Figure 5-29. Straight Beam Contact Search Unit.
Figure 5-30. Angle Beam Contact Search Unit.
Effect of Search Unit on Inspection System Performance.
a. Sensitivity. Sensitivity is the ability of an inspection system to detect small discontinuities. It is
generally rated by the ability to detect a specified size and depth flat-bottom hole in a standard test
block. Sensitivity is unique to each combination of search unit and test instrument. The same model
instruments and search units of the same size, frequency, material, and manufacturing do not always
produce identical indications. For this reason, all inspections must be set up using a series of reference
b. Resolution. Resolution refers to the ability of an inspection system to separate signals from two
interfaces close together in depth. An example of two such signals is the front surface signal and the
signal from a small discontinuity just beneath the surface. The damping or backing material (see
Figure 5-29) affects the time required for the transducer to stop "ringing" after being excited by a pulse
from the test instrument. Low damping causes high "ringing" resulting in a wide, high-amplitude
front surface signal. This would cause a long dead zone. Signals from discontinuities just beneath the
front surface would be masked by the front surface signal.