The Hygrometer

  • TheHygrometer is an instrument used for obtaining the relative Humidity and DewPoint temperature of the air.
  • The onesused at sea on merchant ships is also called 
    • Mason’s Hygrometer
    • or ‘Wet & Dry Bulb Hygrometer
    • or Psychrometer
  • Itconsists of two identical Celsius Thermometers, one is called the Dry Bulb andother the Wet Bulb.
  • Both thethermometers are mounted on a single frame and enclosed in a Stevenson’sscreen.
  • The wetbulb has a single layer of muslin (Starch Free) wrapped around it with otherend dipped in bottle of distilled water.
  • Becauseof capillary action, water is drawn upward keeping the muslin damp.
  • In dryatmosphere, water in the muslin cloth will evaporate and take away heat fromthe wet bulb.
  • Wet bulbwill show lower temperature than the dry bulb, which measures the ambienttemperature.
  • Drierthe atmosphere, faster the evaporation, lesser the temperature shown by the wetbulb thermometer.
  • Thedifference between readings of dry and wet bulb is called the depression of thewet bulb.

Precautions while Using Hygrometer

  • Keep the Stevenson screen on the windwad side in open air, away from artificial source of heat like heaters or blowers, funnel, ventilators of AC etc.
  • Keep at a height of about 1.5 meters for ease of reading.
  • Keep away from bulkheads to prevent re-radiated heat affecting the instruments.
  • Keep muslin free of dust or any salt particles. Such impurities will give higher wet bulb readings. .
  • Use distilled water for the wet bulb to avoid impurities to increase wet bulb readings. Change the muslin cloth and the distilled water once a week
  • Do not allow muslin cloth to become wet which will prevent evaporation and give higher readings at the wet bulb.
  • After changing the muslin cloth, distilled water or instruments position, take readings of the wet bulb after half an hour to let the evaporation be regular and complete.
  • Readings of this instrument are not accurate for ventilatory winds below 7 knots. This shortcomings of the hygrometer has been overcome by the ‘Whirling Psychrometer’.

The meteorological tables are entered with dry bulb temperature on one axis and the depression of the wet bulb on other axis.