Aluminum is a non-magnetic material .  During the entire melting process , the magnetic field is very easy to diverge ,  so the yoke arrangement should be reasonable. First , the yoke must be large enough area , as long as possible , in order to ensure axial and radial magnetic field sensor through a sufficient edge. If the inductor is a two-stage coil ( parallel rewind ),  consider the magnetic flux leakage problem at the indirect slot. Secondly , the distance between the sensors is not too large , and it is suitable for 8 - 12mm .