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Technical Bulletin #10

 

Dew Point Calculator


Definition: Dew Point is the temperature at which condensations forms.


To determine the Dew Point from the charts below, find the temperature of the air in question on the left side of the table. Next, locate the relative humidity of the air in question across the top of the table. The intersection of these two numbers in the matrix identifies the temperature at which Dew Point is reached.


When air comes in contact with a surface that is at or below its Dew Point temperature, condensation will form on that surface.

Example:
If the temperature in a facility is 75º F (24º C) and the relative humidity is 35%, the intersection of the two shows that the Dew Point is reached at a temperature of 45º F (7º C), or below. This means that moisture vapor in the 75º F / 35% RH air will condense on any surface that is at or below the Dew Point temperature of 45º F.
This example could represent the inside of a building that is 75º F and 35% RH during the day. At night the outside temperature drops. It is not likely that the inside air of the building will cool from 75º F to 45º F, but it is quite possible that the framing and any exposed exterior surfaces will reach the Dew Point temperatures, creating condensation.


Air Temperature in Degrees Celsius


Air

Temp °C

% Relative Humidity

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10
43 43 42 41 40 39 38 37 35 34 32 31 29 27 24 22 18 16 11 5
41 41 39 38 37 36 35 34 33 32 29 28 27 24 22 19 17 13 8 3
38 38 37 36 35 34 33 32 30 29 27 26 24 22 19 17 14 11 7 0
35 35 34 33 32 31 30 29 27 26 24 23 21 19 17 15 12 9 4 0
32 32 31 31 29 28 27 26 24 23 22 20 18 17 15 12 9 6 2 0
29 29 28 27 27 26 24 23 22 21 19 18 16 14 12 10 7 3 0
27 27 26 25 24 23 22 21 19 18 17 15 13 12 10 7 4 2 0
24 24 23 22 21 20 19 18 17 16 14 13 11 9 7 5 2 0    
21 21 20 19 18 17 16 15 14 13 12 10 8 7 4 3 0
18 18 17 17 16 15 14 13 12 10 9 7 6 4 2 0
16 16 14 14 13 12 11 10 9 7 6 5 3 2 0
13 13 12 11 10 9 8 7 6 4 3 2 1 0
10 10 9 8 7 7 6 4 3 2 1 0
7 7 6 6 4 4 3 2 1 0
4 4 4 3 2 1 0
2 2 1 0
0 0

Example:
Read the air temperature in the left hand column and the humidity at the top of the chart. If the temperature of the storage unit is 75ºF (24º C) and the relative humidity is 35%, the intersection of the two shows the dew point of the area to be 45ºF (7ºC). If the metal coming in is below 45ºF (7ºC), water will condense on the metal.


Air Temperature in Degrees Fahrenheit


Air

Temp °F

% Relative Humidity

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10
110 110 108 106 104 102 100 98 95 93 90 87 84 80 76 72 65 60 51 41
105 105 103 101 99 97 95 93 91 88 85 83 80 76 72 67 62 55 47 37
100 100 99 97 95 93 91 89 86 84 81 78 75 71 67 63 58 52 44 32
95 95 93 92 90 88 86 84 81 79 76 73 70 67 63 59 54 48 40 32
90 90 88 87 85 83 81 79 76 74 71 68 65 62 59 54 49 43 36 32
85 85 83 81 80 78 76 74 72 69 67 64 61 58 54 50 45 38 32
80 80 78 77 75 73 71 69 67 65 62 59 56 53 50 45 40 35 32
75 75 73 72 70 68 66 64 62 60 58 55 52 49 45 41 36 32
70 70 68 67 65 63 61 59 57 55 53 50 47 44 40 37 32
65 65 63 62 60 59 57 55 53 50 48 45 42 40 36 32
60 60 58 57 55 53 52 50 48 45 43 41 38 35 32
55 55 53 52 50 49 47 45 43 40 38 36 33 32
50 50 48 46 45 44 42 40 38 36 34 32
45 45 43 42 40 39 37 35 33 32
40 40 39 37 35 34 32
35 35 34 32
32 32
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