T.O. 33B-1-15-29Figure 5-29. Straight Beam Contact Search Unit.Figure 5-30. Angle Beam Contact Search Unit.5.2.2.2 EffectofSearchUnitonInspectionSystemPerformance.a. Sensitivity. Sensitivity is the ability of an inspection system to detect small discontinuities. It isgenerally rated by the ability to detect a specified size and depth flat-bottom hole in a standard testblock. Sensitivity is unique to each combination of search unit and test instrument. The same modelinstruments and search units of the same size, frequency, material, and manufacturing do not alwaysproduce identical indications. For this reason, all inspections must be set up using a series of referencestandards.b. Resolution. Resolution refers to the ability of an inspection system to separate signals from twointerfaces close together in depth. An example of two such signals is the front surface signal and thesignal from a small discontinuity just beneath the surface. The damping or backing material (seeFigure 5-29) affects the time required for the transducer to stop "ringing" after being excited by a pulsefrom the test instrument. Low damping causes high "ringing" resulting in a wide, high-amplitudefront surface signal. This would cause a long dead zone. Signals from discontinuities just beneath thefront surface would be masked by the front surface signal.
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