DETERMINING RELATIVE HUMIDITY

WITH A SLING PSYCHROMETER

# GENERAL

This method describes how to determine the relative humidity with a sling psychrometer. A psychrometer measures the ambient air temperature (dry bulb temperature) and the wet bulb temperature. From these temperature measurements, the percent relative humidity is determined.

# APPARATUS

1.   Sling psychrometer

PROCEDURE

1.     Before using, take the end cap off the psychrometer and fill the reservoir with water. Be sure that the wick is saturated with water and covers the mercury reservoir on the wet bulb thermometer. Also, be sure that the mercury reservoir on the dry bulb thermometer is dry.

2.     Pull the tube clear of the body so that the body can swivel.

3.     Hold the tube and whirl the body two to three revolutions per second.

4.     Continue whirling until the temperatures stabilize (should take 1-2 minutes).

5.     Stop whirling and immediately read temperature of the wet bulb thermometer and then the dry thermometer.

6.     Figure the temperature difference between the two thermometers.

7.     Use the attached table to determine the relative humidity.

# EXAMPLE

After following the above procedure, a dry bulb temperature of 86°F and a wet bulb temperature of 71°F were obtained. Thus, the temperature difference is 15°F.

From the “Relative Humidity” table, locate the column at the top headed with 15. Follow this column down to the row headed with dry bulb temperature of 86°F. The intersection of the column and the row gives the number 47. This number is the percent relative humidity.

# REPORTING RESULTS

Record the time, date, dry bulb temperature, wet bulb temperature, and relative humidity.

 Table 1 – Relative Humidity, % Difference Between Readings of Wet & Dry Bulbs in °F Dry Bulb °F 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Dry Bulb °F 40 41 42 43 44 92 92 92 92 93 83 84 85 85 85 75 76 77 77 78 68 69 69 70 71 60 61 62 63 63 52 54 55 55 56 45 46 47 48 49 37 39 40 42 43 29 31 33 35 36 22 24 26 28 30 15 17 19 21 23 7 10 12 14 16 0   3   5   8 10 40 41 42 43 44 45 46 47 48 49 93 93 93 93 93 86 86 86 86 86 78 79 79 79 80 71 72 72 73 73 64 65 66 66 67 57 58 59 60 61 51 52 53 54 54 44 45 46 47 48 38 39 40 41 42 31 32 34 35 36 25 26 28 29 31 18 20 22 23 25 12 14 16 18 19 6   8 10 12 14 2   5   7   9 1   3 45 46 47 48 49 50 51 52 53 54 93 94 94 94 94 87 87 87 87 88 80 81 81 81 82 74 75 75 75 76 67 68 69 69 70 61 62 63 63 64 55 56 57 58 59 49 50 51 52 53 43 45 46 47 48 38 39 40 41 42 32 34 35 36 37 27 28 29 31 32 21 23 24 26 27 16 17 19 20 22 10 12 14 16 17 5   7   9 10 12 0   2   4   6   8 1   3 50 51 52 53 54 55 56 57 58 59 94 94 94 94 94 88 88 88 88 89 82 82 82 83 83 76 76 77 77 78 70 71 71 72 72 65 65 66 66 67 59 60 61 61 62 54 55 55 56 57 49 50 50 51 52 43 44 45 46 47 38 39 40 41 42 33 34 35 37 38 28 30 31 32 33 23 25 26 27 29 19 20 22 23 24 14 16 17 18 20 9 11 13 14 16 5   7   8 10 11 0   2   4   6   7 0   1   3 55 56 57 58 59 60 61 62 63 64 94 94 94 95 95 89 89 89 89 90 83 84 84 84 84 78 78 79 79 79 73 73 74 74 74 68 68 69 69 70 63 63 64 64 65 58 58 59 60 60 53 54 54 55 56 48 49 50 50 51 43 44 45 46 47 39 40 41 42 43 34 35 36 37 38 30 31 32 33 34 26 27 28 29 30 21 22 24 25 26 17 18 20 21 22 13 14 16 17 18 9 10 12 13 15 5   7   8 10 11 1   3   4   6   7 1   2   4 0 60 61 62 63 61 65 66 67 68 69 95 95 95 95 95 90 90 90 90 90 85 85 85 85 85 80 80 80 80 81 75 75 75 76 76 70 71 71 71 72 66 66 66 67 67 61 61 62 62 63 56 57 58 58 59 52 53 53 54 55 48 48 49 50 51 44 44 45 46 47 39 40 41 42 43 35 36 37 38 39 31 32 33 34 35 27 29 30 31 32 24 25 26 27 28 20 21 22 23 24 16 17 19 20 21 12 14 15 16 18 9 10 12 13 14 5   7   8 10 11 2   3   5   6   8 0   2   3   5 1 65 66 67 68 69 70 71 72 73 74 95 95 95 95 95 90 90 91 91 91 86 86 86 86 86 81 81 82 82 82 77 77 77 78 78 72 72 73 73 74 68 68 69 69 69 64 64 65 65 65 59 60 61 61 61 55 56 57 57 58 51 52 53 53 54 48 48 49 50 50 44 45 45 46 47 40 41 42 42 43 36 37 38 39 39 33 33 34 35 36 29 30 31 32 33 25 27 28 29 29 22 23 24 25 26 19 20 21 22 23 15 17 18 19 20 12 13 15 16 17 9 10 12 13 14 6   7   9 10 11 3   4   6   7   8 1   3   4   5 1   3 70 71 72 73 74 75 76 77 78 79 96 96 96 96 96 91 91 91 91 91 86 87 87 87 87 82 82 83 83 83 78 78 79 79 79 74 74 74 75 75 70 70 71 71 71 66 66 67 67 68 62 62 63 63 64 58 59 59 60 60 54 55 56 56 57 51 51 52 53 53 47 48 48 49 50 44 44 45 46 46 40 41 42 43 43 37 38 39 39 40 34 34 35 36 37 30 31 32 33 34 27 28 29 30 31 24 25 26 27 28 21 22 23 24 25 18 19 20 21 22 15 16 17 18 19 12 13 14 16 17 9 11 12 13 14 7   8   9 10 11 4   5   6   8   9 1   3   4   5   6 75 76 77 78 79 80 82 84 86 88 96 96 96 96 96 91 92 92 92 92 87 88 88 88 88 83 84 84 84 85 79 80 80 81 81 75 76 76 77 77 72 72 73 73 74 68 69 69 70 70 64 65 66 66 67 61 61 62 63 64 57 58 59 60 61 54 55 56 57 57 50 51 52 53 54 47 48 49 50 51 44 45 46 47 48 41 42 43 44 46 38 39 40 42 43 35 36 37 39 40 32 33 35 36 37 29 30 32 33 35 26 28 29 31 32 23 25 26 28 30 20 22 24 26 27 18 20 21 23 25 15 17 19 21 22 12 14 16 18 20 10 12 14 16 18 7 10 12 14 15 5   7   9 11 13 3   5   7   9 11 80 82 84 86 88 90 92 94 96 98 96 96 96 96 96 92 92 93 93 93 89 89 89 89 89 85 85 85 86 86 81 82 82 82 83 78 78 79 79 79 74 75 75 76 76 71 72 72 73 73 68 68 69 69 70 65 65 66 66 67 61 62 63 63 64 58 59 60 61 61 55 56 57 58 58 52 53 54 55 56 49 50 51 52 53 47 48 49 50 50 44 45 46 47 48 41 42 43 44 45 39 40 41 42 43 36 37 38 39 40 34 35 36 37 38 31 32 33 35 36 29 30 31 32 34 26 28 29 30 32 24 25 27 28 29 22 23 24 26 27 19 21 22 24 25 17 19 20 22 23 15 17 18 20 21 13 15 16 18 19 90 92 94 96 98 100 102 104 106 108 96 96 97 97 97 93 93 93 93 93 89 90 90 90 90 86 86 87 87 87 83 83 83 84 84 80 80 80 81 81 77 77 77 78 78 73 74 74 75 75 70 71 71 72 72 68 68 69 69 70 65 65 66 66 67 62 62 63 64 64 59 60 60 61 62 56 57 58 58 59 54 55 55 56 57 51 52 53 53 54 49 49 50 51 52 46 47 48 49 49 44 45 46 46 47 41 42 43 44 45 39 40 41 42 43 37 38 39 40 41 35 36 37 38 39 33 34 35 36 37 30 32 33 34 35 28 30 31 32 33 26 28 29 30 31 24 26 27 28 29 22 24 25 26 27 21 22 23 24 25 100 102 104 106 108 110 112 114 116 118 97 97 97 97 97 93 94 94 94 94 90 90 91 91 91 87 87 88 88 88 84 84 85 85 85 81 81 82 82 82 78 79 79 79 79 75 76 76 76 77 73 73 74 74 74 70 70 71 71 72 67 68 68 69 69 65 65 66 66 67 62 63 63 64 64 60 60 61 61 62 57 58 58 59 59 55 55 56 57 57 52 53 54 54 55 50 51 52 52 53 48 49 49 50 51 46 47 47 48 49 44 44 45 46 47 42 42 43 44 45 40 40 41 42 43 38 38 39 40 41 36 36 37 38 39 34 35 35 36 37 32 33 34 34 35 30 31 32 33 34 28 29 30 31 32 26 27 28 29 30 110 112 114 116 118 120 97 94 91 88 85 82 80 77 74 72 69 67 65 62 60 58 55 53 51 49 47 45 43 41 40 38 36 34 33 31 120