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Air - Humidity Measurement from Dry and Wet Bulb Temperature

Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.

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Air humidity can be estimated by measuring

dry bulb temperature, and
wet bulb temperature

Humidity measurement - dry wet bulbe temperature meeter

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

Wet Bulb Temperature - T_wb - 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 19^oC and the web bulb temperature is 12^oC - 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 kg_h2o/kg_{dry_air}

Mollier diagram - state of air based on dry bulb and web bulb temperature

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.

Relative Humidity vs. Dry and Wet Bulb Temperatures - SI Units
T_db- T_wb(^oC)	Relative Humidity - RH (%)
	Dry Bulb Temperature - T_db (^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

Relative moisture - dry and wet bulb temperature diagram - degrees Celsius Download and print Air - Relative Humidity vs. Dry and Wet Bulb Temperatures chart

Example - Relative Humidity

If the dry bulb temperature of air is 18^oC and the web bulb temperature is 12^oC - then the difference between them is 6^oC. 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 75^oF and wet bulb temperature 62.5^oF can be visualized in the psychrometric chart as indicated below.

the relative humidity is approximately 50%
the humidity ratio is approximately 65 grains_h2o/lb_{dry_air}

Psychrometric chart - dry and wet bulb temperatures

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.

Relative Humidity vs. Dry and Wet Bulb Temperatures - Imperial Units
T_db- T_wb(^oF)	Relative Humidity - RH (%)
	Dry Bulb Temperature - T_db (^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 diagram - degrees Fahrenhei

Download and print Air - Relative Humidity vs. Dry and Wet Bulb Temperatures chart

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Related Topics

Air Conditioning
Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort.
Air Psychrometrics
Moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content.
Process Control
Instrumentation and process control systems - design and documentation.

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 20^oF to 120^oF.
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Classification of dehumidifiers.
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Comfort vs. dry bulb, wet bulb and dew point temperature.
Dry Bulb, Wet Bulb and Dew Point Temperatures
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The change in state wwhen mixing moist air - enthalpy, heat, temperature and specific humidity.
Moist Air - Daltons Law of Partial Pressure
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 - 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.
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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 - Properties
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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.
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Comparing advantages and disadvantages of thermocouples, RTDs and thermistors temperature sensors.
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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.

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