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QuintSonic 7
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case, an average sound velocity should be used. To determine the average sound velocity, a
sample cross-section should be made for optical evaluation. If the sound velocity jumps at certain
points, additional sound wave reflections will be produced at such points and lead to erratic
readings if disregarded (see also section 7.3.3.5).
If dramatic changes of the sound velocity are to be expected (e.g. if two material batches vary due
to different production parameters), it is recommended either to readjust the sound velocity within
shorter time intervals or to adjust to the actual batch-related sound velocities. This is to avoid
erratic readings.
Wall thickness measurement
Measuring objects with steel bases mostly exhibit a constant sound velocity in the base, even if the
alloying elements vary. For wall thickness measurement of the steel base, it is important to know
that the variations of sound velocity are so small that they can almost be neglected as far as their
influence on the measuring precision is concerned.
In other materials, however, such as non-ferrous metals or plastics, the sound velocity is subject to
higher levels of variation that may impair the measuring precision. For that reason it is required to
determine the sound velocity by means of a suitable method such as mechanical thickness
measurement of the uncoated sample. For thinner bases, it is recommended to make a material
cross sections.
Also strong sound attenuation may impair measurement. It occurs in some kinds of plastic
materials, for instance. In such materials, sound attenuation becomes stronger with increasing wall
thickness. When reaching a certain material thickness (depending on the type of material), the
sound attenuation is so strong that the amplitude of the reflected sound wave will not be strong
enough for being processed. This may be the case with plastic materials of around 1 to 2 mm
thickness. For such materials, the ultrasonic method is not suitable. If you have a new measuring
tasks, it is recommended first to create an A-scan image (see section 17) to check whether the
amplitude of the receive signal is strong enough to ensure reliable testing.
Variations of the sound velocities as function of material temperature
The sound velocity is also influenced by the temperature in the material layers. The temperature
coefficients also depend on the type of material with some materials exhibiting a non-constant
temperature profile.
The QuintSonic 7 sensor features an integrated temperature compensation for the sound velocities
of layers. The temperature compensation is adjusted to an average temperature coefficient of
typical industrial coating materials.
The temperature compensation will only work if sensor head and measuring object have the same
temperature. Generally, ultrasonic coating thickness measurement systems are not able to record
the current temperature distribution within a layer system that could be used for correction
accordingly.