Air - Humidity Measurement from Dry and Wet Bulb Temperature
Air humidity can be estimated by measuring
- dry bulb temperature, and
- wet bulb 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 continuously air flow around the thermometer 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 a Mollier diagram.
Example - The State of Air in the Mollier Diagram
If the dry bulb temperature of air is 19oC and the web bulb temperature is 12oC - then the state of air can be visualized in the Mollier diagram as indicated below.
- the relative humidity is approximately 42%
- the humidity ratio is approximately 0.0057 kgh2o/kgdry_air
Relative Humidity - Temperature in Celsius
The table below can be used to estimate the relative humidity of air if dry and wet bulb temperatures are known.
| Tdb - Twb (oC) | Relative Humidity - RH (%) | |||||||
|---|---|---|---|---|---|---|---|---|
| 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 |
Example - Relative Humidity
If the dry bulb temperature of air is 18oC and the web bulb temperature is 12oC - then the difference between them is 6oC. By using the table above
- the relative humidity is approximately 49%
Example - the State of Air in the Psychrometric Chart
The state of air with dry bulb temperature 75oF and wet bulb temperature 62.5oF can be visualized in the psychrometric chart as indicated below.
- the relative humidity is approximately 50%
- the humidity ratio is approximately 65 grainsh2o/lbdry_air
Relative Humidity - Temperature in Fahrenheit
The table below can be used to estimate the relative humidity of air if dry and wet bulb temperatures are known.
| Tdb - Twb (oF) | Relative Humidity - RH (%) | |||||||
|---|---|---|---|---|---|---|---|---|
| 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 | 6 |
Related Topics
• Air Conditioning Systems
Design of Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort.
• Air Psychrometrics
Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.
• Process Control Systems
Instrumentation, design and documentation of process control systems.
Related Documents
Air - Humidity Ratio
The mass of water vapor present in moist air - to the mass of dry air.
Air - Psychrometric Chart for Standard Atmospheric Conditions - Imperial Units
Psychrometric chart for air at barometric pressure 29.921 inches of Mercury and temperature ranging 20oF to 120oF.
Dehumidifiers
Classification of dehumidifiers.
Discomfort Index
Comfort vs. dry bulb, wet bulb and dew point temperature.
Dry Bulb, Wet Bulb and Dew Point Temperatures
Dry Bulb, Wet Bulb and Dew Point temperatures can be used to determine the state of humid air.
Humid Air - Heating
Enthalpy change and temperature rise when heating humid air without adding moisture.
Humid Air and the Ideal Gas Law
Pressure, temperature and volume in a perfect ideal gas like moist air (air with water vapor).
Mixing of Humid Air
The change in state wwhen mixing moist air - enthalpy, heat, temperature and specific humidity.
Moist Air - Degree of Saturation
Humidity ratio of moist air to humidity ratio of saturated moist air.
Moist Air - Density vs. Pressure
Density of moist air vs. pressure ranging 75 - 1000 mmHg.
Moist Air - Density vs. Water Content and Temperature
Density of the mix of dry air and water vapor - moist humid air.
Moist Air - Enthalpy
Sensible and latent heat of moist air.
Moist Air - Partial Pressure and Daltons Law
The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.
Moist Air - Properties
Psychrometric table with humid air properties like saturation pressure, specific volume, enthalpy and entropy.
Moist Air - Psychrometric Table for Pressure 29.92 inHg
Dry and wet bulb temperatures, saturation pressure, water vapor weight, specific volume, heat and more.
Moist Air - Psychrometric Terms
Dry and wet bulb temperature, specific volume, relative humidity, enthalpy and more.
Moist Air - Relative Humidity
Relative humidity in moist air is the ratio of partial vapor pressure to air pressure.
Moist Air - Specific Volume
Specific volume of moist air is defined as the total volume of humid air per mass unit of dry air
Moist Air - the Mollier Diagram
The Mollier diagram is a graphic representation of the relationship between air temperature, moisture content and enthalpy - and is a basic design tool for building engineers and designers.
Moist Air - Transforming the Mollier Diagram to a Psychrometric Chart - or vice versa
The relationship between the psycrometric chart and the Mollier diagram.
Moist Air - Water Vapor and Saturation Pressure
Saturation pressure of water vapor in moist air vs. temperature.
STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure
The definition of STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure.
Temperature Sensors - Comparing Types
Comparing advantages and disadvantages of thermocouples, RTDs and thermistors temperature sensors.
Thermocouples
Thermocouples - types, principles and temperature ranges.
Wet Bulb Globe Temperature (WBGT)
The Wet Bulb Globe Temperature can be used to measure the general Heat-Stress index.