How Does a Psychrometer Work?

A psychrometer uses two thermometers, one ordinary and one a wet bulb thermometer, to measure relative humidity. It is a type of hygrometer, and is an older version of the more modern devices that use electronic sensors to measure changes in electrical resistance.

Taken from Greek, the term psychrometer literally means cold measurer. Sir John Leslie invented the device, but the term was actually used in 1818 by German inventor Ernst Ferdinand August. The steps for using a psychrometer are fairly straightforward.

  1. Prepare the device
  2. Make sure both thermometers are at the same starting point. To do this, shake the device for about 30 seconds.

  3. Wet the wick
  4. To get an accurate reading, dip the wick of the wet bulb thermometer in distilled water. Be sure the wick, or sock, is completely soaked with water.

  5. Swing the instrument
  6. Swing or whirl the instrument around in a circle for approximately one minute. This allows the water to evaporate from the wick and, in turn, cools the wet bulb thermometer.

  7. Read the temperatures and calculate humidity
  8. After the water has evaporated, take a reading from both thermometers. The difference between the temperatures indicates the humidity. The drier the air is, the more water will evaporate from the wick. This, in turns, causes the temperature to be cooler. Therefore, the greater the difference in temperatures, the lower the humidity. If there is no difference in temperature, the humidity is 100 percent.