Why Detection Sensitivity Falls as the Aperture Gets Bigger

The relationship is physical, not commercial

The further a contaminant sits from the detection coil, the weaker its signal. Open the aperture and the centre of the opening moves further from the coil on every side, so the smallest detectable object gets larger. This is why any honest needle detector publishes a different figure at every working height, and why a supplier who quotes one sensitivity figure regardless of aperture is not telling you the whole story.

A published example

The HKN-800D needle detector states its capability against height directly: Fe 0.8 mm at a 100 mm working height, Fe 0.8 mm at 120 mm, Fe 1.0 mm at 150 mm, Fe 1.2 mm at 200 mm and Fe 1.5 mm at 250 mm. Nearly doubling the height roughly doubles the smallest detectable fragment. Nothing is wrong with the machine; that is what the geometry does.

The practical consequence

Specify the smallest aperture your product will actually pass through with reliable clearance. Every extra centimetre of unused height costs you sensitivity you have already paid for. Buyers frequently over-specify aperture as a safety margin and then discover their detection limit is worse than the datasheet implied.

Product effect compounds it

Published sensitivity figures are measured on an empty machine. Add wet, salty or frozen product and the product's own signal reduces real sensitivity further. Between the aperture and the product effect, the figure you achieve on a loaded line is meaningfully worse than the datasheet number, and that is normal rather than a fault.

What to do about it

Size the aperture to the pack. Ask for the sensitivity figure at your working height, not the best figure in the range. Get test pieces supplied with the machine and verify at commissioning, then at the interval your quality plan sets. A verification that uses the correct test piece at the correct height is the only figure that means anything.