A typical digital hygrometer uses either resistive or capacitive solid-state sensors to determine humidity levels. In other words, it measures the resistance or capacitance of a particular sample of air, the humidity of which it calculates.
For the capacitive hygrometer you will get two metal plates with air between them. Too much water in the air will affect the capacitance of the plates. By knowing how much load the plates can store, the hygrometer will provide you with an accurate and instantaneous relative humidity value. In the case of the resistive sensor, electricity is fed from a piece of ceramic material that is exposed to air. When the humidity rises, more water vapor condenses inside the ceramic, affecting its durability. The hygrometer will provide accurate humidity measurements by measuring the amount of current flowing through the ceramic.
