The float is replaced by a small displacer. Thedisplacer has buoyancy but does not float on theliquid. The displacer needs to be suspended by a thinwire which is connected to the servo gauge on topof the tank. A weighing system in the servo gaugesenses the tension in the wire, signals from theweighing mechanism control an electric motor in theservo unit andmake the displacer follow the liquidlevelmovements. An electronic transmitter sends thelevel information over field buses to the readout in the control room. Some servo manufacturers claim that the unitmay be used for purposes other than levelgauging. The servo can be used to measureliquid density and water bottom levels, but inboth cases the level gauging is inhibited whilethe servo gauge performs a displacer dip into the product.
