In capacitive tank gauges, the principle of capacitor is used for level measurement.
A constant voltage is applied to a metallic probe within the tank. The corresponding current is then monitored. This current is proportional to the capacitance from the metallic probe to a second electrode. The second electrode is generally a wall of the tank.
Figure: A capacitor interpretation of the elements of a capacitive level measurement system
False high-level indications are a typical failure mode of this type of system. This kind of failure can be caused by one of the capacitive plates becoming coated with a material, such as the product being stored, to such an extent that the sensing element becomes connected to the ground.
Capacitive Tank Gauges
Figure: Simple diagram to demonstrate a capacitance-based level measurement system
Apart from the issue of plate coating, the capacitive level measurement system experiences very little operational degradation. This is because there are no moving parts.
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