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 equals 14 cubic feet of air.) In the standard atmosphere at sea level, one pound of dry air weighs 7,000 grains. (7,000 grains = 1 pound of liquid. A gallon of liquid = 8.34 pounds, and a pint of liquid = 1.043 pounds. So 7,000 grains of moisture is slightly more than 1 pint.)
Relative humidity (RH) is the amount of moisture the air currently is holding at a given temperature. This amount is expressed as a percentage of the maximum moisture air can hold when totally saturated at the same temperature. The amount of moisture air can hold varies with temperature. As air is heated, its volume increases enabling the air to hold more moisture. As air is cooled, its volume decreases reducing the amount of moisture the air can hold.
An example will show how temperature affects relative humidity. Suppose a pound of dry air at 80 degrees can hold 158 grains of moisture when totally saturated. If the air currently is holding 79 grains of moisture, the relative humidity is 50% (79/158=50%). In other words, the air currently is holding one-half the moisture it can hold when totally saturated.
By reducing the temperature of the air 20 degrees, the capacity of the air to hold moisture is cut in about half. Therefore a pound of dry air at 60 degrees can hold only about 70 grains of moisture when totally saturated. If currently this cubic foot of air is holding 79 grains of moisture, the relative humidity is 100% (79/79=100%).
Specific humidity is the actual amount of moisture in the air. This amount is expressed in weight as the number of grains of moisture per pound of air (gpp). A grain is a unit of weight measurement with 7,000 grains equaling one pound (approximately 14 cubic feet of air weighs one pound). Psychrometric charts are used to calculate the specific humidity of air at various temperature and relative humidity conditions. Some electronic moisture meters also convert temperature and relative humidity to specific humidity.
Because of the tremendous effect temperature has on the properties of air, relative humidity is not always a good indicator of the amount of moisture present. Cooler air will hold less actual moisture than warmer air at the same relative humidity. (See the Psychrometric Chart for specific humidity in the following examples.) If air temperature is 80 degrees and the relative humidity is 80 percent, the specific humidity is 126 grains per pound (gpp). If the air temperature decreases to 60 degrees while the relative humidity remains at 80 percent, the specific humidity is 62 gpp.
Understanding specific humidity is critical when deciding whether to use outside air for drying a structure. To use outside air effectively, the outside air should have about 20 gpp less moisture than the inside air. If the outside air contains more moisture than the air inside the structure, using the outside air will not dry the structure, but actually bring more moisture into the building.