Measuring Air Humidity

Measuring relative air humidity with dry and wet bulb temperatures

Air humidity can be estimated by measuring

  • the dry bulb temperature
  • the wet bulb temperature

humidity measurement dry wet bulbe temperature

Dry Bulb Temperature - Tdb - can be measured with a simple thermometer as shown above.

Wet Bulb Temperature - Twb - can be measured with a standard thermometer with some wet clothing, cotton or similar, around the bulb. Note that a continuously air flow is important to evaporate water from the wet clothing and achieve a correct wet bulb temperature.

Sufficient air movement can be achieved with a sling thermometer or similar.

Relative humidity can be estimated from the tables below or alternatively from a psyhrometric or Mollier diagram.

Temperature in Fahrenheit

Relative Humidity - RH (%)
Difference Between Dry Bulb
and Wet Bulb Temperatures
 Tdb - Twb
(oF)
Dry Bulb Temperature - Tdb (oF)
60 64 68 72 76 80 84 88
1 94 95 95 95 96 96 96 96
2 90 90 90 91 91 92 92 92
3 84 85 85 86 87 88 88 89
4 78 80 81 82 83 84 84 85
5 73 75 76 78 79 80 80 81
6 68 70 72 73 75 76 77 78
7 63 66 67 69 71 72 73 74
8 58 61 63 65 67 68 70 71
9 54 57 59 61 63 65 66 68
10 49 52 55 57 59 61 63 64

dry and wet bulb temperature - relative moisture diagram degrees fahrenheit

Temperature in Celsius

Relative Humidity - RH (%)
Difference Between Dry Bulb
and Wet Bulb Temperatures
 Tdb - Twb
(oC)
Dry Bulb Temperature - Tdb (oC)
15 18 20 22 25 27 30 33
1 90 91 91 92 92 92 93 93
2 80 82 83 84 85 85 86 87
3 71 73 75 76 77 78 79 80
4 62 65 67 68 70 71 73 74
5 53 57 59 61 64 65 67 69
6 44 49 52 54 57 59 61 63
7 36 42 45 47 51 53 55 58
8 28 34 38 41 45 47 50 53
9 21 27 31 34 39 41 45 48
10 13 20 25 28 33 36 40 43

dry and wet bulb temperature - relative moisture diagram degrees celsius

Related Topics

  • Air Conditioning - Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort
  • Air Psychrometrics - The study of moist and humid air - air condition - psychrometric charts, Mollier diagrams, air temperature, absolute and relative humidity, moisture content and more
  • Process Control - Instrumentation and process control systems - engineering and documentation

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