Psychometric Chart

A psychometric chart is a graphical representation of the seven physical properties of air. These physical properties are defined and described as follows:

1. Dry-bulb temperature (dbt): The dry-bulb temperature is the temperature of air measured with a standard thermometer. Dry-bulb temperatures (and all other temperatures) usually are expressed in degrees Fahrenheit (°F) with customary units or degrees Celsius (°C) with SI units.

2. Wet-bulb temperature (wbt): The wet-bulb temperature is determined with a standard thermometer having the bulb surrounded by gauze or a sock and a means for keeping the sock wet. As air passes over the wet sock, it will absorb (through evaporation) some of the water from the sock, cooling the bulb of the thermometer. The drier the air is, the greater the evaporation and the greater the cooling effect. The difference between the dry-bulb temperature and the wet-bulb temperature is called wet-bulb depression. Dry-bulb and wet-bulb temperatures can be measured with an instrument called a psychrometer.

3. Relative humidity (rh): Relative humidity is a ratio of the amount of water in the air relative to the maximum amount of water the air could hold if it were fully saturated. Relative humidity is expressed as a percent and the values can range from 0% (dry air) to 100% (fully saturated).

4. Moisture content: The moisture content is a measure of the actual amount of water held in the air in the form of vapor. Moisture content is measured in terms of pounds of water per pound of air (lb water/lb air) or grains of water per pound of air (gr. of water/lb air). (Note: 7000 grains of water equals one pound.) The American Society of Heating and Refrigerating Engineers (ASHRA) measures moisture in pounds per pound, but some psychrometric charts use grains of water.

5. Dew point: Dew point is the temperature at which, as air is cooled, the moisture in the air begins to condense or form droplets that are too large to remain suspended in the air. Condensation occurs on any object with a surface temperature equal to or less than the dew point.

6. The total heat is the total heat energy in the air. It includes heat due to the temperature of the air, heat required to change water vapor present in the air from liquid to vapor, and heat energy in the water vapor itself. The total heat content of air is expressed as BTU per pound of dry air (customary units or Joules per kg of dry air (SI units).

7. Specific volume: Specific volume is the volume of space occupied by a pound of dry air at standard atmospheric pressure (14.7 psi), expressed in cubic feet per pound of dry air (ft3/lb air).

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