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Measuring Relative Humidity - Science Fair Projects

psychrometer
A vintage psychrometer. These devices have been used for more than 100 years to measure the moisture content, or relative humidity, in the air.
Credit: Andrey Savin | Shutterstock

You may have noticed that humidity (the amount of water vapor in the air) makes a difference in how comfortable you feel outside. When the humidity is high in the summer we say it is "muggy." When it is cool and humidity is high we may say that it feels "clammy." Humidity can make a warm day feel warmer and a cool day feel cold. You can observe the difference that humidity makes.

In this experiment, you will determine the relative humidity using wet-bulb and dry-bulb thermometers. Such a device is called a psychrometer.

What you need:

  • 2 identical alcohol thermometers (Celsius)
  • Heavy cardboard rectangle, about 10 inches long
  • Strong tape
  • Fan
  • Cotton balls
  • Rubber band
  • Warm water

What to do:

1. Use tape to secure the thermometers to the cardboard rectangle about 5 inches apart. Make sure the bulb ends of the thermometers hang below the edge of the cardboard by at least 1 inch and the numbered sides of the thermometers can be easily read.

2. Set the thermometers in front of a small fan set to a moderate speed for 5 minutes and record the temperature readings. This is to verify that your thermometers are calibrated and show the same temperature under control conditions.

3. Saturate several cotton balls with warm water and use the rubber band to fix them around the bulb of one thermometer. Set the thermometers in front of the fan as before. After 5 minutes record the “wet bulb” temperature reading and the “dry bulb” temperature reading.

4. To determine relative humidity subtract the lower “wet bulb” temperature from the higher “dry bulb” temperature to determine the difference and compare to the chart.

Relative Humidity (%)
Dry-Bulb
Temp (°C)
Difference between Wet-Bulb and Dry-Bulb Temperatures (°C)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
-20 100 28                            
-18 100 40                            
-16 100 48                            
-14 100 55 11                          
-12 100 61 23                          
-10 100 66 33                          
-8 100 71 41 13                        
-6 100 73 48 20                        
-4 100 77 54 32 11                      
-2 100 79 58 37 20 1                    
0 100 81 63 45 28 11                    
2 100 83 67 51 36 20 6                  
4 100 85 70 56 42 27 14                  
6 100 86 72 59 46 35 22 10                
8 100 87 74 62 51 39 28 17 6              
10 100 88 76 65 54 43 33 24 13 4            
12 100 88 78 67 57 48 38 28 19 10 2          
14 100 89 79 69 60 50 41 33 25 16 8 1        
16 100 90 80 71 62 54 45 37 29 21 14 7 1      
18 100 91 81 72 64 56 48 40 33 26 19 12 6      
20 100 92 82 74 66 58 51 44 36 30 23 17 11 5    
22 100 92 83 75 68 60 53 46 40 33 27 21 15 10 4  
24 100 92 84 76 69 62 55 49 42 36 30 25 20 14 9 4
26 100 92 85 77 70 64 57 51 45 39 34 28 23 18 13 9
28 100 93 86 78 71 65 59 53 47 42 36 31 26 21 17 12
30 100 93 86 79 72 66 61 55 49 44 39 34 29 25 20 16

Results:

Even though the air temperature did not change, you should have noticed a lower temperature reading from the wet-bulb thermometer. This happens because as the cotton balls were drying, some of the water evaporated (changed to water vapor.)

This change of state, from liquid water to water vapor, requires a bit of heat energy from the surrounding atmosphere. The same thing happens when ice melts into liquid water. On the other hand, a tiny bit of heat is released when water vapor condenses back into liquid form or when water freezes to form ice. This explains why you feel cold when you get out of a pool on a windy day, even though the air temperature is high, the water evaporating from your skin is pulling the evaporation heat from your body instead of the air!

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