49 terms — 17 letters

The words on the gauges.

Every term a chapter uses without stopping to explain, in one line each. Aliases are the other names you will hear on the boat; the chapter links go to where the idea is taken apart properly.

49 terms, one line each.

Sorted by the name divers actually use. Bühlmann files under B, the gradient factor under G, and the 50-bar rule under R for reserve.

A

Ambient pressure

Also: absolute pressure

The total pressure around you: 1 bar at the surface plus 1 bar for every 10 m of seawater. 4 bar at 30 m.

Archimedes' principle

Also: Archimedes · buoyancy

A body in water is pushed up by the weight of the water it displaces. A litre of displaced seawater is 1.025 kg of lift.

Ascent rate

Also: 9 m/min · ascent speed

How fast you rise: no faster than 9 m/min on modern computers, and slower in the last 10 m, where bladder air doubles.

B

Backplate & wing

Also: BP/W · wing · backplate

A metal plate, a one-piece webbing harness and a back-mounted bladder — the modular BCD of technical diving. A steel plate doubles as ballast.

Boyle's law

Also: Boyle

At constant temperature, gas volume is inversely proportional to pressure. Halve the ambient pressure and the air in your BCD doubles.

Buddy check

Also: BWRAF · pre-dive check

The pre-dive check of each other’s kit: BCD, Weights, Releases, Air, Final OK. It catches the half-open valve and the loose cam band.

Bühlmann

Also: Buhlmann · ZHL-16 · ZHL-16C

The decompression model behind most dive computers: 16 tissue compartments with half-times from 4 to 635 minutes, each with its own M-value.

Computer

Burst disc

Also: burst disk · rupture disc

A thin metal disc in the cylinder valve that ruptures near hydrostatic test pressure, venting the tank rather than letting it explode.

Tank

C

Cam band

Also: tank band · cam strap

The cam-buckled strap that clamps the cylinder to the BCD. Webbing stretches when wet — tighten it wet, and check it in the buddy check.

Ceiling

Also: deco ceiling · decompression ceiling

The shallowest depth you can ascend to without a tissue exceeding its M-value. Above it, a decompression stop is mandatory.

Computer

Cracking effort

Also: cracking pressure · inhalation effort

The suction needed to open a second stage, measured in centimetres of water — about 2.5–3.8 cm H₂O on a good regulator.

Regulator

Cummerbund

Also: waist band

The wide Velcro waist band on a jacket BCD that keeps the rig from riding up.

D

Dalton's law

Also: Dalton · partial pressure

The pressure of a gas mix is the sum of the partial pressures of its parts. Air at 4 bar is 0.84 bar of oxygen and 3.16 bar of nitrogen.

DCS

Also: decompression sickness · the bends

Bubbles forming in tissues and blood from dissolved nitrogen after ascending too fast or staying too long. Joint pain, rash, numbness — emergency oxygen and a call to the emergency services or DAN.

Computer

DIN

Also: DIN fitting · DIN valve

A screw-in regulator connection that captures the O-ring inside the valve. Rated to 300 bar; standard for technical and cold-water diving.

Ditchable

Also: ditchable weight · ditch weights

Ballast you can release in an emergency to become positively buoyant. Dropping it at depth causes an uncontrolled ascent; it is a surface manoeuvre.

E

EAN32

Also: nitrox · enriched air · EANx · EAN36

Enriched air nitrox with 32% oxygen (36% for EAN36). Less nitrogen means longer no-stop times; more oxygen means a shallower MOD.

Equalise

Also: equalize · equalisation · ear clearing

Matching the pressure in an air space — ears, sinuses, mask — to the water outside. Done early and often on descent.

Mask

F

First stage

Also: 1st stage

The part of the regulator on the tank valve. Drops cylinder pressure to an intermediate pressure referenced to the water around you.

Flutter kick

Also: scissor kick

The vertical scissor kick every swimmer knows: fast and intuitive, but the up-stroke is mostly recovery, and it stirs silt off a bottom.

Fins

Freeflow

Also: free-flow · free flow

A regulator stuck open, dumping gas continuously — usually from freezing or a jammed purge. You can still breathe from it while you ascend.

Regulator

Frog kick

Also: helicopter turn · back kick

Sweep the fins out, snap them together, glide — the recovery hides in the glide with the blades feathered edge-on, so it is efficient and silt-safe. Helicopter and back kicks are its precision cousins.

Fins

G

Glideskin

Also: glide skin · smoothskin · smooth skin

Neoprene with a smooth, unlined surface that seals against skin. Used on wrist, neck and ankle seals to stop water flushing through the suit.

Wetsuit

Gradient factor

Also: GF · GF high · GF low

A dial on the Bühlmann model: how much of the gap between ambient pressure and the M-value you allow yourself to use. GF 85 uses 85%.

Computer

H

Henry's law

Also: Henry

The amount of gas dissolving into a liquid is proportional to its partial pressure. More depth, more nitrogen in your tissues.

Computer

Hydrostatic test

Also: hydro · hydrostatic

A periodic pressure test (typically every five years) in which a cylinder is filled with water and pressurised to about 1.5–1.7× its working pressure. Out of hydro, no fill.

Tank

I

Integrated weights

Also: weight-integrated · weight pockets

Lead carried in quick-release BCD pockets instead of on a belt. Ditchable in one pull — at the surface.

IP

Also: intermediate pressure · interstage pressure

The pressure a first stage delivers to the hoses: about 9.5 bar above ambient. Rising IP — “creep” — is a first stage asking for service.

M

M-value

Also: maximum value · tolerated overpressure

The highest nitrogen pressure a Bühlmann tissue compartment tolerates at a given ambient pressure. Exceed it and you have a ceiling.

Computer

MOD

Also: maximum operating depth

The deepest you can take a gas before its oxygen partial pressure passes the limit (1.4 bar): MOD = (1.4 ÷ FO₂ − 1) × 10 m. 33 m for EAN32.

Tank

N

Narcosis

Also: nitrogen narcosis · narked · rapture of the deep

The anaesthetic effect of nitrogen at raised partial pressure — noticeable from about 30 m on air: slowed thinking, euphoria, tunnel vision. Ascending fixes it in minutes.

NDL

Also: no-decompression limit · no-stop limit · no-stop time

How long you can stay at a depth and still ascend straight to the surface without mandatory stops. About 56 min at 18 m, 20 at 30 m, 9 at 40 m.

Computer

Neoprene

Also: foamed neoprene

Synthetic rubber foamed with nitrogen bubbles: the insulating, buoyant material of wetsuits. About 0.7 kg of lift per millimetre at the surface, compressing with depth.

O

O-ring

Also: o ring

A rubber ring that makes a seal under pressure. The most common cause of a leak anywhere in the kit — and the cheapest to fix.

Octopus

Also: octo · alternate air source · AAS

A spare second stage on a longer hose, usually yellow, so an out-of-gas buddy can breathe from your tank.

Regulator

Oxygen toxicity

Also: CNS toxicity · ox-tox · O2 toxicity

Convulsions from breathing oxygen at a partial pressure above ~1.4–1.6 bar. The reason every nitrox mix has a maximum operating depth.

Tank

R

Reserve

Also: 50 bar · minimum gas · rock bottom

The gas you plan never to breathe: 50 bar for recreational diving. It is the margin for a buddy who runs out, a current, or a slow ascent.

Tank

S

SAC / RMV

Also: SAC · RMV · surface air consumption · respiratory minute volume

Your breathing rate scaled to the surface, in litres per minute. A relaxed diver is around 15–20 L/min; multiply by ambient pressure in bar for the real drain at depth.

Safety stop

Also: 3 minutes at 5 m

A 3-minute pause at 5 m at the end of every dive. Not demanded by the model, always done in practice — it lets fast tissues off-gas before the surface.

Second stage

Also: 2nd stage · demand valve · mouthpiece

The part in your mouth. Opens when you inhale, delivers gas at ambient pressure, closes when you stop.

Regulator

Snell's law

Also: Snell · refraction

Light bends when it crosses between water (n = 1.33) and air (n = 1.00). Through a flat mask lens, objects look 33% larger and 25% closer.

Mask

SPG

Also: submersible pressure gauge · pressure gauge

The gauge on the high-pressure hose that reads cylinder pressure in bar. Check it every few minutes; plan to surface with 50 bar.

Squeeze

Also: barotrauma · mask squeeze · ear squeeze

Pain or injury when an air space shrinks on descent and is not equalised — ears, sinuses, mask, even a drysuit.

T

Trilaminate

Also: trilam · shell suit · membrane drysuit

A thin, non-stretch, three-layer waterproof fabric used for drysuits. No insulation of its own — warmth comes from what you wear underneath.

Wetsuit

Trim

Also: horizontal trim · body position

Your body’s attitude in the water. Flat and horizontal means the smallest frontal area, the least drag, and fins that push you forward instead of up.

V

Valsalva

Also: Valsalva manoeuvre · pinch and blow

Pinch your nose and blow gently: the most common ear-equalising technique. Gently — forcing it can injure the inner ear.

Mask

Venturi

Also: venturi assist · dive/pre-dive switch

Once gas is flowing through a second stage, its own speed lowers the pressure at the diaphragm and helps hold the valve open. The pre-dive switch turns this off to stop surface freeflows.

Regulator

Visual inspection

Also: VIP · visual · annual inspection

The annual look inside a cylinder for corrosion, pitting and thread damage — separate from the hydro, and the only way to find rust you cannot see from outside.