Abstract: Small diameter tubes such as heat exchanger tubes are widely used in industry, and usually require regular inspection in many services. Remote field eddy current testing (RFEC) is one of the effective nondestructive testing methods to detect defects in small diameter tubes, Pulsed remote field eddy current (PRFEC) method combines the rich spectrum of pulsed eddy current with the characteristic of RFEC to detect inner and outer defects simultaneously. PRFEC method is used to detect small diameter tubes, the detection characteristics of the differential probe and the absolute probe in this technology are systematically studied. A novel differential probe is designed with two receiving coils connected in differential mode. One receiving coil of the differential probe is used as absolute coil and its signals are processed with one signal channel. The differential signals from two receiving coils are processed with another signal channel. The detection principle is analyzed through numerical simulations, and three types of defects, corrosion, hole and crack are tested in experiments. The experimental results show that the designed differential probe has a high detection sensitivity for cracks and holes, which is much higher than the absolute coil, but wide defects are detected by the absolute coil. Signals of the absolute coil have obvious spurious peaks, but the spurious peak can be restrained by the differential probe. Three kinds of defects can be effectively detected using designed probe with dual-channel signal reception method, and the defect depth characteristic of pulsed eddy current testing is still valid.

Key words: pulsed remote field eddy current, small diameter tube, differential structure, dual-channel, spurious peak