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An overview of metal detection

The Basic Principles
Metal detectors used in the food industry operate on the concept of a balanced coil, full loop system.
Three coils are wound around the aperture (the opening in a metal detector head) which the product passes through. The transmitting coil lies In the center of the enclosure and broadcasts a radio frequency signal, generating an electromagnetic field.

Equally spaced on either side of the transmitter coil (inside the metal detector head) are two receiver coils.

The field is generally trapped inside the shielded enclosure of the detector but some field escapes from the aperture on both sides of the detector. Anything that enters in this field that is either Magnetic or Electrically Conductive will cause a disturbance in the field strength around it. All metals have either one or both of these characteristics and will be detectable if the size of the signal is large enough.
The signal from the receiving coils are connected in opposition to each other and therefore when no disturbance is occurring there will be a net signal of zero across the coils – they are balanced. This forms the electrical equivalent of a balanced weigh scale.

As metal contaminant passes through the aperture, the balance will be offset. This disturbance is amplified and analyzed by the control electronics and detection will occur if the sensitivity threshold has been exceeded.

But what is the recommended usage? What are the best practices for installation? What are the latest features? Which type of metal detector is right for your application?
Regal Packaging Services has the answer to these and many more in depth questions regarding metal detection technology. Feel free to call us at (630) 942-8461.

Testing Metal Detectors

For any quality control or HACCP (Hazard Analysis & Critical Control Points) system to be successful and no matter how complex or reliable a metal detection system may claim to be, it is crucial that a regular and systematic test and recording procedure be established. This is especially true since there are no industry standards for detector testing and validation.  Each company must determine its own assessment criteria.

Consider the following when writing or altering your HACCP plan:

1) Test Standard
Traditionally, metal detectors have been checked with ferrous, non-ferrous, and stainless steel test samples. The type of test piece used will vary depending upon the product and application. Everything from a ¾” x ¼” puck to a 20 “whip” may be used as well as a variety of cards or testrods. See a full listing here. Some applications will require a custom test piece. Regal Packaging Services has custom designed and manufactured a broad array of test pieces, including a 24 ft “rope,” 6 ft testrods, ½” “top hats” and 1”x5” cylinders.

The size of the metal (seed/standard) sample must be established so that it can be consistently detected inside the product passing through the centerline of the detector – least sensitive point. Placing a test piece on top of or underneath the product moves it closer to the aperture, making it easier to detect. Testing should always be done as close to the center of the aperture as possible. If validation is proven here, it can be safely defined as happening in all other parts of the aperture.

The application for each company and product will be different, so the samples should be adapted to each detector. If the seed (metal) is too small, it will produce unnecessary failures and an unwanted level of frustration level for the operators on the line. If the seed is too large, it will not accurately test the operation of the detector. Using a range of metal sizes, confirm a repeatable performance level.

Typical guidelines for sensitivity:

Aperture Height Dry Product Wet Product
Ferrous & Non Ferrous Ferrous Non Ferrous
Up to 50mm 1 mm 1.5mm 2.mm
Up to 125 mm 1.5mm 2 mm 2.5mm
Up to 200 mm 2 mm 2.5mm 3.mm

2) Test Frequency
The quality control tame needs to determine the frequency of testing the metal detector. Ordinarily, detectors are checked:

    • At Shift Change
    • At Product Change, and
    • Hourly or other interval

A clear tradeoff exists between the cost of testing versus the risk of a detector failure. A company can open itself up to litigation, ROI and sales loss, not to mention the danger to which the end-customer is exposed. An automatic system can boost the regularity of detector performance testing without additional cost.

3) Test Procedure
The method of testing when designed to be as simple as possible, however it must take into consideration that:
a) Test pieces should move through as close to the centerline of the aperture as possible. This is the least sensitive point. If the test is successful at this point, it should be safe at any other point in the aperture.
b) Test pieces should be positioned within the product when possible.
c) Test procedure should include the action of the reject mechanism so that the entire system is checked. This can include:

    • Testing with the test piece at the leading, middle and trailing edges
    • Testing sequential packages or product
    • Testing alternate packages

d) Results of the test must be documented.

4) Test Records
The format for documentation (.docx, .xlsx, .pdf or even handwritten) is not critical, however it should be easily accessible to an auditor and include:
a) Production line or detector identification
b) Date and time
c) Sample used , including metal size, type and certificate number.
d) Identification of the operator
e) Pass or fail result
f) Remedial action taken if result was a fail

5) Automatic Test Systems
Automatic Testing systems are intended to complement, and in some applications replace, manual testing procedures. The effectiveness of these systems should be examined carefully to ensure that the testing is relevant and feasible for the application. When installed effectively, they can offer significant savings through reduced labor and product waste.


The location, including all environmental conditions may influence the performance of the Metal Detector, markedly where high levels of sensitivity are required. Wherever possible, the detector should be situated to avoid or minimize the effect from such considerations.

Sensitivity and performance can be altered by a number of sources:

  • Electrical interference – static, radio, ground loops
  • Vibration – moving metal
  • Temperature variation – ovens, freezing tunnels

While the detector may be capable of filtering some of this interference through such features as “Automatic Balance”, in some instances, the only option is to decrease the sensitivity level. This is a vital consideration when comparing the capabilities of metal detection systems.


See also:




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