Product Effect and Metal Detection
Setting up a metal detector for use on a food production line requires an understanding of “product effect.”
Product Effect is the magnetic and conductive properties of a product. Whether frozen, liquid, solid or powder, when the product travels through the aperture of the metal detector, it will, by nature, have some effect on the coils inside the head of the detector. Metal detectors must somehow factor this in and find a way to “eliminate” it or ignore it. This process should be cared for not only during setup, but on a regularly scheduled basis, especially when a new batch of product is run or during a shift change. In a manner of speaking, the detector needs to “learn” what is the product effect and the vibration or interference that exists in the surrounding environment. The detector will naturally discover these properties and can then be set to a baseline – a setting in which the product (a clean product) and it’s container (paper, cardboard or other non-magnetic housing) moves through the detector without setting of a detection alarm and the associated reject device.
When working to determine and resolve product effect, keep these things in mind:
1) It is not physically possible to detect any contaminant whose value is smaller than the Product Effect value. For instance, if the product effect is equal to metal the size of a ping pong ball, you will not be to achieve a contaminant the size of a marble. Metal detectors are made to set the reject point at a specific threshold. They rely on the simple mathematical formula:
Sensitivity Setting + Product Effect + Contaminant Value = Reject Alarm
In an ideal scenario, the Sensitivity Setting and Product Effect should be just below the Reject Alarm threshold; this means even the smallest contaminant can be detected. Fluctuations in Product Effect can create an undesired “gray area.” To avoid a false positive (the largest portion the gray area), the Sensitivity Setting will have to be lower. But this in turn requires the Contaminant Value to be larger in order to have consistent detection.
2) The larger the deviation in Product Effect, the less sensitive your detector will be.
The orientation of the product in the aperture can also alter Product Effect. Imagine the difference between throwing a pebble in a glass smooth lake and doing a cannonball off the dock. The splash is the Product Effect. Upon entering the magnetic field of the detector, the product creates a ‘splash’ in the field. Given that metal detectors function on a balanced field principle, the amount of ‘splash’ is what determines whether a metal detector is seeing product effect, or product effect AND contaminant.
3) Always run the product which will create the worst-case product effect value to avoid a false positive and reject.
4) onsider the condition of the product. A frozen product that is only partially frozen will affect your Product Effect values considerably. The more frozen your product is, the lower the Product Effect value will be and the greater your sensitivity will be. That is, it will behave more like a dry product. Higher temperature products, or hot products directly from processing, will always show greater product effect values because they contain moisture.
Product Effect and Phasing
The control electronics of a metal detector actually split the received signal into two separate channels: magnetic and conductive. This means there are essentially two balanced scales within the unit (figure 3). These scales constantly measure the magnetic and conductive components of every disturbance. Products that are being inspected can have one or both characteristics.
For example, any iron-enriched product, such as cereals, create a large magnetic interference that the detector must overcome to detect small pieces of metal. These are referred to as “dry” products. On the other hand, products with high moisture or salt content such as bread, meat, cheese, etc. are electrically conductive and produce a conductive error signal and 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.
|Typical ‘Wet’ Products||Typical ‘Dry’ Products|
Food: Meat, Cheese, Bread and Bakery Products, Fish, Dairy Products, Salads
Food: Cereal, Crackers, Flour, Tablets Powders, Biscuits, Frozen Food Products ( < -1.0 Degrees C), Peanut Butter and Margarine (Vegetable oil is not conductive)
|Packaging: Metalized Films||Other: Wood Products, Plastics and Rubber (Products with high carbon black content may be considered ‘wet’), Textiles, Paper Products|
|Other: Plastic and Rubber products with high carbon black content||
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