How to Make a Wet Bulb Hygrometer

Relative humidity is expressed as a percentage of the amount of water vapor in air at a specific temperature as compared to the maximum amount the air could hold at that temperature. A wet bulb hygrometer, also known as a dry-wet bulb hygrometer, is a device used to measure the relative humidity of air. Two glass thermometers are used. The bulb of one thermometer is kept dry and the bulb on the other is kept wet by means of a wick immersed in water.

Wet bulb hygrometers measure humidity by measuring the lowering of the temperature of the wet bulb by evaporation and comparing it to the actual temperature measured by the dry bulb. By referencing a chart called the Relative Humidity Table, in which the actual temperature, as measured by the dry bulb, is compared to the temperature drop of the wet bulb, the relative humidity can be determined.

Things You'll Need

• 2 glass Fahrenheit thermometers
• Backing, 2 inches longer than the thermometers and 3 inches wide (thin piece of plywood, plastic or other suitable material)
• Rubber cement
• Scissors
• Cotton shoelace
• Ball point pen cap with clip
• Power drill and a 1/4-inch bit

Instructions

1. Obtain two glass Fahrenheit thermometers and material for your backing, either a thin piece of plywood, plastic or other suitable material. Make sure that the backing is two inches longer than the thermometers and three inches wide..

2. Attach the thermometers side by side, about 1 inch apart, to the backing with rubber cement. Leave room at the bottom to attach a ball point pen cap, so that its open end is about 1/2-inch below the bulb of one of the thermometers.

3. Using a pair of scissors, cut a 2-inch piece of shoelace between the tips for a wick.

4. Slip the shoelace wick over the bulb of one thermometer so that the upper end is about a half inch above the bulb.

5. Attach a ball point pen cap, using its clip or rubber cement, about 1/2-inch below the bulb and feed the shoelace wick into cap.

6. Drill a hole at the top of the backing to hang the hygrometer, using a power drill and 1/4-inch bit.

7. Fill the ball point pen cap with water

Tips & Warnings

• Place your hygrometer out of the sun and wind.

• Wait about 5 minutes before you take your reading.

• Use an old CD for a backing

References

• Australian Department of Meteorology: How to measure humidity using a hygrometer

Resources

• Jacksonville State University: The Relative Humidity Table
• Analytical Scientific, Ltd.: Thermometers & Hydrometers