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…
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.
Fig. 3 The Search Head (tall product)
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.