How to Use Thermometers to Measure Indoor Humidity

A psychrometer is a weather instrument that uses two thermometers to measure the humidity in a room. Humidity is the amount of water vapor in the air. One of the thermometers measures the actual air temperature; this is the dry bulb temperature. The other thermometer has a wick at the end that is dipped in water before measuring air temperature. This measurement is the wet bulb temperature. The difference between the two temperatures is used to determine the relative humidity.

Make a Psychrometer

• 1

  Wet a piece of gauze with water. Wrap the gauze around the bulb of one thermometer and secure with a rubberband.

• 2

  Place the wet and dry thermometers side by side on a base such as a board or table so that the bulb ends of the thermometers stick out from the end of the base.

• 3

  Fasten the thermometers to the base with tape.

Measure Wet and Dry Bulb Temperature

• 1

  Place the thermometers in front of the fan and turn it on.

• 2

  Observe the temperature on both thermometers. Make sure air continues to blow over the thermometers.

• 3

  Record the temperature of the dry bulb thermometer.

• 4

  Observe the wet-bulb thermometer until the temperature stops changing, after about two to three minutes. The temperature drops as water evaporates from the gauze. Record the wet bulb temperature when it remains constant.

Determine Humidity

• 1

  Subtract the wet bulb temperature from the dry bulb temperature.

• 2

  Locate the dry bulb temperature on the relative humidity chart. Then, locate the dry bulb minus wet bulb value on the chart.

• 3

  Find the intersection of these two values on the chart. This value is the percent of relative humidity in the room.