Hello.We‘re measuring the thickness of an organic coating on carbon steel and we have two probes: one works by magnetic induction, one by the eddy current method. They‘re both right-angle probes designed to measure the inner coating of pipes.When testing on a flat sheet of stainless steel, both probes give accurate measurements. But when tested on a carbon steel cylindrical pipe, the eddy current probe is way off (reading 667 um when put right against the metal)!We will be using the ‘proper‘ probe, but still I‘d like to know how the eddy current probe behaved in such a way. Is it because of the geometry (flat sheet vs. pipe)? I suppose eddy currents would take different paths.I‘m not an engineer as you will have guessed.Also, I know NF/FE means ferrous metals and NC/NF means nonferrous, but what do these acronyms mean exactly?
I've used ultrasonic testing, not eddy current or magnetic induction. However, no one has answered so I'll take a stab at it. I expect two issues: magnetism and calibration. Materials that are strongly magnetic are called ferromagnetic, while those that don't are called non-magnetic. Stainless can be either ferromagnetic or not, depending on the crystaline structure, i.e. type of stainless and (sometimes) heat treat condition. Could it be that the magnetic properties of the underlying material affects the readings? More specifically, do both methods work with both magnetic and non-magnetic materials. Second issue is calibration. If you calibrate for steel, maybe calibration not valid for stainless, or vice versa. Round or flat should not make much difference. Good luck!
