Servo tank gauging system operate on the principle of displacement measurement. A small displacer on a measuring wire is unwound from a drum and accurately positioned in the liquid medium using a servo motor. The weight of the displacer is precisely balanced against a magnetic coupling and the wire drum. When the displacer is lowered and touches the liquid, the weight of the displacer is reduced because of the buoyant force of the liquid.
As a result, the torque in the magnetic coupling is changed and this change is measured by 5 sets of Hall sensors. The signal, an indication of the position of the displacer, is sent to the motor control circuit. As the liquid level rises and falls, the position of the displacer is automatically adjusted by the drive motor. This system provides an outstanding accuracy of +/- 0.7 mm. Servo tank gauging system are ideally suited for light and middle distillates of petroleum.