Signification and Use of A-scan Images
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QuintSonic 7
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methods and steps of echo signal processing. More detailed explanations on that will be given later
on in section 8 “Echo Signal Processing”.
7.3.3.1
Coupling echoes
Example: coupling echoes
The first two echoes (marked “Y” and “Z”) present coupling echoes. As a rule, there are only two
coupling echoes to occur: a positive one (“Y”) for the transfer from the sensor head to the couplant
and a negative one (“Z”) for the transfer from the couplant to the measuring object. As the couplant
is (or should be) thin, the two echo pulses mostly occur in immediate succession. The amplitudes
(signal heights) fluctuate and depend on the nature and distribution of couplant as well as on the
properties of the top layer of coating system.
The two coupling echoes should always be clearly represented. If they disappear through noise or
if they are totally missing, this indicates a strong signal attenuation through coupling (soiled surface
of measuring object or unsuitable couplant). If through signal attenuation the limits of Global
Clipping will be undercut (see section 8.3.3.1), the measuring system will provide no results and an
error message (coupling failure) will appear. In such case, it is recommended to clean the surface
of measuring object at the measuring spot carefully and/ or to apply a suitable couplant.
If, on the other hand, the coupling echo “Y“ is too large, almost reaching the level of the reference
signal, a bad coupling can be assumed. This may be due to an insufficient quantity of or dried out
couplant, an air gap caused by a slope positioning of the sensor or an uneven measuring object. In
such case, please try to improve coupling by cleaning sensor head and measuring spot. After this,
use fresh couplant.
The coupling index displayed during measurement indicates the quality of coupling. The higher the
index, the better the coupling performance. However, the coupling index must not be
misinterpreted as a factor, i.e. a couplig index increasing by 10% does not mean that the coupling
performance has improved by 10%. The achievable maximum coupling index depends on the
measuring object. Though the coupling index gives a useful hint with regards to the quality of
coupling, the A-scan image will provide more detailed information on the conditions.
Coupling echoes
(Y, Z)