Abstract: It is a critical point concerning manufacture quality of the duplex stainless steel weld pipe to control the ferrite-austenite phase equilibrium in the weld metal thereof.Meanwhile,determination of the ferrite volume percentage in the weld metal has so far still remained a technical hard nut.Focused on in the essay are the main aspects related to the performance of the Feritscope MP-30E instrument,covering the measurement accuracy and influence,the calibration for maintenance purpose and the importance of the calibration reference,etc.Moreover practical approaches to control the ferrite content in the weld meal are proposed.