Psykrometry

Psykrometry, also spelled psychrometry, is the science of measuring and analyzing humidity in air and the properties of moist air. It seeks to determine parameters such as relative humidity, humidity ratio, dew point, and enthalpy, which are important in fields like heating, ventilation, air conditioning (HVAC), meteorology, agriculture, and industrial processing.

The primary practical method uses a psychrometer, which consists of two thermometers: a dry-bulb and a wet-bulb. The wet-bulb thermometer has a wick soaked in water. As air passes over the wick, evaporation cools the wet-bulb; the rate of cooling depends on the air’s moisture content. The difference between the dry-bulb and wet-bulb temperatures indicates humidity: a large difference suggests dry air, while a small difference indicates moist air. Using this temperature difference together with tables or a psychrometric chart, one can derive relative humidity and other moisture-related properties.

The psychrometric chart is a graphical representation of moist air properties at a given atmospheric pressure. It typically plots dry-bulb temperature against humidity-related quantities, with contours showing lines of constant relative humidity, humidity ratio, dew point, and enthalpy. Modern practice often relies on digital sensors and software to compute these properties directly, rather than using a physical chart.

Applications of psykrometry include designing and evaluating HVAC systems, assessing indoor air quality, agricultural and greenhouse climate control, food processing, and industrial drying processes. The concept remains a foundational tool in climate control and environmental engineering, bridging thermodynamics and practical moisture management.