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Fundamentals of Psychrometry
Understanding psychrometry is easier if you are familiar with the fundamentals involved in measuring air conditions. 
Grains Per Pound (GPP)
   "Grains per pound" (gpp) is the unit used to measure the weight of moisture in air.  The number of grains per pound of dry air expresses the specific humidityof the air.  Stated another way, specific humidity expresses the weight (in number of grains) of the moisture present in a pound of air.  To determine the "grains per pound" of current air conditions, you must plot the temperature of air and the relative humidity of air on a psychrometric chart.  The intersection of temperature and relative humidity on the chart identifies the grains per pound (gpp).

   The measure of moisture in grains is based on the weight of dry air.  The expression "dry air" refers to air containing no water vapor or contaminants.  Actually, dry air would consist of about 78 percent nitrogen, 21 percent oxygen and 1 percent of various other gases.  "Wet or moist" air then would consist of dry air mixed with water vapor.  GPP (grains per pound) counts the number of grains of moisture in a pound of dry air.  (One pound of dry air ... more

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