A capacitive proximity sensor is a non-contact device that detects both metallic and non-metallic objects by sensing a change in capacitance. Where an inductive sensor sees only metal, a capacitive sensor sees almost anything — which makes it the answer to a large class of problems inductive sensing cannot touch.
How it works
1. Electrostatic field creation
The sensing face carries two conductive plates: one is the active sensing electrode, the other a ground reference. An oscillator applies voltage across them, creating an electrostatic field in front of the sensor.
2. Dielectric change
Any material entering that field changes the capacitance between the plates, because the material's dielectric constant differs from that of air.
3. Oscillator response
That capacitance change alters the oscillator's amplitude. A trigger circuit detects the shift and switches the output.
Because detection depends on dielectric constant rather than conductivity, materials with high dielectric constants — water and most liquids — are detected further away than dry plastics or powders. Water has a dielectric constant around 80; most plastics sit between 2 and 4.
What it can detect
- Metals — detected strongly, as with an inductive sensor.
- Liquids — water, oil, solvents and slurries.
- Plastics — bottles, containers, film and mouldings.
- Glass — jars, bottles and sheet.
- Granular and powder materials — grain, sugar, flour, cement and plastic pellets.
- Wood, paper and cardboard.
Sensing through a container wall
This is the capability that sells capacitive sensors. Mount the sensor against the outside of a non-metallic tank, pipe or hopper, and adjust the sensitivity so the sensor ignores the empty wall but responds to product behind it.
You get level detection with nothing penetrating the vessel, nothing wetted by the process, and no seal that could fail — which matters enormously in food, pharmaceutical and chemical applications where every penetration is a contamination risk and an audit point.
Keep the non-metallic wall under about 4 mm thick for reliable through-wall sensing, and always test with the actual product rather than water. A design that works on the bench with water can fail on a low-dielectric powder.
Setting the sensitivity
Almost every capacitive sensor has a multi-turn trimmer that sets the switching threshold. Setting it correctly is the difference between a reliable installation and a nuisance-trip problem:
- Start with the sensor mounted in its final position, against the empty container.
- Turn the sensitivity up until the sensor triggers on the empty wall, then back off until it releases, and back off a little further for margin.
- Fill the vessel and confirm it triggers reliably.
- Re-check after the plant has run for a while, once real build-up and humidity are present.
Limitations worth knowing
- Sensitive to contamination. Moisture, dust or product build-up on the face changes the capacitance the sensor sees and can cause false triggering.
- Affected by humidity in some installations, particularly at high sensitivity settings.
- Shorter range than inductive for the same barrel size, typically 8–15 mm.
- Needs setting up. Unlike an inductive sensor, it rarely works correctly straight out of the box.
Capacitive or inductive?
Use inductive whenever the target is metal — it is more robust, needs no adjustment and tolerates contamination far better. Reach for capacitive when the target is not metal, or when you need to sense through a non-metallic wall. We compare the two in detail in inductive vs capacitive sensors.
Related reading
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