T.O. 33B-1-1
5-59
5.3.6.5.5
Loose Transducer Element.
A transducer element can separate from the damping material in a search unit. This will cause the initial pulse to
become a long ringing signal (see Figure 5-64. ). Such a situation will cause the search unit to fail the dead zone test.
When this happens, the search unit must be replaced.
Figure 5-64. Example of Ringing Signals Due to a Loose Transducer Element.
5.3.6.5.6
External Noise.
Noise can be indicated on the waveform display when disturbances are created by such sources as follows:
a. Nearby operation of electrical machinery or radio or radar transmitters.
b. Machining on the test part (grinding, cutting, filling, etc.) during the inspection.
c. Ground loop.
Noise from causes listed above are more likely to be encountered when using equipment with a
broadband receiver amplifier and/or long cables between the search unit and the instrument. Sometimes a double
shield on the cable as, shown in Figure 5-65, will help reduce this noise. In this case, the ground electrode of the
transducer element is not connected to the metal case of the search unit and the external shield of the connecting cable.
The ground electrode is connected to the instrument ground via a second internal shield of the cable. Ground loops are
created when instruments, cables, alarm boxes, etc. are not grounded to a common ground point. Also a good earth
ground is essential, not only for preventing stray electrical interference, but also for safety reasons. If a ground loop is
suspected, tie all grounds together, and connect them to a good earth ground. Portable a/c. units can be operated with
