offshore platform, dockside or other platform, and lifting it back on board.

### lay barge

    Also: pipelay barge See: Pipe-laying ship and Submarine pipeline Barge on which pipeline sections are assembled, welded and laid on the seabed as the barge is moved forward.

### laying line

    See: Cave diving#Line management To run line (unreel under light tension while advancing) and place it

### lazy shot

    See: Diving shot#Lazy shot A shot line (q.v.) which does not reach the bottom

### LDS

    Local dive shop. A retailer of recreational diving equipment, which may also sell cylinder fills, service equipment, provide training or book or organise recreational dives for customers.
Subsection: Top, La, Le, Li, Lo, Lu

## Le
### LED

    See: Light emitting diode A semiconductor device for producing light from an electrical current commonly used in dive lights.

### lens port

    Transparent window on an underwater camera housing through which light reaches the lens.
Subsection: Top, La, Le, Li, Lo, Lu

## Li
### LID

    See: low impact diving

### lifeline

    See: Lifeline (diving) A line connected securely to the diver at one end and anchored at the other end at the diving control point, which is handled by a line tender, and is used to communicate with the diver and provide a means of finding the diver for a surface standby diver, and for assisting the diver to the surface and back to the control point if necessary.

### life support system

    See: Life-support system#Underwater and saturation diving habitats Equipment vital to the short term survival of the diver. Most notably the breathing gas supply, and for saturation diving, equipment for providing a correctly pressurised environment. In some cases thermoregulatory equipment is also considered life support, and in saturation diving, all of the peripheral systems essential to maintaining a habitable saturation system.

### life support package (LSP)

    Also: "fly-away package" A collection of equipment and supplies kept in a suitable location such that when a hyperbaric rescue chamber or self-propelled hyperbaric lifeboat arrives at the safe haven it is available to support or complete decompression by using the LSP components to maintain the decompression environment by way of power, gas mixtures, heating and cooling.

### life support supervisor

    A senior life support technician (q.v.) appointed by the diving contractor to supervise the operation of saturation life-support systems.

### life support technician

    Also: "LST" See: Life support technician A person who operates and maintains the life support systems of a saturation diving system.

### lifting bag

    Main article: Lifting bag A robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy when filled with air.

### light

    Alternative term for a viewport of a standard diving helmet.

### light head

    See: Dive light#Canister light The part of a canister light (q.v.) which emits light, and is held in the hand or mounted on the helmet.

### lightweight helmet

    Main article: Diving helmet#Lightweight demand helmets Low volume, close fitting diving helmet, usually with demand valve. Neutrally buoyant, and moves with the diver's head.

### line marker

    See: Line marker Line arrows, cookies and sometimes clothes pegs. Tags used to indicate the direction to an exit, midway point between exits, jumps and personal markers to identify divers on a guide line in an overhead environment.

### line tender

    See: diver's attendant

### line trap

    Narrow gap where the guide line passes but divers can not get through.

### lipid pneumonia

    Main article: Lipid pneumonia A lung inflammation (pneumonia) that develops when lipids enter the bronchial tree

### live-aboard

    Main article: Liveaboard A large boat which provides transport, accommodation and services for vacationing recreational divers.

### live-boating

    1. Diving from a boat which is under way (not moored). The major implication is that the engines will be running and propellers or thrusters may be engaged while divers are in the water, a significant hazard.
    2. (offshore diving) Diving from a vessel underway, making way, but not using dynamic positioning. In offshore diving, diving from a DPV is not considered live-boating.

### living depth

    See: storage depth
Subsection: Top, La, Le, Li, Lo, Lu

## Lo
### lock

    Also: "air-lock", "airlock" and "air lock" Main article Airlock Compartment of a hyperbaric habitat or chamber which can be entered through two or more openings which can be closed and sealed, and the pressure adjusted relative to the adjacent chambers. Used to transfer personnel or equipment between areas of different pressure, and for transfer under pressure between hyperbaric chambers.

### lock-in

    Enter a pressurised environment through an airlock

### lock-off

    Disconnect a mobile hyperbaric chamber from another hyperbaric chamber after sealing the doors between them and venting the connection space.

### lock-off time

    The time at which a diving bell under pressure is locked off (disconnected) from the compression chamber(s) on deck.

### lock-on

    Connect a mobile hyperbaric chamber to another hyperbaric chamber so that the pressures can be equalised to allow transfer under pressure between them.

### Lock-on time

    The time at which a diving bell under pressure is locked on (reconnected) ready for equalisation to the compression chamber(s) on deck.

### lock-out

    Exit from a pressurised environment through an air-lock

### lockout

    See: Lockout–tagout Safety procedure when working on a hazardous energy source.

### lock-out mode

    See: Dive computer#Management of violations Factory set function of some dive computers which disables the computer from decompression calculation after a violation of a factory set limit for depth, decompression ceiling or other violation of approved operating range for a period sufficient for tissues to fully desaturate if the diver survives uninjured (48 hours in some models). Some models will lockout immediately, usually to gauge mode which provides depth and time data but no decompression information, leaving the diver without some safety-critical information, others will continue to provide the diver with best estimate decompression information until the end of the dive.

### log book

    Record of dives kept as proof of experience. Optional for recreational divers, but legally required for professional divers in many jurisdictions.

### long-hose

    See: Doing It Right (scuba diving)#Hoses, and Human factors in diving equipment design#Hose routing 5 to 7 feet (1.5 to 2.1 m) interstage hose used on one of the regulators used by cave and other technical divers, which allows gas sharing through narrow spaces where the divers must pass through in single file.

### longshore current

    Main article: Longshore current Mass transport of water along a shoreline, often due to wave action at an angle to the shoreline.

### loop

    Also: "breathing loop" Main article Rebreather#Components The ambient pressure circuit in a rebreather through which the breathing gas passes during a breathing cycle.

### loop flush

    1. Manual addition of gas mixture to the breathing loop of a semi-closed circuit rebreather to ensure that the loop mixture has enough oxygen to support consciousness during an ascent. This is most necessary with passive addition rebreathers with a low discharge ratio, but can be relevant with active addition rebreathers, depending on the mass flow rate.
    2. (Also loop purge, or diluent flush) Manual addition of diluent mix to the breathing loop of a closed circuit rebreather, to check calibration of oxygen sensors, or when there is uncertainty as to the current loop mix.

### loop mixture

    1. The breathing gas in the loop of a rebreather.
    2. Composition of the breathing gas in the loop of a rebreather, usually after passing through the scrubber, where it has had the carbon dioxide removed and is analysed for oxygen partial pressure.

### loop volume

    Volume of the breathing loop of a rebreather.

### loop vent valve

    An over-pressure relief valve in a rebreather loop.

### lost buddy procedure

    See: Buddy diving#Buddy separation procedure, Cave diving#Lost buddy standardised procedure followed when a diver realises that their buddy is not where they should be. Procedures may vary depending on the circumstances and training organisations.

### lost line procedure

    See: Cave diving#Lost line Standardised procedure to be followed when the guideline to open water is lost in a penetration dive, often in conditions of low visibility and darkness.

### low impact diving

    Main article: Low impact divingSee also: Environmental impact of recreational diving Diving with low environmental impact. Diving in a way that avoids contact with or disturbance of sensitive organisms and adversely affecting the environment. Usually applied to recreational diving.

### low pressure

    1. Also: "intermediate pressure" In diving, low pressure usually refers to the gas pressure provided from the first stage regulator to the demand valve for reduction to ambient pressure, the pressure provided by a low-pressure breathing air compressor to the gas panel for supply to the diver's umbilical or airline, or the pressure supplied to buoyancy compensator, dry suit or supply for lift bag inflation gas or pneumatic tools. These pressures are generally less than 30 bar, and in the case of scuba regulators are usually about 9 to 11 bar above ambient pressure.
    2. In the context of diving cylinders, in the US, a low pressure cylinder has a working pressure below 2,500 pounds per square inch (170 bar)

### low viz

    See: Underwater vision#Low visibility Water where, regardless of illumination, the distance over which objects can be seen is small. It is a term with highly variable and often relative meaning, but it would be almost universally accepted that less than 0.5 metres (1.6 ft) visibility would be considered low visibility. For operational purposes NOAA define it as "When visual contact with the dive buddy can no longer be maintained", and DAN Southern Africa suggest less than 3 metres (9.8 ft).

### LP

    See: low pressure

### LP compressor

    Low-pressure compressor. Used for breathing air supply for surface supplied air diving.

### LP cylinder

    Low-pressure cylinder (US) with working pressure less than 2,500 pounds per square inch (170 bar).

### LP port

    Opening on the first stage of a regulator through which regulated gas is supplied at low/intermediate pressure.

### LST

    See: life support technician
Subsection: Top, La, Le, Li, Lo, Lu

## Lu
### lung-packing

    See: glossopharangeal insufflation

## M

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |
Subsection: Top, Ma, Me, Mi, Mo, Mu

## Ma
### mammalian diving reflex

    Also: "diving response" A reflex response to breathhold and chilling of the face diving response expressed by the cardiovascular system, which exhibits hypertension, bradycardia, oxygen conservation, arrhythmias, and contraction of the spleen.

### manifold

    See: twin cylinder manifold

### manifold (breathing gas supply)

    Also: "gas panel" Panel for the distribution of diver breathing gas.

### manifold operator

    Also: "gas man" A person such as a life support technician (LST), diving supervisor, or mixed-gas diver, who is designated to perform the duties of gas distribution on a surface-supplied mixed gas (HeO$_2$) diving operation, who is competent in the operation of the manifold and whose primary responsibility is to operate the manifold.

### manual bypass valve

    A valve that allows the diver to manually inject gas into the breathing loop of a rebreather, bypassing the valve operated by the control system.

### manual closed circuit rebreather

    Also: "mCCR", "MCCR". See also: diver controlled closed circuit rebreather See: Rebreather#Closed circuit mixed gas rebreathers A closed circuit rebreather which relies on the diver to monitor and control the gas mixture in the loop.

### manufacturing standard

    Also: "manufacturing code" Set of design and manufacturing rules intended to produce uniform and safe products by several manufacturers in an industry.

### Marsh Marine connector

    One of the popular underwater plug connector systems for diver communications cables.

### Martini's law

    Rough rule of thumb for estimating nitrogen narcosis effects based on equivalence to consumption of dry martinis: Variously quoted as one martini per 10 m or one martini per 50 ft depth.

### master link

    (Rigging): The large heavy duty link to which the legs of a chain sling are attached, and which is the attachment point on the sling for the lifting hook or shackle

### maze cave

    Cave structure characterised by multiple branches and changes in direction.

### MCCR

    See: manual closed circuit rebreather
Subsection: Top, Ma, Me, Mi, Mo, Mu

## Me
### medical lock

    See: Diving chamber#Access doors Small lock on a decompression chamber used for transfer of medical equipment and other supplies into and out of the chamber while the chamber remains under pressure.

### metres sea water

    Main article: metres sea water Unit of pressure equal to 1/10 bar. Not a linear measure of depth.
Subsection: Top, Ma, Me, Mi, Mo, Mu

## Mi
### microbubbles

    Microscopic bubbles which are not detectable by ultrasound or Doppler ultrasound, yet can affect the likelihood of DCS by slowing off-gassing.

### micronuclei

    Microscopic cavities that function as bubble seeds by absorbing dissolved gas.

### midwater

    Significantly distant from both the bottom and the surface.

### mix

    Short for trimix breathing gas

### mixed gas

    Breathing gas for diving other than air, but usually implies a helium based mixture.

### mixed mode diving

    A mixed mode dive team is a buddy team where the divers use different modes of diving on the same dive, such as one diver on open circuit and the other on rebreather

### mixed platform diving

    Mixed platform rebreather diving refers to the use of different makes or models of rebreather on the same dive.
Subsection: Top, Ma, Me, Mi, Mo, Mu

## Mo
### mobile diving system

    Also: "portable diving system" A diving system (q.v.) which is installed on a vessel or installation on a temporary basis and that is not fixed, i.e. can be demobilised, transported and re-sited. Includes surface supplied air, nitrox, heliox and saturation diving systems.

### mobile offshore drilling unit

    See: Mobile offshore drilling unit A generic term for several classes of self-contained floatable or floating drilling rigs such as drilling vessels, semisubmersibles, submersibles, jack-ups, and similar facilities that can be moved without substantial effort. These facilities may have self-propulsion equipment on board and may require dynamic positioning equipment or mooring systems to maintain their position.

### MOD

    Maximum operating depth: Limiting depth for safety based on partial pressure of oxygen of a breathing gas mixture.

### modified flutter kick

    Version of the flutter kick finning style which reduces risk of silting by directing thrust more directly backwards. Two techniques exist: One version has the legs bent at the knees so that the fins are placed relatively high and on average are aligned more horizontally. The other version has one fin stationary below the moving fin to deflect downwash. Leg movement is restrained, and ankle movement used for precision manoeuvring.

### modified frog kick

    Version of the frog kick finning style which reduces risk of silting by directing thrust more directly backwards. Performed with bent knees and fins raised above the line of the torso.

### MODU

    See: mobile offshore drilling unit

### monofin

    Main article: Monofin A type of swimfin typically used in finswimming and free-diving. It consists of a single surface attached to footpockets for both of the diver's feet.

### molecular sieve

    Main article: Molecular sieve Material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids. Molecules small enough to pass through the pores are absorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level.

### monkey diving

    See: Sidemount diving#Terminology The use of sidemount configuration and procedures with a single cylinder.

### moonpool

    Main article: Moonpool An opening in the floor or base of the hull, platform, underwater habitat, or chamber, giving access to the water below.

### msw

    See: metres sea water
Subsection: Top, Ma, Me, Mi, Mo, Mu

## Mu
### muck diving

    See: Muck diving Recreational diving in low visibility, generally because there are some organisms of interest which live in that habitat.

### multi-level dive

    Also: "multilevel dive", "multi level dive" See: Multi-level dive A dive profile in which the diver remains in more than one distinct depth ranges (excluding decompression stops) for a significant period before beginning final ascent to the surface.

### multi-stage compressor

    See: Compressor#Reciprocating compressors Compressor in which gases are compressed more than once, usually with cooling between stages. Used to improve efficiency and reduce temperatures.

### mung

    A brown organic deposit usually found on the ceilings of caves which is easily dislodged by diver's exhaust bubbles and then drifts down through the water.

### mushroom valve

    A rubber non-return valve flap which is circular or oval, with a stem in the middle to attach it to the holder in the centre of the grating over the orifice. Also sometimes known as umbrella valve.

### M-value

    See: Decompression theory#M-value At a given ambient pressure, the M-value is the maximum theoretical value of absolute inert gas pressure that a tissue compartment can take without presenting symptoms of decompression sickness.

## N

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |
Subsection: Top, Na, Ne, Ni, No, Nu

## Na
### NACD

    National Association for Cave Diving

### narcs

    See: nitrogen narcosis

### Navy Experimental Diving Unit

    Main article: Navy Experimental Diving Unit The United States Navy Experimental Diving Unit, the primary source of diving and hyperbaric operational guidance for the US Navy, located in Florida.

### Navy SEAL

    Main article: U.S. Navy Seals A US Navy trained combat diver.

### NDL

    See: No decompression limit
Subsection: Top, Na, Ne, Ni, No, Nu

## Ne
### neck dam

    See: Neck dam The lower part of a lightweight diving helmet which includes a neoprene or latex neck seal similar to the neck seal on a dry suit, to prevent ingress of water.

### necklace

    Bungee loop attached to the secondary regulator second stage, worn around the neck to store the secondary close under the chin, where it is protected and the diver is immediately aware of a free-flow. With a little adjustment and practice it is possible to pick it up by head and mouth movement alone, not needing use of a hand. The bungee is attached to the second stage by a breakaway connection, often a close-fitting loop over the mouthpiece, so that it can be moved away from the diver's head in an emergency without disturbing the primary second stage or the mask.

### neck thread

    See: Diving cylinder#Cylinder neck Internal screw thread in the bore of the cylinder neck to fit a cylinder valve, which controls flow of gas into and out of the cylinder.

### NEDU

    See: Navy Experimental Diving Unit

### negative buoyancy

    Also: just "negative" in context See also diving heavy Buoyancy less than weight. Insufficient upward force due to buoyancy to keep afloat or remain at constant depth

### negative entry

    See: Scuba skills#Negative entry Entry into the water in a buoyancy condition that will sink by default. When intentional, generally after reducing buoyancy of BC and, if applicable, dry suit by venting to ensure that the diver will not float back to the surface, but will continue to descend.

### negative static lung load

    See: Human physiology of underwater diving#Positive and negative pressure breathing Breathing against an external pressure slightly less than the relaxed pressure in the lungs. More effort is needed to inhale, less to exhale. This can occur when using back mounted counterlungs on a rebreather, in a steep head up position with a single-hose demand valve, or when snorkelling.

### neo-Haldanian

    Also: "neo-Haldanean" Main article Decompression theory#Critical difference hypothesis Decompression models based on later modifications of the principles described by John Scott Haldane.

### neoprene

    Main article: Neoprene Synthetic elastomer used in the form of foamed sheets as the material for most wetsuits and some drysuits.

### neutral buoyancy

    See: Buoyancy Having a fully immersed buoyancy exactly equal to weight, so that the forces are balanced and the person or object statically remains at a constant depth. Effectively average density is equal to that of the surrounding fluid medium. The state of neutral buoyancy is typically metastable for a compressible system.

### Newtsuit

    Main article: Newtsuit An atmospheric diving suit designed and originally built by Phil Nuytten.

### net cutter

    A handle with a hooked blade used to cut netting or cordage to free the diver from entanglement

### NFCI

    Non-freezing cold injury: Permanent tissue damage due to low temperature exposure without any freezing damage.
Subsection: Top, Na, Ne, Ni, No, Nu

## Ni
### niggles

    See: Decompression sickness#Signs and symptoms Minor symptoms characteristic of mild decompression sickness.

### night diving

    Main article: Night diving Diving during the hours of darkness.

### NiMH

    Nickel-metal hydride. A technology for rechargeable battery cells.

### nitrile

    Main article: Nitrile rubber A synthetic elastomer used for most standard O-ring seals.

### nitrogen

    Main article: Nitrogen The major component gas of air and many breathing gas mixtures used in diving. Important in diving as an active agent in nitrogen narcosis and decompression sickness.

### nitrogen narcosis

    Main article: Nitrogen narcosis Also known as narcs, inert gas narcosis, raptures of the deep, Martini effect: A reversible alteration in consciousness that occurs while breathing gases containing nitrogen under elevated partial pressure similar to alcohol intoxication or nitrous oxide inhalation, and can occur during shallow dives, but usually does not become noticeable until greater depths, beyond 30 meters.

### nitrox

    Also: "Enriched Air Nitrox (EAN)" Main article Nitrox Mixture of nitrogen and oxygen for use as breathing gas. Usually with oxygen percentage higher than air.

### nitrox stick

    See: Gas blending for scuba diving#Mixing the gases A mixing tube used to blend oxygen with air before compressing to make nitrox breathing gas.
Subsection: Top, Na, Ne, Ni, No, Nu

## No
### NOAA

    National Oceanic and Atmospheric Administration (US)

### no decompression limit

    See: Decompression practice#No decompression limit No decompression limit. The maximum time which a diver can remain at a specified depth without incurring a stage decompression obligation in terms of the specified decompression tables or algorithm.

### nominal capacity

    Volume of free gas contained by a scuba cylinder when filled to charging pressure at constant temperature.

### No-mount diving

    See: Side mount diving#Terminology A specialized overhead-environment strategy for dealing with particularly tight restrictions which may involve divers wearing a very basic harness or simply hand-carrying cylinders.

### normoxic

    1. A breathing gas mixture with oxygen content approximating atmospheric air.
    2. A breathing gas for diving which contains sufficient oxygen to minimise risk of hypoxia at atmospheric pressure.

### NSS

    National Speleological Society
Subsection: Top, Na, Ne, Ni, No, Nu

## Nu
### nystagmus

    Main article: Nystagmus An oscillation of the eyes alternating a slow eye movement in one direction, and a fast eye movement in the other direction.

## O

| Contents: Top | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | References |

### octopus breathing

    Sharing air using an octopus regulator

### octopus regulator

    Also: "octo reg", "safe second" See: Diving regulator#Octopus A secondary demand valve fitted to a diving first stage for use as an alternative air source for another diver in case of an emergency. Also occasionally useful as a backup in case of some kinds of malfunction of the primary

### offboard

    Not an integral part of the scuba unit. Usually applied to gas cylinders carried additional to the onboard gas of a rebreather.

### offboard gas

    Gas carried in cylinders not integrally mounted on a rebreather, but plumbed into the unit.

### off-gassing

    See: outgassing

### offshore

    1. A significant distance away to seaward from the shoreline.
    2. Outside the national maritime border, in international waters, though generally still inside the exclusive economic zone.

### offshore diver

    Main article Commercial offshore diving Colloquial term for a diver who works in the offshore oil and gas industry. A professional diver who works in regions outside the jurisdiction of national occupational health and safety and labour laws.

### O-lay

    Method for installation of sub-sea pipeline for the oil and gas industry. The pipe is constructed in an onshore construction yard, moved into the water and bent into a spiral without causing plastic deformation of the material. When the pipeline is sufficiently long it is transported with the help of tugs to the installation area. At the installation area the pipeline is unwound and pulled over a simple lay barge with stinger and installed on the bottom.

### OLED

    Main: OLED Organic light-emitting diode: A low energy high contrast light source commonly used on instrument displays such as later generation dive computers.

### omitted decompression protocol

    See:Omitted decompression procedure Procedure for managing a diver who surfaces without completing required decompression. If a chamber is available, the diver is recompressed in it, otherwise if no symptoms have presented, the diver may be recompressed as soon as practicable in the water. Various schedules for further decompression depend on the dive history.

### on at the back, off at the hat

    See: Bailout to emergency gas supply Commercial diving pre-dive check response by the diver confirming that the bailout cylinder valve is open at the cylinder (on at the back) and closed at the helmet bailout block (off at the hat).

### onboard

    Mounted directly onto the unit as an integral component. Usually applied to gas cylinders mounted on a rebreather frame or housing, or emergency gas supply cylinders mounted on a diving bell or stage.

### onboard gas

    Gas carried in cylinders mounted as part of a unit, usually a rebreather or diving bell or stage.

### on-gassing

    See: ingassing

### onshore

    See: inshore

### open circuit

    See: Underwater breathing apparatus# Breathing apparatus which discharges exhaled gas into the environment, without any further use.

### open water

    1. See: List of diving environments by type#Environments by confinement. Water where there is no physical obstruction to a direct ascent to the surface.
    2. Water which is open to influences by weather and climatic conditions.

### optrode

    Main article: Optode Optical sensor device to measure a specific substance, usually with the aid of a chemical transducer.

### OOA

    Out of air. An emergency situation where the supply of breathing gas to the diver has stopped.

### OPV

    Over-pressure valve. A pressure relief valve which automatically opens at a set pressure to allow excess gas to escape.

### O-ring

    Main article: O-ring A mechanical gasket in the shape of a torus; a loop of elastomer with a circular cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the contact surfaces.

### ori-nasal mask

    A breathing mask that covers the mouth and nose only. It may be an independent item, such as an oxygen mask or BIBS mask, or it may be a component inside a full face diving mask or helmet to reduce the amount of dead space

### OTU

    See: oxygen toxicity unit

### outgassing

    See: Physiology of decompression#Outgassing of tissues Diffusion of gas out of the tissue into the blood, and transport to the lungs where it diffuses into the lung gas and is eliminated by exhalation.

### out-of-gas incident

    See: Diving hazards Underwater diving emergency in which the breathing gas supply to the diver is interrupted.

### overhead

    A physical or procedural obstruction to a direct ascent to the surface. Physical overheads include cave, cavern or culvert ceilings, fishing nets, ship hulls, and wreckage. Procedural overheads are generally a decompression obligation.

### overrun

    Excessive rotation of a reel or spool by inertia, causing the line to unwind beyond the amount paid out, and lie in loose coils on the spool which can jam or tangle.

### overweighting

    See also: diving heavy Carrying more ballast weight than is necessary to achieve neutral buoyancy at all times during a dive. In scuba diving usually a dangerous error, but used in surface-supplied diving to stabilise the diver when working on the bottom.

### oxygen

    Main article: Oxygen Important component gas of atmospheric air and essential component of any breathing gas. Required to sustain life.

### oxygen analyzer

    Main article: Oxygen analyzer Instrument for measuring the partial pressure of oxygen in a gas mixture

### oxygen cell

    Electro-galvanic oxygen sensor for measuring partial pressure of oxygen in the breathing gas using an oxygen analyser

### oxygen clean

    See: Oxygen compatibility#Oxygen cleaning Cleaned for oxygen service by appropriate methods and materials and tested for contaminants. Verified that particulates, fibres, oils, greases and other contaminants are absent.

### oxygen clock

    See: Oxygen toxicity#Underwater A notional alarm clock, which accumulates hyperbaric oxygen exposure at a rate which increases with higher oxygen partial pressure toward the maximum single exposure limit recommended. This function is implemented as cumulative oxygen exposure for acute (CNS) oxygen toxicity in some dive computers.

### oxygen compatible

    Main article: Oxygen compatibility Made from materials which are suitable for oxygen service. Capable of coexisting with elevated oxygen concentrations and a potential source of ignition without flashing, based on a system’s maximum operating pressure and temperature.

### oxygen compatible air

    Air which has been filtered to reduce contaminants to a level suitable for blending with high pressure oxygen. Air with a reduced level of condensable hydrocarbon mist or vapour.

### oxygen service design

    See: Oxygen compatibility#Oxygen service design Design that minimizes any tendency for heat generation, ignition of particulates, or the accumulation of contaminants for an intended partial pressure of oxygen and temperature.

### oxygen fraction

    See: Breathing gas#Classification by oxygen fraction Fraction by volume or pressure of the gas mixture made up by oxygen

### Oxygen Pete

    Imaginary monster associated with CNS oxygen toxicity, also a CNS oxygen toxicity seizure.

### oxygen rebreather

    See: Rebreather#Oxygen rebreathers A closed circuit rebreather that uses only oxygen as a gas supply.

### oxygen sensor

    A component that produces a signal in response to the presence or concentration of oxygen. In diving this is usually a galvanic cell that generates a current and voltage proportional to the partial pressure of oxygen in a breathing gas.

### oxygen service

    See:Oxygen compatibility#Oxygen service Suitable for operating with significantly higher levels of oxygen than normal atmospheric air. Often implies special cleaning procedures, use of oxygen compatible materials, and design to reduce ignition risk. System or component that has been designed and tested for oxygen use, has been tested as oxygen clean and is oxygen compatible.

### oxygen toxicity

    Main article: Oxygen toxicity A condition resulting from the harmful effects of breathing molecular oxygen (O$_2$) at elevated partial pressures.

### oxygen toxicity unit

    Unit of exposure to toxic concentrations of oxygen in breathing gas, primarily in connection with pulmonary oxygen toxicity and oxidative damage to cell membranes. The accumulation of OTUs depends on time and partial pressure of exposure, and dissipation is a function of recovery time at non-toxic partial pressures.

### oxygen window

    Main article: Oxygen window in diving decompression Inherent unsaturation due to metabolic reduction of total gas pressure in the tissues.