The Dew point temperature or Saturation temperature can be defined as the temperature at which Water vapour begins to condense. The quantity of any gas in a mixture can be expressed as a pressure. The major components of compressed air are nitrogen, oxygen and water vapor, so total atmospheric pressure is composed of the partial pressures of these three gases. While nitrogen and oxygen exist in stable concentrations, the concentration of water vapor is highly variable and must be measured to be determined. Gas of unknown water vapor concentration is passed over a temperature controlled surface.