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RF capacitance

The liquid acts as an isolator between two conductors (probe and tank wall). When level rises, there is more gain of capacity into an analog or digital signal.

Kotron® RF capacitance electronic level measurement utilizes the same fundamentals as any electronic capacitor. A capacitor is formed when an AC signal is applied across two conductive plates separated by an insulating medium, or dielectric. The value of a capacitor is determined by the:

  1. area of the conductive plates (A)
  2. distance between the plates (D)
  3. dielectric of the insulating medium between the plates (E)

Capacitance = E*A/D (The AC frequency is fixed as part of the design.)

Capacitance is measured in Farads and the value we utilize in industrial measurement is an extremely small unit called a pico Farad (pF). A pico Farad is one trillionth of a Farad.

The capacitance value increases as the dielectric increases, the plate size increases, or the distance between the plates decreases.

In industrial applications, the probe is one plate of the capacitor, the tank wall is the other plate (ground reference). The insulating material that separates them is the dielectric. A tank that is empty is actually filled with air that has a dielectric of 1, the baseline of the dielectric scale. All materials have a dielectric higher than air, e.g. oil = 2.5, alcohol = 18, tap water = 80, etc. As a tank is filled with media, the air (dielectric 1) is being displaced by the media having a higher dielectric; consequently, the amount of capacitance being generated between the probe and the tank wall increases. This capacitance change, corresponding to the rising or falling of media in a tank, can be detected by either a point level device yielding a contact closure; or, a transmitter yielding a continuous output.