higrometer

A hygrometer is an instrument used to measure the humidity of air—the amount of water vapor present. The word derives from Greek hygros, moisture, and metron, measure. The term higrometer is a common misspelling.

Hygrometers can measure relative humidity, dew point, or absolute humidity, depending on the sensor design. Mechanical hygrometers rely on materials that change dimension with moisture, such as human hair or bi-metal strips. Electronic hygrometers employ capacitive sensors (humidity affects dielectric properties) or resistive sensors (humidity changes electrical resistance). Dew-point hygrometers determine humidity by cooling a surface until condensation forms and identifying the dew point. Psychrometers use the temperature difference between a dry-bulb and a wet-bulb thermometer to infer relative humidity.

Calibration and accuracy are essential for reliable readings. Devices are typically calibrated against known humidity levels using saturated salt solutions or reference instruments. Consumer devices often achieve roughly ±3–5% relative humidity, while laboratory-grade instruments can reach ±1% or better under controlled conditions.

Applications are diverse. Hygrometers are fundamental in meteorology and climatology, HVAC and cleanroom control, industrial processes (such as food, pharmaceuticals, and electronics), agriculture, and museums or archives where tight humidity control helps protect materials.

History notes that humidity measurement has roots in early modern science, with various primitive devices using hair or condensation. Over time, advances in chemical and electrical sensing led to the modern range of hygrometers in common use today.