The Whirling Psychrometer

  • Principle of operation is same as the Hygrometer.
  • To operate, the frame is held horizontal and one drop of water is dropped on to the muslin cloth.
  • The frame is whirled in open air for at least two minutes.
  • Rate of evaporation at the Wet Bulb depends upon the speed of air flowing past the bulb.
  • It reaches maximum at 7 knots wind speed.
  • So readings unchanging and most reliable at speeds of 7 knots and above.
  • In normal hygrometer, the speed of wind may vary, but in Whirling Psychrometer, speeds in excess of 7 knots are guaranteed when whirled at 1 revolution per second.
  • Immediate reading is taken of the dry and the wet bulb.
  • Essential that readings are taken immediately or else the wet bulb will give wrong readings.
  • Obtain RH and the Dew Point from the meteorological tables meant for whirling Psychrometer.
  • Very simple, and very accurate.