Capacitance

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The electrical characteristic which determines the magnitude of electrostatic charge between two oppositely charged plates.

Certain models of electronic in-tank probes utilize capacitance type liquid measurement as a means of detecting changes in the depth of liquid in a storage tank. A hollow metal tube, with a smaller electronic tube running down its center, is installed vertically in an underground storage tank. The outside surface of the inside tube and the inside surface of the outside tube form the two plates of a capacitor. The space between them is a dielectric that serves essentially as an insulator between the two plates. 

The outer tube is open at the bottom, and liquid in the tank rises in this tube. A small electric charge is stored on the inner tube. This electric charge seeks to pass through the dielectric to the opposite plate represented by the outer tube of the probe. Air (where no liquid is present) may be assigned a dielectric value of 1.

Gasoline, on the other hand, could be assigned a dielectric value of 2. Thus, the electric charge on the inner tube encounters a different resistance when it seeks to pass through the air than when it seeks to pass through the gasoline. The capacitance probe is capable of precisely sensing the amount of air versus the amount of liquid fuel present between the tubes.

This electronic property is then converted to a measurement of the liquid level in the tank.

The measurement is translated on a gauging instrument, outside the tank, to the volume of liquid present in the tank. When set in a leak detection mode, over a period of time the system can detect the presence of a leak in the tank.

See also Magnetostrictive probe.