How to Read a Hygrometer

Hygrometers measure the humidity of the surrounding air, and have various related applications including weather forecasting and air conditioning. While there are new, easy-to-read hygrometers available, the classic double-thermometer hygrometers are still in use and can reveal a great deal about what constitutes humidity and how it is determined.

  1. Identify the wet bulb and dry bulb thermometers in the hygrometer handle. The wet bulb thermometer should have a muslin sleeve or wick at its tip for water absorption.

  2. Dip the tip of the wet bulb thermometer into distilled water. Make sure the temperature of the distilled water is close to the temperature of the surroundings you will be measuring for humidity. Cold water requires extra energy to evaporate and hot water requires less. Either way, your wet bulb temperature will be off.

  3. Swing the thermometer holder around the handle. A velocity of at least 2 meters/second will usually produce a good reading. You can convert to angular velocity by taking into account the distance of the wick from the axis of rotation. Wait a few minutes.

  4. Write down the dry bulb temperature.

  5. Record the wet bulb temperature after it has settled. Water has been evaporating from the tip, so the temperature may still be changing after the apparatus has stopped spinning.

  6. Determine the relative humidity using a humidity table or conversion program.