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Metal detection - Where is the least sensitive place?


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#1 GMO

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Posted 15 February 2011 - 08:06 PM

Ok, would you accept this...

The centre of a metal detection aperture is the least sensitive place but also some products may not fill the aperture being too "low". I've always thought the best place for a test stick is in the product as it mimics a contaminant but, in this case is this fair? Would the worst case scenario be on the product and so it's better to tape the stick to the top? But this is a case which will never occur so is it "right"?



#2 Madam A. D-tor

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Posted 15 February 2011 - 10:51 PM

Dear GMO,

I have always understood that it does not matter of the test piece is in the product or on the product.
I have, somewhere, a very good guide from MT. Unfortunately, I can not find it. (it is printed copy, I received at a symposium)

I just saw that you can order a free exemplare on their site.


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#3 Charles.C

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Posted 16 February 2011 - 01:13 AM

Dear GMO,

Try this thread for some related comments. Ultimately inconclusive though IMO. :smile:

http://www.ifsqn.com...dpost__p__18012

Rgds / Charles.C


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#4 Jason H.Z.C.

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Posted 16 February 2011 - 09:22 AM

Dear GMO,

Did you do the test and successfully verified that a sample with a metal test piece inside go through the MD but no alerming occure; meanwhile when position the metal test piece at the top of such sample triggered the alerming system?

If in your test this is the repeatable situation, I guess it is either a systematic erro of the MD, or a new milesone discovered by you :whistle:

If you did meet this in your internal test, it is propsed you to write a great thesis and make it public :smarty:

Ha, just a small proposal.

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Jason


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Jason

#5 GMO

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Posted 16 February 2011 - 10:08 AM

Always good to be referred back to a thread you'd heavily contributed to! Brain has obviously gone to sleep. Posted Image

So I'll ask it in a different way. If you were auditing and a site only put the stick on the top of the product, would you accept that? If so would you only accept that if it was the centre of the aperture or accept it in any circumstances?

I remember one site for example with 20kg blocks of cheddar who used to put the stick on the top, nowhere near the centre of the aperture but it was impractical to put it in the centre. My advice was to buy an x-ray where it's less important as it "sees" in 2D but not an instant or cheap solution.



#6 Madam A. D-tor

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Posted 16 February 2011 - 10:07 PM

So I'll ask it in a different way. If you were auditing and a site only put the stick on the top of the product, would you accept that? If so would you only accept that if it was the centre of the aperture or accept it in any circumstances?


Yes, I accept the test piece on top of the product.
I do not accept testing with only the metal test piece (without product)
And I do not accept test packagings. An empty packaging or filled with paper, PU foam or salt, with the test piece exactly in the middle of this packaging. (unless the finished produt is empty packaging, paper, PU foam or salt)
Kind Regards,

Madam A. D-tor

#7 Charles.C

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Posted 16 February 2011 - 11:30 PM

Dear Madam A.D-tor,

And I do not accept test packagings


What do you mean by "test packagings" ?,eg embedding a metal reference within a product block ? I thought this is a quite common setup since, for example, I believe glaze on frozen items can reduce sensitivity. Added - or was it induce false positives :unsure: ?

Rgds / Charles.C

added - after reading next post, perhaps i interpreted previous post in reverse way to intended. :smile:

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#8 Jason H.Z.C.

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Posted 17 February 2011 - 03:55 AM

Dear all,
:rolleyes:
Well, by reading the theority papers, I think putting a test piece at the top of a packed product is feasible if the height of the product is not excess the centre of the aparture.

However, I am trying to guess what GMO worrying is whether the filled/packed products could block and reduce the magnetic or electric conductivity of the test piece once it is embadded into the materials. If so, the level of magnetic/conductivity from test piece might be decreased to a smaller signal and finally could not be detected by the metal detector? If I understand towards a wrong direction, please pull me back, GMO.

So based on the above guessing, I tried to ask you if you conducted a test to demonstrate the existing of this possibility. If it does exist, at least I should consider the re-position of the test piece inside the packed materials.(Maybe my asking manner is not so well due to my poor English level :doh: )

If you are thinking some materials have the properties including conductivity/magnetic themselves and might trigger the allerming system eventhough no test piece installed in it, I propose that before doing the calibration test, we could use an end-product ascertained not combined with a test piece to test the possibility of mis-alerming of the MD. Then we could add the test piece.

About the views of Madam A D-tor referred to " not accept test packaging", I guess she insist on the combination of "test piece" with "tested materials". Here tested material means material which has been filled/packed into the primary/secondary packages. And the empty packages with a test piece inside is not accepted.

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Jason


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Jason

#9 Rex

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Posted 17 February 2011 - 11:29 AM

Always good to be referred back to a thread you'd heavily contributed to! Brain has obviously gone to sleep. Posted Image

So I'll ask it in a different way. If you were auditing and a site only put the stick on the top of the product, would you accept that? If so would you only accept that if it was the centre of the aperture or accept it in any circumstances?

I remember one site for example with 20kg blocks of cheddar who used to put the stick on the top, nowhere near the centre of the aperture but it was impractical to put it in the centre. My advice was to buy an x-ray where it's less important as it "sees" in 2D but not an instant or cheap solution.



Hi GMO
For the metal detector, when a metal passed through it, electromagnetic field of metal detector will be distrubed, center of the metal detector was lowest sensitive. and sensitivity of metal detector is inversely proportional to the center high.
I think that the size of the samples are more important. what size would you acceptable? Sensitiveness of the metal detector could be adjusted. But higher sensitiveness will promote mis-alarming.
So acceptable level of the samples size shall be take into account according to risk of the potential metal foreign body hazard.
General, three type samples were used (Fe, No-Fe, SUS) in the factory, when you adjust the sensitiveness of the metal detector, For the best sensitiveness of the MD, balance point shall be found, because three samples weren't always rejected by the MD in the highest sensitiveness!

#10 mgourley

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Posted 17 February 2011 - 03:53 PM

In bakeries (bread, buns, rolls) here in the USA, I believe it's pretty much an industry standard that the test wand is placed inside the test product and the test product is placed on the conveyor so that is passes through the metal detector in the normal "flow" of the product.



#11 Madam A. D-tor

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Posted 17 February 2011 - 10:52 PM

In bakeries (bread, buns, rolls) here in the USA, I believe it's pretty much an industry standard that the test wand is placed inside the test product and the test product is placed on the conveyor so that is passes through the metal detector in the normal "flow" of the product.


Hello mgourley,

I think this practice is the right way to test metal detector for bakery products. These products are not expensive and can be easy opened to cotaminate with the test piece.

Dear Madam A.D-tor,
What do you mean by "test packagings" ?


Dear Charles,

With a test packagings (I suppose this is incorrect English), I mean a simulate product, which is not appropriate. For example: A company producing meal salads and ready to cook meals (combination of cooked pasta, fresh vegetables, fresh marinated meat and sauce), used a product packaging which was filled with the stuff on the picture (i can not find a good english translation), with a test piece in it. The size of this test packaging was identical to the product produced, however the material meals or foam are not compareable. Also seen packagings filled with only a test piece and no product and packaging filled with dry uncooked rice and test piece, where they were producing fresh soups.

Attached Files


Kind Regards,

Madam A. D-tor

#12 GMO

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Posted 18 February 2011 - 09:33 AM

Hello mgourley,

I think this practice is the right way to test metal detector for bakery products. These products are not expensive and can be easy opened to cotaminate with the test piece.


Yes but say that the pack is low and so the top of the product would be nearer the aperture centre?

#13 mgourley

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Posted 19 February 2011 - 12:46 AM

Yes but say that the pack is low and so the top of the product would be nearer the aperture centre?


I'm not sure I completely follow the question.
I would assume that your metal detection equipment would be specified for the type of product you are producing.
I'm not an engineer, but again I would assume that if the package or product that passes through the detection equipment is roughly X by Y by Z in size, your detection equipment would be calibrated to be most sensitive for that size of product or package.
Almost every newer metal detector that I know of can be set up for multiple products based upon size, density, etc.
It's then simply a matter of calibration for each product (or group of products) and then having the operator or supervisor change the pre-sets on the detector to match the product (or group of products) that are run through it.

Marshall

#14 Charles.C

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Posted 19 February 2011 - 05:39 AM

Dear All,

Looks like this thread is similar to previous, ie plenty of confusion. Part of the problem is the number of possible variables, eg package contents-shape-presentation / test piece shape / magnetic array shape / other geometrical aspects / false positives. Plus the inability to draw pictures in a thread post.

Objective : To ensure that the occurrence of a defined( eg 3D shape / sized metallic object) contaminant “in” a defined product unit (eg 3D s/s package) is “adequately” detected when occurring at a position within / at the surface of forementioned package such that the likelihood of detection is a minimum. ( presumably it may be required to establish whether placement on the surface gives a significantly different result as compared to being “in” the surface.) From memory the theory ideally prefers to (initially) nominate the position to be a point location (as per usual ideal).

So the question is apparently something like – assumptions - (a) that the geometry of the detector/conveyor configuration is fixed and in the conventional array style, (b) a uniform rectangular box is involved with its long line parallel to the belt flow and placed symmetrically about the centre of belt. Then - where is the point (see 1st para.) of minimum likelihood of contaminant detection, ie minimum sensitivity of detector. Answer from memory is at some point on a hypothetical line within the product in box/in the surface of product within box / parallel to conveyor belt and which achieves closest approach to analogous line through geometrical center of coil array. Don’t remember if any significance as to where the point is along this line, eg near end which enters field first or last or in the middle (I recall this aspect was discussed in published (Tesco?) test procedure in previous thread) ?

And therefore, as GMO initially postulated, for the “low” box scenario, the desired line is presumably “in” the top surface of box, ie at the intersection of this surface and a vertical plane which passes through the box’s geometrical center and is parallel to long box axis. Or maybe (to be validated) can be “on” the box top surface also.
And the easiest way to validate is presumably to simply test, ideally perhaps initially with a spherical standard size object of required diameter, eg a ball bearing, but most commercial test pieces seem to be strips, eg wands. Yet another variable to take into account.
And as already mentioned, the available detector strength may be limited by possibility of false positives.

Now even more confused ? :biggrin:

Rgds / Charles.C


Kind Regards,

 

Charles.C


#15 GMO

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Posted 09 March 2011 - 01:54 PM

Dear All,

Looks like this thread is similar to previous, ie plenty of confusion. Part of the problem is the number of possible variables, eg package contents-shape-presentation / test piece shape / magnetic array shape / other geometrical aspects / false positives. Plus the inability to draw pictures in a thread post.

Objective : To ensure that the occurrence of a defined( eg 3D shape / sized metallic object) contaminant “in” a defined product unit (eg 3D s/s package) is “adequately” detected when occurring at a position within / at the surface of forementioned package such that the likelihood of detection is a minimum. ( presumably it may be required to establish whether placement on the surface gives a significantly different result as compared to being “in” the surface.) From memory the theory ideally prefers to (initially) nominate the position to be a point location (as per usual ideal).

So the question is apparently something like – assumptions - (a) that the geometry of the detector/conveyor configuration is fixed and in the conventional array style, (b) a uniform rectangular box is involved with its long line parallel to the belt flow and placed symmetrically about the centre of belt. Then - where is the point (see 1st para.) of minimum likelihood of contaminant detection, ie minimum sensitivity of detector. Answer from memory is at some point on a hypothetical line within the product in box/in the surface of product within box / parallel to conveyor belt and which achieves closest approach to analogous line through geometrical center of coil array. Don’t remember if any significance as to where the point is along this line, eg near end which enters field first or last or in the middle (I recall this aspect was discussed in published (Tesco?) test procedure in previous thread) ?

And therefore, as GMO initially postulated, for the “low” box scenario, the desired line is presumably “in” the top surface of box, ie at the intersection of this surface and a vertical plane which passes through the box’s geometrical center and is parallel to long box axis. Or maybe (to be validated) can be “on” the box top surface also.
And the easiest way to validate is presumably to simply test, ideally perhaps initially with a spherical standard size object of required diameter, eg a ball bearing, but most commercial test pieces seem to be strips, eg wands. Yet another variable to take into account.
And as already mentioned, the available detector strength may be limited by possibility of false positives.

Now even more confused ? :biggrin:

Rgds / Charles.C


Perhaps the answer is to use the x-ray cards for this situation? Or am I being to pedantic and as long as it's validated, does it matter?




#16 Jules

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Posted 16 March 2011 - 07:57 AM

Sorry folks, found the information but lost the topic. The position of least sensitivity in a metal detector is at half the height of the apperture. Measure your apperture height, halve that number and that gives you the height of least sensitivity. That is the approximate height that the metal detector test pieces should be placed when checking the detector sensitivity. Then take an opinion on the product height, if it is taller than the mid way point, make up test packs that have the test pieces inserted to the correct height. MARK UP YOUR TEST PACKS WELL AND ENSURE THEY ARE MAINTAINED AT THE SAME TEMPERATURE AS THE PRODUCT. If your product pack is shorter than the mid point, open a pack and place the test piece on top of the product. You may choose to do this for each test.

Hope this is helpful.

BTW go to your customer requirements regarding test packs, they may want one made each test you conduct and may want to dispose of the product inside once used.


Kind Regards

Julie

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#17 Charles.C

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Posted 16 March 2011 - 12:33 PM

Dear Julie,

Sorry folks, found the information but lost the topic. The position of least sensitivity in a metal detector is at half the height of the apperture. Measure your apperture height, halve that number and that gives you the height of least sensitivity. That is the approximate height that the metal detector test pieces should be placed when checking the detector sensitivity.


Yes, this was the starting assumption (see post#1) for the previous posts. Actually there is another forum thread on the basic theory (including technical links) but, from memory, the recommended "best" handling of mismatched practical situations as introduced here by GMO was confused / confusing. Presumably a variable aperture (or conveyor belt) height would be nice. :smile:
( added - http://www.ifsqn.com...dpost__p__18012 }

Then take an opinion on the product height, if it is taller than the mid way point, make up test packs that have the test pieces inserted to the correct height. MARK UP YOUR TEST PACKS WELL AND ENSURE THEY ARE MAINTAINED AT THE SAME TEMPERATURE AS THE PRODUCT. If your product pack is shorter than the mid point, open a pack and place the test piece on top of the product.


Yes, this was the (sort of)conclusion from the preceding posts.

The question is - can you validate (technically and in practice) the previous conclusion ?? That would be interesting since, so far, a definitive link has IMO proven elusive.

Rgds / Charles.C

Kind Regards,

 

Charles.C


#18 Brian Meek

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Posted 22 June 2012 - 10:45 AM

To test a metal detector:

1 Ensure that the test is worst case, test sample should be placed as near to centre of the aperture as possible.
2 Use actual product and not dummy packs, make up a new pack from each new batch.
3 Place the test packs in the actual flow of running product.
4 Test at front, middle and rear of the pack to eliminate product effect from the pack.
5 Ensure that each pack is rejected each time.

Use this guide and you wont go far wrong.

Kind regards

Brian






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