Could metal be going undetected on your production line?

Sensitivity is the metal detector’s ability to detect a specific type and size of metal contaminant. A small difference in spherical sensitivity performance can mean a big difference in the length of wires or other irregular-shaped contaminants that can be detected by your metal detection system. Investing in a metal detector that delivers the highest possible sensitivity can mean the difference between detecting a contaminant before product leaves your factory, and potentially facing a costly product recall due to undetected metal.

According to the Grocery Manufacturers’ Association1, the average product recall can cost a food manufacturer £7.5m in direct costs – far more than the cost of investing in the right metal detection system for your production line.

Understanding sensitivity can assist you in making a decision about installing a new metal detector. A difference in spherical sensitivity of 0.2 – 0.5mm may not sound like a lot. However, when you equate this to the capability of each system to detect non-spherical or irregular-shaped contaminants, the difference in performance can be significant. Upgrading older equipment, or replacing an existing metal detector with one that delivers better sensitivity can pay for itself simply by avoiding even a single product recall. There may be additional benefits such as virtually eliminating false rejects which can also have a positive impact on your bottom line.

Factors That Impact Sensitivity

Metal detector 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.

It’s important to understand these factors to get the most out of your metal detection programme and minimise your risk of a product recall due to undetected metal.

  1. Metal type

Industry standards state that metal detectors must be able to detect all types of metal 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.

  1. 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.

  1. 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 conveyorised inspection, the dimensions and orientation on belt.

  1. 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 metallised film is used, a solution may be available which overcomes the potential issues caused by the packaging material.


  1. 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.

  1. 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.

  1. 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.

  1. 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.

Understanding Sensitivity Standards – For Comparison Purposes

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, as highlighted earlier, there is a significant difference between the test piece’s spherical sensitivity and the actual length of an irregular or wire type contaminant that can be detected due to the metal contamination’s ‘orientation effect’.

When comparing the performance of different metal detectors, you can examine the individual sensitivity for specific types of metal.

In some cases, the clear winner is the metal detector with the best spherical sensitivity across all metal types, when measured in the centre of the aperture. However, in other cases, it may not be so clear, with one detector outperforming the other on one metal type, and vice versa for another.

When there is no clear winner at first glance, it may be better to use the ‘mean sensitivity’ performance measure. Download our guide to understanding sensitivity for more details on this approach


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 their 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, or packaged in metallized film, METTLER TOLEDO’s 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 an effective food safety programme to minimise metal contamination. However, it is equally important to ensure your metal detection system is installed, set up, operated and maintained correctly, so it continues to perform as expected.

Partnering with an experienced product inspection solutions provider such as METTLER TOLEDO, with a local sales and service presence in 39 countries, and coverage in over 80 countries through their third party network, ensures you have access to the expertise, training and full lifecycle support you need to maximize your return on investment.

Free Download

To learn more about metal detector performance and how to reduce your risk of product recalls due to metal contamination, download their free guide on “Understanding Sensitivity in Metal Detection