The Science and Problems With Metal Detectors

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May 29, 2013

The function of metal detectors is rather obvious: they detect metal as it passes through an aperture, so a product can be accurately rejected if it contains contaminants.

How they do this isn’t quite as obvious to most.

Metal detectors consist of an aperture that uses a set of electrically charged coils to detect metal in a product.

metal detector

A transmitter coil produces an electromagnetic field which the product passes through.

When metal comes in contact with this field, depending on the type of metal, it gives off an irregularity or disturbance in the electrical field which is picked up by receiving coils on either side of the aperture.

There are many little things that make using and calibrating metal detectors difficult.

Size of the metal contaminants is obviously a concern, but metal detectors need to be finely tuned because orientation of contaminants factors in as well.

To explain this, imagine that a whole paper clip made its way into some meat being inspected by the metal detector.

metal detector with divert

Something like that seems large and easy to detect, but it has a small cross section because it is a wire bent into a certain shape. The metal detector only sees one slice of this cross section at a time, which can cause difficulties in long, thin shards of metal.

Different metals are also more difficult to detect than others because of the way that they affect the electromagnetic field.

Ferrous metals: Iron and other metals with high iron composition and magnetic properties. These are the easiest to catch because they push the magnetic field outward. A well-calibrated detector should be able to catch this down to a two millimeter  wide cross section.

metal detector

Non-ferrous metals: Natural non-iron metals like aluminum and copper. These are more difficult because they don’t strongly affect the field. A detector is expected to find this with a 2.5-mm cross section.

Stainless steel: Certain types of stainless steel and other man-made metals, even though they contain a high percentage of iron, have different effects on magnetic fields. This is the most difficult thing to catch as it can tend to pull the electromagnetic field away from the coils. A good detector can catch this at about four millimeters.

These are just some things to watch out for when looking for metal detectors.

Metal detectors are an important piece of equipment to have and can be synced with an automated reject system.

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