Electromagnetic Testing Chapter 3- Electromagnetic Testing

Flux leakage sensors have small diameters, some as small as 0.02”, in order
to have adequate sensitivity for detecting short-length or small diameter
defects. Because of their small size, the scanning head may have 16 or more
sensors in order to achieve satisfactory throughput speeds. Probes are
springloaded against the tube surface to provide fixed lift-off; they are lowered
after the leading end of the tube is detected and raised just before the lagging
tube end is reached. Signals from the probes on the inner and outer surfaces
of the tube are transmitted through springs to the electronics unit where they
are filtered and analyzed by a continuous spectrum analyzer. Inside and
outside flaws are automatically marked by different-colored dyes that indicate
the size and type of flaws detected. Transverse flaws are detected by passing
the tube through a ring yoke that produces longitudinal magnetization. In this
case, the tube surface is surrounded by and scanned with a ring of small
probes. Signal processing and flaw marking is the same as previously
described.
Charlie Chong/ Fion Zhang