Last edited 22 Sep 2020

Psychrometric charts

Psychrometric charts are complex graphs that can be used to assess the physical and thermodynamic properties of gas-vapour mixtures at a constant pressure. They are often used to assess the properties of moist air. This can be useful in the design of heating, ventilation and air-conditioning systems for buildings, and psychrometric charts often include a zone in the middle that represents the range of conditions that people find comfortable under different circumstances (such as summer and winter).

Typically, the properties represented on psychrometric charts are:

Dry-bulb temperature: A measure of air temperature recorded by a thermometer exposed to the air but shielded from radiation and moisture.
Wet-bulb temperature: The temperature recorder by a thermometer that has its bulb wrapped in cloth and moistened with distilled water. The rate of evaporation from the wet bulb, and so the temperature it records varies depending on the humidity of the air it is exposed to.
Relative humidity: The ratio of the actual vapour pressure relative to the vapour pressure of saturated air at the same temperature, expressed as a percentage.
Specific volume: The volume of a unit weight of dry air.
Dew point temperature: The highest temperature at which water vapour will condense.
Humidity ratio: The dry-basis moisture content of air expressed as the weight of water vapour per unit weight of dry air.
Enthalpy: The energy content of air.

The state of moist air can be determined from any two of these properties, from which all other properties can then be determined.

Atmospheric pressure varies with altitude and so a number of psychrometric charts are available for different atmospheric pressures, however, for altitudes of less than 600m the sea-level psychrometric chart is often considered adequate.