In this article we’re going to cover how we should validate the test pieces we use to monitor our CCP of metal detection. When an auditor asks for metal detection validation, it’s quite common for them to be given the servicing records for the metal detector, or the metal detection ‘calibration’ as it’s sometimes known.  What we really should be doing is carrying out a proper metal detection validation, which proves that the monitoring procedure that we have applied is the most robust that it can be.

So, we need to make sure that when we are doing our metal detection tests, that:

  • The test pieces are placed in the right place – on or in the product
  • The test pieces are the optimum size

Validation of the test piece equipment

Where you put the test piece is really important, because we need to make sure that it is in the hardest place for the metal detector to ‘see’ it. In the previous article we talked about the search head – if you’ve not read that article you can find it here.

Here’s the picture of the search head again…

metal detection validation

Fig. 1 The Search Head

The signal comes from the search head (See Fig. 1) and it’s the signal creates the rejection when metal is detected. So, if we look at the picture again of the search head and draw (green) lines from the corners of the search head, we can see where the weakest part of the detector is.

This spot is where the detector finds it most difficult to see metal (See Fig. 2). Therefore, this is where the test piece of metal should go – so we can make sure we are really challenging the machine.


Fig. 2 The Search Head (weakest point)

The size and shape of the product

This is where the size and shape of the product comes in.  If your product is tall, the test piece would need to be in the centre of the product, to make sure it was at the weakest point, as shown in the diagram (See Fig. 3).

Whereas, if your product was small, the hardest place for the detector to be able to see metal would be on the top of the product – so this is where your test piece should be (See Fig. 4). So that’s the first thing you need to validate – where the test piece should be.

metal detection validation

Fig. 3 The Search Head (tall product)

metal detection validation

Fig. 4 The Search Head (small product)

To do this, measure the detector head height from the conveyor and record it on your validation report.  Then, record the product sizes you make and the size of each – to show where the height of the product sits against the detector head.  Then work out, from this, if the test piece should be in the centre of the product or on top of the product (depending on which is closes to the weakest point). Then record this for each size of product.

You may find, that if you only have one detector that is used for a wide range of products, which are different sizes, you may need to vary where the test piece goes, depending on what product you are producing.

Test piece size

The next step in the validation is to work out what test piece sizes you can achieve.  Many retailers will have set test piece sizes for you to use, but you can use this validation to prove that either:

  • You can achieve a smaller size
  • You cannot achieve the size they have specified, as it will cause false rejects (learn about false rejects in our previous article

In order to do this effectively, you will need a range of test piece sizes to use and so, it’s generally a good idea to get your metal detector manufacturer to come in and help you – as they will have a test pieces of all the sizes you’ll need, as ideally you don’t want to be buying them! You will need to carry out trials.  You can do this on each product you make (if you don’t make many) or you can group your products into similar groups and just pick one product from each group to trial.

Then, with your trial product being produced, learn the detector to the product – to make sure the sensitivity is correct.  Then, make up test packs with your current size of test piece and send these through the detector 30 times (I use 30, but if you want to do more or less that’s up to you). If all 30 test packs pass, record this and then move on to the next smaller test piece and repeat.

Repeat this process, until you find you get at least 1 fail (i.e. it is not rejected) out of your 30 test packs.  At this point, this test piece has failed, and so, you have proven that the test piece size, one up from this one, is the one that has been validated as successful. This is the test piece size you should use to check the machine on an ongoing basis, for that product or product group. You would then need to repeat this for the other types of test piece.

Record all of your trial results on your validation report. You need repeat this validation when something changes; e.g. the product recipe, size, weight or the equipment producing the product. Ideally, it should also be carried out yearly following the servicing of the machine.

In the next article we’re going to start going through the tests, why we do them and what the test is challenging in the machines functionality. As always, if you have any questions or if you would like to add to the information provided in this article, please do so in the comments box below.

Have your say…

9 thoughts on “How to carry out metal detection validation

  1. So glad you did this article on validation. It is something we have found very little information on.

    I am confused about your statement, “If all 30 test packs pass, … move on to the next smaller test piece and repeat.” Shouldn’t all 30 test packs FAIL (i.e., the machine detects the metal test piece and sounds the alarm/rejects the product)? And when you say, “Repeat this process until you get at least 1 rejection out of your 30 test packs. At this point, this test piece has failed …,” shouldn’t this actually be at least one pass where the test piece is NOT rejected (i.e., it gets through the metal detector without being detected)?

    Thanks for clarifying.

    1. Hi, I think it’s just a case of terminology. “If all 30 test packs pass, … move on to the next smaller test piece and repeat.” A metal detection test that passes to me, is one where the test pack is rejected. When I’ve said ‘rejection’ – yes, you’re right I mean it hasn’t been rejected as it should! I agree this is confusing! I’ll amend the post so it’s clearer, thank you for pointing it out 🙂

  2. An excellent and informative article, yet straight forward

    Having recently advised on this very topic it is surprising how many people in our industry do not have a background knowledge in metal detection, and unfortunately validation is even less understood.

    I will certainly be referencing your articles during any future training. Keep up the good work

  3. Kassy

    Usual praise for this article but any comment on when Ghosts appear!! Eg high density foods, high moisture or poorly distributed moisture, high levels of minerals etc??

    False positives in a way…..

    1. Hi Chris,
      Thank you! ‘Ghosts’ as in false rejects – it’s true, these will occur if the water content of the product varies, or if there is condensation on the pack. Making sure the product going through the metal detector is consistent and also that the machine is set with each product, where the products differ – rather than trying to use one product setting for all products. Where products are inconsistent, or where the normal process cannot be followed (e.g. stops in production which cause chilled product packs to get consendation on them) may need the product to be ‘learnt’ for each run, or each specific problem run.

  4. Hello Kassy here in Canada a lot of the work is done by the metal detection company which includes organizations such as Fortress, Loma, etc. They work with the site to deliver a system of a stated aperture configuration ( height X width) based on the product(s) and for me packing frozen bread my aperture hight is quite large whereas for my associate checking packages of Canadian bacon that aperture is reduced drastically. With that detection head as you would call it, they provide the control limit meaning, for example, they could say the system is optimized around 2.5mm Ferrous, 2.5 mm non- ferrous and 3.0 mm stainless.
    Now my work begins using ferrous as an example I would find or purchase additional test pieces lets say a 2.0 mm ferrous and I would challenge the system to see if we could demonstrate accuracy and repeatability at 2.0 mm vs 2.5. Assuming that in over 10 -15 passes the detection provided sporadic failures or total failures I would conclude that 2.5 mm was the best choice and would then run the 2.5 test piece and verify that it passed every time.
    Briefly let me also say that precursor to the challenge test is the preparation of a test pack which means that I obtain a case ( corrugate) that is metal-free ( yes we find shipping cases with detectable traces of metal), that the product as presented to the MD is frozen ( this is a process requirement) and that we pass the case down the midline of the detector head ( worse case). Your placement diagram ( small package vs large package) raises some questions because for a stated aperture and a site testing 5 different package configurations the center line approach works better. The test piece is taped to the test case at the trailing at a point equivalent to where it would hit the imaginary center line. Keep up the good work.

  5. Hello Kassy,
    I found your this valuable article by searching accidentally! Many thanks.
    We are a service company for metal detectors. I am honored to be allowed to discuss this very important and complex issue with you here.
    We are currently helping more users improve the detection capabilities of their devices.
    The current highest record is 260,000 consecutive successful validation.
    This number is calculated backward based on the customer’s production time. I am very confident of this result and have our test basis.
    Our method is to record 200 signal values for the product, Fe, NFe and SS respectively, and then analyze the law of these signal values, and then calculate the maximum possible number of successful verifications.
    This is very important for mass production in factories and to ensure the high reliability of metal detectors.
    I look forward to more users getting further improvements!

    Many thanks.

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