TITLE: BUILDING A PSYCHROMETER
AUTHOR: John Cowens; Greenwood Elementary,
La Grande, OR 97850
GRADE LEVEL/SUBJECT: Appropriate for Grades 3-12
OVERVIEW:
Humidity is moisture in the air. When air feels damp, the humidity
is high. When air feels dry, the humidity is low. The amount of
water the air can hold is determined by the temperature. Warm air
can hold much more moisture than cold air. (See Relative Humidity
Table for specific examples.)
When air can hold more moisture at a given temperature it is said
to be saturated. Relative humidity is the percentage of saturation.
A psychrometer is one tool for measuring relative humidity. The bulb,
or rounded end of one thermometer is covered with a wet cloth.
After the thermometers are fanned the wet bulb temperature will
usually be lower that the dry bulb. The water evaporating from the
cloths lowers the temperature of the wet bulb because evaporation is
a cooling process. The relative humidity can then be read from
the chart (provided at the end of this lesson).
OBJECTIVES:
Students will demonstrate their knowledge of the atmosphere and
humidity by building and using their own psychrometer.
MATERIALS/RESOURCES NEEDED:
Tape, water, 2 identical thermometer, gauze (2" X 2"), rubberband,
piece of cardboard (8" X 11"), Relative Humidity Table
ACTIVITIES AND PROCEDURES:
1. Wrap the gauze around one of the bulbs and tie it firmly in
place with a rubber band.
2. Wet the gauze.
3. Place the thermometers side by side with the two bulbs just
hanging over the edge of a desk or table.
4. Use the cardboard to fan the thermometers. (Be careful not
to bump or hit the thermometer while they are taped down, or they
could break).
5. Fan vigorously until the temperature of the thermometer with
the wet bulb stops going down.
6. Record the temperature readings on both thermometer.
7. Subtract the wet bulb temperature from the dry bulb temperature
and record the difference.
Note: With the more advanced classes use Fahrenheit
thermometers and require them to convert from Fahrenheit to Celsius
and then find the difference!!
8. Look at the Relative Humidity Table. The numbers in the left
hand column represent the dry bulb reading. The numbers across the
top represent the difference between the wet and dry bulb readings.
This is the answer you obtained from step #7. Follow your dry bulb
temperature across and follow your difference column down until
they intersect. Record the number at this point. The number you
have recorded is the relative humidity of your room.
Extension:
1. Find the Relative Humidity of another room (i.e. an empty
classroom). Any difference? Why?
2. Find the Relative Humidity outdoors
RELATIVE HUMIDITY TABLE
(SHOWN IN %)
_______________________________________________________
DRY BULB DIFFERENCE BETWEEN WET AND DRY BULB READINGS IN
CELSIUS DEGREES
_______________________________________________________
C 1 2 3 4 5 6 7 8 9 10
_______________________________________________________
10 88 77 66 55 44 34 24 15 6
11 89 78 67 56 46 36 27 18 9
12 89 78 68 58 48 39 29 21 12
13 89 79 69 59 50 41 32 22 15 7
14 90 79 70 60 51 42 34 2 18 10
15 90 80 71 61 53 44 36 27 20 13
16 90 81 71 63 54 46 38 3 2 15
17 90 81 72 64 55 47 40 32 25 18
18 91 82 73 65 57 49 41 34 27 20
19 91 82 74 65 58 50 43 36 29 22
20 91 83 74 67 59 53 46 39 32 26
21 91 83 75 67 60 53 46 39 32 26
22 92 83 76 68 61 54 47 40 34 28
23 92 84 76 69 62 55 48 42 36 30
24 92 84 77 69 62 56 49 43 37 31
25 92 84 77 70 63 57 50 44 39 33
_________________________________________________________
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John Kurilecjmk@ofcn.org