In the framework of the JET Magnetic Diagnostic Enhancement, a set of pick-up coils (UC subsystem) wound on metallic Inconel® 600 former was manufactured. For cross-validation purposes, two different calibration methods were used. A discrepancy in the range of 3% was observed, which can be explained when considering the dependence of the calibration coefficients on the field strength, which in turn is mostly due to the nonlinear behaviour of the Inconel formerr. For this reason a specimen of Inconel was analysed by means of a magnetometer, which showed a nonlinear and hysteretic behaviour occurring at low field level (below 5mT). The calibration coefficients are also measured at low field (0.1-2 mT) and so are affected by such peculiar ferromagnetic behaviour. Moreover, the ferromagnetic behaviour might be sensitive to mechanical and thermal treatment performed during probes manufacturing and testing. Therefore the achievable accuracy for the calibration of coils wound on Inconel formers is limited by the following effects: i) the field level in operation can be completely different from the field used in the calibration procedure; ii) measurements of the magnetic properties on Inconel specimens can not be extrapolated to the former, because of unpredictable effects of mechanical and thermal treatments made on the coil; iii) residual magnetization; iv) temperature variations during operation.