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Metal detection - single frequency vs. multi-frequency

Source:Heat and Control     Date:2022-08-16
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METAL is one of the most commonly found contaminants in food and can be introduced at all stages of food manufacturing. By identifying metal contaminants during processing or packaging, a metal detector helps ensure the safety and the integrity of a wide range of unpackaged, packaged, or bulk goods and helps to eliminate metal contaminants from a food manufacturer’s finished product.


How metal detectors work

There are several types of metal detectors, and each uses a different detection method to operate. In the food industry, metal detectors usually apply the balanced coil method. These metal detectors are equipped with a transmitter coil, which generates an electromagnetic field, and two receiver coils. The receiver coils have an equal but opposite output, creating a balanced state.

 

When a conductive object passes through the detector, it interrupts the balanced state of the electromagnetic field and causes a signal that is detected by the receiver coils. The electronic unit in the metal detector then analyses this signal, evaluates it, signals a metal detection, and either activates the automatic reject units that separates the contaminated product from the production line or alerts the operators.

 

Metal detectors are strategically placed based on areas where foreign materials can enter the production stream. If consumer protection is the goal, several inspection points are useful. Inspection of raw material catches metal particles before they are broken into smaller pieces which tend to be harder to detect. If the metal detector is used for machinery protection, it is installed directly before the machine requiring protection.


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A metal detector helps ensure the safety and the integrity of a wide range of unpackaged, packaged, or bulk goods.

 

Inspections at critical control points (i.e., hazard analysis critical control points) during the production process are recommended so operators are alerted of machinery failures (e.g., broken blades, missing hardware) in a timely manner.

 

Consequently, after packaging, food products often are inspected at their final stage with a metal detector (and/or an X-ray system) to fully exclude contamination before being sent to the retailers’ shelves.

 

For optimal product protection, an inspection of raw materials at several critical control points and a final inspection of the finished product are recommended.

 

Factors that lead to false detections

Some products contain salt, iron, and moisture — each of these can alter the detector’s field and can trigger a false detection. This effect increases with higher inspection frequencies that are typically used to detect smaller particles and can lead to significant levels of wasted product and downtime. It is not uncommon to use a reduced frequency to minimise the number of false detections, but this procedure also reduces the ability of the detector to identify non-ferrous and stainless-steel contaminants.

 

How many frequencies are necessary?

 

Single frequency

The majority of metal detectors on the market use only one frequency when inspecting product. The selected operating frequency depends primarily on the product being inspected. This is satisfactory when the product is dry and non-conductive. Single frequency metal detectors have limited abilities to work with different products and are difficult to optimise for varying conditions of temperature, water content, and packaging types.

 

Multi-frequency

Using a metal detector that offers more detection frequencies can result in more sensitive metal detection and fewer product effect errors, as well as being able to use the machine on products with differing characteristics.

 

There are multi-frequency metal detectors that offer several operating frequencies. However, these machines can only use one of the available operating frequencies at a time. The operator may need to switch from one frequency to another when changing products, or in more advanced multi-frequency systems, the metal detector will switch back and forth between two or more frequencies in rapid succession. However, only one frequency (one electromagnetic field) is capable of ever being on at any given time, and to prevent false rejections on more difficult product, reducing the sensitivity typically is still required. 



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The CEIA THS/MS21 is the world’s only multi-spectrum metal detector


Multi-spectrum

Finally, there are multi-spectrum metal detectors. These metal detectors operate over an entire spectrum of frequencies that are simultaneously analysed and applied to completely eliminate product effect and allow for an improved inspection of the product.

 

CEIA are the world’s largest manufacturer of metal detectors — and the only company to make metal detectors using a patented cutting-edge multi-spectrum technology. This innovative technology uses multiple frequencies simultaneously — meaning that they can increase sensitivity — reduce false rejects — and detect thin metal fragments.

 

The CEIA THS/MS21 is the world’s only multi-spectrum metal detector. It has unique detection capabilities and extreme sensitivity of magnetic, non-magnetic, and even 316 stainless-steel metal contaminants, and is available in a USDA-approved design. It is suitable for nearly all variations of food product characteristics — and can detect foreign objects while operating simply, efficiently and at high speed — at the same time, collecting and retaining important production run data.


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