Skip to content

Suddenly false rejects?

Five factors to check before you buy a new metal detector

Metal detectors are designed to protect products from contamination particles of iron, non-ferrous metal or stainless steel. If metal is detected, the contaminated products are discharged from the product stream. Unfortunately, it can happen that detectors erroneously “detect” metal where there is none. Actually saleable products are destroyed – an annoying and avoidable damage. The metal detector is not always the source of error, some problems can be easily and inexpensively remedied.

False rejections cannot be completely ruled out, but there are important factors with which you yourself can significantly reduce such errors. To understand how false rejections occur, a quick look on how metal detectors work is helpful.

How a metal detector works

There are different types of metal detectors based on different detection methods. Most detectors for private use work with only one search coil. In industry, however, usually such metal detectors are used which operate on a simple transmitter-receiver principle with so-called “balanced coils”. The device is equipped with a total of three coils. The transmitter coil builds up an electromagnetic alternating field similar to a radio transmitter. The other two coils receive the signals of this magnetic field. In an undisturbed situation, this field “looks” the same for both revievers – it is balanced. 

As soon as a metal part passes the detector, a disturbance of the magnetic field occurs, which is detected by the receiver coils, they register an imbalance.

This disturbance depends on the conductivity and magnetic properties of a material.

  • Ferrous metals are magnetic and conductive, they generate a strong interference signal.
  • Non-ferrous metals such as aluminum or brass are good conductors, even if they are not magnetic. They also generate a strong interference signal.
  • Stainless steel is usually non-magnetic and does not conduct electricity very well. Its interference signal is therefore significantly weaker than that of iron or non-ferrous metals.
Balanced Receiver coils for metal detectors

But not only metals are “seen” by a detector. Other substances, such as salt or even water, are also conductive and influence the magnetic field. The software of a good metal detector must therefore evaluate the different interferences of the magnetic field and distinguish the specific interference signal of a metal particle from other influences. To do this, every industrially used detector “learns” the specific signals of the products it is to examine later.  If, during the production process, the signal of a product deviates from the previously “learned” structure, the detector recognizes this as metallic contamination.

The smooth functioning of a metal detector can be disturbed by various influences, which then lead to false rejections. However, some of these can be easily identified and quickly eliminated.

5 factors to check

If you should have the impression that your detector conspicuously often erroneously displays metal, then you can check the following points.

1. Interference from other machines

A frequent and very easy to correct error is interference from other machines or metal parts in the vicinity of the detector. Metal detectors work with electromagnetic signals that radiate in waves around the detector. The field generated by this can easily extend beyond the detector into the environment. Accordingly, the receiver coils detect devices in their environment that work with similar signals. Conveyor belt parts or other metallic parts that are too close to the detector can also cause interference.

If false rejects occur “all of a sudden” check for:

  • Frequency converters, cables or drives that were not there before in the vicinity of the detector
  • Metallic parts like railings or conveyors that protrude into the “metal-free zone” around your detector
  • Metallic equipment like pallet trucks stationed too close to the metal detector
Full length portrait of a manual worker pushing a fork pallet truck stacker isolated on white background
Lift trucks are valuable helpers, but should not be used or parked directly next to detectors.

2. The packaging: Metallized foil

You changed the packaging of your products to metallized foil? Good decision for storing and presenting products, but products that are packaged in metallized films are difficult to test for metallic contamination. Some detectors can ignore the interference effects via the foil (e.g. the METAL SHARK BD with ferrous in foil option), but for these products an X-ray scanner might be the better option for reliably checking your products for foreign objects.

3. The environment: Temperature and humidity

Metal detectors are quite stable against environmental influences such as temperature or humidity when correctly adjusted. Nevertheless, these factors can play a role if they fluctuate strongly or change significantly after the installation and setup of the detector. So, if suddenly increased false rejections occur, it may be useful to check:

  • Does the ambient temperature fluctuate strongly during the day since the metal detector was installed?
  • Is it generally significantly higher or significantly lower than when the device was installed?
  • Has the air humidity increased noticeably, e.g. due to cleaning cycles?

If so, it may be useful to “teach-in” the tested products again under the new ambient conditions.

4. The product

Gefrorener Fisch kann Metalldetektion stören
Frozen fish can be difficult to inspect, especially if it is salted or starts melting.

Products usually have their own conductivity, albeit very low. Different ingredients influence this conductivity. If this influence is too strong, it can lead to disturbances of the detector sensitivity.

  • Product moisture
    Animal products such as fish, meat or cheese have a high water content. This can vary over the course of a day which also changes the strength and conductivity of the products. This can influence the “speed” at which the magnetic waves can penetrate the product sufficiently to create defects. Good detector software compensates for such effects. Nevertheless, it is worthwhile to control factors such as temperature and humidity.
  • Salt
    Salt is more conductive than many other substances that are usually found in food and can influence a magnetic field accordingly. If you are testing salty products such as chips, nuts or almonds for metallic impurities, the detector should be specially adjusted for this type of product. If false rejections occur more frequently than before, check the settings of your detector for salty products.
  • Product temperature: Hot or chilled products
    When warm products cool down along the production line or frozen products thaw, they also change their conductivity. This can go so far that these changes affect the product signal. Check whether significant temperature changes could be caused by longer transport distances or fluctuations in ambient temperature.

5. The detector itself

Advanced metal detection devices are designed to provide maximum sensitivity to contaminants while maintaining high immunity to interference. They can also be trained for specific products, so that later product changes can be carried out quickly and easily – with settings perfectly adapted to each product.

For example, all METAL SHARKs have a product memory for up to 250 products. In most cases, in case of suddenly increased false rejections, it is sufficient to “train” the detector again for the specific product and to store the settings for the future.

The above mentioned tips did not bring any improvement?
In this case ask our service professionals. They will be happy to help you match your equipment perfectly to your products.