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Metal Detectors

Metal detectors are often one of the last steps in processing food products. This simple step assures that food products do not contain any metal that could injure a consumer. Metal detectors work in very specific and precise ways and in today’s Tech Talk, we are going to explain how they work.

Metal detectors consist of three basic parts that are critical to the function of the metal detector. These parts are the send coil, receive coil and sensor. In normal operation, the send and receive coils create a magnetic field through the flow of electrons by electromagnets. The sensor reads this magnetic field through its own magnets and translates the changes in magnetic field strength to changes in electrical current.

When nothing is in the metal detector, the sensor reads a baseline current. When a product with metal inside is passed through the metal detector, there are two ways the sensor can detect it. If the metal is ferrous (iron, copper magnetic), it has a high conductivity and when passed through the magnetic field it will interact and cause the detector to read the change in magnetic field strength as an increase. Conversely, if a metal is non-magnetic (such as stainless steel commonly in food plants) it has a low conductivity relative to the ferrous metals, so when it is passed through the magnetic field it will not interact positively. It will however interact by blocking a small section of the field from reaching the detector due to its high resistance. This will cause the sensor to detect less magnetism than the baseline which results in a negative output from the sensor.

Metal detectors have built in toleration limits; if the change in the current, either positively or negatively, from metal in the product is above the tolerance level the product is rejected. In many plants the conveyor will simply stop, but in large fast-moving operations stopping a conveyor belt would cause a huge bottleneck in production so the product that failed the metal detection check is removed from the conveyor by a compressed air piston.

The primary function of the metal detector is to be able to detect metals present in the product and/or packaging and keep that product from entering the food supply and potentially harming consumers. Therefore, the metal detector is the final step before product is sealed and shipped. For this reason, metal detectors are one of the most common Critical Control Points in HACCP plans throughout the food industry.


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