Remote field eddy current flaw detector for metal pipes having a pair of receiver coils providing a differential offset amplitude signal

A remote field eddy current flaw detector for detecting flaws in a metal pipe, comprising:

• a plurality of coils inserted axially into said metal pipe including:
• an exciting coil for receiving an alternating current and generating field eddy current signals;
• a first receiving coil, spaced axially from said exciting coil by a predetermined axial distance, for detecting remote field eddy current signals that have passed through said pipe;
• and
• a second receiving coil, spaced apart from said first coil such that said second coil is spaced from said exciting coil by an axial distance greater than said predetermined distance, for detecting said remote eddy current signals, wherein said first coil has a greater number of turns than said second coil;
• means connected to said first and second receiving coils for obtaining a differential signal representing the vectorial difference between detected remote eddy current signals produced in said first and second receiving coils, wherein said greater number of turns of said first coil and said first coil being axially spaced apart from and positioned closer to said exciting coil than said second coil cumulatively provide an offset amplitude to said differential signal;
• and
• means for comparing the phase of said differential signal to that of a reference signal to determined the presence of locally flawed, gradually flawed, and nonflawed regions of said pipe based on said cumulative offset amplitude.