Amontons' Law
“For any gas at a constant volume, the pressure of the gas is directly proportional
to its absolute temperature.” – Guillaume Amontons
P1 x T1 = P2 x T1
P and T are pressure and temperature.
Amontons’ Law describes the relationship of pressure and temperature at a constant volume. It states that when temperature increases the pressure increases if the volume stays the same, as in a rigid/inflexible container. When the molecules are excited by the increase in temperature, and begin to move faster, the pressure increases.
In theory, if a cylinder’s temperature were increased sufficiently, a catastrophic explosion would occur. The over-pressure relief disk or ‘burst disc’ prevents this from occurring. Located in the SCUBA cylinder valve , this relief mechanism is designed to rupture and vent excess pressure in the event the cylinder’s pressure exceeds it’s safe operating limit.
It’s still considered prudent not to expose a pressurized cylinder to excessive temperature. In the event the disk fails, the cylinder is unusable until it’s repaired.
SCUBA cylinders are relatively constant in volume; if the temperature of the gas increases, then the pressure in the tank increases.
GENERAL DIVE RULE: For each degree Fahrenheit change in temperature, the pressure in the cylinder would increase or decrease by 6 psi. A diver entering water that is significantly colder than the air temp. would see a corresponding drop in tank pressure.
Also referred to us Gay-Lussac’s Law, this gas law is considered generally more relevant to SCUBA diving than Charles’ Law.