Psychrometric Chart

PSYCHROMETRY is the science of studying the thermodynamic properties of moist air and the use of these properties to analyze conditions and processes involving moist air.

Moist air is a mixture of dry air and water vapour. In atmospheric air, water vapour content varies from 0 to 3% by mass.

Is often used in the design of refrigerating system by engineers to determine properties of moist air by plotting in a Psychrometric Chart

How to use the Psychometric Chart

A psychrometric chart can be used in two different ways.

The first is done by plotting multiple data points, that represent the air conditions at a specific time, on the chart.

Then, overlaying an area that identifies the “comfort zone.”  The comfort zone is defined as the range within occupants are satisfied with the surrounding thermal conditions. After plotting the air conditions and overlaying the comfort zone, it becomes possible to see how passive design strategies can extend the comfort zone.


  • This lines represent measured  Dry Bulb Temperature (DBT)
  • Temperature increased from Left to Right


  • This lines represent measured  Wet Bulb Temperature (WBT)
  • Temperature increased from Left corner  to Upwards


  • This lines represent Relative Humidity (%RH)
  • These lines are curved, and begin at 100% along the top of the chart, and decrease moving downward. It is fairly common for these lines to be indicated in intervals of ten

How to plot and read Psychrometric the Chart (SAMPLE  PLOTTING

Consider an example of air where the given values of DB and WB temperature are 78 degrees F and 65 degrees F respectively. We will find out various values from these given values of DB and WB temperatures.

  • Step1) On the psychrometric chart locate value 78oF on the DB temperature scale located at the bottom of the chart.
  • Step 2) Locate WB temperature of 65oF on the saturation curve scale..
  • Step 3) Extend the vertical line (constant DB temperature line) from 78and the diagonal line (constant WB temperature line) from 65oF and get the point of intersection of the two lines, which indicates the condition of the given air.
  • Step 4) Find out the values of various parameters at this point. The relative humidity line passing though this point indicates RH of 50%. The horizontal line passing though this point and meeting the saturation curve indicates DP temperature of 58oF. The specific volume of this air is 13.7 ft3/lb and the enthalpy of air is 30 Btu/lb.

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