A method of operating a sensor 112 having a first transmit plate 114, a second receive plate 115 and a dielectric material between the two plates 114, 115. The method comprises the steps of: applying an alternating across the transmit and receive plates 114, 115, thereby to create an alternating electric field, which applied voltage results in a current Iz flowing through the two plates 114, 115; producing a voltage signal corresponding to the resultant current Iz; determining the average value of the product of the corresponding voltage signal and a reference voltage signal Vref3; and adjusting the phase of the reference voltage Vref3 until a null condition is achieved, at which condition the average value is approximately zero. A sensor 112 is also disclosed.