Sensitivity is the measure of a metal detector’s ability to detect different types and sizes of metal contaminants. Performance is usually expressed in terms of the diameter of a test sphere made from a specific type of metal, such as ferrous, non-ferrous, aluminum, or stainless steel.
To assess sensitivity, a test piece must be detectable when passed through the center (the least sensitive part) of the detector's aperture. Higher sensitivity means smaller metal pieces can be detected. When comparing industrial metal detectors, the best performance is usually indicated by the lowest spherical sensitivity across all metal types at the center of the aperture.
Download our detailed guide, "Understanding Sensitivity in Metal Detection", to learn more about the factors that can affect the sensitivity of metal detection systems.
What Can Affect the Sensitivity of a Metal Detector?
There are many factors that can affect metal detector sensitivity, including:
- Metal type
- Orientation effect
- Aperture size and position
- Packaging material
- Environmental conditions
- Product characteristics
- Process speed
- Detector frequency
A product’s own characteristics (such as high salt or moisture content) can mimic the product signal of a metal contaminant. This is called product effect. Product effect can lead to a high false reject rate if the correct frequency technology is not used, so it's important to understand it to get the most out of your metal detection program.
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Metal Type
The sensitivity of an industrial metal detector varies depending on the type of metal contaminant present, such as ferrous, non-ferrous, and stainless steel. Typically, ferrous metals are the easiest to detect, while stainless steel is the most difficult. However, there are exceptions to this general rule. A HACCP audit can help manufacturers identify their own potential risk areas, as well as the most likely type of metal contamination.
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Orientation Effect
A metal detector’s ability to identify non-spherical contaminants, such as wire or swarf, is affected by the type of metal (e.g., ferrous, non-ferrous, or stainless steel) and the orientation of the metal object. This "orientation effect" is only observed when the contaminant’s cross-sectional area, such as the diameter of a wire, is less than the spherical sensitivity of the metal detector.
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Aperture Size and Position
To maximize metal detection sensitivity, the smallest possible aperture size should be used. The optimal aperture size will depend on the products being inspected. For conveyorized inspection, it will depend on the dimensions and orientation of the product on the belt as it passes through the metal detector.
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Packaging Material
The packaging material used for a product can affect metal detector sensitivity if the material is conductive. For this reason, the production process should be evaluated to determine the best inspection point for metal detection. In some cases, metal detectors are ideally placed immediately before the packaging stage. In other cases, such as when using metallized film, a metal detection solution that addresses any potential issues that may be caused by the packaging material should be considered.
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Environmental Conditions
Factory conditions can affect the performance of a metal detector, which is why it is important to use a metal detector with built-in Noise and Vibration Immunity. This helps to minimize the risk of false rejects caused by airborne electrical interference or vibrations from within the plant.
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Product Characteristics
Some products with a high moisture or salt content, such as meat or bakery items, can conduct electricity and behave like metal when they pass through a metal detector. This phenomenon is commonly known as “product effect”. The latest metal detection solutions combat product effect using Multi-Simultaneous Frequency and Product Signal Suppression technology, which virtually eliminates the active product signal.
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Process Speed
Speed is not necessarily a limiting factor for most metal detection systems. However, it is important to confirm that the metal detector can operate at optimal performance levels. This includes considering any possible variations in speed or product flow on the production line.
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Detector Frequency
Metal detectors can operate at different frequencies, and the optimal frequency depends on the product being inspected. For dry products like snack foods, metal detectors work best at high tuned frequencies. For wet products such as meat and poultry, a combination of Multi-Simultaneous Frequency and Product Signal Suppression technology is recommended to achieve the best results and minimize false reject rates.
