Hygrometers measure the level of moisture in the air, also known as relative humidity (RH), by detecting changes in electrical resistance or capacitance. Hygrometers can be classified into two main types based on their sensing mechanism
Electrical resistance hygrometers are typically composed of two metal electrodes separated by a moisture-sensitive material. As the relative humidity of the surrounding air changes, the moisture content of the sensing material fluctuates, which in turn alters the electrical resistance between the electrodes. This change in resistance is measured and converted into a RH reading.
Capacitance hygrometers utilize a capacitor with a moisture-sensitive dielectric material. When the relative humidity of the air changes, the dielectric material absorbs or releases moisture, altering the capacitance of the capacitor. This change in capacitance is measured and converted into a RH reading.