The servo gauge system uses the small displacer. The displacer has buoyancy, but on the liquid it does not float. The displacer must be held on top of the tank by a thin wire linked to the servo gauge.
A servo weighing system senses the tension in the wire, signals from the weighing mechanism control an electrical motor in the servo unit and the displacer follows the liquid level movements.
An electronic transmitter sends the field bus level data to the control room reader. A still pipe is needed wherever a servo gauge is installed to prevent the displacer from drifting in the tank. In fixed roof tanks, this is also required.