Single magnetic stray field sensor whose signals are differently attenuated in two channels and then compared

Apparatus for defects testing of metal tube, comprising:

• means for magnetizing the tubes;
• a single magnetic sensing element scanning the tube surface and producing signals responsive to defects in the tubes;
• a first channel connected to the single sensing element, said channel including first signal storage means;
• a second channel connected to the single sensing element, said second channel including filter means and a second signal storage means connected with the filter means, said filter attenuating the signals of external defects and internal defects to a different degree;
• and
• said signal storage means consisting of analog storage elements and the outputs of the first and second channels being connected to a comparator.

Apparatus for receiving electric signals from a single magnetic sensing element responsive to changes in flux in a magnetized hollow metal tube produced by defects in said tube and providing signals identifying the location of the defects, comprising:

• a first channel interconnected with the single sensing element having, in the order stated, an amplifier and peak signal storage means serially connected;
• a second channel interconnected with the single sensing element having, in the order stated, an amplifier, a filter for passing signal frequencies characteristic of tube defects existing adjacent the tube exterior, and a peak signal storage means;
• a comparator connected to the first and second channels, said comparator providing a first output when the first channel output exceeds the second channel output, and a second output when the second channel output exceeds the first channel output;
• a first AND gate connected to provide an output on coincidence of a first channel output and one of the comparator outputs;
• and
• a second AND gate connected to provide an output on coincidence of a first channel output and the other of the comparator outputs.

A method of inspecting metal tubes for defects in which the tubes are magnetized and the tube surfaces are scanned by a single magnetic sensing element which produces electric signals responsive to the presence of magnetic stray fluxes at said tube surfaces, said electric signals having first and second frequency ranges characteristic of defects located adjacent the tube exterior and interior, respectively, which comprises the steps of:

• passing the stray flux signals along first and second channels to respective outputs;
• filtering out one of said first and second frequency ranges in the second channel;
• attenuating the transmission of signals differently in the first and second channels;
• forming a third signal corresponding to a ratio of the signals at the outputs of the first and second channels;
• comparing the third signal with a predetermined standard of magnitude corresponding to unity for the third signal such that identification of the location of the defect producing the stray flux signals as internal or external corresponding to whether the third signal is greater than or less than unity.