68
MAY 2017
FOOD FOCUSTHAILAND
STAND OUT
TECHNOLOGY
Wet productsexhibit somethingcalleda ‘product effect.’Thismeans
that an element of the product is capable of conducting electricity and
creatingamagneticdisturbancewithin theapertureof themetaldetector.
Aproduct packed in brine is a good example. Salt water is a good
conductor of electricity. Luckily it’snot aseasilymagnetisedas iron– in
technical language, it has lowmagnetic ‘permeability’ – which means
that themagneticeffect ismuch less than that generatedwithinapiece
of iron. But metal detectors are highly sensitive items of equipment
designed todetect tinypiecesofmetal inabigproduct. If the volumeof
saltwater is largeenough, itwill createamagneticeffect that’sasstrong
as that createdbyasmall pieceofmetal.That’swhen theproduct effect
becomesaproblem. Itmeans that theproduct itself can createasbiga
disturbance in themagneticfieldaswouldbecreatedbyaflakeofmetal
contaminant.
In practical terms, the product effect reduces inspection sensitivity
because the metal detector cannot distinguish between the magnetic
effects caused by the actual product and those caused by metal
contaminantssmaller thanacritical size. For production linemanagers,
it’sadilemma:do theyaccept reducedsensitivityand let relatively larger
contaminants through undetected (compromising consumer safety) or
maintain sensitivity and live with a metal detector that registers more
false rejects (compromisingOEE (Overall Equipment Effectiveness))?
Awide rangeof productsexhibit theproduct effect, includingbread,
dairyproducts, freshmeat, poultryand fish. Inmany cases, theproduct
effect can vary from item to item. Products that have beenmarinated,
for example, are not identical, even when they weigh the same. The
proportionofmeat tomarinade can vary fromone item to thenext. This
makes it even harder for the software in ametal detector to determine
whether amagnetic signal is the result of a contaminant or an above-
average volume of marinade.
Other Factors that Create aProduct Effect
Temperature:
The conductivity of a product varies with its
temperature. If theambient temperature inaproductionunit varies, the
product effect can vary too.
When frozenproductsareexposed towarmdampair, condensation
formson theouter packagingwhichcreatesaproduct effect.Defrosting
couldpotentiallyconvertadryproduct intoawetone.When frozenmeat
(adryproduct) thaws,
it becomes a wet
product.
An even greater
effect is the way that
c h a n g e s i n
temperature alter the
phase angle of the
voltage in the metal
detector’s receive
coil. A change of just 5°C will create a shift in phase angle that’s
significant enough tomake detectionof contaminantsmuchharder.
Size and Shape of the Products:
Variations in the size and
shapeofproducts, theirdensity,and theirpositionororientationwithin
the metal detector also affect detector sensitivity. A chicken that
passes through the aperture lengthways presents a much smaller
cross-section than one that passes through sideways. From the
viewpoint of the metal detector, the lengthways chicken appears
smaller than the sideways one.
PackagingMaterials:
Mostnon-metallicpackagingmaterialsdo
not createaproduct effect.Theexception ismetallisedfilm.The thin
layer (roughly half amicron) of aluminium on the surface of the film
is enough to generate eddy currents and disturb themagnetic field.
Historically, ifyouwerepackingproduct inmetallisedfilm itwasoften
best to inspect theproduct prior topackagingaswas thenorm in the
snack food industry with the use of “throat” type metal detectors,
howevernowwith recentdevelopments inmetaldetection technology
inspection in the final pack is a real alternative as sensitivity levels
aremuchcloser aligned to thoseobtainedwhen inspectingproducts
in plain “nonmetallised” film.
In conclusion, Productswith high product effect aremuchmore
challengingand the choiceofmetal detector and themetal detector
supplier ismuchmorecomplex.Forsuchapplications,we recommend
that you consult with an expert supplier with the technology and
service support to take thisworry away.
Agoodmetal detectorsuppliershouldofferawide rangeofmetal
detectors for every application, sowhen choosing ametal detector,
be sure to assess the supplier equally carefully
The
“ProductEffect”
ReducestheSensitivityof
TraditionalMetalDetectors
Metal detectors have traditionally worked best on what are
known as dry products. Dry products aren’t necessarily
completelymoisture-free–although theydo typicallyhave low
levels of moisture. ‘Dry’ is a catch-all term for food and
pharmaceutical products that respondwell to inspection by a
metal detector. They’re easy products to work with because
they don’t generate false or ambiguous signals.
