Polymer humidity sensor SJ The series is a new generation of composite resistance-type humidity-sensitive components. The complex impedance is exponentially related to the relative humidity of the air. The DC impedance (measured by ordinary digital multimeter) is almost infinite. It has ionic conduction in the traditional resistance where water molecules in the air participate in the moisture sensing of the membrane. Since water molecules are polar molecules, they will ionize and decompose in the presence of DC, which will affect the conductivity and the life of the component. Therefore, an AC circuit is required to power the sensor.