The Orchid House
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.
| 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 | |
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
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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