Sensitivity is not a single number. It changes with the metal, with the height of the aperture and with the product itself. Unless all three are asked together, two quotations are not describing the same thing.
Sensitivity is a sphere diameter
The sensitivity of a detector is the diameter of the smallest test sphere it can find, and it is stated separately for each type of metal. The GJ-VIII in the catalogue is a clean example: Φ 0.5 mm for iron, Φ 1.0 mm for stainless steel. Whenever a quotation shows a single figure, ask which metal it refers to.
Why is iron easy and stainless steel hard?
A metal detector looks for two effects that upset the coil balance: magnetic permeability and electrical conductivity. Iron shows both strongly, so even a small piece is caught. Austenitic stainless steel is effectively non-magnetic and its conductivity is comparatively low – what remains is a weak signal.
The catalogue values confirm the ordering: for the GJ-III, iron Φ 0.6–2.0 mm against stainless steel Φ 1.2–3.5 mm. The roughly twofold gap follows from that physical difference.
The larger the aperture, the lower the sensitivity
The GJ-III figure being a range, stated in the record as “depending on model”, says exactly this. The centre of the detection field is far from its edges; as the tunnel gets taller, the signal at the weakest point falls.
The practical consequence: do not choose a tunnel larger than your product needs. The NWMC2000 thick-textile detector can be customised over a tunnel height of 100–300 mm and a width of 600–2000 mm; its stated Fe 0.8 mm figure only means something together with that geometry.
Product effect: wet and salty products
Moist, salty or acidic products are conductive in themselves and generate a signal in the detector. This is called product effect, and it may leave no option but to reduce sensitivity. Aluminium foil packaging makes a metal detector unusable altogether.
That is the reason the GJ-XF X-ray unit exists in the catalogue; its record names as the leading advantage “highly salted products and aluminium-foil packaging that are difficult to inspect with a metal detector”. Because X-ray looks at density differences, it also finds glass, ceramic, stone, bone and hard plastic.
A needle detector is not a food metal detector
Both look for metal, but their jobs differ. A needle detector finds a broken sewing needle in a garment and must show where it is – the JXH-3 in the catalogue does this with an 8-point position indicator. A food metal detector finds metal on the line and rejects the product; the GJ-III record gives automatic stop as standard, with a rejection system as an option.
One caution: the JXH-3 records state sensitivity as “twice that of standard automatic needle detectors”. That is a relative claim, not a value in mm. Do not treat it as a number when comparing quotations.