Checkline TI-MVX - Manual (Page 24)

Echo-Echo Mode: The echo-echo mode measures
between two reflections. This technique is common-
ly used to eliminate errors from surface coatings
and also to make measurements in multiple layered
materials. The disadvantage is that two echoes are
needed which requires a much stronger echo
(reflection).
Searching for small defects Dual element delay
line transducers are especially useful in searching for small defects. In the pulse-
echo mode with high amplifier gain, very small defects can be measured. The A-
Scan display of the TI-MVX can be used to see the defect and optimize the
transducer placement.
Sometimes a grain particle or air bubble will cause a reflection. Without the A-
Scan display it is impossible to know if this is a thin area or just a harmless
defect. The A-Scan allows the user to see the backwall as well as the defect sim-
ilar to a flaw detector.
-24-
Dual Element Transducer
in Echo-Echo mode
Waveform showing echo
from corroded backwall
Waveform showing echo
from solid backwall
signal amplitudes, we would also lose our detection of both back wall echoes.
Notice that the amplitude of the cycle just to the left of (C). The amplitude of the
cycle, at (C), is less than the amplitude of the cycle at (B), in terms of overall
height. If we decreased our gain, we would lose the first cycle (C) long before
losing the second cycle (B). Therefore, adjusting the gain or threshold levels will
not help us in this example. Our final option is adjusting the GATE2 hold-off,
shown at point (C) in the incorrect diagram, to point (B) in the correct diagram. If
we adjust the GATE2 hold-off further to the right of the first reflection, the ring
down noise from the first back wall echo no longer interferes with the true detec-
tion (D), shown in the correct diagram.
The adjustment considerations in the example above will typically be used for all
thru paint applications respectively. In some applications the hold-off may be suf-
ficient, while a gain or threshold adjustment will solve the problem. A similar
process of elimination should be considered for all thru paint applications.
-57-