Capacitance Level Terms

GENERAL

Simply, capacitance is a measure of how much energy can be stored in an insulating material sandwiched between two plates. The main variables are related by the formula:

Capacitance = Dielectric Constant * ( Area of the Plates)/(Distance between the Plates)

For the ranges that we are discussing capacitance is measured in  pico Farads (pF)

    The dielectric constant is a physical property of the material, typical values are:

Air = 1, Oil = 2.0-2.5, Alcohol = 18, Tap Water = 80

For level measurement the arrangement usually consists of a probe located in a vessel. The probe serves as one side of the capacitor and the vessel wall is the other. In an empty tank the air serves as the dielectric. Filling the tank replaces the air and changes the dielectric and the area and thus the capacitance. Remembering that the capacitance increases with the dielectric constant, increases with the surface area and increases as the distance between the plates becomes less will help in capacitance applications.

LIQUIDS

NON-CONDUCTIVE
Low Dielectric - dielectric < 10 - Best solution with liquids that are free of solids is to use a concentric probe.
Medium Range - dielectric  12 to 35 - Standard probes can be used. A "stilling well" is advised in applications where there are surges in the level
CONDUCTIVE
Conductive liquids will "short out" a bare metal probe and therefore an insulated probe is used.

SOLIDS - Capacitance probes will work well withmost solids. Care should be taken when the solid has a low dielectric (plastics and flours). Also changes in density and moisture will change the dielectric constant and change  capacitance value.