NDT Systems TG400 Operator's Manual page 28

Dual Element Multiple Echo Interval Measurement Procedure.
With careful optimization and setup, multiple back echo intervals can be
used for thickness measurement using dual element transducers. In the
example shown to the right, multiple back echo intervals as far as the 4th
back echo can be seen. In this case, the first back echo was set to the left-
hand start of the windowed A-trace.
Go to GATES, MULT, 1-2,
st nd
Got to ECHO-BLK and set to a minimum point between the 1 and 2 echo
where the gage will trigger reliably on the proper second echo. As a point of
reference, compare the two screens to the right. In the upper screen, a
"standard" fingertip dual transducer is connected. Not how the waveform
Typical "Standard" Dual Element Echo
nd
extends in a decaying fashion between the first and 2 echo. In this case
Pattern
the ECHO-BLK will need to be extended out far enough to pass the
multiple echo cycles of the first echo. This will limit the low end thickness
capability. In the lower image, a TG560P transducer is attached. In this
case the transducer is optimized to allow for thinner material
measurement.
This technique can be used with many dual element transducers to
measure painted or coated objects or object at elevated temperatures.
With the ZEROing technique described in the preceding procedure, when
the transducer delay lines contact a hot surface, they rapidly expand.
Expansion changes the V-path significantly, and consequently produces
sizeable errors. Using a multi-echo mode eliminates this error.
In general, multi echo measurements in a dual element mode are limited to
Typical "specialized" dual element
optimized for echo-echo mode
a top end thickness of about 3", probe, material, coating and application
specific.
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