The Disc level gauges are based on servo powered null-balance technique. A disc serves as a continuous level sensing element.
A disc with a relative density higher than that of the product to be measured, is suspended from a stainless steel wire B, that is attached to a measuring drum. A two phase servo meter controlled by a capacitive balance system winds or unwinds the measuring wire until the tension in the weighing springs is in balance with the weight of the displacer partly immersed in the liquid.
The sensing system in principle measures the two capacitance formed by the moving center sensing rod E provided with two capacitor plates and the side plates. In balance position the capacitances are of equal value. A level variation will a difference in buoyancy of the displacer. The center sensing rod will move in the direction of one of the side capacitor plates. This causes a difference in value of these capacitances.
By an electronic circuit this change is detected and integrated. During the rotation of the servo motor the cam driven transmitter continuously change the voltage pattern to a remote indicator of which the receiver motor drives a counter indicating level variation.