Psychrometry

Psychrometry is the study of moist air and its properties, focusing on humidity and the thermodynamic relationships between temperature, moisture content, and pressure. It provides methods for measuring humidity and for describing the state of air–water vapor mixtures in a consistent framework.

Key quantities in psychrometry include dry-bulb temperature (the ordinary temperature), wet-bulb temperature (the temperature read on a thermometer with a wet wick), relative humidity (the ratio of actual vapor pressure to the saturation vapor pressure at a given temperature), dew point (the temperature at which air becomes saturated and condensation begins), humidity ratio w (mass of water vapor per mass of dry air), vapor pressure e, and the enthalpy h of moist air. Relative humidity is defined as RH = e/e_s(T), where e_s(T) is the saturation vapor pressure at temperature T. The humidity ratio can be approximated by w = 0.622e/(P − e), with P the total pressure. A common approximate expression for moist-air enthalpy is h ≈ 1.006 T + w(2501 + 1.805 T), using temperatures in Celsius and h in kilojoules per kilogram of dry air.

A psychrometric chart graphically represents these relationships at a given pressure. Moving horizontally at constant humidity or vertically at constant enthalpy shows how temperature and humidity interact. The chart is built from lines of constant RH, constant w, constant dew point, and constant enthalpy, enabling rapid assessment of processes such as heating, cooling, humidification, or dehumidification.

Instrumentation includes psychrometers (such as sling or aspirated devices) that measure dry-bulb and wet-bulb temperatures to derive humidity, along with modern hygrometers and chilled-mirror devices for direct vapor-pressure measurements. Applications are widespread, especially in heating, ventilation, and air-conditioning design, meteorology, industrial drying, food storage, and greenhouse climate control.