Basic definitions

Dewpoint temperature

The temperature to which the gas must be cooled in order that it should be saturated with water vapour (i.e. 100% relative humidity).

For practical reasons it is referred to water above 0 °C (+32 °F) and ice below 0 °C (+32 °F).

Dewpoint is expressed in degrees Celsius (°C) or degrees Farenheit (°F).

Water vapour pressure

The pressure exerted by the water vapour contained in any mixture of gases. The total pressure exerted by the gas mixture is the sum of the pressures exerted by its components – including the water vapour.  Water vapour pressure varies in direct proportion to the total gas. Expressed in units of pressure such as pascals (Pa), millibar (mbar) or millimeters of mercury (mmHg).

Saturation water vapour pressure

The maximum pressure of water vapour that can exist in a gas at a given temperature.  Expressed in units of pressure as partial water vapour pressure.

Parts per million by volume

The ratio of the water vapour pressure to the total gas pressure, expressed as ppm(v) or vpm.

Parts per million by weight

The figure is modified according to the ratio of the molecular weight of water vapour to the molecular weight of the carrier gas mixture, expressed as ppm(w).

Absolute humidity

The mass of water vapour in a unit volume of moist gas at a given temperature and pressure. Expressed as grams per cubic metre (g/m³), milligrams per litre (mg/L), milligrams per cubic metre (mg/m³) or pounds of water per million cubic feet of gas (lb/MMSCF).

Relative humidity

The ratio of actual water vapour pressure in the gas to the saturation water vapour pressure at the same temperature.