DETERMINING THE DEW POINT

WITH A SLING PSYCHROMETER

GENERAL

This test method describes how to determine the dew point using a sling psychrometer. A psychrometer measures the ambient air temperature (dry bulb temperature) and the wet bulb temperature. From the dry bulb and wet bulb temperature the dew point is calculated. The dew point is the temperature at which moisture in the air will start condensing.

When painting structural steel, specifications may require the dew point to be measured. This is to ensure that no moisture is condensed on the surface to be painted.

APPARATUS

1.   Sling psychrometer

2.   Replacement thermometer

3.   Replacement wicks

INSTRUMENT OPERATION

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

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

3.   Holding the tube, whirl the body two to three revolutions per second.

4.   Continue whirling until temperatures stabilize (1 1/2 minutes is usually ample).

5.   Immediately read the wet bulb thermometer and then the dry bulb thermometer.

6.   Figure the difference between the dry bulb and wet bulb temperature. On Table 1, locate the column, which is headed with this temperature difference. Then locate the row labeled with the appropriate dry bulb temperature. The value at the intersection of the column and row gives the dew point.

7.   The dew point should be at least 5°F less than the surface temperature of the steel being painted.

EXAMPLE

If the dry bulb temperature is 71°F and the wet bulb temperature is 64°F, the difference between the dry bulb and wet bulb is 7°F. At the top of Table 1, locate the column headed with 7. Follow this column down to the row headed with the dry bulb temperature of 71°F. The intersection of the column and row gives the number 60, which is the dew point in degrees Fahrenheit. Add 5 degrees to the dew point to get 65°F. Sixty-five is the minimum surface temperature of the steel before painting should begin to ensure that there is no condensation on the surface.

NOTES

1.   The surface and air temperature will vary depending on the location of the temperature measurements. For instance, a beam exposed to direct sunlight will be much warmer than a beam in the shade. Therefore if painting work is done in the shade, temperature measurements should also be done in the shade.

2.   Moisture problems are most likely to be encountered in the morning, so temperature measurements should be made in the morning before painting begins.

REPORTING RESULTS

The following information should be reported: Time, dry bulb temperature, wet bulb temperature, dew point, and surface temperature.

Table 1 - Dew Point Temperature - °F

Difference Between Dry Bulb and Wet Bulb - °F

 Dry Bulb °F 0 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 40 40 38 35 33 -- -- -- -- -- -- -- -- -- -- -- -- 41 41 39 36 34 -- -- -- -- -- -- -- -- -- -- -- -- 42 42 40 38 35 33 -- -- -- -- -- -- -- -- -- -- -- 43 43 41 39 36 34 -- -- -- -- -- -- -- -- -- -- -- 44 44 42 40 37 35 -- -- -- -- -- -- -- -- -- -- -- 45 45 43 41 38 36 34 -- -- -- -- -- -- -- -- -- -- 46 46 44 42 40 37 35 -- -- -- -- -- -- -- -- -- -- 47 47 45 43 41 38 36 33 -- -- -- -- -- -- -- -- -- 48 48 46 44 42 40 38 35 -- -- -- -- -- -- -- -- -- 49 49 47 45 43 41 38 36 33 -- -- -- -- -- -- -- -- 50 50 48 46 44 42 40 37 34 -- -- -- -- -- -- -- -- 51 51 49 47 45 43 41 38 36 33 -- -- -- -- -- -- -- 52 52 50 48 46 44 42 40 37 34 -- -- -- -- -- -- -- 53 53 51 49 47 45 43 41 38 36 33 -- -- -- -- -- -- 54 54 52 50 48 46 44 42 40 37 34 -- -- -- -- -- -- 55 55 53 51 50 48 45 43 41 38 36 33 -- -- -- -- -- 56 56 54 53 51 49 47 44 42 40 37 34 -- -- -- -- -- 57 57 55 54 52 50 48 46 43 41 39 36 -- -- -- -- -- 58 58 56 55 53 51 49 47 45 42 40 37 -- -- -- -- -- 59 59 57 56 54 52 50 48 46 44 41 39 33 -- -- -- -- 60 60 58 57 55 53 51 49 47 45 43 40 35 -- -- -- -- 61 61 59 58 56 54 52 50 48 46 44 42 36 -- -- -- -- 62 62 60 59 57 55 53 52 50 47 45 43 38 -- -- -- -- 63 63 61 60 58 56 55 53 51 49 47 44 39 34 -- -- -- 64 64 62 61 59 57 56 54 52 51 48 46 41 35 -- -- -- 65 65 63 62 60 59 57 55 53 51 49 47 42 37 -- -- -- 66 66 64 63 61 60 58 56 54 52 50 48 44 38 -- -- -- 67 67 65 64 62 61 59 57 55 53 52 49 45 40 34 -- -- 68 68 67 65 63 62 60 58 57 55 53 51 46 42 36 -- -- 69 69 68 66 64 63 61 59 58 56 54 52 48 43 37 -- -- 70 70 69 67 65 64 62 61 59 57 55 53 49 44 39 33 -- 71 71 70 68 67 65 63 62 60 58 56 54 50 46 41 35 -- 72 72 71 69 68 66 64 63 61 59 58 56 52 47 42 37 -- 73 73 72 70 69 67 66 64 62 60 59 57 53 49 44 38 -- 74 74 73 71 70 68 67 65 63 62 60 58 54 50 45 40 34 75 75 74 72 71 69 68 66 64 63 61 59 55 51 47 42 36 76 76 75 73 72 70 69 67 66 64 62 60 56 53 48 43 38 77 77 76 74 73 71 70 68 67 65 63 62 57 54 50 45 39 78 78 77 75 74 72 71 69 68 66 64 63 58 55 51 46 41

Table 1 - Dew Point Temperature - °F (cont.)

Difference Between Dry Bulb and Wet Bulb - °F

 Dry Bulb °F 0 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 79 79 78 76 75 73 72 70 69 67 66 64 59 57 53 48 43 80 80 79 77 76 74 73 72 70 68 67 65 61 58 54 50 44 81 81 80 78 77 75 74 73 71 70 68 66 63 59 55 51 49 82 82 81 79 78 77 75 74 72 71 69 67 64 60 57 52 50 83 83 82 80 79 78 76 75 73 72 70 69 65 62 58 54 52 84 84 83 81 80 79 77 76 74 73 71 70 66 63 59 55 53 85 85 84 82 81 80 78 77 75 74 72 71 68 64 61 57 54 86 86 85 83 82 81 79 78 76 75 73 72 69 65 62 58 56 87 87 86 84 83 82 80 79 78 76 75 73 70 67 63 59 57 88 88 87 85 84 83 81 80 79 77 76 74 71 68 64 61 59 89 89 88 86 85 84 82 81 80 78 77 75 72 69 66 62 60 90 90 89 87 86 85 83 82 81 79 78 76 73 70 67 63 61 91 91 90 88 87 86 85 83 82 80 79 78 75 71 68 65 63 92 92 91 89 88 87 86 84 83 81 80 79 76 73 69 66 64 93 93 92 90 89 88 87 85 84 83 81 80 77 74 71 67 65 94 94 93 92 90 89 88 86 85 84 82 81 78 75 72 68 67 95 95 94 93 91 90 89 87 86 85 83 82 79 76 73 70 68 96 96 95 94 92 91 90 88 87 86 84 83 80 77 74 71 69 97 97 96 95 93 92 91 89 88 87 85 84 81 78 75 72 70 98 98 97 96 94 93 92 90 89 88 87 85 82 79 76 73 72 99 99 98 97 95 94 93 92 90 89 88 86 83 81 78 74 73 100 100 99 98 96 95 94 93 91 90 89 87 85 82 79 76 74 101 101 100 99 97 96 95 94 92 91 90 88 86 83 80 77 75 102 102 101 100 98 97 96 95 93 92 91 89 87 84 81 78 76 103 103 102 101 99 98 97 96 94 93 92 91 88 85 82 79 78 104 104 103 102 100 99 98 97 95 94 93 92 89 86 83 80 79 105 105 104 103 101 100 99 98 96 95 94 93 90 87 84 82 80 106 106 105 104 102 101 100 99 98 96 95 94 91 88 86 83 81 107 107 106 105 103 102 101 100 99 97 96 95 92 90 87 84 82 108 108 107 106 104 103 102 101 100 98 97 96 93 91 88 85 84 109 109 108 107 105 104 103 102 101 99 98 97 94 92 89 86 85 110 110 109 108 106 105 104 103 102 100 99 98 95 93 90 87 86