Table 7-1. Uses of Eddy Current Inspection
sensitivity for defect detection. When using the ED-520 for purposes other than defect detection, frequencies may be
selected to minimize variations in parameters other than lift-off. The magnitude of the current change corresponding to a
specific change in test coil impedance is regulated by the position of the function switch LO, MED, HI and the
Changing magnitude of Current. In the simplest type of instrumentation, analysis of the signal consists of
measuring the change in relative magnitude of the current flowing through the bridge. Changes in the ac current are
amplified and converted to a dc current prior to readout. In more sophisticated instrumentation, both amplitude and
phase are determined. In this case, a reference signal is provided as a basis of comparison with the amplified signal
from the bridge or compensating circuit. Phase is then measured in relation to the reference signal.
Needle Deflections. Acceptance of an eddy current probe should be based primarily on crack or flaw detection
capability or sensitivity. With the ED-520 adjusted for 3 mils of lift-off and maximum sensitivity, probes should exhibit a
static meter deflection of at least 100 units from a 0.60 inch long x 0.020 inch deep Electrical Discharge Machined (EDM)
notch in aluminum in order to exhibit adequate sensitivity. Probes that are also to be used on steel or titanium should
meet minimum sensitivity requirements for those materials. Response from a 0.060 inch long x 0.010 inch deep EDM
notch in steel should be a minimum of 60 meter units. A 0.100 inch long x 0.030 inch deep notch in titanium should yield
a meter deflection of 30.