How to Build a Sling Psychrometer
A sling psychrometer is a simple device to measure relative humidity. When the air is dry, water evaporates more easily than when it is humid, cooling off wet objects. By comparing the temperature of a wet thermometer and a dry thermometer, a scientist can determine how easily water is evaporating and, thus, how humid the air is.
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Things You'll Need
- Thermometers
- Block
- Glue
- Cotton
- Thread
- Drill
- String
- Paper
- Pencil
- Psychrometric Chart
Instructions
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1
Get two high-quality, mercury thermometers. Make sure they show the same temperature when you buy them. If they are not accurate, your sling psychrometer will be worthless.
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2
Glue one thermometer to each side of a block of wood so that the numbers are visible and the bulbs of each thermometer is hanging off the end of the block. The exact size of the block doesn't matter as long as the thermometers are at least three inches apart.
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3
Wrap a small piece of cotton around one of the thermometer bulbs. Tie it one with a piece of thread so that it is firmly in contact with the bulb.
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4
Drill a hole through the end of the block away from the thermometer bulbs and tie a string through it. When you hold the string, the thermometer bulbs should more or less hang straight down.
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5
Dip the cotton in cool or room-temperature water. Be sure to not get the other thermometer wet.
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6
Swing the psychrometer around in a circle for at least three minutes. Then slow down the swinging until it hangs in front of you. Record the temperatures of the two thermometers.
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7
Compare the two different temperatures to a psychrometric chart such as the one linked to below. This will tell you the relative humidity.
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