Water vapour behaves as a real gas and follows the Gas Laws.

Boyle’s Law relates pressure (P) and volume (V):

P x V =  Constant

Charles’s Law relates pressure (P) and volume (V) to temperature (T) and can be stated at constant volume:

P = Constant x T

Dalton’s Law of Partial Pressures states that the total pressure is the sum of the partial pressures of the gases present:

 P = P1 + P2 + P3.....

Combining these laws means we can relate the pressure exerted by water as a gas (vapour pressure) to volume and temperature. Relative humidity can be expressed as the ratio of the vapour pressure (p) of the actual amount of water present to its vapour pressure at saturation (ps):

  % RH = 100 x p/ps

In Relequa the volume is held constant and by simple substitution in Charles’s Law, relative humidity (p/ps) is directly related to temperature:

% RH = Constant x T

Relequa can very simply be used to demonstrate the absolute dependence of %RH on temperature and measure an important property called Equilibrium Relative Humidity