Loma IQ2 Service Manual page 70

Sensitivity
The sensitivity of a detector is normally specified as the smallest diameter of
sphere that can be detected when passing through the centre of the aperture
(the least sensitive area). Sensitivity varies with aperture size, so siting the
detector to give minimum aperture height is an important factor in obtaining
good performance. For instance, a detector required for large cartons will give
poor sensitivity when compared to one designed for inspecting individual small
packs.
A sensitivity of 1mm (ferrous and non-ferrous) is normally possible per 100mm
of aperture height (or width if smallest) for most dry applications.
For other applications where product effect may be a limitation the achievable
sensitivity that is possible depends greatly upon the choice of operating mode
and operating frequency.
Shape and Orientation
So far we have been referring to the detection of spherical samples. These are
commonly used as a standard for testing and comparing the performance of
metal detectors because they present a regular shape to the aperture
irrespective of their orientation.
With irregular shapes the detectability will vary greatly with orientation since the
sample will present a different cross-section to the magnetic field in the search
head according to its orientation. Furthermore, these effects differ in relation to
non-ferrous metals compared with ferrous metals.
For magnetic materials the effect is related more to bulk, but with non-ferrous
metals it is the area of the magnetic field interrupted by the cross-section of the
material that matters. Thus a long thin non-ferrous wire lying along the axis of
travel has little more effect than a sphere of the same diameter.
Placed at right-angles to the direction of motion, the same wire will be much
more detectable, since it is now presenting a much larger cross-section. The
same applies to small scraps of aluminium foil. Lying flat, the cross-sectional
area presented to the field is minimal and a small piece may pass undetected.
Face on, however, the same small piece may give an enormous signal.
To overcome these problems Loma offer differently constructed units, such as
cross field and twin angled systems, where necessary.
Module 7026
Configuration and Set up Information
5-48