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Psychrometry is the study of the

properties of moist air (dry air + water

vapor). It includes the study of various

temperatures, humidity and Psychrometric


  1. Dry Air

 Mixture of gases (mainly oxygen and nitrogen) which does not contain water vapor.

2. Wet air

Wet air or moist air is a mixture of dry air and water vapor.

3. Saturated air

Saturated air is one which contains maximum water vapors at a particular temperature.

For saturated air,

Dry bulb temperature= wet bulb temperature= dew point temperatures

 Relative humidity is 100 %.

4. Dry air temperature

Temperature of dry air is the temperature of atmospheric air (moist air). A thermometer measures dry bulb temperature.

5. Wet bulb temperature

 Temperature of fully saturated air. Wet bulb thermometer measures it. Sling Psychrometer has the wet bulb thermometer..

6. Sling Psychrometer

Sling psychrometer measures dry and wet bulb temperatures.

7. Dew point temperature

Dew point temperature is that temperature at which first condensation of water vapor takes place on cooling the moist air. Dry bulb temperature is greater than wet bulb temperature. Wet bulb temperature is greater than dew point temperature.

9. Specific humidity 

Specific humidity (w) is the amount of water vapor per kg of dry air. Mathematically

w = 0.622 pwv/(pt–pwv)

10. Relative humidity

Relative humidity is at a particular temperature on volume basis.

 mass of  H2O vapor in moist air/ mass of H2O vapor when saturated

 Expressed as ‰ .

Its symbol is ‘Ф’.


Ф = pv/pvs

Where pv is pressure of water vapors. Read from steam tables at the dry bulb temperature.

pvs  pressure of water vapors when saturated

Read from steam tables at the wet bulb temperature.

11. Degree of saturation

Degree of saturation is at a particular temperature.

It is on mass basis.

Its symbol is ‘ μ’.

μ = mass of H2O vapor/mass of H2O vapor when saturated

It has no units.

Example, its value is like 0.56

12. Enthalpy of moist air

Enthalpy of moist air = enthalpy of dry air + enthalpy of water vapor portion in 1 kg of dry air

Let w be the mass of water vapor in 1 kg of moist air. Mathematically

Enthalpy of moist air/kg of dry air, hma

hma = 1 cp dt + w [hg +cpv (tdb -tdp)]