Reading the Psychrometric Chart: Part One

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Dr. John Watt gave the following helpful explanation for reading and understanding the  psychrometric chart.

“To simplify calculations, psychometric properties of nondry air are plotted on standard psycrometic charts…Given values for any two psychrometric properties, all others can be found without computation.  The chart is arranged as follows:

  1. Lines of constant dry-bulb temperature are vertical.
  2. Lines of constant dew point temperature and specific humidity are horizontal.
  3. Lines of constant enthalpy , the heat content per pound of air, and wet0bulb temperature slant diagonally down to the right.
  4. The saturation line of 100 percent relative humidity curves down from right to the left.  The temperatures printed along it  indicate the coinciding dew point, dry bulb, and wet-bulb temperatures in saturated air.
  5. Curves of constant relative humidity are spaced below the saturation line, generally parallel to it.
  6. Lines of constant specific volume, in cubic feet per pound, are widely spaced and run downward from left to right, more steeply than wet-bulb lines.”*

*  Watt, John R., Richard L. Koral, and John R. Watt. Evaporative Air Conditioning Handbook. New York: Chapman and Hall, 1986.