False rejects — seven usual causes
False rejects are not merely irritating. They are dangerous, because they always end the same way: somebody turns the sensitivity down so the line can keep running. At that point the detector is no longer protecting what it was installed to protect.
These are the causes we find most often when we go out, in roughly the order of likelihood.
1. Product effect
By far the most common. Wet, salted, warm or acidic product conducts and therefore produces its own signal in the detector's field. The detector cannot distinguish it from metal.
Signs: rejects occur on specific products, at specific temperatures, or after a product change. Fix: a correctly learned product programme for each variant, and a detector with adequate product effect compensation. Always change programme at product change — one programme for everything is among the most widespread errors in the industry.
2. Temperature and moisture changes in the product
The same product frozen and thawed behaves differently. So does a product straight out of the oven. If both states run on one programme, one of them will produce false rejects.
Fix: separate programmes, learned on the product in the state it is actually in when it passes the detector.
3. Moisture in the enclosure
After a few years of washdown, moisture works in through seals and cable glands. It shows up as instability and random rejects long before it shows up as an actual breakdown.
Signs: rejects start after cleaning and diminish through the day. Fix: inspect seals and glands — and in the longer term, a detector with an ingress rating that matches your cleaning regime. IP69K is the class that covers high-pressure washdown with hot water.
4. Vibration and mechanical movement
The coil is sensitive to movement within its own field. A worn roller bearing, a belt that slaps, a pump starting nearby, or a detector mounted on a frame that is not stiff enough — all of them produce signals without any metal present.
Signs: rejects coincide with specific machines starting and stopping. Fix: mechanical inspection of the conveyor and of how the detector is mounted.
5. Metal too close
Moving metal in or near the field — a stainless chute, a handrail, a chain, a forklift passing close — affects the field. So does fixed metal that has come closer after a modification.
Signs: the problem started after a change to the line. Fix: respect the metal-free zone around the detector. If it is not respected, no amount of adjustment will help.
6. Earthing and electrical noise
Variable-frequency drives, welding, failing fluorescent fittings and missing or poor earthing all produce electrical noise the detector picks up.
Signs: rejects in clusters with no relation to the product, often when particular equipment runs. Fix: check earthing and cable screening. It is often a cheap fault to correct once it has been found.
7. Wrong or outdated programme learning
The programme was learned three years ago. Since then the raw material changed, the salt content was adjusted, the packaging was altered or the belt was replaced. The programme no longer matches the product.
Fix: re-learn programmes when the product changes — and as a routine part of periodic calibration.
What not to do: turn the sensitivity down until the rejects stop. That treats the symptom and removes the protection. If the critical limit in your HACCP plan says 2.0 mm stainless and the detector is actually running at 3.5 mm to avoid false rejects, the control point is out of control regardless of what the paperwork says.
Cannot find the cause?
We do fault finding on metal detectors, including makes we did not build. We can often narrow the cause down over the phone from when the rejects occur — which is faster and cheaper than a call-out.
Related: What HACCP and BRCGS require · Product effect in meat production