The Orchid House

Determining Relative Humidity

Relative humidity represents the extent to which air is saturated with moisture at a given temperature. When raising plants, you should have some idea of the relative humidity, especially in the winter. If you get a shock from a spark every time you touch something, it is certainly too dry. If water is condensing on the windows, the air is too moist (at least for the health of your house).

A hygrometer is used to measure the level of humidity. The usual type that you can buy is typically made from a fibre (originally horse hair) that changes length with changes in humidity. Because the fibre hygrometer can be affected by many different conditions, it should be calibrated quite regularly. They are especially quite inaccurate at very high and low humidities.

A simple way to get a value for the relative humidity is to use a wet-and-dry bulb hygrometer (psychrometer). The value can then be used to calibrate a "hair" hygrometer. To make one is relatively simple. First you need a thermometer (two would be better) shaped such that you can wrap a small piece of tissue or cloth around the bulb. First measure the air temperature. Next wrap a small piece of tissue around the bulb and wet it. Wait about ten minutes until the temperature stabilizes - it will be lower due to evaporation. Subtract the wet temperature from the dry temperature and look up the humidity in the table below.

A more permanent setup would use two thermometers and a small water reservoir. The thermometers would be fixed on a card. One thermometer would have a piece of cloth wrapped on the bulb with the end dipping into a small reservoir of water similar to the image left.

A sling psychrometer is similar portable device used to measure relative humidity. An example and explanation is found here. Obviously easy to fabricate using a couple thermometers.

Relative Humidity from Wet and Dry Bulb Thermometer (Fahrenheit Scale)

Relative Humidity from Wet-and-Dry Bulb Readings
(Values below are in per cent.)
Fahrenheit Degrees difference between dry-bulb and wet-bulb readings
04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22
38 66 58 50 42 33 25 17 10 02
40 68 60 52 45 37 29 22 15 07
42 69 62 55 47 40 33 26 19 12 05
44 71 63 56 49 43 36 30 23 16 10 04
Dry-bulb 46 72 65 58 52 45 39 32 26 20 14 08 02
reading 48 73 66 60 54 47 41 35 29 23 18 12 07 01
in De- 50 74 67 61 55 49 43 38 32 27 21 16 10 05
grees 52 75 69 63 57 51 46 40 35 29 24 19 14 09 04
Fahren- 54 76 70 64 59 53 48 42 37 32 27 22 17 12 08 03
heit 56 76 71 65 60 55 50 44 39 34 30 25 20 16 11 07 02
58 77 72 66 61 56 51 46 41 37 32 27 23 18 14 10 06
60 78 73 68 63 58 53 48 43 39 34 30 26 21 17 13 09
62 79 74 69 64 59 54 50 45 41 36 32 28 24 20 16 12 08 04 01
64 79 74 70 65 60 56 51 47 43 38 34 30 26 22 18 15 11 07 04
66 80 75 71 66 61 57 53 48 44 40 36 32 29 25 21 17 14 10 07
68 80 76 71 67 62 58 54 50 46 42 38 34 31 27 23 20 16 13 10
70 81 77 72 68 64 59 55 51 48 44 40 36 33 29 25 22 19 15 12
72 82 77 73 69 65 61 57 53 49 45 42 38 34 31 28 24 21 18 15
74 82 78 74 69 65 61 58 54 50 47 43 39 36 33 29 26 23 20 17
76 82 78 74 70 66 62 59 55 51 48 44 41 38 34 31 28 25 22 19
78 83 79 75 71 67 63 60 56 53 49 46 43 39 36 33 30 27 24 21
80 83 79 75 72 68 64 61 57 54 50 47 44 41 38 35 32 29 26 23
82 84 80 76 72 69 65 61 58 55 51 48 45 42 39 36 33 30 28 25
84 84 80 76 73 69 66 62 59 56 52 49 46 43 40 37 35 32 29 26
86 84 81 77 73 70 66 63 60 57 53 50 47 44 42 39 36 33 31 28
88 85 81 77 74 70 67 64 61 57 54 51 48 46 43 40 37 35 32 30
90 85 81 78 74 71 68 65 61 58 55 52 49 47 44 41 39 36 34 31

Relative Humidity from Wet and Dry Bulb Thermometer (Centigrade Scale)

This table gives the approximate relative humidity directly from the reading of the air temperature (dry bulb) (t °C) and the wet bulb (T °C). It is computed for a pressure of 74.27 cm Hg. Errors resulting from the use of this table for air temperatures above 10°C and beween 77.5 and 71 cm Hg will usually be within the errors of observation.
Condensed from Bulletin of the U.S. Weather Bureau No. 1071

t-T
   t  2.0  2.2  2.4  2.6  2.8  3.0 3.2 3.4  3.6 3.8 4.0  4.5  5.0  5.5  6.0  6.5  7.0  7.5  8.0 8.5 9.0 9.5 10.0  10.5 11.0
       
   2   68   65   62   59   55   52  49  46   43  40  37   29   22   14    7        ..   ..   ..  ..  ..  ..    ..   ..   .. 
   3   70   67   64   61   58   55  52  49   46  43  40   33   26   19   12    5   ..   ..   ..  ..  ..  ..    ..   ..   ..
   4   71   68   65   62   60   57  54  51   48  46  43   36   29   22   16    9   ..   ..   ..  ..  ..  ..    ..   ..   ..
   5   72   69   67   64   61   58  56  53   51  48  45   39   33   26   20   13    7   ..   ..  ..  ..  ..    ..   ..   ..
   6   73   70   68   65   63   60  58  55   53  50  48   41   35   29   24   17   11    5   ..  ..  ..  ..    ..   ..   ..
   7   74   72   69   67   64   62  59  57   54  52  50   44   38   32   26   21   15   10   ..  ..  ..  ..    ..   ..   ..
   8   75   73   70   68   65   63  61  58   56  54  51   46   40   35   29   24   19   14    8  ..  ..  ..    ..   ..   ..
   9   76   74   71   69   67   64  62  60   58  55  53   48   42   37   32   27   22   17   12   7  ..  ..    ..   ..   ..
  10   77   74   72   70   68   66  63  61   59  57  55   50   44   39   34   29   24   20   15  10   6  ..    ..   ..   ..
  11   78   75   73   71   69   67  65  62   60  58  56   51   46   41   36   32   27   22   18  13   9   5    ..   ..   ..
  12   78   76   74   72   70   68  66  64   62  60  58   53   48   43   39   34   29   25   21  16  12   8    ..   ..   ..
  13   79   77   75   73   71   69  67  65   63  61  59   54   50   45   41   36   32   28   23  19  15  11     7   ..   ..
  14   79   78   76   74   72   70  68  66   64  62  60   56   51   47   42   38   34   30   26  22  18  14    10    6   ..
  15   80   78   76   74   73   71  69  67   65  63  61   57   53   48   44   40   36   32   27  24  20  16    13    9    6

   t  2.0  2.5 3.0  3.5  4.0  4.5  5.0  5.5  6.0  6.5  7.0  7.5 8.0 8.5  9.0 9.5 10.010.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.515.0 16.0 17.0 18.0 19.0
 
  16   81   76  71   67   63   58   54   50   46   42   38   34  30  26   23  19  15   12    8    5    ..            ..   ..   ..  ..   ..   ..   ..   ..
  17   81   76  72   68   64   60   55   51   47   43   40   36  32  28   25  21  18   14   11    8    ..            ..   ..   ..  ..   ..   ..   ..   ..
  18   82   77  73   69   65   61   57   53   49   45   41   38  34  30   27  23  20   17   14   10    7             ..   ..   ..  ..   ..   ..   ..   ..
  19   82   78  74   70   65   62   58   54   50   46   43   39  36  32   29  26  22   19   16   13   10    7        ..   ..   ..  ..   ..   ..   ..   .. 
  20   83   78  74   70   66   63   59   55   51   48   44   41  37  34   31  28  24   21   18   15   12    9    6   ..   ..   ..  ..   ..   ..   ..   ..
  21   83   79  75   71   67   64   60   56   53   49   46   42  39  36   32  29  26   23   20   17   14   12    9    6   ..   ..  ..   ..   ..   ..   ..
  22   83   80  76   72   68   64   61   57   54   50   47   44  40  37   34  31  28   25   22   19   17   14   11    8    6   ..  ..   ..   ..   ..   ..
  23   84   80  76   72   69   65   62   58   55   52   48   45  42  39   36  33  30   27   24   21   19   16   13   11    8    6   .   ..   ..   ..   ..
  24   84   80  77   73   69   66   62   59   56   53   49   46  43  40   37  34  31   29   26   23   20   18   15   13   10    8   5   ..   ..   ..   ..
  25   84   81  77   74   70   67   63   60   57   54   50   47  44  41   39  36  33   30   28   25   22   20   17   15   12   10   8   ..   ..   ..   ..
  26   85   81  78   74   71   67   64   61   58   54   51   49  46  43   40  37  34   32   29   26   24   21   19   17   14   12   10   5   ..   ..   ..
  27   85   82  78   75   71   68   65   62   58   56   52   50  47  44   41  38  36   33   31   28   26   23   21   18   16   14   12   7   ..   ..   ..
  28   85   82  78   75   72   69   65   62   59   56   53   51  48  45   42  40  37   34   32   29   27   25   22   20   18   16   13   9    5   ..   ..
  29   86   82  79   76   72   69   66   63   60   57   54   52  49  46   43  41  38   36   33   31   28   26   24   22   19   17   15  11    7    ..  ..
  30   86   83  79   76   73   70   67   64   61   58   55   52  50  47   44  42  39   37   35   32   30   28   25   23   21   19   17  13    9    5   ..
  31   86   83  80   77   73   70   67   64   61   59   56   53  51  48   45  43  40   38   36   33   31   29   27   25   22   20   18  14   11    7   ..
  32   86   83  80   77   74   71   68   65   62   60   57   54  51  49   46  44  41   39   37   35   32   30   28   26   24   22   20  16   12    9    5
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