Capacitive probes can be used to measure the depth of liquids and solids. A rod, normally coated with PVC or PTFE, is inserted into the tank and the capacitance measured to the tank wall. The capacitance has two components C1 above the surface and C2 below the surface. As the level rises C1 will decrease and C2 will increase. These two capacitors are in parallel, but as liquids and solids have higher dielectric constant than vapour, the net result is that the capacitance rises for increasing level.
Since the effect is small, the change is best measured with an AC bridge circuit driven about 100 kHz as illustrated in the figure below:
The small capacitance also implies that the electronics must be close to the tank to avoid errors from cable capacitance.
The response is directly dependent on the dielectric constant of the material. If this changes, due to say, bubbling, errors will be introduced.
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