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Metal Detection: Product Effect And Metal Free Area

Source: Fortress Technology

The control electronics actually split the received signal into two separate channels: magnetic and conductive. This means there are effectively two balanced scales within the detector. These scales continuously measure the magnetic and conductive signal component of every disturbance. Products that are being inspected can also have one or both of these characteristics.

Metal detectors detect metal based on measuring electrical conductivity and magnetic permeability. Many products to be inspected inherently have one or both of these characteristics within their makeup. For example, any product that is iron enriched such as cereals, create a large magnetic signal that the detector must overcome in order to detect small pieces of metal. These are referred to as “dry” products. Conversely, products with high moisture and salt content such as bread, meat, cheese, etc. are electrically conductive and produce a conductive error signal. These are referred to as “wet” products. The table below shows typical product error signals and categorizes them as wet or dry.  

The detector must remove or reduce this "product effect" in order to identify a metal contaminant. Most modern detectors will have some form of automatic calibration to do this ‐ it is often referred to a phasing.