The SJ series polymer humidity sensors are a new generation of composite resistance type humidity sensitive components. Their complex impedance is exponentially related to the relative humidity of the air, and the DC impedance (measured by an ordinary digital multimeter) is almost infinite. Unlike traditional resistance, water molecules in the air participate in ion conductivity in membrane humidity sensing. As water molecules are polar molecules, they will ionize and decompose in the presence of DC electricity, As a result, it affects the conductivity and the lifespan of the components, so it is required to use AC circuits to power the sensors.