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Understanding sensitivity in Metal Detection

There are a number of factors that affect the detection sensitivity of a metal detector and the ability to find different types of metal. It’s important to understand these factors to get the most out of your metal detection program. Sensitivity is the measure of a metal detector’s ability to detect a specific type and size of metal contaminant. The better the sensitivity of the metal detector, the smaller the pieces of irregular-shaped metal it can detect. 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, aluminium or stainless steel. The higher the sensitivity of the metal detector, the smaller the pieces of metal it can detect. It is a clear ideal, but one that is complicated by a number of factors. We regularly refer to the diameter of a ball size when discussing detection sensitivity. For example, a highly sensitive detector could detect a 0.6mm ferrous ball. The effect of the ball size is small but real life, metal contaminants can be much larger. Therefore, small changes in detecting ball size can make big changes to the size of metal contaminants that can be found.

How Metal Detector Sensitivity is Measured When measuring the sensitivity of a metal detector, a test piece must be reliably detectable when passed through the centre of the aperture of the metal detector. However, there is a significant difference between the test piece's spherical sensitivity and the actual length of an irregular-shaped or wire-type contaminant that can be detected. A number of factors can affect the sensitivity of a metal detector.

Metal type A HACCP audit may identify a risk of various metal types, including ferrous, non-ferrous and stainless steel. However, the sensitivity of the metal detector can vary depending on the type of metal contaminant present. Typically ferrous is the easiest to detect, and stainless steel is the hardest to detect. However, as with many “rules” there are exceptions to this.

Orientation effect A metal detector’s ability to identify a non-spherical contaminant such as wire or swarf, is partially determined by the type of metal contaminant (ferrous, non-ferrous or stainless steel), as well as the metal object’s orientation. Orientation effect is only observed when the contaminant’s cross-sectional area (i.e. diameter of a wire) is less than the metal detector’s spherical sensitivity.

Aperture size and position To maximize sensitivity the smallest possible aperture size should be used. Optimum aperture size will be dependent upon the products being inspected, and in the case of conveyorized inspection, the dimensions and orientation of the product on the belt.

Packaging material The packaging material used to pack a product can affect sensitivity if the material is itself conductive. The production process should be assessed to determine the best inspection point. In some cases, this may be immediately prior to packing; in other cases, e.g. when metallized film is used, a solution may be available which overcomes the potential issues caused by the packaging material.

Environmental conditions Factory conditions can also affect the metal detector’s performance. It is important to use a metal detector which has built-in Noise and Vibration Immunity to minimize the risk of airborne electrical interference and local plant vibration affecting the metal detector.

Product characteristics Some products are electrically conductive and as such behave in the same way as metal when passing through the detector. For example, products with high moisture or salt content such as meat and poultry, exhibit this phenomenon, often referred to as Product Effect. The latest innovative metal detection solutions, which combine Multi-Simultaneous Frequency and Product Signal Suppression technology are able to overcome this issue by minimizing the active product signal.

Process speed This is not necessarily a limiting factor for most metal detection systems. However, it is important to ensure the metal detector can operate at optimal performance levels, taking into account any potential variations in speed or product throughput on the line.

Detector frequency Metal detectors can be run on different operating frequencies – the optimum frequency depends on the type of product being inspected. For dry products such as snack foods, metal detectors are more effective at high, tuned frequencies, but for wet products such as meat and poultry, a combination of Multi-Simultaneous Frequency and Product Signal Suppression technology is recommended to get the best results, while also minimizing false reject rates.

How to Achieve Optimum Operating Sensitivity

To protect consumers, maximize efficiency and meet industry standards, manufacturers and processors have an important role to play in identifying, implementing and maintaining an optimum level of metal detector sensitivity.

METTLER TOLEDO metal detectors have a range of different technologies to suit the product being inspected. For example, the optimum technology for inspecting dry products such as snack foods is our Profile metal detector with ultra high-tuned frequency to deliver exceptional sensitivity to detect more and smaller pieces of metal.

In challenging applications where products are hot, wet, chilled or cooling, our  Profile Advantage metal detector with multi-simultaneous frequency and product signal suppression technology is far more effective.

Choosing a stable, reliable metal detector that delivers enhanced sensitivity levels is an integral part of a food safety program to minimize metal contamination.

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Source: Mettler-Toledo

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