Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.2.2 Bar and tube testing using encircling coils.
Bars and tubes are often eddy current tested during manufacture to detect flaws,
or conductivity variations, and to measure their diameter. Both are tested using
encircling coils, which can be absolute or differential. A common setup is to use
differential coils with an external reference. A reference bar, known to have the
required properties, is inserted in one coil, and the bars to be tested are passed
through the balance coil. Any difference between the two bars gives a signal,
which can be analyzed as required to determine its cause. Eddy current bar
testing is usually highly automated, with signals recorded for analysis, or
automatic marking of the product at locations showing signals from
unacceptable conditions. The following outlines bar testing using encircling
coils, but it is a specialized application, and further information should be sought
by those responsible for setting up or modifying such test systems. a The
approach to testing bars with encircling coils is similar, but complicated by
differences in the ratio of inside diameter to outside diameter, and so will not be
considered further here.
Charlie Chong/ Fion Zhang