Over the next few articles I’m going to talk about the CCP metal detection; how metal detectors work, where the test pieces should go and why we carry out the tests we do. The purpose of the CCP metal detection is to ensure that any detectable metal, of a detectable size and orientation, which may have contaminated the product, is detected and rejected.
In order to understand how to manage your metal detector and get the most out of it, we really need to understand how the metal detector works and how it reacts with the product(s) that we are manufacturing. So, let’s go through how it works…

Fig.1 The Search Head

The search head

In order to achieve the successful detection of metal in the finished product, the metal detector generates an electric field.  When metal passes through it this electric field, it distorts it.  The metal detector recognises this distortion and uses this to trigger the rejection system.

This field is generated in and around what we call the ‘search head’. The field of the detector is generated by the interaction between three coils within the search head.  The area called the search head is shown in red in Fig. 1, the arrows show the direction of the field.

The size of the search head

The sensitivity of the metal detector will be limited by the size of the opening of the search head.  To achieve the best sensitivity, the search head should be of a size appropriate for the specific product being produced on that line. i.e. just big enough to get the product through the opening of the search head.  The bigger the search head, the further the product is away from the search head, the less sensitive the detector will be on that product.

Reducing interference

The metal detector unit must be capable of ‘blocking out’ any potential interference within the factory around the machine. Interference may be caused by vibration, heavy vehicular traffic or from other equipment. Similarly, electrical “noise” from adjacent/connected equipment should be avoided and metal items should not be stored on top or near the metal detector unit, including metal detectable pens and test pieces.

The signal

When a conductive material (metal contaminant) passes through the search head, it creates a distortion in the field – this is known as the signal.

There are two types of distortion (signal):

  1. a reactive value – caused by metal of ferrous (Fe) composition,
  2. a resistive effect – caused by metal of a non-ferrous (NF) composition.

Stainless steel also exhibits a resistive effect but less strongly than a non-ferrous metal – making stainless steel (SS) more difficult to detect.

The signal that is produced from a reactive value and a resistive value can be displayed as a chart (see Fig. 2) to help to explain the effect it has.  By drawing it on a chart, we can see the size and position of the signal that is produced by the 3 test pieces (NF: non ferrous, F: ferrous, and SS: stainless steel) and also the signal produced when nothing is passing through the search head.

CCP Metal Detection

Fig. 2 The Signal

The closer to the signal is to R, the less distortion (either +ve or -ve) to the field, the less signal is produced and so, it is more difficult for the detector to ‘see’ the metal. You can see in Fig. 2, that non-ferrous (NF) and ferrous (F) metals are quite some distance from ‘R’, therefore the signal is different to that from ‘R’, and also the arrows is quite long, so the signal is strong.  Which means, the detector would recognise the signal as metal.

Stainless steel however, is quite close to ‘R’ and the arrow is only short, and so weak.  This is why stainless steel is more difficult to detect.  You will find that if any test piece is going to fail to reject when carrying out your tests, it’s normally  stainless steel – for this very reason.

The other important thing to understand, is that the product causes a distortion to the field as well.  This is because the product has its own conductivity.

Water is a good conductor and is found in most products.  But the additional of other ingredients such as sugar, can actually increase the conductivity. This means that when the product itself passes through the search head, it produces a signal. In Fig. 3, we’ve added a product signal to the chart.

CCP Metal Detection

Fig. 3 The Product Signal

False rejects

A false reject is one where there is no metal in the product, but the detector rejects it anyway, as it thinks it contains metal because of the signal it produced. We don’t want false rejects, because it’s annoying to the operators in the area, it causes waste product and also it means that people lose faith in the machine, because they don’t think it’s working.  The risk with this, is that if the machine fails its 3 piece test, the operators may decide to continue on anyway, because it’s just the machine ‘playing up’.  Or, if the machine actually rejects product because it really has got metal in it, nobody notices, because it’s one of so many rejections.  A rejected product, needs to be a really big deal to the operator, so it’s taken seriously – that’s what we need to aim for.

So, to prevent false rejects the metal detector needs to be programmed to ignore the signal created by the product. The machine does this by knowing what part of the signal to ignore.  This is known as the metal detector ‘learning’ the product. To explain how the metal detector does this, Fig. 4 Product Window, below shows the product window (the area of signal that the machine ignores).

If the product window is set up incorrectly, as in, not wide enough, it will cause false rejects – because the detector will still ‘see’ the signal from the product.  The diagram (See Fig. 5) shows this.

CCP Metal Detection

Fig. 4 The Product Window

Fig. 5 The Product Window (too narrow)

Fig. 6 The Product Window (too wide)

Or if the product window is too wide, the detector will think any signals generated within it are good product (i.e. do not contain metal).  This may mean that stainless steel signal is within the area to be ignored, which will allow product containing stainless steel to pass through the detector without being rejected. The diagram (See Fig. 6) shows this.

So, you can see that getting the ‘ignore’ area of the signal right is really important.  We need it to be as narrow as possible, to make sure that only the product is ignored, but wide enough to make sure we don’t reject good product.

This is why, we have to make sure that we have settings of each type of product that we are producing.  If we have one setting for all products and the signal that those product produce is wide, then we are at risk of not rejecting product that contains metal.

Some metal detectors will have the functionality to ‘learn’ the product once and then store these settings, under the product name (or product group), allowing the operator to retrieve them on the next run – by picking the required product from the menu.

You don’t need a setting for every single product you make, if they are similar in size and recipe.  Go through the products and work out how they can be grouped best and then get the machine to learn each group.  If you’re not sure, you can get your metal detector company to come in and help you to work out what the variation in signal is, that each product produces.

The operators of the metal detector then need to make sure that they select the right product, when they are using the detector.  There are so many times I’ve gone to a metal detector and checked what product has been selected, and found that it’s not the one that’s being produced at the time.

Some metal detectors may not have this functionality however, or the product may be so variable that storing the settings is not practical.  In this situation, the metal detector would need to be set up to ‘learn’ the product prior to each run.  And so, this would need to be part of the change over procedures for setting up the metal detector.

Test piece size

The smaller the piece of metal, the weaker the signal and also, the closer to the product window the signal may be.  Therefore, it is key to work out what test piece size can be used, to test the machine.  There is a balance to be made here, to find the smallest test piece possible, but without triggering false rejects.  To do this, trials must be carried out to find out what the smallest test piece size can be used, that is consistently detectable, without causing excessive product rejects.  These trials need to make up part of your metal detection CCP validation.  In the next article we’ll look at how we should validate your test pieces.

If you have any questions about the CCP metal detection, please add them to the comments below and I’ll answer them for you. Or, if this is a speciality subject area of yours, I’d love you to add to this article, by sharing your knowledge with us!

Have your say…

15 thoughts on “How does a metal detector work?

  1. this insight guides are great training tools for younger members of the team. Are there plans to do a “how an x-ray works” ?

    1. Hi,
      No not always. Most customers and auditors would expect end of line metal detection to be a CCP, but if there are additional metal detectors within the process (pre packing/end of line) then these would obviously not be CCPs, as the last point of defence would be the CCP.

    1. Hi, I’d not thought of doing that, but we definitely could. I’ll look at doing this when I’m next writing. Thanks, Kassy

  2. Hi!

    Recently we have had a non conformance in the BRC audit regarding the validation of the metal detector. Despite our MD service carried out a validation by passing different sizes of the 3 types of metal in order to determine the correct size of the test pieces for our operation, the auditor raised that in order to validate that our detector is capable of detecting metal contamination coming in different shape and orientation, we must also validate the detector by passing potential foreign bodies in our factory. The auditor found a piece of wire (about 15 mm long) on the floor and she asked us to put it in the product and pass it through the detector. The outcome was that it was detected in one orientation but not the other. She said that we must include a test of this type in our validation. I would just like to know your opinion on this, since our MD service and we agree that that kind of test can’t be standardized and the industry uses steric balls to eliminate the effect of the shape and orientation.
    Thank you

    1. Hi Cristian
      Oh dear… I totally agree this isn’t a valid non-conformance. The reason the wire wasn’t detected in all orientations is because of it’s shape and yes, that’s why balls are used. Validation is meant to prove a theory, and putting a piece of wire through the metal detector like this wouldn’t prove anything – it would be good to see what the non-conformance actually says, to understand what she was wanting you to prove.
      I would definitely challenge the non-conformance wtih your Certification Body. If you would like any help in doing this, please let me know. And, please let us know the outcome!

      1. Hi Kassy,
        Although I am a provider and service provider of metal detectors, I still think it is very meaningful for users! Even as a service provider, I still recognize the concerns of users.
        Because we may have overlooked one of the most important tasks:
        If a metal detector is not professionally maintained, its detection capability may not be keep or even attenuated.
        Three standard test pieces are used for verification. Even if 100 successful verifications are carried out, the issue cannot be accurately explained, for example:
        The signal value of the three metal test pieces may all exceed 1000, but the threshold set is only 200.
        However, the detection capability of the MD may be slowly decreasing (almost certainly exists).
        If the MD has not been professionally maintained, and when verifying, only observe whether it is rejected without paying attention to the specific signal value.
        Then even if the detection ability is reduced to a signal value of only about 400, you can still perform at least 1,000 successful verifications ( I am very sure that I am confident enough on this issue! ).
        Therefore, it is not whether the wire itself can be detected, but more importantly, we need to pay attention to the specific signal value, not just how many successful verifications.

        I don’t know if I have described this issue accurately, if anyone has further questions, I would be happy to discuss with you.

        Many thanks.


  3. I need to clarify whether the metal detector is a Critical Control Point or is it a monitoring tool for monitoring the Critical Control Point For example, there is a production line on which a critical control point has been placed, which is heating to 100 degrees C, and that point is monitored by a thermometer, so is the thermometer in this case equivalent to a metal detector

    1. Hi Moshen,
      Metal detection is a CCP, if your HACCP determines it to be. A metal detection is a process step and the metal detector is a CCP monitoring tool.
      Heat treatment to 100C can also be a CCP if your HACCP determines it to be. Cooking is the process step and the probe is a CCP monitoring tool.
      Does that help?

  4. Hi Kassy,
    What is the real difference between calibration, verification and validation? My auditor wanted to see different documents for each. If you can help us. Thanks a lot!

    1. Hi Ronak
      Calibration of metal detectors are commonly confused with service records. So that would most likely be what your auditor was looking for. Verification records, would be your routine metal detector tests. Validation records would be the report you create (or it can be done by your metal detector provided) which details how you came up with the verification testing routine that you use, e.g. test piece sizes.
      I hope that helps.

  5. Hi Kassy,
    the question is about food standard clause Is it acceptable to use some control packs of product with test pieces inside prepared month ago, not those being produced on the line during verification of metal detector?
    Thank you!

    1. Hi Mila
      No – because the test packs wouldn’t be reflective of what is being packed currently.

Share your thoughts…

Your email address will not be published.

We've tagged this article as: ,