What Is a Psychrometric Chart?

A psychrometric chart is a type of graph that relates the properties of air to moisture and temperature. It is a complex diagram that maps the relationships of several thermodynamic and physical properties of air for a given pressure, often normalized to one standard atmosphere, or standard, sea level air pressure. As many as seven different parameters can be shown on a typical chart, and as long as any three of them are known, all of the others may be read or calculated from the chart and its data.

The parameters of a typical psychrometric chart include dry and wet bulb temperatures, humidity and dew point. Also included are enthalpy, specific volume and humidity ratio. All of these properties are represented on the chart and are interrelated, each affecting the others as the values change. A psychrometric chart is useful for predicting the way masses of air will behave based upon 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 troubleshoot various problems.

A great deal of data is contained in a psychrometric chart, all relating to properties of moist air. The dry bulb temperature is the temperature read from any standard thermometer, while the 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 difference in perceived temperature when wearing wet clothing and is influenced by relative humidity, which is the amount of moisture suspended in air.

The dew point of a body of air is the temperature at which the air can hold no more moisture, or at which humidity reaches 100%. The ability of air to hold moisture is determined by temperature and pressure. As temperature goes up, a given body of air is able to hold more moisture, and as temperature decreases, the air is able to hold less moisture. Enthalpy is the total amount of energy contained within a volume of air and is influenced by temperature and dissolved moisture.

Specific volume indicates how much space is occupied per unit of mass. It is influenced by the amount of dissolved moisture in the air and its temperature at a given pressure. The humidity ratio is the mass of dissolved water per unit volume of air, and is tied closely to relative humidity and specific volume. All of these properties are shown on the psychrometric chart and their relationships to one another allow any one of them 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.