Low-power EMAT measurements for wall thickness monitoring

Electromagnetic acoustic transducers (EMATs) have been a subject of research in NDT for almost 50 years, but their use has not been as extensive as piezoelectric transducers. This is in part due to their relative inefficiency for ultrasonic generation and their relative insensitivity as ultrasonic detectors. Nevertheless, for some applications they can provide a unique solution to a particular inspection problem, with their ability to operate without physical contact with the sample. EMATs are also capable of generating ultrasonic wave modes that can be difficult to generate using piezoelectric transducers, and they inherently have wide electrical and acoustic bandwidths. To compensate for their low efficiency as detectors, but particularly as generators, they are often driven using high voltages and currents, and in some applications this is absolutely necessary. This has led to the perception that EMATs are high-power devices that are not easily battery operated, which is not strictly true. In this paper, some measurements are presented that demonstrate that a radially-polarised EMAT can operate at relatively low powers, from a battery-powered pulser and amplifier. This paper demonstrates how, with trivial and quick signal averaging, an SH-wave EMAT has been used successfully to measure the thickness of low-carbon steel with an unfavourable, unoxidised surface, driven by a flaw detector producing 20 ns-wide pulses of less than 11 V at 620 mA. For low-carbon steels with a magnetite surface oxide coating, such as those found in many petrochemical applications, even lower power pulses can be used. The operation of a practical EMAT adapter for use with a conventional flaw detector is also explained and demonstrated, which can facilitate the operation on samples such as stainless steel that would usually be considered too inefficient to inspect with an EMAT.