Dive Theory
What the exam tests
The Dive Theory domain tests physics, physiology, equipment, dive planning (RDP/computer), and environment. Expect calculations and 'which is true' questions that mix correct facts with plausible but wrong units or assumptions.
Pressure and gas laws
- Absolute pressure = depth/10 + 1 in metres of seawater; 10 m is approximately 1 atm/bar.
- Boyle's law: P1V1 = P2V2. Use absolute pressure, not gauge pressure.
- Dalton's law: total pressure is the sum of partial pressures; ppO2 = fraction O2 × absolute pressure.
- Henry's law: gas solubility increases with pressure and decreases with temperature; this underpins nitrogen absorption and DCI.
Physiology
- Lung overexpansion injuries: pneumothorax, mediastinal emphysema, subcutaneous emphysema, AGE. Never hold your breath on ascent.
- DCI: 100% O2, lay the diver flat, call EMS, monitor, and do not re-immerse.
- Nitrogen narcosis: impaired judgement at depth; ascend to relieve.
- Oxygen toxicity: CNS and pulmonary; keep ppO2 at or below 1.4 atm for normal diving (1.6 atm maximum for decompression/contingency).
- Barotrauma: squeeze; equalise early and often.
Worked example
A diver has a 12 L cylinder at 200 bar and a SAC of 20 L/min. The dive averages 20 m and you want a 50 bar reserve. How long can the dive last?
Reasoning: usable gas = 12 L × (200 − 50) bar = 1,800 L. At 20 m, absolute pressure is 3 atm, so consumption is 20 × 3 = 60 L/min. Time = 1,800 ÷ 60 = 30 min. If the diver's SAC is higher under workload or the average depth is greater, recalculate; never plan to the exact reserve.
Traps that cost marks
- Using gauge pressure in gas-law calculations.
- Mixing up volume and pressure in Boyle's law.
- Confusing DCI and AGE signs, or re-immersing a DCI victim.
- Assuming ppO2 is safe because the mix is 'nitrox'; depth matters.
- Forgetting wetsuit compression reduces buoyancy at depth.