higrometre

Higromètre, or hygrometer in English, is an instrument used to measure the moisture content of the air. The most common reading is relative humidity, the percentage of water vapor present in the air relative to the maximum amount the air can hold at a given temperature. Some hygrometers also indicate dew point, absolute humidity, or mixing ratio.

Historically, hygrometers evolved from simple, physical methods. Early devices often used natural fibers such as human or animal hair, which change length with humidity and can be correlated to relative humidity. The term hygrometer dates to the 18th century, and modern developments have expanded to a range of sensors and principles.

Today, hygrometers fall into several categories. Mechanical or hair-based hygrometers rely on the flattening or stretching of a fiber as humidity changes. Electronic hygrometers use sensors whose electrical properties vary with humidity: capacitive sensors detect changes in capacitance as a dielectric material absorbs moisture, while resistive sensors measure changes in resistance. Thermo-hygrometers combine temperature and humidity sensing to derive dew point or absolute humidity. Psychrometers use a wet-bulb and a dry-bulb thermometer to infer humidity, often operated as a sling device or a humidity sensor arrangement. Dew-point hygrometers determine humidity by cooling a surface until condensation forms.

Applications span meteorology, HVAC and building automation, agriculture, museums and archives, electronics manufacturing, and laboratory environments. Calibration and maintenance are important for accuracy, with traceability to standards. Common readings are given as percent relative humidity, with supporting data such as dew point, temperature, or absolute humidity.