How it Works

Features - Foreign Object Detection

December 11, 2015

The Metal Detector

In very basic terms, a metal detector consists of three coils wound around a rectangular or circular supporting frame (a “former”) to create three loops through which the product is passed:

  • The middle coil is charged with an electrical current and acts as a transmitter.
  • The other two coils, positioned before and after the middle coil, act as receivers.
  • This arrangement creates an electro-magnetic field within the former through which the product passes. The electro-magnetic field remains undisturbed until something disturbs it.
  • When this happens, the voltage in each coil changes by just a few nanovolts, but it is enough to be detected and interpreted by the detector’s electronic circuitry and software algorithms.
  • The software generates an electronic signal, which can be used to raise an alarm and activate an automated product rejection mechanism, or to stop the production process by de-activating the conveyor or other packaging or processing machine.
  • Because products vary in density, moisture, acid, etc., they can have a “product effect” on the detector, so the frequency must be specifically calibrated for each product.


X-rays are a form of electromagnetic radiation with short wavelengths which can pass through materials that are opaque to visible light. As may be expected, the denser a material, the fewer the x-rays that pass through it. So, an x-ray system is essentially a scanning device:

  • The system is set up to recognize the density aspects of the specific product and alert on non-conformance.
  • An x-ray beam is generated at the top of the unit and travels in a straight line through the product and onto the detector.
  • When a product passes through the unit, the system captures a greyscale image of it, showing density differentiations in the product. Because contaminants such as glass and metal are of high density, they absorb more x-rays, allowing fewer to pass though.
  •  So when the system’s software analyzes the image, comparing it to a pre-determined acceptance standard, it either accepts or rejects the image.
  • A rejection sends a signal to an automatic reject system, which removes the product from the line.
  • The x-ray assembly is encased in a stainless steel x-ray cabinet with a highly visible lamp stack that signals the system status. The lamp stack is wired to a safety circuit; if the lamps fail, the x-ray source automatically switches off.

Detection Deficiencies

Metal detectors can only detect ferrous and non-ferrous materials. X-rays can only detect contaminants that are denser than the product in which they are embedded.

Thus, non-metal, low-density contaminants such as insects, wood, bone, and plastics cannot be readily detected by either means. It is for this reason that many plastic-product manufacturers embed metal-detectable materials in their products and/or color code products for visual detection.

Practices for Prevention

Given these facts about metal detectors and X-ray equipment what other steps can food processors take to protect their products from foreign object contamination?

In relation to the equipment, Detectapro Marketing Manager Paul Gaertner said, “The best practices we’ve seen in regards to detection and prevention come through due diligence in maintaining calibration of equipment based on the product being scanned.”

There have been cases where line operators actually turned off detection equipment because it was slowing down the line with false positives, Gaertner said, adding, “Surely the QA manager would not like to hear this is happening.”

To help deter such practices and ensure continual protection, constant calibration and testing is essential to reduce false positives; it should be part of every shift change and made an integral part of the facility’s programs. “Practices of foreign object detection and prevention will succeed only when the actual line operators, trained in the calibration of the equipment they use, buy into the effectiveness of such a program,” he said.

Additionally, “detection of foreign objects in the line can be a bit tricky as the head height of the unit has to be calibrated to match that of the product going through the detection system,” Gaertner said.

There is not a specific standard for frequency setting of metal detectors from different manufacturers, however the height and width of the opening of the aperture is critical to the size of the piece of material you are trying to pick up, he said. Because sensitivity decreases as height increases, “the smaller the piece you desire to pick up the closer to the line the head needs to be. Most agree that 300KHz is common for dry foods and 25 KHz for wet foods, he said.

“Aside from implementing metal detection or x-ray inspection systems to detect potential physical contamination, it is important to have robust root-cause analysis programs in place once a contamination has been identified,” said Mettler Toledo Senior Advisor Food Safety Robert Rogers. “The goal is to identify the source of the contamination and put a preventive measure in place to prevent future contamination from that source,” he added.

Beyond the basic reliance on equipment, a facility needs to look at its general practices. “When a potential hazard for physical contamination has been identified, the strategy should consist of: first eliminating the risk, second preventing the risk, and third detecting the risk,” Rogers said. However, eliminating the risk or preventing the hazard is not always possible, so facilities implement metal detection and x-ray systems for detection. 

“As food processing companies continue toward zero tolerance in foreign object contamination to its final product, tools that are detectable by both X-Ray and metal detection systems can be of significant benefit,” Gaertner said. Adding color coding to the tools can add further protection by designating specific colors for specific areas and aiding in preventing cross contact allergen contamination.

In relation to foreign object detection and prevention, beyond equipment use, Rogers said, “Best practice includes having a good hazard analysis and risk identification, a robust reporting and documentation control system, and good root-cause analysis and corrective action program.”

[How it Works descriptions adapted from Mettler-Toledo/Safeline White Paper Metal Detection, X-ray or Both? Making the Right Choice,]


The author is Editor of QA magazine. She can be reached at