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Determine the humidity or characteristics parameters of air using Psychrometer

Aim: To determine the humidity or characteristic parameters of air using Psychrometer or dry and wet bulb temperature.


Humidity(HA): Humidity is defined as the amount of water vapour present in the moist air per unit mass of the dry air in a given volume.

Humidity, HA = Mass of water vapour persent in the air (kg) / Mass of dry air (kg).

Relative humidity(RH): It is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air.

Absolute Humidity: It is defined as the amount of water vapour present per unit volume of the gas.

Humidity at Saturated Gas (Hs): It represents the maximum possible amount of water vapours. At a given temperature and pressure, if the gas in vapour (air) is in equilibrium with the gas in liquid, then that gas is said to be saturated.

Percent Humidity: It may be defined as the ratio of actual humidity to saturation humidity as expressed in percentage, at a certain temperature and pressure.

Percent Humidity = (actual humidity / saturation humidity) x 100

Psychrometer: It is a set of dry and wet and bulb thermometers.


Dry Bulb Temperature: It is the dry bulb temperature of moist air when it is measured at rest (by an instrument i.e. psychrometer), which is not affected by the moisture content of air or by radiation.

Wet Bulb Temperature: Wet bulb temperature is the dynamic equilibrium temperature attained by a water surface when exposed to air under adiabatic conditions.

Psychrometry: It is the science of study of various properties of air, method of controlling its temperature and moisture content or humidity and its effects an various materials and human beings.

Psychrometric chart: It is a graphical representation of the psychrometric processes of air which includes physical and thermodynamic properties such as dry and wet bulb temperature, humidity, enthalpy and air density.

Dew point: It indicates the amount of moisture present in the air. The higher the dew points, higher will be the moisture content of air at given temperature.


Psychrometer, Psychrometric chart, stopwatch, water.


  1. Fill the psychrometer wet bulb container with water, up to the hole/mark on the container bulb and fix it on the psychrometer.
  2. Place the psychrometer at a specific location named as Location A and leave it for 15 minutes for saturation of temperature.
  3. Note the temperature readings for dry and wet bulb thermometer after saturation and again allow them to saturate on same place for 15 minutes and take another readings same as previous.
  4. Repeat the Step again to take at least three reading for Location A.
  5. Similarly, shift the location to Location B about 5 to 20 meters away from Location A and take again three reading using same procedure.
  6. Shift at least three location and note the readings using similar procedure.
  7. Plot the psychrometric chart using the reading noted in previous steps.
  8. Find the different parameter of air by from the intersection point of dry and wet bulb temperature on the psychrometric chart.


Different characteristics parameter of air for different location are as follow _______ .


Subrahmanyam C.V.S., Setty J.T., Suresh S., Devi V.K., ‘Pharmaceutical Engineering – Unit Operation – I’, Vallabh Prakashan, 2nd Edition, 2011, Page No 291-302.


Relative Humidity Calculator is a simple tool by using which you can easily calculate instant Dew Point and Relative Humidity from Dry and Wet Bulb Temperature.

Sample Readings and Chart:

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