Expansion
Pressure rises by one atmosphere for every 10 metres of seawater
Expansion
Ambient pressure rises by 1 atmosphere per 10 metres of seawater.
Boyle’s law (volume inversely proportional to pressure)
- Gas spaces compress on descent and expand on ascent
- Breath-holding during ascent risks pulmonary barotrauma and arterial gas embolism
- Middle ear and sinus barotrauma occur if equalisation fails
Henry’s law (dissolved gas proportional to partial pressure)
- Nitrogen dissolves into tissues at depth
- Rapid ascent allows it to come out of solution as bubbles: decompression sickness, with joint pain (the bends), neurological signs and the chokes
- Treatment is recompression and hyperbaric oxygen
Partial pressure effects
- Nitrogen narcosis below about 30 metres, resembling alcohol intoxication
- Oxygen toxicity above about 1.6 atmospheres partial pressure, causing convulsions
- Helium is substituted for nitrogen in deep diving to avoid narcosis, although it causes high pressure nervous syndrome
Breath-hold diving carries a separate hazard: hyperventilation before diving lowers carbon dioxide and delays the urge to breathe, so hypoxia can cause shallow water blackout before any warning.