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In this post we’re going to go through the last two types of test and why we do them; the memory test and the large metal piece test…

The memory test

This test is where you have your 3 test packs and in-between them, you have good product, as shown below.

metal detector memory test

The purpose of this test, is to prove that the metal detector can see metal in the ‘test pack 1’, reject it and then reset itself, so that it can see that there’s no metal in ‘good pack 1’.  Then it has to be able to see and reject ‘test pack 2’, then reset itself so it doesn’t reject ‘good pack 2’ and so on.

If the machine cannot reset itself fast enough, it may also reject the good packs. Plus, we are testing the machine, to make sure it does actually reject the test packs and not, the good packs by mistake, allowing the test packs to go through. So, this means that if the machine does not reject all 3 test packs AND, does not allow all 3 good packs to go through, the test has failed.

It is a common mistake to think that the purpose of the test is only to reject the test packs and that it’s ok for the good packs to be rejected too.  Rejecting the good packs shows that the machine is not set up correctly and so, you’ll get false rejects.

Also note, how there are 3 good packs – the last pack is there to prove that the machine resets itself after test pack 3, if it’s not there, then you’ve not proven that the reset mechanism worked for all 3 packs.  Also, it’s good practice to always have a good pack at the end of the test, as you may find you need this pack to ensure that all the products go through in normal product flow.  You may find that, that last pack is needed to ‘nudge’ the test pack through the detector.

Large piece of metal test

One of the most recent tests to be introduced is the large piece of metal test. This test was introduced to replicate a large piece of metal going through the detector, like a spanner for example.

It seems odd that we’d check that the machine can see a large piece of metal, when we’re already checking the machine with smaller pieces of metal – surely, it’s obvious it’s going to reject?  Well yes, you’re right! But this is where this test is very often misunderstood… 

When a large piece of metal goes through the detector, it causes such as large signal, that this can sometimes send the machine haywire!  This means, that it will reject the large piece of metal, but once that’s gone through the machine may not work as it should anymore.  Which then means, that if a product was to contain metal and go through the machine, it may not be detected and rejected.

So, the key part of this test is that the machine not only sees and rejects the large piece of metal, but actually the most important check – is that it will still see and reject your normal 3 test pieces afterwards.

Passing the 3 test pieces through the machine after this test, is the part that’s very commonly forgotten.  Make sure your procedure is to do this, because otherwise putting a large piece of metal through the machine on its own could cause the machine to break and you wouldn’t even know it!

As always, please add your thoughts to the comments box below – I’d love to hear them.   Please, if you are experienced in this topic too, please add any points that others can learn from.

13 Comments

  • Dilip Kumar Chowdhury says:

    We are BRC Certified shrimp processing plant. So we need BRC news letter.

    • Kassy Marsh says:

      Hi, thank you for your comment, we will be writing about BRC V8 in the forthcoming articles – so I think these should help you. Thanks, Kassy

  • Nikki says:

    This is really helpful. I have never really know what is meant by the memory test and have never even considered the effect a large piece of metal could have on the units! I think a rewrite of the metal detection procedure is now on the cards!

  • Paul Taylor says:

    Surely a saturation test gives a better idea of if your metal detector is working correctly along with this more than the annual test carried out by companies employed to come in and do exactly what operators activities are showing every hour/half our.
    also seeing if the detectors can be challenged to detect smaller particulates below the thresh hold of the current test pieces.

  • Kate says:

    Hi!What I need to do if during my large piece test I have 2 or 3 standard packs rejected which follows after pack with large piece?Count them as metal rejects?

    • Kassy Marsh says:

      Hi,
      Thanks for your question!
      You need to do your 3 piece test (packs with test pieces in) after the large metal piece – to prove that the machine is still working. If it starts rejecting standard packs then there is something wrong with it and it needs attention.
      Thanks
      Kassy

  • Julie-Ann Liggett says:

    Hi Kassy
    There are 2 customer codes of practice which specify the order the test pieces should be in- s/s last in trailing position which contradicts another customer COP which states it should be in the middle pack in the centre. What would your advice be on this? Does one challenge the system more effectively?
    Many thanks in advance
    Julie-Ann

    • Kassy Marsh says:

      Hi, I’m not aware of any reason why testing the machine in a particular order would make any difference (other than s/s is the most difficult to ‘see’). If you could email me the COPs you’re talking about, I can look into it further for you.
      Thanks
      Kassy

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