Inductive sensors are the default in factory automation and they have one hard limitation: they only see metal. For everything else — liquid in a tank, granulate in a hopper, powder in a silo, a cardboard box on a conveyor — the equivalent device is capacitive, and it behaves differently enough to catch people out.
How it differs
An inductive sensor generates a magnetic field and detects eddy currents in conductive targets. A capacitive sensor forms one plate of a capacitor at its face and detects changes in capacitance as material approaches.
Because capacitance depends on the dielectric constant of whatever is in the field, and everything has a dielectric constant, a capacitive sensor responds to almost any material. That’s its strength and the entire source of its difficulties.
Dielectric constant decides your sensing distance
The higher a material’s dielectric constant, the stronger the response and the further away it can be detected.
Water sits around 80, which is enormous. Anything water-bearing — liquids, wet product, damp grain — is easy to detect.
Glass is roughly 4 to 8. Plastics are typically 2 to 4. Dry powders and granulates often come in between 1.5 and 3, which is close to air at 1, and that’s the hard end of the range.
The practical consequence: detecting water in a tank is straightforward, detecting dry plastic pellets in the same tank is marginal, and the same sensor at the same setting will do one and not the other.
Sensing through a wall
The property that makes capacitive sensors useful for level detection is that they see through non-conductive container walls. Mount one on the outside of a plastic or glass vessel and it detects the material inside.
The field has to pass through the wall first, and the wall consumes part of the sensing range. As a rough guide, wall thickness up to about 4 mm is workable with a standard M18 or M30 sensor detecting a high-dielectric material like water; thicker walls or lower-dielectric contents need a longer-range sensor or a different approach.
Metal walls don’t work at all. The wall is conductive, the sensor detects the wall, and nothing inside makes any difference.
Sensors intended for this job have a sensitivity adjustment — a potentiometer or a teach input — so you can set the threshold above the wall’s contribution and below wall-plus-product. That’s the whole setup procedure, and it must be done with the vessel both full and empty.
The failure mode everyone meets
Capacitive sensors respond to anything with a dielectric constant, and that includes things you didn’t want them to respond to.
Product build-up on the wall. A layer of residue where the sensor is looking reads as permanently full. This is the single most common capacitive level fault, and it’s why sensors get mounted where flow keeps the wall clean rather than where the bracket was convenient.
Condensation and washdown water. Water’s dielectric constant of 80 means a film of condensate can outweigh the product entirely. In humid or washdown environments this produces a sensor that works all day and reads full every morning.
Foam. Foam on a liquid surface is detected as product by some settings and not others, and the level fluctuates accordingly.
The general answer is to run the sensitivity as low as will reliably detect the product, not as high as the sensor allows. High sensitivity feels safer and is the direct cause of most false-full faults. Sensors built for this application publish the adjustment range and the recommended wall thickness per series — going through a sensor manufacturer’s catalogue you’ll find capacitive parts split by barrel size and by whether they’re intended for level detection through a wall or for direct object detection, because the two jobs want different sensitivity characteristics.
When to use something else
If the vessel is metal, if the product is a low-dielectric powder that cakes, or if the process involves regular washdown, capacitive sensing will fight you. Vibrating fork, radar and guided-wave level devices exist for those cases and cost more for good reason.
Capacitive is the right answer for a clean plastic tank of liquid, and it’s a cheap answer. Outside that, check the failure modes before you commit the bracket.