Metallic inclusions are the number one contaminant in food products, causing product quality and consumer safety issues. Fortress Technology scrutinizes the rationale of using metal test spheres to ascertain a food inspection system’s sensitivity level and why size is important.

Q1. Why use spheres?
A1. Like all metal detector manufacturers, spherical test samples are used to showcase advances in sensitivity. Spheres are used industry-wide because they are the same shape from every orientation when passing through the metal detector.

Typically expressed by diameter in millimeters, test spheres provide machine suppliers and buyers with a comparative benchmark. It provides a solid and reliable gauge on machine sensitivity. So, when a supplier reports a sensitivity improvement of 0.5mm, this is actually quite a big deal. A 0.5mm sphere could equate to a wire length contaminant measuring 2.5cm.

In a conventional single-planar-field metal detector, a spherical shaped metal would be relatively easy to spot. However, realistically metal contaminants are more likely to be non-spherical or an irregular shape. In fact, most are likely to be swarf, flat flakes or wires than globular in shape.

Q2. Are there set industry thresholds?
A2. The food metal detection industry has general sphere size guidelines. These are based on whether the product being inspected is wet or dry, as well as the overall size of the product. For a wet block of cheese measuring approximately 75mm high, the sphere size parameters are currently ferrous 2.0mm, non-ferrous 2.5mm and stainless steel 3.5mm, which Fortress metal detectors easily exceed.

Q3. What conditions can impact a metal detectors performance?
A3. Many variables can determine the sensitivity of a food inspection metal detector. Among them the aperture size - the smaller the aperture, the smaller the piece of metal that can be detected, the type of metal - ferrous, non-ferrous or stainless steel, product effect, and the orientation of metal contaminants as they pass through the detector. Environmental conditions, such as airborne electrical interference - static, earth loops - vibration and temperature fluctuation may also affect performance.

However, since size, shape and symmetry of metal contaminants cannot be controlled, operating a metal detector at the highest possible sensitivity setting is often viewed as the best method to address the challenges of product and orientation effect.

Q4. How does the orientation of metals impact performance?
A4. Orientation effect is a result of asymmetrical metal contaminant shards being more easily detected if they pass through the metal inspection system in one direction rather than another. A typical scenario occurs when equipment is calibrated to detect a stainless-steel sphere that’s 2mm in diameter. While it may identify and reject this contaminant, the machine may fail to detect a stainless-steel wire that is smaller in diameter, but longer than 2mm, depending on the orientation of the wire as it travels through the detector.

Q5. Are some metal contaminants easier to detect?
A5. Most industrial food detectors will exhibit a different level of sensitivity for the three main groups - ferrous (such as iron or steel), non-ferrous (including aluminum foil) and stainless steel. Because metal detectors work by spotting materials that create a magnetic or conductive disturbance as they pass through an electromagnetic field, stainless steel is typically the most difficult to identify.

Often, it is easier to detect stainless steel and non-ferrous wires when they pass through the aperture space sideways or upright, rather than in alignment with the conveyor. The reason for this is down to the magnetic permeability of the metal, which for stainless steel is much lower than other metals.

Q6. What solutions are available?
A6. Reducing aperture size is a simple and effective way to increase metal detector sensitivity. Because sensitivity is measured at the geometric center of the aperture, the ratio of the aperture to the size of the product should be considered. Maximum sensitivity occurs when the belt and food item is closest to the edge of the metal detector portal, so it makes sense that the smaller the aperture, the more failsafe a system is.

To identify a metal contaminant within conductive products, the detector must remove or reduce the ‘product effect’. The solution is to change the frequency of operation to minimize the effect of the product. The downside is this can impact your ability to find different metals. When the frequency drops this tends to enhance the ability to find ferrous metals yet limits performance when it comes to stainless steel. The reason - the lower end of the frequency is more responsive to magnetic effects of the contamination.

By the same token, the reverse happens when the frequency is taken higher - it starts to limit the ferrous detection capability but enhances stainless-steel detection.

In the early Fortress days, technology was generally limited to single fixed frequency ‘balanced coil’ systems. However, there are now solutions such as simultaneous multi-frequency and multi-directional scanning metal detectors on the market to address these challenges.

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