Metal detectors are an important part of a comprehensive contamination-control program. If they are specified, installed, operated and maintained correctly, they will contribute to improv­ing product quality and reducing losses. A properly used metal detector should not be considered insurance against metallic contamination. Rather, it is a diagnostic device that can help to discover accidental metallic contamination. Plans and procedures should be in place to prevent contamination.

Purchase and installation

Some of the information a processor needs to specify a metal detector includes:

  • Thorough definition of product (or ingredient) to be inspected, including an ingredient list, physical properties and a descrip­tion of the package if the product is packaged. One of the most important physical properties is moisture content.
  • List of available utilities at installation site (e.g. compressed air and electric).
  • Description of the operating environment (temperature, humid­ity, corrosive materials, etc.).
  • Description of the process (how product is being made and han­dled in the area where the metal detector is to be installed).
  • Description of controls desired.
  • Description of the desired reject mechanism.
  • Installation of the metal detec­tor will require a qualified electrician and a mechanical contractor familiar with metal detectors and the type of conveyors used in the operation. If there is a chance the metal detector or rejection mechanism could impede the normal flow of production, it may be desirable to install and test the unit during a scheduled down time to avoid production losses. An alternative is to install and test the unit on a mock production line.

Metal detection program

Sensitivity standards should be set for the entire facility. An important aspect of this is to identify an agreed-upon minimum particle type and size. For example, a typical detection standard for finished product might be to remove all spherical, non-magnetic particles larger than 2 millimeters and all spherical, mag­netic particles larger than 1.5 millimeters. Only detectors that meet these standards would be considered for purchase and installation. The conditions should be clearly marked on the side of any installed detector, and samples of the correct diameters should be available for testing the unit.

Metal detectors should be operated at the maximum sensitivity setting for a given product. The maximum acceptable sensitivity setting will allow the detector to perform reliably for extended pe­riods of time without excessive false rejects. Scheduled testing of the detector and reject device (with ferrous and non-ferrous metal samples) will confirm proper operation. Intervals between tests can be determined by the consequence of a failed test. Testing every two to four hours is typical, and a testing procedure should be established and followed. Furthermore, every effort should be made to identify, document and correct the source of any detected metal. For a detailed list of suppliers of metal detectors, visit The Sourcebook on, and browse By Category for Metal Detectors.

Tim Bowser, food processing engineer at Oklahoma State University’s Robert M. Kerr Food & Agricultural Products Center, can be reached at (405) 744-6688 or This story was excerpted from a Food Technology Fact Sheet published by the center. For more information, contact Mr. Bowser.