Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

The instrument should be standardization using a steel reference block with
artificial flaws. Commonly, these are spark eroded slots 0.5 mm, 1 mm, and 2
mm deep. If the welds are painted, the thickness of the paint should be
measured first, then standardization performed with plastic shims of the
thickness of the paint over the artificial discontinuities. Because the probes
are differential, there is no clear lift-off signal to use as a reference signal for
adjusting the phase, although the balance points with the probe in air and the
probe on a ferromagnetic material usually differ slightly because the different
impedance of the coils on the ferromagnetic material changes the impedance
of the bridge circuit. After the instrument is balanced on the reference block,
the phase is normally adjusted so that flaw signals are vertical. The gain is
then adjusted to give the sensitivity required by the procedure being used.
Charlie Chong/ Fion Zhang