Scuba diving glossary

A typical boat briefing: check your SPG, mind your NDL, and on nitrox keep your ppO2 at or below 1.4 bar (1.4 ata). That’s three abbreviations in one sentence. This glossary explains them and 104 other terms, from absolute pressure to yoke. For the physics behind the terms, see dive theory explained; for the math, the dive theory formulas sheet; for what each level covers, the courses.

A

Absolute pressure OW

The total pressure on you: the air above the water plus the water itself. It is about 1 bar (1 atm) at sea level and rises by 1 bar every 10 m (1 atm every 33 ft) of seawater, so 3 bar at 20 m (3 ata at 66 ft).

Alternate air source OW

A spare second stage, often yellow and on a longer hose, that you carry to give air to a buddy who runs out; divers call it the octopus. Clip it on your chest, where a buddy can see it and pull it free.

Altitude OW

For divers, any place where air pressure is lower than at sea level: an airliner cabin, a mountain road, a lake above about 300 m (1000 ft). Going to altitude soon after diving raises your DCS risk, and diving there needs special procedures.

Apparent weight DM

How heavy an object is in water: its weight minus the weight of the water it displaces. A negative result means it floats; to raise an object, a lift bag must cancel its apparent weight.

Archimedes’ principle DM

Any object in water is pushed up by a force equal to the weight of the water it displaces. If that push is greater than the object’s weight, it floats; if smaller, it sinks; if equal, it is neutrally buoyant.

Automated external defibrillator (AED) Rescue

A portable device that analyzes the heart’s rhythm and, if needed, gives an electric shock, guiding you with voice prompts. Use it as soon as one is available during CPR, after moving the diver out of standing water and drying the chest.

B

Bezel AOW

The rotating ring on a compass, with marks you set around the north end of the needle to hold a heading. Many bezels also carry a reciprocal mark for the way back.

Blackout DM

A sudden loss of consciousness during a breath-hold dive, often on the way up, with no warning. Hyperventilating beforehand delays the urge to breathe while oxygen keeps falling, so breathe normally first and never breath-hold alone.

Boyle’s law DM

At a constant temperature, the volume of a gas varies inversely with absolute pressure: P1 × V1 = P2 × V2. Double the pressure and the volume halves; the biggest changes happen near the surface.

Buddy check OW

The check you and your buddy make of each other’s gear before every dive: valve open and both second stages breathing easily, BCD inflating and deflating, weights and how they release, straps and buckles, then a last look.

Buoyancy control device (BCD) OW

The jacket that holds your tank and lets you adjust your buoyancy: its inflator adds air from your tank, and the deflate button and dump valves let it out. At the surface, an inflated BCD keeps you afloat without effort.

Buoyant emergency ascent OW

The last-resort way up when you are out of air, too deep to swim up and unable to reach your buddy: drop your weights and rise, exhaling all the way. It is fast and risky, but drowning is worse.

Burst disc DM

A small safety disc in the cylinder valve that ruptures if the pressure rises dangerously, for example when a cylinder overheats in the sun or is overfilled. It protects the cylinder but releases all of its gas.

C

Carbon dioxide (CO2) OW

The waste gas you breathe out; rising CO2 in your blood, not a lack of oxygen, is what makes you want to breathe. Hard work and fast, shallow breathing let it build up, so you feel short of air.

Carbon monoxide (CO) DM

A poisonous gas that can contaminate a fill when a compressor draws in exhaust fumes or overheats its oil. It binds to hemoglobin far more readily than oxygen and blocks oxygen transport; depth multiplies its effect.

Cardiopulmonary resuscitation (CPR) OW

Chest compressions and rescue breaths for someone who is unresponsive and not breathing normally. For an adult: 30 compressions, then 2 breaths, at 100 to 120 compressions a minute, 5 to 6 cm (2 to 2.4 in) deep, on a firm surface out of the water.

Ceiling AOW

When your computer shows a required decompression stop, the shallowest depth you may ascend to until the stop clears. Stay at that depth or slightly below it, never above, and check that your gas covers the stop.

Charles’s law DM

At constant pressure, the volume of a gas is proportional to its absolute temperature. In a rigid cylinder, pressure follows temperature the same way (often credited to Gay-Lussac), so a hot fill reads lower once it cools.

Compartment DM

In a decompression model, a theoretical tissue that takes on and releases nitrogen at its own speed, set by its half-time. It is a mathematical tool, not a real organ; the one closest to its limit controls the dive.

Conservatism OW

A dive computer setting that adds a safety margin by giving you shorter no-decompression limits. Learn how yours works before relying on it; in a buddy team, follow the more conservative computer.

Controlled buoyant ascent Rescue

The way to bring an unresponsive diver up from depth: hold them close with the airway open, add small bursts of air to their BCD to start up, then vent as the air expands. If the ascent runs away, let go.

D

Dalton’s law DM

The total pressure of a gas mix is the sum of the partial pressures of its gases, each equal to its fraction times the absolute pressure. Breathing air at 30 m (4 bar), your ppO2 is 0.84 bar.

Dead space DM

The part of each breath that stays in your airways, snorkel or mouthpiece and never reaches the tiny air sacs of your lungs. Fast, shallow breaths are mostly dead space and clear little CO2; slow, deep breaths work best.

Decompression sickness (DCS) OW

Injury caused by nitrogen bubbles forming in your body during and after the ascent; signs such as joint pain, tingling, rash or weakness usually appear within 6 hours. First aid: oxygen, lying down, emergency services and a diving hotline.

Decompression stop OW

A stop at a set depth and for a set time that your computer requires once you have stayed beyond the no-decompression limit. Unlike a safety stop, it is mandatory: skipping it sharply raises your risk of DCS.

Demand valve Rescue

An oxygen valve with a mask that gives close to 100% oxygen, but only when the diver breathes in. It suits a breathing diver who can hold the mask on and keep a good seal.

DIN fitting OW

A regulator connection that screws into the tank valve, trapping the O-ring, instead of clamping over it like a yoke. It holds higher pressures than a yoke.

Dive computer OW

The instrument that measures your depth and time, runs a decompression model and shows your no-decompression limit, ascent rate, safety stop and surface interval. It is personal: one per diver, worn on every dive of the day.

Divers Alert Network (DAN) OW

A diving safety organization that runs emergency hotlines for divers. After a suspected diving injury, call local emergency services and a diving hotline such as DAN’s: they advise on care and help arrange recompression treatment.

Downstream valve DM

The valve design used in most second stages: it opens with the gas flow. If the intermediate pressure creeps too high, the regulator free-flows instead of locking shut, so you can still breathe from it.

Drift dive AOW

A dive on which you let the current carry you along the site while the boat follows, often tracking a buoy towed by the guide. Stay close to the group, and deploy your surface marker buoy before you surface.

Dump valve OW

A valve that lets air out of your BCD quickly, usually opened by pulling a cord or handle; air escapes from the highest point, so use the valve that is highest in your position. Lift bags have one too, to slow their rise.

Duty of care DM

The reasonable care a dive professional owes divers: dives planned for their abilities and the conditions, warnings about known hazards, attentive supervision and help in an emergency. In many legal systems, breaching it and causing harm is negligence.

E

EAN32 AOW

Enriched air nitrox with 32% oxygen, the most common blend; the number after EAN is the oxygen percentage. At a ppO2 of 1.4, its maximum operating depth is 33 m (111 ft); for EAN36, it is 28 m (95 ft).

Emergency action plan Rescue

A written plan for each site or trip: local emergency numbers and how to call, the location to give, the nearest medical facility and recompression chamber, a diving hotline, where the oxygen and first aid kit are, and how to evacuate.

Emergency swimming ascent OW

An out-of-air option when your buddy is too far away and you are shallow, roughly 6 to 9 m (20 to 30 ft) or less: swim up at a normal rate, regulator in your mouth, exhaling continuously, one hand above you.

Equalize OW

To add air to an air space, such as your middle ears, sinuses or mask, so its pressure matches the water’s. Do it early and often on the way down, gently: blow against a pinched nose, swallow or move your jaw.

Equivalent air depth (EAD) DM

The depth at which air would give the same nitrogen partial pressure as your nitrox mix: EAN32 at 30 m has an EAD of about 24 m. Don’t use it to plan for narcosis.

Eustachian tube OW

The narrow tube that links your middle ear to your throat. When you equalize, you send air up it to balance the pressure on your eardrum; a cold or an allergy can make it swell shut.

Exhaust valve AOW

The valve that lets air out of a dry suit, usually on the upper left arm. To vent, make it the highest point, for example by raising that arm; some valves vent automatically.

Expanding square AOW

A search pattern that starts at the last known position: swim short legs with 90° turns, and make the legs longer as you go. It suits a fairly flat bottom and reasonable visibility.

F

First stage OW

The part of the regulator that mounts on the tank valve. It lowers the tank’s high pressure to an intermediate pressure for the hoses to your second stages and inflator; a high-pressure hose feeds your pressure gauge.

Free-flow OW

A regulator delivering air nonstop, often in cold water or after a knock. Keep it in your mouth with your lips loose, sip the air you need, and start an ascent with your buddy: it empties a tank fast.

Frog kick AOW

A kick that pushes water backward rather than down, with your knees bent and fins up. Near the bottom or a reef, it avoids lifting silt, unlike a flutter kick, which drives water downward.

G

Gas embolism OW

Arterial gas embolism: air forced into the bloodstream by a lung overexpansion injury, which can reach the brain. Signs such as confusion, weakness or unconsciousness come suddenly, within minutes of surfacing; it is the most urgent diving injury.

Gauge pressure DM

Pressure counted from the water alone, leaving out the atmosphere: at 25 m, it is 2.5 bar, while the absolute pressure is 3.5 bar. Cylinder gauges show it; gas laws need absolute pressure.

Gradient factor DM

A setting that keeps your computer’s limits to a percentage of the margin between ambient pressure and the M-value. Lower numbers are more conservative: with a GF high of 70, you surface using at most 70% of that margin.

H

Half-time DM

The time a compartment takes to cover half the difference between its nitrogen tension and the surrounding partial pressure. After six half-times, it is about 98% of the way, which models treat as saturated.

Head tilt and chin lift Rescue

The way to open the airway of an unresponsive person: tilt the head back with a hand on the forehead and lift the chin with your fingertips, so the tongue no longer blocks the throat.

Heading AOW

The compass direction you swim, in degrees. Point the lubber line at your target and set the bezel marks around the needle; underwater, keep the needle between the marks and swim where the lubber line points.

Henry’s law DM

At a given temperature, the amount of gas that dissolves in a liquid is proportional to the partial pressure of that gas. It is why your tissues take on nitrogen at depth and release it as you ascend.

Hypercapnia DM

Too much carbon dioxide in the blood, causing breathlessness, headache and confusion, and at high levels unconsciousness. In diving, it comes from hard work, skip breathing, dense gas at depth or a regulator that breathes hard.

I

Inflator OW

The valve, fed by a low-pressure hose, that adds air from your tank to your BCD at the push of a button; most also have a deflate button and a mouthpiece for inflating by mouth. A dry suit has its own inflator.

Intermediate pressure DM

The pressure the first stage delivers to the hoses, kept a set amount above the surrounding water pressure, often about 9 to 10 bar. If it creeps too high, a downstream second stage free-flows.

K

Kick cycle AOW

One complete cycle of your fin kicks, counted to estimate distance underwater. Count your cycles over a known distance at a relaxed pace beforehand; current, fatigue and gear change the count.

L

Lift bag AOW

A bag you fill with air to raise an object: each liter of air lifts about 1 kg. Fill it from your alternate air source, control it with its dump valve, and never attach yourself to it.

Lubber line AOW

The fixed line on a compass that shows the direction you are facing. Hold the compass level with the lubber line along the center of your body, and always swim where it points.

Lung overexpansion injury OW

Lung damage caused by expanding air when you hold your breath while ascending: gas embolism, a collapsed lung (pneumothorax), or air in the chest or under the skin. All are emergencies needing oxygen and medical care.

M

M-value DM

In a decompression model, the highest nitrogen tension a compartment may hold at a given ambient pressure. Your computer keeps you below it, often with extra margin such as gradient factors or a conservatism setting.

Maximum operating depth (MOD) AOW

The depth at which a nitrox mix reaches your ppO2 limit. Divide the limit by the oxygen fraction, subtract 1, multiply by 10 m (33 ft) and round down: at 1.4, EAN32 gives 33 m (111 ft).

N

Narcosis OW

Nitrogen narcosis: the alcohol-like effect of breathing nitrogen under pressure, with slow thinking, overconfidence and poor judgment. Most divers notice it around 30 m (100 ft); it fades when you ascend a few meters.

Natural navigation AOW

Finding your way with clues from the site itself: sand ripples, depth, reef edges, landmarks, light and current. Combine it with your compass, and look back on the way out to see the view you will need for the return.

Neutral buoyancy OW

The state in which you neither sink nor rise, because the water’s upward push equals your weight. A bigger push gives positive buoyancy and you float; a smaller one gives negative buoyancy and you sink.

Nitrogen tension DM

The pressure of the nitrogen dissolved in a tissue. If it is lower than the surrounding partial pressure, the tissue takes on gas; if higher, it releases gas, and the bigger the difference, the faster the exchange.

Nitrox AOW

Enriched air: a breathing gas with more oxygen than the 21% in air, usually 32% or 36%. Less nitrogen gives you longer no-stop times, but the oxygen sets a maximum depth, and nitrox doesn’t let you go deeper.

No-decompression limit (NDL) OW

How long you can still stay at your current depth and then ascend directly to the surface without mandatory stops. Your computer shows it; it gets shorter as you go deeper, so end the dive well before it runs out.

No-fly time OW

The time your dive computer tells you to wait before flying. Follow the longer of it and the DAN guidelines: at least 12 hours after a single no-decompression dive, 18 hours after several, longer after decompression dives.

No-stop time AOW

The time left before you would need decompression stops: your no-decompression limit, counted down by your computer. It drops quickly when you go deeper, so start up with a margin rather than waiting for zero.

Non-rebreather mask Rescue

An oxygen mask with a reservoir bag, for a breathing diver who can’t use a demand valve. Run oxygen at 15 L/min (15 lpm) and fill the reservoir bag before placing the mask on the face.

O

Octopus OW

Divers’ name for the alternate air source, the spare second stage you carry for a buddy who runs out of air. It is often shortened to octo.

Offgassing DM

The release of dissolved nitrogen from your tissues, mostly during and after the ascent: the blood carries it to the lungs and you breathe it out. A slow ascent and a safety stop help it happen without forming bubbles.

Overhead AOW

Any place where you can’t swim straight up to the surface, such as the inside of a wreck. Going under an overhead needs specific training: you can get lost, silt can wipe out visibility, and your gas must cover the way out.

Oxygen toxicity AOW

Harm caused by breathing oxygen at a high partial pressure. Above a ppO2 of about 1.4 bar (1.4 ata), it can affect the nervous system and cause a convulsion, often without warning; long exposures can also irritate the lungs.

P

Panic OW

The loss of reasoned control when stress takes over: the diver acts on instinct, often bolting for the surface or clinging to anyone in reach. Spotting stress early, in yourself and others, stops it from turning into panic.

Panicked diver Rescue

A diver who has lost reasoned control: wide eyes, mask and regulator often off, fighting to keep the head high, not answering you. They may grab a rescuer, so approach from behind or below and make them buoyant fast.

Partial pressure AOW

The part of the total pressure due to one gas in a mix: its fraction times the absolute pressure. Your body responds to partial pressure, not percentage, so the same gas can be safe at one depth and harmful at another.

Partial pressure of oxygen (ppO2) AOW

The oxygen fraction of your gas times the absolute pressure. Recreational divers keep it at or below 1.4 bar (1.4 ata) during the dive, and treat 1.6 bar (1.6 ata) as an absolute maximum for contingencies only.

Patent foramen ovale (PFO) DM

An opening between the upper chambers of the heart, found in about a quarter of adults. It can let bubbles from the veins bypass the filter of the lungs, and it is linked to some forms of DCS.

Pneumothorax OW

A collapsed lung: air leaks into the space around a lung, causing chest pain and breathlessness. In diving, it is one of the lung overexpansion injuries caused by holding your breath while ascending, and an emergency.

Pocket mask Rescue

A small folding mask for giving rescue breaths. It gives the easiest seal, its one-way valve keeps water and vomit away from you, and its inlet lets you add oxygen at 15 L/min (15 lpm).

Pressure gauge (SPG) OW

The submersible pressure gauge, which shows how much air is left in your tank, in bar. Check it often, and more often the deeper you go, so you reach the surface with your reserve.

Purge button OW

The button on a second stage that sends a burst of air through it to blow water out. After a dive, you also press it to release the pressure in the hoses once the tank valve is closed.

R

Reciprocal AOW

The opposite of a compass heading, used to come back the way you went: add or subtract 180°, so out on 70°, you come back on 250°. Many compass bezels have a reciprocal mark.

Recompression chamber OW

A sealed chamber in which medical staff raise the pressure around a diver to treat decompression sickness or gas embolism, usually while the diver breathes oxygen. Never put an injured diver back in the water instead.

Recovery position Rescue

A stable position on the side for a person who is unresponsive but breathing, or who is vomiting. It keeps the airway open and lets fluids drain from the mouth; keep checking their breathing.

Regulator OW

The device that lowers your tank’s high pressure so you can breathe from it. Its first stage mounts on the tank valve; hoses carry air to the second stage in your mouth, which delivers air only when you inhale.

Repetitive dive OW

Any dive made while your body still holds extra nitrogen from an earlier dive. Your computer takes that nitrogen into account and gives you shorter limits, so make the deepest dive of the day first.

Rescue breath Rescue

A breath you blow into a person who isn’t breathing: mouth to mouth, mouth to nose or through a pocket mask. In the water, keep giving them while you tow; on land, combine them with chest compressions in CPR.

Reserve OW

The air you keep for the end of the dive and the unexpected: plan to reach the surface with at least 50 bar (500 psi), or the reserve set in the briefing. Signal your buddy as soon as you reach it.

Residual nitrogen DM

Nitrogen still dissolved in your body from earlier dives. Your computer keeps tracking it between dives and shortens the limits of your next dive; slow compartments can hold it for many hours.

Reverse block OW

Air trapped in your ears or sinuses that can’t escape on the way up, often when a decongestant wears off. You feel pressure or pain while ascending: stop, go down slightly, then come up more slowly.

Roll call DM

The headcount taken before a dive (who goes in) and after it (who is back), with each diver confirming their buddy by name. The boat never leaves until the count matches; if it doesn’t, the missing-diver procedure starts.

S

Safety stop OW

A recommended stop of 3 minutes at 5 m (15 ft) before surfacing, on every dive unless an emergency prevents it. It gives your body extra time to release nitrogen and helps you control the last part of the ascent.

Second stage OW

The part of the regulator you hold in your mouth, which delivers air only when you inhale; its purge button blows water out of it. Your alternate air source is a spare second stage.

Second victim OW

A would-be rescuer who gets into trouble too, for example by searching alone underwater or swimming out without anything that floats. Your own safety comes first: a rescuer in trouble can’t help anyone.

Silent bubble DM

A small bubble in the veins of a diver who stayed within the limits and feels fine, detected with Doppler ultrasound. The lungs usually filter them out; they are one reason for slow ascents and safety stops.

Skip breathing DM

Holding each breath to make your air last longer. It saves little air, lets carbon dioxide build up and often ends in a headache: breathe slowly and continuously instead, and never hold your breath while ascending.

Squeeze OW

Pain or injury when an air space, such as your middle ear, a sinus or your mask, isn’t equalized as you descend and the water pressure squeezes it. Equalize early and often; for your mask, breathe out through your nose.

Supersaturation DM

The state of a tissue whose nitrogen tension is higher than the surrounding partial pressure, as during the ascent. Some supersaturation is how you offgas; too much, too fast, and the gas forms bubbles.

Surface air consumption (SAC) DM

Your breathing rate at the surface, in liters per minute, also called RMV. Multiply it by the absolute pressure to get your consumption at depth: 20 L/min becomes 60 L/min at 20 m.

Surface interval OW

The time you spend out of the water between two dives. During it, your body releases extra nitrogen; your computer counts it and uses it to set the limits of your next dive.

Surface marker buoy OW

A tall inflatable tube that shows boats and your crew where you are. Deploy it before you surface in a current or in boat traffic, so the boat can see you and keep clear.

T

Thermocline AOW

A layer where the water temperature drops sharply with depth. Below it, the water can be much colder, so dress for the temperature at the bottom, not at the surface.

Tired diver Rescue

In rescue terms, a diver in difficulty who still responds and follows instructions, but is out of breath, cramping or exhausted. Make them buoyant, reassure them and help them back before they become a panicked diver.

Trim AOW

Your body angle in the water. Horizontal trim lets you move with less effort and keeps your fins off the bottom; moving weight toward your head or your hips changes it.

Turn pressure OW

The tank pressure, agreed before the dive, at which you and your buddy start back; also called turn-around pressure. It must leave enough air for the way back, the ascent, the safety stop and your reserve.

Y

Yoke OW

A regulator connection that clamps over the tank valve, sealing on the valve’s O-ring; also called A-clamp or INT. The other common type is the DIN fitting, which screws into the valve.