psikrometer

A psikrometer, or psychrometer, is a hygrometric instrument used to measure relative humidity in air by comparing temperatures recorded by two thermometers: a dry-bulb thermometer and a wet-bulb thermometer. The wet-bulb thermometer has its bulb wrapped in a wick saturated with distilled water. As air passes over the bulbs, evaporation from the wick cools the wet bulb; the amount of cooling depends on the moisture content of the surrounding air.

The readings from the two thermometers are used to determine humidity. The difference between the dry-bulb and wet-bulb temperatures correlates with relative humidity, dew point, and humidity ratio when atmospheric pressure is known. This relationship is typically read from a psychrometric chart or calculated with equations. A sling psychrometer is a common form in which the paired thermometers are rotated to produce airflow over the bulbs, while an aspirated or mechanical version uses a fan to provide stirring air.

Applications for psikrometers include meteorology, HVAC design and monitoring, agriculture, and laboratory humidity control. They are inexpensive and straightforward but require proper technique: adequate and consistent airflow, a clean, saturated wick, and regular wick replacement to maintain accuracy. Readings can be affected by wind speed, water temperature, wick condition, and variations in atmospheric pressure.

Modern variants may use digital sensors to measure dry-bulb and wet-bulb temperatures and compute relative humidity automatically. These electronic psychrometers offer faster readings and easier data interpretation while retaining the same underlying principle.