Capacitive Proximity Sensors

True or False: An inductive prox is for sensing metal, and a capacitive prox is for sensing everything else.

It's correct in spirit, but technically it is not true on two points. One, capacitive switches are great at sensing metal. Why? Because metal has high dialectric constant. Since inductive prox sensors cost less, capacitive switches are only used to sense metal is special applications, such as wire-break detection, which would be difficult for photo eyes. But they can sense metal quite well. Secondly, capacitive switches certainly do not "sense everything else".

There are a lot of materials other than metal which do not have good dialectric constant. Therefore, they are poor targets for capacitive switches.

If it weren't for that, however, they would not be near so useful. Capacitive prox switches are great for fill/no fill applications, in fact, for that very reason. Many products, like wood products or many plastics, have a very low dialectric constant rating and make poor capacitive targets. These products are often used to -make containers such as bottles or cardboard boxes.
Therefore, as is the case with our example, above left, capacitive switches can be used to determine if product is inside the container. They "tune out" the outside wall of bottles made of plastic, boxes made of cardboard, vats made of fiberglass, etc., yet they can often sense the product inside, because the product often is made up of liquid, which is full of "mass" or solids with high dialectric constant. The switch is not "burning through" the outer wall, but rather tuning it out. In other words, the product inside has a higher dialectric constant than the container wall. So it works!