Air-supply methods
A diver cannot breathe water, so air must come from somewhere. There are two main ways.
- Surface-supplied: a compressor or big tank on the boat sends air down a hose. The diver can work for a long time and can talk to the boat. Helmet and full-face mask kits use this.
- Self-contained (scuba): the diver carries a cylinder on the back. No hose, so the diver moves freely, but the air runs out sooner.
Good teams always keep a spare air source ready.
Why pressure matters
Water is heavy. At the surface, air presses with 1 bar. Going down, pressure = 1 + depth ÷ 10 bar (depth in metres, sea water). At 30 m the pressure is 4 bar.
Gas gets squeezed when pressure rises. If pressure becomes 4 times bigger, the same air takes only one quarter of the space. This is Boyle's law: pressure × volume stays the same.
Descent techniques
Go down slowly and in control, often feet first along a descent line (a rope with a weight). Every metre or two, equalise your ears: pinch the nose and gently blow, so the pressure inside your ears matches the water. Never push down with pain in the ears. Stop, go up a little and try again. Check your gauge and your partner often.
Ascent techniques
Coming up is the risky part. As pressure drops, air in the body grows. If you hold your breath, the lungs can be hurt. So breathe out gently all the way up.
Also, the body soaks up extra gas at depth. If you rise too fast, the gas forms bubbles in the blood and joints. This is decompression sickness (the bends). Rise slowly, follow a table or computer, and do a safety stop (a short pause in shallow water).
Underwater surveys and work
Plan the work before you go in: who does what, how long, which signals. Common survey ways are walking a measured line along the bottom or swimming a circle around a fixed rope. Note depth, what you see and where. For work, use lifted tools, a work platform and a partner who watches the clock. A tender on the surface holds the hose and rope and answers signals. Stop at once if anything feels wrong.
Try it
Fill a small sealed bag with air in a room. Squeeze it gently: it gets smaller and pushes back. That is pressure on gas. Then in the 3D, drag the depth slider and write down the air percent at 10 m, 20 m and 30 m before you read the numbers.
Key formulas and definitions
- Pressure (bar) = 1 + depth (m) ÷ 10
- P × V = constant (Boyle's law, same temperature)
- Air used = surface rate × pressure × time
- Rule: breathe out on ascent, rise slowly
Worked examples
1. Find the pressure at 20 m depth.
P = 1 + 20 ÷ 10 = 3 bar.
2. A bag holds 12 L of air at the surface. What is its volume at 30 m?
P at 30 m = 4 bar. V = 12 ÷ 4 = 3 L.
3. A diver uses 20 L of air per minute at the surface. How much per minute at 20 m?
P = 3 bar, so use = 20 × 3 = 60 L per minute.
4. A cylinder has 2400 L of usable air. How long does it last at 20 m for the diver above?
Use is 60 L per minute. Time = 2400 ÷ 60 = 40 minutes.
Common mistakes
- Holding your breath while coming up. The air grows and can hurt the lungs.
- Thinking pressure only counts the water. At the surface the air already gives 1 bar.
- Rising fast because the air is running low. Use the spare air and rise slowly.
- Diving alone or without a surface helper.