A capacitive proximity sensor detects anything with a dielectric constant higher than air — water, oil, plastic granules, powder, grain, glass. That is why it can be strapped to the outside of a non-metallic tank and sense the liquid through the wall, something an inductive sensor can never do.
Quick reference
- Senses dielectric change, not conductivity — so it detects metals and non-metals alike.
- Water (εr ≈ 80) is easy. Dry plastic granules (εr ≈ 2–3) are hard.
- Through-wall sensing works on plastic or glass walls up to roughly 4–10 mm, depending on sensor size.
- Sensing distance is typically 1–25 mm depending on thread size and target dielectric.
- The main enemy is build-up on the face, followed by condensation and foam.
How dielectric sensing works
The sensing face of a capacitive sensor is one plate of a capacitor; the machine frame and surroundings form the other. An oscillator drives that capacitance. Bring any material with a dielectric constant higher than air into the field and the capacitance rises, the oscillator amplitude changes, and the trigger stage switches.
Because the mechanism is dielectric rather than eddy-current, there is no distinction between ferrous and non-ferrous, and no reduction factor table for metals as there is with inductive sensors. What matters instead is the dielectric constant of whatever you are trying to detect.
Dielectric constants of common process materials
| Material | Relative dielectric constant (εr) | Ease of detection |
|---|---|---|
| Air / vacuum | 1.0 | Reference — not detected |
| PTFE | ≈ 2.0 | Difficult |
| Polypropylene, polyethylene | 2.0 – 2.3 | Difficult |
| Dry paper, cardboard | 2 – 4 | Difficult |
| Mineral and hydraulic oil | 2 – 4 | Moderate |
| Nylon, PVC, acrylic | 3 – 5 | Moderate |
| Glass, quartz | 3.7 – 10 | Moderate |
| Wood (dry to damp) | 2 – 7 | Varies with moisture |
| Flour, sugar, cement powder | 3 – 10 | Moderate — varies with packing density |
| Alcohols | 16 – 33 | Easy |
| Water, aqueous solutions | ≈ 80 | Very easy |
| Any metal | Effectively infinite | Very easy — detected at maximum range |
The practical consequence: anything water-based is straightforward, dry plastics are the hard case. If you are detecting polypropylene pellets in a hopper you are working at εr ≈ 2, which is only twice air, and you should expect to need a larger sensor, a shorter gap and careful sensitivity setting.
Sensing level through a tank wall
This is the capacitive sensor’s signature application. Mount the sensor on the outside of a non-conductive tank at the level you want to monitor. The wall is part of the dielectric path; the sensor’s field passes through it and responds to what is on the far side.
It works when three conditions hold:
- The wall is non-conductive — plastic, glass, ceramic. A metal tank shields the field completely and the sensor will only ever see the wall.
- The wall is thin enough. As a rule of thumb, wall thickness must be well inside the sensor’s rated sensing distance, since the wall consumes part of it. Small sensors manage 2–4 mm; larger M30 units with 20 mm+ range can handle 8–10 mm walls, but only for a high-dielectric medium like water.
- There is a clear dielectric step between “full” and “empty” at that point. Water against plastic gives a very large step. Oil against plastic gives a modest one. Dry powder against plastic gives a small one.
Setting it up
The reliable procedure is to set the threshold with the tank empty at that level, not full:
- Fit the sensor firmly against the wall with no air gap — an air gap between sensor face and wall is wasted sensing distance. Use the supplied bracket or a bonded pad.
- With the level below the sensor, turn the sensitivity up until the sensor just operates on the empty wall, then back it off until it releases.
- Back it off a further margin — typically 10–20% of the adjustment range.
- Fill above the sensor and confirm solid operation with margin.
- Cycle the level several times, and check again after the tank has been through a full thermal cycle.
What goes wrong, and what to do about it
| Symptom | Likely cause | Fix |
|---|---|---|
| Output stays on after level drops | Product clinging to the inside of the wall, or build-up on the sensor face | Reduce sensitivity; move to a location with flow across it; specify a build-up-resistant variant |
| Chatters near the switch point | Insufficient hysteresis for a slow-moving, turbulent or foaming level | Choose a unit with greater hysteresis; add a short off-delay in the PLC; fit a stilling arrangement |
| Trips when an operator walks past | Sensitivity set far too high — the sensor is seeing the room | Reduce sensitivity; the correct setting operates on product, not on ambient |
| Works in the morning, fails by afternoon | Condensation on the outside of a chilled tank, or thermal drift | Shield and insulate the sensing area; set the threshold at worst-case temperature |
| Never detects the product | Dielectric constant too low, wall too thick, or metallic tank | Move up a sensor size, sense through a plastic sight tube, or change technology |
| Erratic on a metal tank | The tank is shielding the field | Capacitive is the wrong choice — use an internal probe, a level switch, or an external ultrasonic device |
Mounting rules
Capacitive sensors follow the same flush and non-flush logic as inductive units, but they are more sensitive to their surroundings because any dielectric affects them, not just metal. A plastic guard 20 mm from a non-flush capacitive sensor is not neutral the way it would be for an inductive one.
- Flush (shielded) units can be embedded in metal and are the right default for tank-wall and machine-mounted duties.
- Non-flush units reach further but need a metal-free and dielectric-free zone around the head.
- Side-by-side spacing should be at least twice the barrel diameter, more for non-flush units — the same principle described in our guide to shielded versus unshielded mounting.
- Keep cable runs short and away from VFD power cables. Capacitive front ends are high-impedance and pick up electrical noise readily.
Capacitive or something else?
| Application | Best choice |
|---|---|
| Metal target, dirty environment, need robustness | Inductive |
| Liquid level through a plastic tank wall | Capacitive |
| Powder or granule level in a plastic or glass hopper | Capacitive, sized generously |
| Detecting product inside a sealed metal vessel | Not capacitive — use an internal probe or ultrasonic |
| Presence of a box or label at a distance | Photoelectric — see fundamentals of photoelectric sensing |
| Detecting through a stainless steel cylinder wall | Magnetic, with a magnet on the piston |
| Continuous level measurement, not a switch point | Radar, ultrasonic or hydrostatic — capacitive proximity is a switch, not a transmitter |
Frequently asked questions
Can a capacitive sensor detect water through a plastic tank?
Yes, and it is one of the most reliable capacitive applications there is. Water’s dielectric constant of about 80 gives an enormous signal step against an empty wall. Practical limits are wall thickness relative to the sensor’s rated range, and product clinging to the inside of the wall after the level drops.
Will it work through a metal tank wall?
No. A conductive wall completely shields the electric field, so the sensor responds only to the wall itself. For metal vessels you need an internal probe, a float or paddle switch, or a non-contact technology such as radar or ultrasonic mounted through a nozzle.
Why does my capacitive sensor stay on after the tank empties?
Almost always product residue on the inside of the wall, or build-up on the sensor face. The sensor is doing exactly what it should — there is still high-dielectric material in its field. Reduce sensitivity so it needs bulk product rather than a film, and if possible relocate to a point where the product flows past rather than sits.
What is the maximum sensing distance?
Typically 1 to 25 mm depending on thread size, mounting style and the target’s dielectric constant. The catalogue figure is quoted against an earthed metal target, which is the easiest possible case. Detecting dry plastic granules you should expect a fraction of that, so size the sensor from the real target, not the headline number.
Can I use one to detect a plastic part on a conveyor?
Yes, if the part is close and reasonably substantial. Thin dry plastic at εr ≈ 2 is a hard target, and a moving belt of varying moisture content is a shifting background. For general presence detection on a conveyor, a photoelectric sensor is usually more repeatable — our guide to selecting the right photoelectric switch type covers the options.
Have a level or non-metallic detection problem? Tell us the material, the wall construction and thickness, and the switch point you need. Accent Controls manufactures capacitive proximity switches in Mumbai and our engineers will tell you honestly whether capacitive is the right technology for the job.