The content of Fe is not the same as the value of FN. The following measurements are normally used.
(1) PMI including Fe : portable X-ray fluorescence (Niton?, X-Met?, Texas Nuclear?, etc.), portable optical emission spectroscopy (Arc-Met, Spectroport Model TP-07 or TFO-02, SpectroTest F), laboratory chemical analysis, etc..
(2) FN on welds : Ferrite Scope, Microscope, etc.
Please find the previous forum for the relation between Fe and FN below.
(1) PMI including Fe : portable X-ray fluorescence (Niton?, X-Met?, Texas Nuclear?, etc.), portable optical emission spectroscopy (Arc-Met, Spectroport Model TP-07 or TFO-02, SpectroTest F), laboratory chemical analysis, etc..
(2) FN on welds : Ferrite Scope, Microscope, etc.
Please find the previous forum for the relation between Fe and FN below.
http://www.technonet.co.kr/qa/files/FN.pdf