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Will you be able to pass a current through the tube or measure a voltage difference over a length of the tube? How does the diameter of the tube compare to the diameter of the loop? Are the tube and loop fixed in position or free to move? Any other physical parameters that you can supply will help us come up with suggestions.
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Not all metals respond to EM detectors.
Ferromagnetic material will cause a change in the loops magnetic field.
Diamagnetic material, like aluminum, has a different effect when passing through the loop.
It will take a bit of experimenting and some sensitive instruments to get a reliable detection.
Note that diameter, total metal content, and stray EM will affect your results.
Why not just use a metal detector?
I have an application in which I have a loop of flexible cable (braided steel, designed for physical strength, not for any particular electrical properties, though it could have wire added to it for that purpose). The cable forms a closed loop, about 8 inches in diameter. I need to be able to detect whether a metal tube is passing through the loop.
For the metal tube, think of the steel post designed for a chainlink fence as the primary example, though there's a fair amount of variability in material and diameter.
It would be good enough for now to be able to detect steel, though in a future redesign, detecting an aluminum tube might become important.
I'm not an expert on electromagnetic sensors, but my basic understanding is that if I go that route, my options are an inductive sensor (but this may limit applicability to aluminum in the future) or a capacitive sensor (but this may have challenges related to the presence of nearby things outside the loop.
Can anybody comment on what makes sense here and how to approach it?