- Absolute humidity is the mass of water vapour present in a given volume of gas at
defined temperature and pressure (expressed as g of H2O/m3 of gas). - Relative humidity is the mass of water vapour present in a given volume of gas,
expressed as a percentage of the mass of water vapour required to saturate the same volume of gas at identical temperature and pressure. - The amount of water vapour required to saturate a known volume of gas increases with temperature, i.e. a gas saturated at 20°C contains less water than the same volume, saturated at 37°C
- Relative humidity (RH), can be calculated from the ratio of the mass of water vapour present (mP), to the mass required for satruation (mS) as
RH = mP/mS - If droplets are present, supersaturation has occurred and relative humidity exceeds 100%.
- From the gas laws, mass of a gas in a mixture is proportional to the partial pressure it exerts, thus: RH = water vapour pressure/ saturated water vapour pressure
- Instruments used to measure humidity are called hygrometers. Examples include:
Regnault’s hygrometer
Hair hygrometer
Wet and dry bulb thermometers
Humidity transducers
The Lay Medical Man 