Capacitive Sensors for Powder, Granule and Grain Level

A capacitive proximity sensor responds to a change in dielectric near its face. It does not need a metal target, which means it detects plastic, glass, wood, grain, powder, granules and liquid. For dry bulk level in a hopper or silo it is usually the simplest answer available.

Sensing through the wall

The useful property is that a capacitive sensor can be mounted on the outside of a non metallic vessel and detect product on the inside. Nothing penetrates the vessel, nothing contacts the product, and there is nothing inside to clean or to break off into the batch.

Three conditions have to hold:

• The wall must be non metallic and reasonably thin. Plastic, glass or fibreglass. A metal vessel shields the field completely and the sensor sees only the wall.
• Wall thickness eats sensing distance. Our D32C capacitive sensor is M30 at 10 mm. If the wall is 5 mm thick you have roughly 5 mm left for the product, so this works on thin walled hoppers and not on thick ones.
• The product must have enough dielectric contrast with air. Most do. Low density, dry, fine powders are the marginal case.

Setting sensitivity

Capacitive sensors have a sensitivity adjustment and it is the whole job. Too low and the product is not detected through the wall. Too high and the sensor responds to the wall itself, to a hand near it, or to moisture on the outside.

The procedure that works:

1. With the vessel empty at the sensor height, turn sensitivity up until the sensor just switches on, then back it off until it switches off.
2. Back it off a little further, so there is margin against the empty condition.
3. Fill past the sensor and confirm it switches on with margin.
4. Empty it again and confirm it releases cleanly.

Do this with the vessel in its normal state, including any coating already on the inside wall, not on a clean new vessel. A sensor set against a clean wall drifts as the wall coats.

Build up, the recurring failure

Most capacitive level installations that fail do so because product has stuck to the inside of the wall at the sensor position. The sensor reads permanently on and the plant concludes the sensor has failed.

Defences, in order of effectiveness:

• Mount where material does not stick. On a sloping wall rather than a vertical one, and away from the fill stream where material is thrown against the wall.
• Set sensitivity with a realistic coating present, as above.
• Use the sensor as a point level, not a continuous indication. Capacitive gives you high level or low level, which is what a hopper control actually needs.
• Check it at cleaning intervals. Any level sensor in a sticky product is a maintenance item.

If the product is genuinely adhesive, wet, hygroscopic or changes moisture content between batches, capacitive is the wrong technology and you should be looking at a different measurement principle.

Moisture changes everything

Water has a very high dielectric constant compared with dry grain or plastic granules. A product whose moisture content varies between batches will present quite differently to the sensor, so a sensitivity set on dry material can read a damp batch at a different level, and condensation on the outside of the wall can trigger it with the vessel empty.

If moisture varies, set sensitivity on the driest material you run, which is the hardest condition to detect, and verify the empty state on the wettest day.

Typical applications

• Grain, pulses and seed in plastic hoppers on cleaning and grading lines
• Plastic granules at moulding machine hoppers, high level and low level
• Powders in pharmaceutical and food dosing hoppers, see packaging and pharmaceutical machinery
• Cement, fly ash and additives in small day bins, see cement plant sensors
• Sugar and bagasse handling, see sugar mill sensors

For liquids and larger tanks, see tank level sensing.

Ordering

D32C-3010-PM-3S is M30, 10 mm, PNP, normally open, three wire with short circuit protection. Substitute NM for NPN, PB or NB for normally closed. Choosing NC for a low level alarm means a broken wire raises the alarm rather than hiding it, which is usually what you want on a level interlock.

The ordering code guide covers each position, and capacitive sensing of plastics and non metals covers the non level applications.