hygrometers

A hygrometer is an instrument used to measure the moisture content or humidity of the surrounding air. Most hygrometers report relative humidity (RH), but some can determine dew point, specific humidity, or vapor pressure. They are used in meteorology, HVAC (heating, ventilation, and air conditioning), agriculture, museums and archives, and industrial processes where control of moisture is important.

There are several main types. Mechanical hygrometers rely on materials that physically respond to humidity changes, such as human or animal hair, which lengthens or shortens with moisture variation. Electronic hygrometers use capacitive or resistive sensors; a dielectric material or printed polymer changes its electrical properties as humidity changes, producing a readable RH value. Dew-point hygrometers, including chilled-mirror devices, determine humidity by cooling a surface until condensation forms on the surface, with the temperature at onset indicating the dew point. Psychrometers use a wet-bulb and a dry-bulb thermometer to derive RH from the difference in readings.

Calibration and accuracy are important for reliable measurements. Many hygrometers are calibrated with reference humidity standards, often using saturated salt solutions or traceable humidity generators. Regular calibration and temperature compensation are common requirements because readings can drift with time and temperature.

Applications range from monitoring indoor air quality and preserving artifacts to ensuring proper conditions in horticulture, manufacturing, and scientific research. While electronic devices offer convenient digital output and logging, certain high-precision tasks rely on specialized methods like chilled-mirror dew-point measurement.