Dealing with different
product densities
The performance of food inspection conductivity. Indicators for stainless steel, it may not detect the sample in the same
machines will be affected by product on the other hand, can easily be swamped way it would when landing on a higher
characteristics and density as they pass by product effect. density raisin.”
Director of Fortress Technology and Sparc With an x-ray machine, light waves pass Similarly, there will be a different
Systems Phil Brown examines x-ray and through the product to be inspected. response if an odd shaped piece of metal
metal detectors to explain why. Because of this, products of differing passes through a machine in different
densities impact an x-ray’s performance. directions. This is why spheres are used
There are two distinct components to To put this into context, Phil uses a box to test a metal detector – it reflects the
metal detection: magnetic permeability of cereal as an example “Although an same amount of signal no matter where
and conductivity. Most food products x-ray machine will be calibrated for the it’s pointing. Yet, flatten out the metal or
will leave a ‘fingerprint’ that combines product’s total density, cereal ingredients, roll it into a needle or wire shape, there’ll
elements of both, though the overriding raisins and bran for example, have be a significant difference in signal,
component is likely to be conductivity. variable densities. Speckles of raisin will depending on how it passes through. This
Metals also exhibit telltale combinations be quite high, the bran low and the box is due to the physics of how it’s breaking
of both types of effect. For example, they’re in, the same density. As the x-ray the lines in the field. The general rule is
the majority of ferrous metals display is measuring the collective density, if ‘if any of the dimensions are less than
stronger magnetic permeability, while there’s a contaminant in the box and the the detectable metal’s sphere size, the
non-ferrous metals will signal higher machine lands on a low density bran flake, machine may have trouble detecting it’.
through the aperture. European Managing
28 FDPP - www.fdpp.co.uk