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Lab #5 Humidity



Purpose: To experimentally determine the absolute and relative humidity of air.


Apparatus: Thermometer, 250-mL beaker, stirring rod, tap water, and ice cubes.

Introduction:


The air in the atmosphere can "hold" water vapor. The effects of this can easily be felt by humans. A muggy, humid day can be much more uncomfortable than a dry day. Hence, 95 F in Phoenix will typically feel much different than 95 F in Atlanta due to the difference in humidity at these two locations.


By definition, humidity is the amount of water vapor (moisture) in the air. The absolute humidity refers to the weight of water vapor in a known volume of air. Since the amount of water vapor contained in a volume of air is usually very small, the weight is typically expressed in grains (gr) where





or


1 lb = 7000 gr.


The air volume containing the water vapor is usually expressed in cubic feet (ft3). Thus, absolute humidity has units of


.


Relative humidity (expressed as a percentage) is more commonly used than absolute humidity when discussing the moisture content of air. The relative humidity is the ratio of the absolute humidity to the maximum moisture capacity of a given volume of air at a known temperature. In symbols,




The maximum moisture capacity per unit volume at a given temperature can be looked up in reference tables. The dew point can easily be used to determine the absolute humidity of an air sample. The dew point is simply the temperature at which dew begins to form.


Exercise:


1. Determine the air temperature in degrees Celsius. Record this value in the data table below. Convert this temperature to degrees Fahrenheit using


TF = 1.8TC + 32.


Record the room temperature in degrees Fahrenheit in the data table below


2. Obtain a 250-mL glass beaker and fill it approximately half full with tap water. Place a thermometer in the water. Add several ice cubes and gently stir with a glass stir rod. Carefully observe the beaker as the temperature of the ice/water mixture begins to fall. Record the temperature at which you first notice moisture collecting on the outside of the beaker in the data table below.


3. Carefully remove the ice from the beaker but keep the water in the beaker. Do not dry the outside of the beaker. Stir the water gently. The temperature of the water inside of the beaker will begin to increase. Note the temperature at which the moisture on the outside of the beaker disappears. Record this temperature in the data table below.

4. Calculate and record the average dew point in degrees Celsius for the glass beaker measurements.


5. Convert the average dew point in degrees Celsius to degrees Fahrenheit using


TF = 1.8TC + 32.


Record the average dew point in degrees Fahrenheit in the data table below.


6. Referring to Table 1at the end of this experiment, use the average dew point (in Fahrenheit) for the glass beaker measurements to determine the absolute humidity. For example, if the dew point temperature is 45 F, the absolute humidity is 3.4 gr/ft3. Record this value in the data table below.


7. Referring once again to Table 1 determine the maximum moisture capacity of the air in the room at the temperature determined in Step #1 (in degrees Fahrenheit). Record this value in the data table below.


8. Using the glass beaker data, calculate the percent relative humidity by dividing the absolute humidity value from Step #7 by the maximum moisture capacity of the air determined in Step #9. Multiply this quotient by 100%. Record this value in the data table below.


Data Table


Room Temperature

C

Room Temperature

F

Glass Beaker Measurements

Temperature, C

Trial #1




Trial #2




Average Glass Beaker Dew Point

C

Average Glass Beaker Dew Point

F

Glass Beaker Absolute Humidity

gr/ft3

Maximum Moisture Capacity

gr/ft3

Glass Beaker Relative Humidity

%



Questions:


1. Assume that the air temperature in our classroom decreases by 5 F. If the absolute humidity remains constant, how would the relative humidity change? Show your work.


References:


1. An Introduction to Physical Science , 10th Edition, James T. Shipman, Jerry D. Wilson, and Aaron W. Todd, Houghton Mifflin Company (2003).


2. Laboratory Guide for an Introduction to Physical Science, 8th Edition, James T. Shipman and Clyde. D. Baker, Experiment 45, "Humidity", Houghton Mifflin Company (1997).

Table 1


Temperature

°F

Moisture Content

gr/ft3

0

0.5

5

0.6

10

0.8

15

1.0

20

1.2

25

1.6

30

1.9

35

2.4

40

2.8

45

3.4

50

4.1

55

4.8

60

5.7

65

6.8

70

8.0

75

9.4

80

10.9

85

12.7

90

14.8

95

17.1

100

19.8



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