Amontons law

Discovered by Guillaume Amontons (1663-1705) and published in detail by Thomas Graham (1805-1869), it is also called 'Graham's Law'. In simple words, it states:

'The quotient of pressure (p) and temperature (T) in a confined amount of gas at equal volume (V) remains constant.'

In the above formula, temperature is to be expressed in Kelvin [K]. The Kelvin scale starts at the lowest conceivable temperature (0K = -273°C) and has the same graduation as the Celsius centigrade scale. For conversion from Celsius centigrade [°C] to Kelvin [°K] you just have to add +273 (ie 273°K = 0°C, or 373° K = 100°C).

Practical relevance: During (rapid) compression the compressed gas inside a hyperbaric chamber warms up (see below under adiabatic compression). When the target pressure is reached and all valves have been closed, the compressed gas is slowly cooled down to ambient temperature by temperature exchange through the chamber wall. According to Amontons' Law, this is accompanied by a drop in pressure, for which a correction must be made in order to keep the pressure at therapeutic levels.

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