Metal Detector Sensitivity: Food Industry Applications

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meat bag on conveyor metal detector machine

Metal detectors might seem straightforward, though striking the correct sensitivity levels for your production line needs isn’t always easy. A metal detector system is just as sensitive and vital as a smoke detector: you need it to locate problems, though you don’t want alarms going off for no reason. Adjusting your metal detector’s performance helps locate metal contaminants inside your food products without wasting safe items.

At TDI Packsys, we provide high-quality industrial solutions for various food industry functions, ranging from x-ray inspection systems to product sealers. We want to teach you how to use your metal detectors properly by finding the optimum sensitivity for your product needs. Continue reading to learn everything you need to know about metal detector sensitivity in the food industry.

Types of Contaminants Detected by a Metal Detector

Before diving into sensitivity, let’s discuss what a metal detector does. Industrial food-grade metal detectors examine product packages for unsafe contaminants so you can remove faulty products at critical control points. Most metal detectors use a balanced system with one transmitter coil and two receiver coils equally spaced apart and joined with earth loops (closed conductive loops formed through electrical equipment interconnections).

Contaminated products may have metal particles inside the food or packages. Metal detectors can locate the following raw materials:

  • Stainless steels: Stainless steels contain alloying elements that resist corrosion.
  • Ferrous metals: Ferrous metals include steel or iron-enriched materials.
  • Non-ferrous metals: The non-ferrous detection capability locates aluminum, lead, copper, zinc, tin, gold, and silver. Non-ferrous metals also include copper alloys like bronze and brass.

Note that x-ray systems can detect the above materials, plus non-metal items such as stone, glass, bone, rubber, and plastic. If you need to reliably detect such items, we recommend investing in a comprehensive x-ray inspection system rather than a metal detector.

What Is Metal Detection Sensitivity?

food worker adjusting the sensitivity of food detector

Metal detection sensitivity refers to your system’s ability to locate each metal contaminant without false rejects. In food applications, your products usually don’t have random giant hunks of metal. Instead, a contaminated product may have trace amounts of different metals from various stages of the production process.

Many people incorrectly assume you can set the machine to maximum sensitivity to find every metal particle. However, oversensitivity limits performance, and leads to product waste. Instead, you must find the right balance based on the factors below.

Metal Detector Sensitivity Factors

To measure and adjust your metal detector’s sensitivity, you must use a test piece, though many factors affect your machine’s detection abilities in practice. Metal detection machines analyze spherical testers differently than random scraps of stainless steel. The factors listed below impact how your product test results vary from the sensitivity performance.

Contaminant Material

Sensitivity depends on metal type because certain materials have increased magnetic properties, and are more electrically conductive under metal detectors. A large magnetic signal can increase electrical conductivity. For example, metal detection machines can locate ferrous metals more easily than non-ferrous materials.


The orientation of the metal contaminant also impacts the detection capability. Tester conductive products come in spherical shapes because they offer equal magnetic permeability from all angles due to the shape’s physical laws. The metal detector might not notice the contaminants if you have wire-shaped metal particles.

Aperture Size and Positioning

food maintenance adjusting aperture size

Lens diaphragm (aperture) size and positioning impact results. The smaller you set the aperture, the more you increase metal detection sensitivity. You also must adjust products on the conveyor belt according to the aperture positioning to adequately locate metal contaminants.

Product Packaging

Some product packaging materials have magnetic effects, which can impact sensitivity. For example, a stainless steel container could throw off the metal detector, making it hard for the system to locate actual contaminants.

Rather than changing your packaging, you could inspect your products at a different time before they reach the packing stage. If you sell canned goods in metal containers, you could examine the food items before the canning process.

Manufacturing Environmental Conditions

The climate inside your manufacturing facility may affect how metal detectors operate. A temperature fluctuation or airborne electrical interference can throw off results. We recommend monitoring your building’s interior climate to avoid any thermal changes.

Product Material

Different product characteristics can affect metal detection if the food contains conductive characteristics. For example, foods with a high salt or moisture content, like meat, create what industry insiders call the “product effect.” Many products like meat, cheese, poultry, cereal, and bread fall into this category.

Metal detection systems function well for fresh and frozen products, though wet products affect the frequency requirements.


Metal detector systems use various frequency levels to detect contaminants inside different types of food. Dry products require high frequencies, while wet products work best with multiple simultaneous settings that combine high- and low-frequency waves.

Single-fixed frequency technology options have limited product scopes, though they offer better hazard analysis abilities. Multi-frequency metal detector settings offer product flexibility but reduce sensitivity when noticing contaminants like stainless steel.

Frequency is the largest factor when adjusting the sensitivity of your machine. Other than aperture size, you may not be able to control many of the factors above, though you can always manage frequency.

Common Guidelines When Testing Detector Sensitivity

food controller looks around in metal detector machine section

Use ferrous, non-ferrous, and stainless steel testing balls to test and set your metal detector. Spheres appear the same from every aspect, making them easily detectable. Because real contaminants usually come in irregular forms, you should practice placing the testers in various positions and orientations.

The general guidelines for sphere size depend on the food’s size and composition. For example, a 75-mm-high wet block of cheese should require the following spheres:

  • Ferrous tester: 2 mm
  • Non-ferrous tester: 2.5 mm
  • Stainless steel tester: 3.5 mm

The shape of your object creates a huge impact. For example, if you flattened a 3.5-mm metal ball, it could become as long as 30 mm, which you would likely notice without a metal detector.

Most contaminants you need to locate are smaller and more irregular. If you want to test for a 2.5-cm wire, consider using a 0.5-mm sphere.

How To Achieve the Right Sensitivity for Your Production Line

Setting your metal detector to achieve the best sensitivity for your needs requires adjusting the frequency settings. Finding the right balance is challenging because low sensitivity settings won’t find contaminants, and high sensitivity settings will result in too many rejections. Taking the time to get the settings right is crucial.

Reducing False Metal Contamination Rejects

False rejects frequently happen, especially with wet, high salt-content, or iron-enriched foods that experience the “product effect.” Items like meat, cheese, bread, and enriched cereals all have electrical conductivity properties that cause sensitivity levels to suffer. The different item temperatures and densities also impact the final results.

Lowering the frequency waves to reduce the “product effect” limits the metal detector’s ability to find non-ferrous metals. You could ramp up the metal detector to find all contaminants, though your products may react poorly and create inconsistent results. So, what’s the solution?

No magic frequency setting exists for all products. Instead, you must strike the proper balance for each item you wish to examine. As a general rule of thumb, we recommend picking lower frequencies for dry foods and simultaneous frequencies for items within the “product effect” category.

Simultaneous frequency machines use both low- and high-frequency waves in tandem to target multiple sensitivity ranges. The simultaneous solution allows you to escape the “product effect.” Many assume this algorithm is best for all products, though you should still customize low-frequency settings for dry items to acquire the best results.

Tips on Buying Food Metal Detectors

food metal detector machine at food production factory

You might think every metal detector does the same thing, but you must consider frequency-setting abilities and a few other factors. When shopping for a metal detector, we recommend keeping the following in mind:

Product material: The types of products you wish to examine impact which machine you need. For example, liquid products require flow-through systems, and powdered products require gravity-fed detectors.

Packaging material: Products with conductive packages require more attention. You may need a machine that offers better sensitivity settings.

Product variations: Are you examining one type of food or multiple? If you need to detect contaminants across various food types, your machine should be adaptable to different settings.

Top potential contaminants: You might already know the main contaminants that occasionally appear in your products. You can narrow your search accordingly if you know which type of metal to look out for.

Machine cost: Consider your budget and potential savings. A metal detector could reduce product waste and improve your returns.

Metal detector size: If your manufacturing facility doesn’t have much space, pick a machine that will fit where you need it.

Multi-frequency abilities: Modern technology usually includes simultaneous frequency settings. If you need to examine complicated products, pick a machine with this ability.

Ease of use: Adjusting sensitivity and operating a metal detector can be challenging. We recommend finding a machine that everyone on your team can operate easily.

Shop High-Quality Metal Detection Systems Today

At TDI Packsys, we offer a wide range of high-quality metal detectors for your inspection needs, including conveyorized, flow-through, gravity-fed, and throat-style drop-through systems. We also offer combination systems that perform both metal detection and checkweighing. Call TDI Packsys today at (877) 834-6750 to speak with our associates about the best option for your needs.

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