Psychrometer

Psychrometer is an instrument to measure the wet-bulb and dry-bulb temperature of an air-water vapour mixture. This instrument uses the principle of adiabatic saturation. The specific humidity and relative humidity of air-water vapour mixture can be determined with knowledge of dry-bulb and wet-bulb temperatures figure 36.1

• The dry-bulb temperature is the temperature of the incoming mixture.
  
  
• The wet-bulb temperature is the temperature of the saturated air-water vapour mixture.
  
  
• For air-water vapour mixtures, the wet- bulb temperature is found to be approximately equal to the adiabatic Saturation temperature (AST).
  
  
• If and are known, & can be found out.
  
  
• The enthalpy of air-water vapour mixture is expressed as

(36.1)

where is the enthalpy of the accompanying water vapour, and

Refer to figure 36.2

We can write

(36.2)

In the above equation, is vapour enthalpy, is also enthalpy of vapour. The term is meant for enthalpy of liquid water     

Also

For the above reasons, we can rewrite (36.2) as

(36.3

Invoking (36.1), the above equation becomes

is the conserved property in an adiabatic saturation process. (Remember that state 2 need not be saturated). We can further write

(36.4)

Where

and

(36.5)

If is small constant and we get

(36.6)

A long an adiabatic miniaturization process will remain constant

(36.7)

or

(36.8)