Sartorius Intec explains detection sensitivity in freefall metal detectors

There are no uniform standards for performance validation of freefall metal detection systems so decisions regarding the machine of choice and accuracy of its performance validation, are made on a case-by-case basis by users and the quality management team.

In food production, reliable separation of foreign objects from the product flow must be shown via replicable validation checks, said the firm.

Factors influencing the result from metal detectors include non-homogeneous detection sensitivity within their search coils. This is due to the field distribution in the detector tunnel: the sensitivity decreases as the distance from the edge of the search tunnel increases.

Sartorius Intec said it had proven this through measurements taken by its product development team.

Elmar Oberdörffer, head of R&D detection at the firm, said: "If a test sphere representative of maximum detection sensitivity, made from AISI 304 (Stainless steel) for example, is guided along the edge of the search coil, the signal could quite easily be 2.5 times greater than in the centre."​

The least sensitive position is in the centre of the outlet, which is in the centre of the tube for a free fall metal detector. So, the metal piece to be checked should be guided through the centre of the search coil opening as any other position bears a high risk of producing false detection sensitivity results.

For reproducibility of test results, spheres should be used as detection sensitivity test pieces, said the firm.

Due to their radial symmetry, spheres produce a signal which depends on their position within the search coil but not on direction. The carrier material of the test sphere must not produce any significant false signal and as long as this is ensured, plastic cards, spheres, rods, tablets etc. can be used.

Sartorius Intec said tests in which the test piece is guided into the side of the search coil opening yields less reliable information than where it is introduced in the middle of the tube.