<here's what I'm basing my answer on:
When you test conventional metal detection systems, you need to use both ferrous and nonferrous test packs. These should be made up from packs that are proven to be free from metal,
and be clearly marked and labelled so they cannot be packed inadvertently for despatch.
You need to make up fresh test packs at a frequency that reflects the nature, durability and shelflife of the product concerned. If you use "stale" test packs, they will not reflect the same properties as the products which the metal detector is inspecting.
In the case of Loma Systems metal detectors, your system comes with a ready supply of plastic "wands" which contain pieces of various metals in the specified test sizes. These are convenient
and easy to place into your test packs, and also make the testing process even more efficient.
When you are testing finished packed products on a conveyor system, place the test piece of metal, where possible, at the extreme end of the pack. If this is impractical - e.g. when you are
testing individual small packs or sandwich wedges - place the test piece of metal in the centre of the product.
Next, pass both ferrous and non-ferrous test packed individually through the search head twice - first with the metal test piece at the leading edge of the pack, and then with the test piece at the
trailing edge. In each case, you must observe that the test piece successfully enters the reject bin.
http://www.loma.com/docs/Guide_to_Metal_Detection.pdf