Principle of Servo Tank Gauging
Varec servo tank gauges 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 (U.S. patent). 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 gauges are ideally suited for light and middle distillates of petroleum.
Servo Tank Gauging Solution
Varec's servo tank gauging solution consists of a 6005 STG that is able to measure level to +/- 0.7 mm accuracy. It is also able to measure two interface levels, spot density and density profiling (when the product is ordered with these options). The 4532 ATC (or 4539 ATC) can be selected for temperature measurement suitable for inventory management (or custody transfer, NMi approved) applications. If a tank side (local) display is required, the 4560 SGM can be seamlessly integrated with the 6005 STG. The 4560 SGM also provides local operation of the 6005 STG. The servo tank gauging solution can be selected with FM, ATEX or CSA approvals and the appropriate field communications protocols for data transfer to the control room system.
