Theoretically, many variables will 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, food products come in all shapes, sizes and density. What’s more, products don’t always travel consistently in the same direction when passing through the metal detector aperture. Since size, shape and symmetry of metal contaminants cannot be controlled, operating a metal detector at the highest possible sensitivity setting is generally viewed as the best method to tackle product and orientation effect.
Reducing the aperture size is also widely regarded as a simple and effective way to increase metal detector sensitivity. That’s because sensitivity is measured at the geometric centre of the aperture, therefore the ratio of the aperture to the size of the product is an essential consideration. Maximum sensitivity occurs when the belt and food item is closest to the edge of the metal detector portal.
Putting sensitivity to the test
Metal detectors are generally tested with ferrous, non-ferrous and 316 stainless steel test samples. Testing with these three types of samples ensures that the detector is capable of discovering all metals.
The standard technique for measuring the sensitivity of food inspection metal detectors is to use metal test spheres because they are the same shape from every aspect when passing through the metal detector. “Using a spherical object means that the signal they give off is the same in any orientation,” explains Phil Brown, Managing Director at Fortress Technology Europe.
Although this type of spherical testing tool is typically expressed by diameter in millimetres, realistically metal contaminants are more likely to be non-spherical or an irregular shape.
During regular testing of food metal detectors, manufacturers should insert test pieces in various locations and orientations within products, for example in the front, centre and back, and run consecutive tests. “This provides extra assurance that your metal detectors are performing as they should and picking up contaminants regardless of metal type, size or orientation,” emphasises Phil
Test sphere thresholds
The food metal detection industry has general sphere size guidance figures and 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 would be approximately 2.0mm for ferrous metals such as iron or steel, 2.5mm for non-ferrous, including copper or brass and 3.5mm for stainless steel.
However, if you were to roll out a 3.5mm stainless steel metal sphere between your hands, it could equate to a wire length of 30 cm, depending on the diameter of the wire. Realistically, something of this size is likely to protrude from a product and would be relatively easy to spot with the naked eye. It’s when smaller wires or swarf slivers are embedded into a product where the sensitivity of your metal detector really comes into play.
Machinery manufacturers often quantify advances in metal detection sensitivity by the size of the sphere size. Yet, visualising what a percentage or millimetre improvement in metal detection actually equates to can be challenging for food factories.
Putting an example into context, Phil says that an improvement in sphere size of 0.5mm can equate to wire length contaminant measuring 2.5cm. The smaller the spherical size, the shorter the wire length.
Striking a balance
It’s equally important to recognise how ramping up the sensitivity in conventional metal detectors can lead to higher volumes of false product rejects, particularly in wet, moist and high salt content foods. Again, this is linked to product effect.
Large volumes of food applications that are inspected inherently have electrical conductivity and/or magnetic permeability within their makeup. For example, any product that has a high moisture and salt content, such as bread, meat and cheese, is electrically conductive. This means sensitivity levels suffer, as the metal detector has to deal with the signature of the product.
Even wet products will exhibit a very different product effect. For example bread and meat are both conductive, but meat typically has higher water content and thermal changes caused by thawing or warm products cooling can affect the products signal quite significantly and cause a false reject. In addition, meat cuts are different densities, so again the product effect will differ.
Conversely, any product that is iron enriched, such as fortified cereals, supplements and breakfast bars, creates a large magnetic signal that the detector must overcome in order to detect small pieces of metal. These are referred to as ‘dry’ products and tend to be a lot easier in terms of detection capability, because there is no product effect to worry about.
To identify a metal contaminant within conductive products, the detector must remove or reduce this product effect. The solution is to change the frequency of operation to minimise the effect of the product. The downside is this can impact your ability to find different metals. When the frequency is dropped it can enhance your ability to find ferrous metals, yet this limits performance for non-ferrous metals, since 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 the non-ferrous detection.
At Pack Expo International 2018 in Chicago, Fortress Technology unveiled its latest advancement - the Interceptor DF (Divergent Field). The multi-orientation, multi-scan food metal detector reports a 100 per cent increase in sensitivity compared to single- or-dual frequency food metal detectors and an improvement in identifying and rejecting contaminants that are non-spherical.
Critically, the Interceptor DF addresses several previous limitations, including the orientation, size, geometry, and position of metals, especially ultra-thin contaminants that might be more prevalent in high value, low side profile foods, such as confectionery or sliced meats. Rather than missing a metal contaminant because it hasn’t aligned with a specific frequency, the Interceptor DF uses multiple field patterns to inspect products both horizontally and vertically simultaneously as they pass through the detector. This increases the probability of finding a small swarf, shaving, or flake of metal, regardless of the orientation.
Phil concludes: “There are many variables which can affect a metal detector’s performance, including orientation of contaminants, the type of product passing under the detector, product size and the speed of the line. However, machine sensitivity remains a solid and reliable gauge.”