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03 / 07 — Life-support stack

Buoyancy Control Device

A wearable balloon that makes you weightless on command.

The BCD is an inflatable bladder in a harness. Add air to float, vent air to sink — it lets you hover mid-water like an astronaut, and it is why the same diver can be neutral at 5 m and at 30 m.

A litre of air lifts a kilogram — count your buoyancy

1 L 1 kg

Lift capacity
10–25 kg
Lift per litre
~1 kg (seawater 1.025)
Dump valves
2–3
Fed from
LP inflator hose, 9.5 bar

A balloon with a harness

Drag or use the arrow keys to orbit. Press plus or minus to zoom, Home to reset. Use the parts manifest to move between callouts.

Buoyancy Control Device. This 3D plate needs WebGL; the parts list below carries the same content.

Drag sideways to orbit

FIG 3.1 — Jacket, 50% inflated, Assembled

Scale: the bar is 10 centimetres on the model.

Try the variations

Where the lift lives

50%

Flip between jacket and back-inflate to see where the lift lives — and how that changes your body position.

01Air bladder

The lung of the BCD — a urethane-coated nylon bag that inflates around your torso (jacket) or behind you (wing). Displaced water is lift: 1 litre of air gives 1.025 kg of buoyancy in seawater.

Parts list5 parts

  1. Air bladder

    The lung of the BCD — a urethane-coated nylon bag that inflates around your torso (jacket) or behind you (wing). Displaced water is lift: 1 litre of air gives 1.025 kg of buoyancy in seawater.

  2. Power inflator

    Where the low-pressure hose from your first stage connects. One button feeds gas in; the mouthpiece lets you inflate orally — a skill every beginner practises, and the backup if the hose fails.

  3. Dump valve

    Over-pressure and rapid-exhaust valves at the shoulder and lower back. Pull the cord (or they vent automatically) so expanding air can escape on ascent — this is what prevents an uncontrolled rocket to the surface.

  4. Cam band

    The strap that clamps the tank to the BCD. One band on recreational rigs, two on technical setups. Webbing stretches when wet, so tighten it wet — a loose cam band is a classic pre-dive-check catch.

  5. Harness & weights

    Shoulder straps, cummerbund and quick-release buckles hold everything on. Many BCDs carry lead in ditchable integrated pockets — releasable in one tug so you can get positive fast at the surface. Ditching is a surface manoeuvre or a genuine emergency: dropping lead at depth produces an uncontrolled ascent.

Displace water, rise; vent, sink

  1. Archimedes in a jacket

    Buoyancy is displaced water. Inflate the bladder by 2 litres and you displace 2 kg more seawater — you rise. Vent 2 litres and you sink. Divers tune this continuously, in sips of air.

    Boyle's law, in reverse

    Air in the bladder grows by half again from 20 m to 10 m — then doubles over the last 10 m to the surface. The shallows are where runaway ascents happen; experienced divers start dumping before they even begin to rise.

  2. Descend: vent everything

    At the surface you dump all the air and exhale fully — a full chest of air is 4–5 litres of lift you are trying to lose. As you sink, your wetsuit compresses and loses lift, so you add short bursts to the BCD to stay neutral.

    Archimedes: 1 litre 1 kilogram

    Fresh water weighs 1.00 kg/L, seawater 1.025. So a diver who is 2 kg negative needs about 2 litres of air in the wing to go neutral — you can count your buoyancy in litres.

  3. Hover: the neutral point

    At depth you add just enough air that you neither rise nor sink. Breathe in — you rise slightly; breathe out — you fall. That lung-level fine control is the sensation divers call weightlessness.

    The lead budget

    You are weighted for the two ends of the dive: enough to sink a full wetsuit at the surface, and enough to hold a 3-minute stop at 5 m with a near-empty tank. Everything in between, the BCD makes up. A 7 mm suit loses roughly half its 5 kg of lift by 30 m; an AL80 gains 2.7 kg of lift as you breathe it down. The bladder absorbs both.

  4. Ascend: dump as you go

    Going up, ambient pressure drops and the air in your bladder expands. Left alone it would accelerate you upward — so you vent as you go, holding the ascent to 9 m/min or slower.

Feel the expanding bubble

Dive profile — one timeline, seven instruments

T+12 minAL80 · SAC 18 L/min · 5 mm suit

Cylinder

136bar

Wing

1.6L

Suit

2.5mm

Mask air

113mL

NDL left

10min

Ceiling

0m

Ascent

0.0m/min

Depth 30.0 m · ambient 4.00 bar. Drag the needle, or use the arrow keys.

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A diver hovering in a marked water column, rising and sinking with the gas in the wing. This view needs WebGL.

Neutral — the wing is holding

Archimedes, integrated

Chase the neutral point

The wing holds a fixed mass of gas, but its volume obeys Boyle. Rise and it swells, lifts harder and rises faster — the green band is where the gas you are carrying holds you still, and it moves every time you change depth.

Challenge

Get neutral at 25 m, then take your hand off the dump and let it run: the ascent alarm fires without you adding a single litre.

Where the lift lives

10m

Bladder now

3.1L

Lift

3.2kg

Net

+0.0kg

Descent rate

0m/min

Gas aboard

6.2surface L

Lift needed here

3.2kg

Ballast

6.6kg of lead

Ascent limit

9m/min

At 10.0 m the 7 mm suit still floats 3.5 kg of its 4.9 kg, so the wing has to find 3.2 kg — 3.1 litres of gas at this depth. You are carrying 6.2 surface litres, which Boyle squeezes to 3.1 L here.

Takeaway — locked

Let the diver run past 9 m/min to unlock what this proves.

7 mm suit · 6.6 kg of lead · 16 L bladder · seawater at 1.025 kg/L. Drag and body mass are approximated; the pressure terms are not.

Where should the lift live?

Three shapes, one question: where do you want the lift to live? Around your torso for a stable, upright float at the surface, or behind you for flat, quiet trim underwater. Beginners are steered to the jacket; almost everyone who keeps diving ends up moving the lift onto their back.

Jacket style

What it is
The classic rental shape — the bladder wraps around your torso like a vest, front and back.
For
  • Very stable and upright at the surface
  • Familiar, easy to don, pockets everywhere
  • Forgiving for beginners
Against
  • Squeezes your torso when inflated
  • Tends to tip you head-up underwater
  • More drag
Best for
New divers and anyone who values a high, face-up surface float.

Back-inflate

What it is
All the lift lives behind you, leaving your chest free — the recreational diver’s upgrade path.
For
  • Nothing squeezing your chest
  • Naturally horizontal trim underwater
  • Less drag, cleaner profile
Against
  • Can tip you face-forward at the surface
  • Slightly pricier
Best for
Divers past their first ~20 dives who want better trim and comfort.

Backplate & wing

What it is
The technical diver’s modular system: steel or aluminum plate, donut-shaped wing, one-piece webbing harness.
For
  • Bombproof — almost nothing to break
  • Perfectly customisable fit
  • Scales from single tank to doubles
  • A steel plate is 2–3 kg of ballast you do not have to carry as lead
Against
  • No padding or pockets by default
  • Setup learning curve
  • Travel weight (steel plate)
Best for
Technical, cave and wreck diving — and minimalists who want one rig for life.

What fails, and what you do

Why buoyancy is the skill

An over-inflated BCD at the start of an ascent is one of the most common causes of uncontrolled ascents — and of the decompression sickness that follows them.

Jacket BCDs hold you vertical and face-up at the surface; wings hold you horizontal underwater. The trade-off is real and personal.

Integrated weights you can ditch in one pull are a genuine life-safety feature at the surface — and a hazard at depth. Ditching 4 kg at 20 m is a rocket ride to the surface.

A leaking dump valve slowly deflates you all dive — you will feel yourself working harder to stay off the bottom. Check them in the pool, not on the boat.

Where this chapter connects