What is the basic operating principle of a capacitive level sensor, and what are typical limitations?

• A. Capacitance changes with liquid dielectric between probe and tank; limitations include foaming liquids, dielectric variability, temperature effects, and dirty media.
• B. It uses a magnetic float to move a reed switch.
• C. It measures hydrostatic pressure at the bottom to infer level.
• D. It relies on ultrasonic echoes to measure distance.

Answer: (C)

Capacitive level sensing works because capacitance changes as the dielectric between the two conductors changes. The sensing probe and the tank (or another plate) form a capacitor, and the liquid between them has a different dielectric constant than air. As the liquid level rises, more of the space between the conductors is filled with liquid, increasing capacitance. The sensor electronics measure this change and convert it to a level reading. Typical limitations include foaming liquids, which create air pockets that make the dielectric environment unstable; dielectric variability between different liquids, which requires recalibration for each liquid; temperature effects, since permittivity and geometry can change with temperature; and dirty media, such as scale or deposits on the probe, which alter the effective dielectric path and cause drift or inaccurate readings.