Exploration Lab
Determining Relative Humidity
To measure relative humidity, you can use a sling psychrometer. This device contains two thermometers. An absorbent wick keeps the bulb of one thermometer wet. As you spin the psychrometer, air flows over the wet bulb, increasing the rate of evaporation. In this lab, you will discover how asling psychrometer measures the relative humidity of the air.
Problem How can you measure the relative humidity of the air?
Materials
sling psychrometer
relative humidity chart from Appendix G
clock or watch with second hand
For the probeware version of this lab, see the Probeware Lab Manual, Lab 10.
Skills Using Tables, Formulating Hypotheses
Procedure 
On a separate sheet of paper, make a copy of the data table shown above.
Wet the cotton wick of the sling psychrometer with water.
Observe the temperature of the wet-bulb thermometer. Then, spin the psychrometer for 30 seconds. CAUTION Take care not to spin the psychrometer near anyone or anything.
Repeat Step 3 until the temperature of the wet-bulb thermometer remains constant. Then, record the temperatures of both thermometers in your data table.
Calculate the difference between your drybulb and wet-bulb temperatures.
In the relative humidity chart, find the row and column that list the dry-bulb temperature and the difference in temperature that you calculated. The number located where this row and column meet is the relative humidity of the classroom. Record the relative humidity in your data table.
Place the psychrometer on a flat surface. Observe the temperature of the wet-bulb thermometer every 30 seconds until it remains constant. Then, repeat Steps 2 through 5 at the location your teacher indicates.
Analyze and Conclude
Analyzing Data Why did the two thermometers have different temperatures?
Formulating Hypotheses Explain how the relative humidity of the air affected the difference between the temperatures of the two thermometers.
Drawing Conclusions What do you think caused the difference in relative humidity between the two locations?
Predicting How would the relative humidity change if you cooled the air in the classroom?
Go Further
Measure and record the temperature and relative humidity outside your school or at home at various times of day for several days. Graph these data and explain any patterns that you observe.
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