Physics and physiology · Lesson 10
Boyle’s Law for Divers: Pressure, Volume and Depth
Boyle’s law connects pressure with gas volume. It explains why flexible air spaces change as you descend or ascend, and why the shallow part of a dive deserves careful control.
Copperband learning sheet
Pressure up. Flexible gas volume down.
Fixed gas amount, constant temperature: P₁V₁ = P₂V₂.
Logic sheet: connect the idea with the decision
Use these prompts to check your understanding before discussing practical decisions with your instructor.
Remember: Approximate seawater model. The example is not a sealed-lung model or a target diving depth.
Educational information only. This sheet does not replace official training, practical instruction or certification through a recognised scuba diving organisation such as PADI. It is not an emergency protocol, personal medical assessment or technical dive plan.
Use absolute pressure
Absolute pressure includes atmospheric pressure as well as the pressure of the water. For a simplified seawater example, use approximately 1 bar at the surface and add approximately 1 bar for every 10 metres of depth. Actual water density and atmospheric conditions vary.
At 10 metres this model gives 2 bar absolute, not 1 bar absolute. Confusing gauge pressure with absolute pressure produces incorrect gas-law calculations. State your units and assumptions before substituting numbers.
The relationship
For a fixed amount of ideal gas at constant temperature, pressure multiplied by volume remains constant: P1 × V1 = P2 × V2. If pressure doubles, the volume halves. If pressure halves, the volume doubles.
Imagine a flexible sealed gas space of 6 litres at the surface. At 2 bar absolute, its theoretical volume is 3 litres: 1 × 6 ÷ 2. At 3 bar absolute it is 2 litres. This is a simplified illustration, not a model of a freely breathing diver’s lungs or a personal buoyancy instruction.
Connect the formula with diving
Gas in a flexible BCD space changes volume as ambient pressure changes. A mask and middle ears need appropriate equalisation. Gas supplied by a regulator can expand on ascent, which helps explain why breath-holding on scuba is dangerous.
Different systems have different constraints. A rigid cylinder does not simply shrink to half its physical size at depth. An open, breathing respiratory system is not a sealed balloon. The equation is useful only when its assumptions describe the situation.
Go a little deeper
compare equal depth changes
Using the same model, travelling from 10 metres to the surface changes pressure from 2 to 1 bar, so a sealed flexible gas space doubles in volume. Travelling from 20 to 10 metres changes pressure from 3 to 2 bar, so volume increases by a factor of 1.5. Equal depth changes therefore do not create equal proportional volume changes. This explains the importance of controlled shallow-water ascents and buoyancy practice, without prescribing an ascent procedure.
| Depth | Approximate absolute pressure | Gas volume |
|---|---|---|
| Surface | 1 bar | 6 litres |
| 10 m | 2 bar | 3 litres |
| 20 m | 3 bar | 2 litres |
Check your understanding
Does Boyle’s law alone calculate a safe dive?
No. It explains a pressure–volume relationship under stated assumptions. It does not calculate decompression, gas reserves, personal medical suitability or safe operating procedures.
Connect theory with training
For a Pattaya trip, tell the team your qualification, last-dive date and learning goals so the practical programme can be matched to you.
Sources and further reading
This is an independently written educational guide, not an official PADI course manual or certification assessment. Ask your instructor about unclear concepts before applying them in the water.