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A psychrometric chart maps the relationships between various properties of air, including temperature, humidity, and pressure. It can be used to predict how air masses will behave and solve problems in closed systems. The chart includes dry and wet bulb temperatures, humidity, dew point, enthalpy, specific volume, and humidity ratio. Each property affects the others, and all are related on the chart. The chart is calibrated for a specific atmospheric pressure and can be used to read or calculate values if at least three other values are known.
A psychrometric chart is a type of chart that relates the properties of air to humidity and temperature. It is a complex diagram that maps the relationships of various thermodynamic and physical properties of air for a given pressure, often normalized to a standard atmosphere, or standard atmospheric pressure at sea level. Up to seven different parameters can be displayed in a typical graph, and if three are known, all others can be read or calculated from the graph and its data.
The parameters of a typical psychrometric chart include dry and wet bulb temperatures, humidity, and dew point. Enthalpy, specific volume and humidity ratio are also included. All these properties are represented in the graph and are related, each affecting the others as the values change. A psychrometric graph is useful for predicting how air masses will behave based on certain known characteristics, as well as for determining unknown properties when some properties are known. Environmental management of closed systems, such as greenhouses and other buildings, can benefit from a psychrometric chart to solve various problems.
A large amount of data is contained in a psychrometric graph, all relating to the properties of moist air. Dry bulb temperature is the temperature read by any standard thermometer, while wet bulb temperature is the temperature shown by a thermometer when moist air moves over a wet temperature sensor. The difference is analogous to the temperature difference felt when wearing wet clothing and is affected by relative humidity, which is the amount of moisture suspended in the air.
The dew point of a body of air is the temperature at which the air can no longer hold moisture or at which the humidity reaches 100%. The ability of air to hold moisture is determined by temperature and pressure. As the temperature increases, a given body of air is able to hold more moisture, and as the temperature decreases, the air is able to hold less moisture. Enthalpy is the total amount of energy contained in a volume of air and is affected by temperature and dissolved humidity.
The specific volume indicates how much space is occupied per unit of mass. It is influenced by the amount of moisture dissolved in the air and its temperature at a given pressure. Humidity ratio is the mass of dissolved water per unit volume of air and is closely related to relative humidity and specific volume. All these properties are shown on the psychrometric chart, and their mutual relationships allow one to be read directly from the chart or calculated using established scientific formulas if at least three other values are known. Each chart is calibrated for a specific atmospheric pressure, with standard sea level being the most common, although it is possible to compile a chart for any pressure.
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