The standard technique for measuring the sensitivity of food inspection metal detectors is to use metal test spheres
Although this type of spherical testing tool is typically expressed by diameter in millimetres, in the real world metal contaminants are more likely to be non-spherical or an irregular shape. Phil Brown, Sales Director at Fortress Technology explains why spheres are used and why a size detection improvement as little as 0.5mm can dramatically cut the risk of metal fragments entering the production chain.   
“The rationale for using spheres to ascertain the system’s sensitivity level is because they are the same shape from every aspect when passing through the metal detector,” explains Phil. “Realistically, metal contaminants are more likely to be a piece of swarf, metal shard or narrow wire than a perfectly formed ball.
“Furthermore, the signal produced from a wire shape will vary greatly depending on the type of metal it is and on its angle when it passes through the detector. This is known as orientation effect. For example, a stainless steel wire that passes through the aperture upright or sideways generates a higher signal than a straight.  In the worst case a wire may produce a signal no bigger than a sphere of the same size as the diameter of the wire.”