Temperature Sensors for Linux

This comes from DigiTemp user Robert Graham, instructions on how to measure humidity using the wet bulb method.

I know there is a humidity sensor out there but for those of you who like to build things you can make a wet/dry bulb hygrometer similar in operation to a sling Psychrometers used to calibrate humidity sensors.

Using two DS18S20 one old fan from your junk pile and some white plastic PVC pipe. I made mine out of 5 PVC couplings and a 5-6 reducer seal at one end for the water reservoir.


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For the wick for the wet bulb, I used a new cottonshoelace. You do not have to have the fan but you will get very fast response if you do.

I have a chart showing the relation ship between the temperature of the drybulb and the temperature of the wet bulb.

......Relative Humidity Table.......
Dry Bulb
Temperature ........Wet Bulb reads °C lower than Dry Bulb
°C      1  2  3  4  5  6  7  8  9  10 12 14 16 18 20
 2 >>>> 84 68 52 37 22 8
 4 >>>> 85 70 56 42 29 26 3
 6 >>>> 86 73 60 47 34 22 11
 8 >>>> 87 75 63 51 39 28 18 7
10 >>>> 88 76 65 54 44 33 23 14 4
12 >>>> 89 78 67 57 47 38 29 20 11 3
14 >>>> 89 79 69 60 51 42 33 25 17 9
16 >>>> 90 80 71 62 54 45 37 29 22 14
18 >>>> 91 81 73 64 56 48 41 33 26 19 6
20 >>>> 91 82 74 66 58 51 44 37 30 24 11
22 >>>> 91 83 75 68 60 53 46 40 34 27 16 5
24 >>>> 92 84 76 69 62 55 49 43 37 31 20 9
26 >>>> 92 85 77 70 64 57 51 45 39 34 23 14 4
28 >>>> 92 85 78 72 65 59 53 47 42 37 26 17 8
30 >>>> 93 86 79 73 67 61 55 49 44 39 29 20 12 4
32 >>>> 93 86 80 74 68 62 56 51 46 41 32 23 15 8  1
34 >>>> 93 87 81 75 69 63 58 53 48 43 34 26 18 11 5
36 >>>> 93 87 81 75 70 64 59 54 50 45 36 28 21 14 8
38 >>>> 94 88 82 76 71 65 60 56 51 47 38 31 23 17 11
40 >>>> 94 88 82 77 72 66 62 57 52 48 40 33 26 19 13
42 >>>> 94 88 83 77 72 67 63 58 54 50 42 34 28 21 16
44 >>>> 94 89 82 78 73 68 64 59 55 51 43 36 29 23 18

For example @20° C if there is a 1 degree difference that would = 91% relative humidity @20° C if there is a 10 deg difference that would = 24% relative humidity.

This design was based on one I have seen at some green houses, it is rugged and can get fertilizer on it and keeps working. When both sensor read the same you need to add water.


Copyright 2008 by Brian C. Lane