Conventional eddy current bobbin probes are known to be ineffectual in detecting circumferential cracks in tubing. Rotating pancake and/or multi-pancake probes are required to detect circumferential cracks. However, it has been demonstrated in CANDU (CANada Deuterium Uranium nuclear reactor) steam generator tubes with deformation and ferromagnetic deposits that probes with surface transmit-receive (T/R) coils are superior to those using surface impedance coils. No subsurface cracks in this tubing were detected with rotating impedance pancake coil probes and multiplexed impedance probes. In the same tubing, circumferential cracks as shallow as 50
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