Metal
‘Watchers’
With regulatory bodies recommending inspection of food products, metal
detectors are fast catching up as safety bugs to reduce metal contamination,
updates Andrew Lock
Metal
detectors are now accepted as the most essential equipment by most food
and pharmaceutical processors as many companies have been laying down
strict inspection standards in terms of detector sensitivity. However,
for metal detectors to be efficient, they need to form part of an effective
overall metal detection program.
Why Metal Detectors are installed?
• To prevent damage to processing equipment
• To comply with stringent quality standards by major customers
including high volume retailers, fast food chains, food service and
vendor certification programs
• To avoid the cost and implications of consumer complaints, adverse
publicity, product recall and litigation
• To win new markets and customers with high quality products
• To comply with legislation such as 'Due Diligence' and FDA/U5DA
Directives.
Basic Principles
The most common types of metallic contamination
in a broad range of industries include ferrous (iron), copper, aluminum,
lead and various types of stainless steel. Of these, ferrous metal is
the easiest to detect, while stainless steel is the most difficult.
The nonferrous metals such as copper and lead fall between these two
extremes. Only metal detectors using a balanced threecoil system have
the ability to detect small particles of non-ferrous and stainless steel.
The three coils are wound on a non-metallic frame or former, each exactly
parallel with the other. The centre coil is connected to a high frequency
radio transmitter. The two coils each side of the centre coil act as
radio receivers or aerials. As these two coils are identical and the
same distance from the transmitter, they pick up the same signal and
an identical voltage is induced in each. When the coils are connected
in opposition, they cancel out resulting in zero output. When a particle
of metal passes through the coil arrangement, the high frequency field
is disturbed under one coil, changing the voltage generated by a few
microvolts. The state of perfect balance is lost and the output no longer
zero. The resulting signal is processed and amplified. It is this phenomenon,
which is used to detect the presence of unwanted metal. Additionally,
to prevent airborne electrical signals, or nearby metal items and machinery
disturbing the detector, the complete coil arrangement is mounted inside
a metal case with a hole in the centre to allow the passage of product.
Aluminum is normally used for the case, but on some applications where
frequent wash down is required, stainless steel may be used.
Mechanical Techniques
Metal detector need to be totally rigid and stable system, unaffected
by vibration from motors, temperature changes, and transportation and
close-by machinery. Thus, the selection of former material, coil specifications,
and case design, are crucial. To increase mechanical rigidity further,
some manufacturers put the detector with a material to prevent relative
movement of the metal case to the coils. Electronic Techniques Temperature
changes, build up of product in the aperture, aging of electric components
...
....C
O N T D
TO
READ FURTHER... SUBSCRIBE TO YOUR
COPY TODAY!!!
