Sensitivity Is A Key Contributor To Metal Detection Effectiveness
There are a number of factors that affect the sensitivity of a metal detector and the ability to find different types of metal. It is important to understand these factors to get the most out of your metal detection program.
This guide explains the concept of metal detector sensitivity, key factors that affect it, and why 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.
What Is Sensitivity? How Is It Measured?
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.
Metal detector performance is usually expressed by the diameter of a test sphere made from a specific type of metal, such as ferrous, non-ferrous (aluminum, brass), or stainless steel.
Sensitivity should always be measured as near to the center of the metal detector’s aperture as possible, as this is the least sensitive point. When comparing the performance of different metal detectors, the clear winner is usually the one with the best spherical sensitivity across all metal types, when measured in the center of the aperture.
However, in some cases, it may not be so clear. One metal detector could outperform the other on one metal type, and vice versa for another. In this case, you should consider which contaminant type is most likely to occur (often this is stainless steel) and could base your decision on which system performs best for this metal contaminant type.
Ultimately, it is best to discuss the situation with a metal detection system expert who can guide you through the process of choosing the best overall system, as there are many other factors to consider.
Factors That Contribute To Metal Detector Sensitivity
When measuring the sensitivity of a metal detector, a test piece must be reliably detected and rejected when passed through the center 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. An experienced provider can help you select the right solution to overcome these challenges.
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 becomes a limiting factor 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 product position: To maximize sensitivity, the smallest possible aperture size should normally 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 metalized film is used, several alternatives are available, including alternative/multiple frequencies or using a slightly larger aperture. The ultimate critical control point (CCP) is after final packaging.
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’s performance.
Product characteristics: Some products are electrically conductive or magnetic 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 combine two or more frequencies simultaneously along with Product Signal Suppression technology 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.
