EMAT ultrasonic probes are now available for boiler wall thickness checks and similar applications.
The big advantage of Emat probes is their ability to produce accurate readings on heavily scaled surfaces. This is because the probes induce ultrasound in the steel itself rather than by transmitting it via a coupling medium. In fact the probes will work at a small distance from the material surface. This allows the probe to function when the test surface is heavily corroded, or has a large oxide scale build up.
The probes do not require special equipment and are used with a conventional UT set. Calibration is simple other than for needing a special block. Probe wear is negligible as they are essentially non-contact so frequent calibration checks are no longer necessary.
The operation of the probes requires that scale is present and they will only work on carbon steel that has operated 350oC to 400oC for some time. These conditions favour build-up of the magnetic oxide on the tube surface necessary for generation of electromagnetically induced ultrasound.
A major cost benefit of these probes is that expensive and time consuming cleaning and surface preparation is no longer necessary.
Readings are faster and more comprehensive as the non-contact nature of the probes facilitates scanning rather than spot readings. The probes are also more accurate through ignoring scale and returning the true metal thickness.