Pressure and temperature
Air above us has weight and pushes with 1 atmosphere (1 atm). Water is much heavier. Every 10 m of sea water adds about 1 atm. So the total (absolute) pressure at depth d metres is P = 1 + d/10 atm. At 30 m it is 4 atm.
Water also gets colder with depth. There is often a thin layer where the temperature drops quickly. Water takes heat from the body about 25 times faster than air does, so divers wear a wetsuit or drysuit.
Properties of gases for divers
Boyle's law: if the temperature is the same, the volume of a gas is smaller when the pressure is higher. P × V stays constant. At 2 atm the volume is half; at 4 atm it is one quarter. This is why air in lungs and masks gets squeezed on the way down and expands on the way up.
Dalton's law: in a mixture, each gas pushes with its own share, called its partial pressure. Air is about 21% oxygen and 79% nitrogen, so at 3 atm oxygen pushes with 0.63 atm. Too much oxygen pressure is poisonous, and nitrogen at depth can make a diver dizzy (nitrogen narcosis).
Henry's law: the higher the pressure, the more gas dissolves in liquid. More nitrogen dissolves in the blood at depth. If the diver rises too fast, it comes out as bubbles, like fizz in a shaken bottle.
Buoyancy
Archimedes' principle: water pushes up an object with a force equal to the weight of the water it pushes aside. If this push is more than the object's weight, it floats. If less, it sinks. If equal, it is neutrally buoyant, which is what a diver wants.
Divers use a weight belt to sink and a buoyancy vest (BCD) filled with a little air to fine-tune. Sea water is denser than fresh water (it has salt), so it pushes up more. As the wetsuit squeezes at depth, buoyancy falls, so the diver adds air.
Seeing and hearing underwater
Seeing: light bends when it goes from water to the air in the mask. Things look about one-third bigger and closer than they are. Water absorbs colours: red disappears in the first few metres, then orange, yellow, green. Blue lasts longest. That is why deep water looks blue and a torch shows true colours. Murky water scatters light and cuts the view.
Hearing: sound travels about four to five times faster in water than in air. The ears cannot tell the direction well, so a diver cannot easily tell where a boat engine is. Calling out does not work, so divers use hand signals.
Ocean currents
A current is moving water. Causes are tides (the daily rise and fall), wind pushing the surface, and differences in temperature and salt. Narrow channels and river mouths can have strong flow. A diver should never fight a strong current: swim across it, or let it carry you and surface signalling. Plan the dive for slack water, when the tide is changing and the flow is weak.
Marine organisms
The sea has friendly and harmful life. Learn which to avoid.
- Stinging: jellyfish, some corals and anemones.
- Spines: sea urchins, stonefish, lionfish. Never step on the sea bed.
- Bites: moray eels, barracuda, and sharks (rare, but keep calm and still).
- Venom: sea snakes and cone shells.
The rule is: look, do not touch, wear gloves and shoes, and tell the team if you are stung.
Try it
Fill a clear plastic bottle with a little air, cap it, and push it into a bucket. Watch if the sides squeeze. Float a coin on a sponge in water and notice buoyancy. In the 3D, predict the pressure at 25 m before you slide there.
Key formulas and definitions
- P (absolute, atm) = 1 + depth (m) / 10
- Boyle: P1 × V1 = P2 × V2 (same temperature)
- Partial pressure = fraction of gas × total pressure
- Buoyant force = weight of water displaced
- Neutral buoyancy: buoyant force = weight
Worked examples
1. Find the absolute pressure at 20 m in sea water.
P = 1 + 20/10 = 3 atm.
2. A 6 L balloon of air at the surface is taken to 20 m. What is its volume?
P1V1 = P2V2. 1 × 6 = 3 × V2, so V2 = 2 L.
3. At 30 m a diver breathes air (21% oxygen). Find the oxygen partial pressure.
Total P = 1 + 3 = 4 atm. Oxygen = 0.21 × 4 = 0.84 atm.
4. A diver with gear weighs 80 kg and displaces 85 L of fresh water (1 L weighs 1 kg). Does the diver float? How much weight should be added to be neutral?
Buoyant push = weight of 85 L water = 85 kg. Diver weight = 80 kg. 85 > 80 so the diver floats. Add 5 kg.
Common mistakes
- Using gauge pressure and absolute pressure as the same thing. Absolute pressure includes the 1 atm of air.
- Thinking pressure doubles every 10 m. It rises by 1 atm each 10 m (2, 3, 4, ...), but the volume of air halves from 1 to 2 atm.
- Thinking red things look red at depth. Red light is absorbed first.
- Holding the breath on the way up. The air expands and can burst the lungs.