In my situation I have frozen product in laminated bags. And my Metal Detector head has an aperture size of 450mm wide x 250 mm high.
Mu supplier recommendation is 2.5-3.0mm Non Ferrous. I read somewhere that there is a relation between Aperture size to Sensitivity limit. It says one you have more than 200mm high, then the recommended sensitivity should be 3.0 for Non Ferrous.
I agree with Snookie, can be a real PITA.
There seem to be 2-3 issues –
(1) Choice of wand-sphere diameter with respect to sensitivity.
(2) Understand the auditor’s method of checking
Plus perhaps one Canadian aspect –
(3) AFAIK, Canada defines significant metal hazards as >2mm. whether this is relevant to yr MD situation I do not know. I assume (hope) it is not.
I am in no way a MD expert but here is one suggested solution (IMO).
I assume 3.0mm for non-Fe fully works for you ?
Re – (1)
AFAIK, yr product will be categorised as wet.
You are using a substantial aperture size (250mm).
Both of the above reduce the sensitivity.
Assuming the above, numerous publications / tables will support yr use of 3.0mm non-ferrous wand, for example –
met1, Tesco.doc 631.5KB
Re – (2)
Again there appear to be 2 issues –
(a) front and back
(b) above and below
Regarding (a). The reason for front/back does not seem to be explained in either the Tesco or Safefood white paper as far as I could see. (The Mettler 60+ page document mentioned by Caz appears only available by post (!?) but looks impressive from the on-line preview). Perhaps the b/f logic is supposed to be obvious but it wasn't to me.
Here is a previously contributed explanation (credit Alan Johnson) which at the time seemed plausible to me. However I hv yet to see any further explanations to compare –
In terms of placing metal test wand at the front and rear of the pack, most UK retailer codes of practice recommend this in one form or another. Primarily it is because a metal detector on its own does not know the position of the test wand in the pack. It can only sense (not very accurately) the position of the metal sphere as it passes through the coils. To ensure accurate rejection of the product on a conveyor based system there normally has to be a photo eye that senses the leading edge of the pack and this is wired so that the pack is positively rejected wherever the test wand is positioned. Metal detectors have been known not to reject positively without photo eyes or if the reject timing has been set badly, so the test is worse case. There are even additional test routines that check the functionality of these additional failsafe devices at the start of a production shift
Regarding (b) the typical response with height is theoretically something like –
I assume the curve shown is (in total) "symmetrical" so if MD detects OK in the middle, should also work above and below but not necessarily vice-versa. Hence the methodology suggested in most texts to test close to the centre-line.
This is all approximate logic due sensitivity is governed by a host of factors additional to the above, eg as stated in the previous attachment –
Product effect, metal free area, type and orientation of contaminant and other factors can affect the practical sensitivity in any application.
Accordingly the above suggestions are welcome to be criticised by any experts around (or non-experts for that matter ).
Rgds / Charles.C