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QuintSonic 7
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5 Calibration
5.1 Particularities of ultrasonic coating thickness
measurement systems
In ultrasonic measurement, the primary measured variables are the differences in travel times of
the various components of the transmit and receive signal. In ultrasonic coating thickness
measurement, travel times range from a few nanoseconds to several hundreds of nanoseconds (a
nanosecond (ns) is one billionth of a second). The precision at which such travel times can be
measured directly influence the accuracy that can be achieved for measurement. The SIDSP
®
-
sensor of the QuintSonc 7 is a crystal oscillator based time measurement system with a typical
accuracy of
50 ppm (
0.005%). This value is so small that, in practical operation, it can be
neglected against all other factors that might impair accuracy. Unlike the conventional measuring
methods used for coating thickness measurement, the QuintSonic 7 measuring system does not
need any calibration. However, there is one decisive factor of influence resulting from the
measuring object itself that you must be calibrated for: the sound velocity in the material to be
measured.
When converting the differences of travel times as determined in the measuring object into coating
thickness, the sound velocities in the individual layers act as constants of proportionality. These
variables are properties of the measuring object, not of the measuring system and depend on the
material and temperature involved. Most times, these variables are not exactly known. As a
consequence, their determination through indirect methods is required. Most times this is done
through optical evaluation of a material cross-section. Once determined, the variables can be
made available to the measuring system in the form of a sound velocity calibration. This explains
why the sound velocity calibration is vital for the precision of an ultrasonic coating thickness gauge.
5.2 How to calibrate for the sound velocity
QuintSonic 7 offers several methods for a sound velocity calibration. The methods are described in
the following section. For the practical operation, please refer to section 12.1.4.4.
a)
Enter numerical values for the sound velocities
A prerequisite for this method is that the numerical values of the sound velocities are know for
each individual layer. For calibration, simply enter these values manually into the sound
velocity table of the current batch. This method is very simple. In practical operation, however,
this method is not applicable, as in most cases, the sound velocity of the materials to be
measured is not know.