Capacitive sensing is using the dielectric effect to detect the presence of target materials. This is specifically alteration of the E, or Electric field, that is emitted by the magnetic field of the capacitive sensor, by the target material. It is non-contact sensing, with a relatively short sensing range. The choices of product outputs are the typical normally open, and normally closed, as well as PNP or NPN.
The technology benefits from its ability to sense unusual targets such as liquids, and the human touch. Since it can sense liquids, yet ignore some plastics, it is often used to see through the plastic walls of a tank or reservoir and sense the (discrete) level of liquid in within. This can be an advantage since discrete or analog ultrasonics, which are excellent for tank level sensing, require an opening in the top of the tank, so they can be looking downward into the tank. Ultrasonics cannot see through a plastic or metal tank wall.
Because capacitive sensing can sense moisture or water, its reliability is somewhat humidity dependent.
Capacitive sensors may need to be purchased and evaluated for any given application, to determine suitability for both the target materials and incidental target materials that it will encounter. Interestingly, and maybe for this reason of uncertainty, some major manufacturers of various other industrial sensors, e.g. photoelectric, ultrasonic, inductive, etc.…., appear to have discontinued their offerings of capacitive sensors.
The underlying message may be that if you were thinking of using a capacitive sensor, ask if some other technology sensor would perform the task as well?