Don't just measure one item
Many buyers size a needle detector or metal detector by measuring the thickness of a single product and adding a small margin. In real production, products are often stacked, piled up or bounce slightly on the conveyor, so the height they actually occupy is much greater than one item. If the aperture is too tight, products rub against the channel walls, which affects conveying stability and can push products away from the center of the detection zone, weakening sensitivity performance.
Sensitivity varies inside
Sensitivity is not uniform across the detection channel; it is usually weaker near the edges and the upper and lower openings. When calculating aperture height, base it on the maximum outline height of products in their actual passing state, then allow for stacking layers, bulging packaging and conveyor fluctuation, and keep a reasonable margin. An excessive margin is also unfavorable: the larger the channel space, the relatively weaker the signal from small metal contaminants, so a balance must be struck according to product characteristics.
Describe the real passing state
When talking to suppliers, provide the single-item dimensions, stacking method, packaging form and conveyor operating conditions together, so the manufacturer can recommend a channel specification. After the equipment arrives, verify passability with actual products, check whether products stay centered and whether there is rubbing, then confirm sensitivity with test samples. Getting the aperture size right makes later commissioning and maintenance much easier.