SADP_Operating_ Instructions_0114
10 BASIC
DEFINITIONS
WATER VAPOUR PRESSURE: is 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 pressure.
DEWPOINT TEMPERATURE: is defined as 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 deg C and ice below 0 deq C.
PARTS PER MILLION BY VOLUME: PPM(v) or VPM is the ratio of the water vapour pressure to the
total gas pressure.
PARTS PER MILLION BY WEIGHT: PPM(w) is the same as VPM, except that the figure is modified
according to the ratio of the molecular weight of water vapour to the molecular weight of the carrier gas
mixture.
RELATIVE HUMIDITY: is the ratio of the actual water vapour pressure in the gas to the saturation water
vapour pressure at the same temperature.
