Humidity Measurement

Atmospheric humidity is measured using a dry bulb and wet bulb hygrometer or psychrometer or an electrical hygrometer. The dry bulb and wet bulb hygrometer or psychrometer comprise two thermometers. The dry bulb thermometer gives the ambient temperature. The wet-bulb thermometer has, around its bulb, a muslin cloth the other end of which is in a reservoir of distilled water. The water rises up the muslin and evaporates drawing heat from the bulb and hence reducing its temperature. So the wet-bulb thermometer gives the lowest temperature to which the air can be cooled by the evaporation of water. The rate at which the water evaporates depends on the relative humidity. With high relative humidity, the rate of evaporation will be slow so the wet bulb temperature will be relatively high. Conversely, if the air is dry the evaporation will be rapid and the wet bulb temperature will be much lower than the dry-bulb temperature. 

Dry-bulb and Wet-bulb Hygrometer or Psychrometer 

  • If air is dry, water will evaporate from the muslin covering the wet bulb and latent heat will lower the temperature. 
  • If air is saturated, no evaporation will occur and thermometers will read the same. 
  • Dew point, relative humidity and HMR are read from tables or slide rule by entering with the two temperatures obtained. 

Dew Point Temperature 

Dew point (DP) is the temperature to which air must be cooled at constant pressure for saturation to occur. 

Note that the dew point temperature is not the same as the wet-bulb temperature (except at saturation).

The dew point has a lapse rate of 0.5°C/1000 ft Wet bulb = dry-bulb (= dew point) – 100% RH (saturation)

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