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7. The key is pressed and the display shows 1OF2. The steps 3. to 6. are now
repeated for the second calibration point.
8. The key is pressed so that the M/S (or IN/ µS) starts to flash. The instrument
now displays the sound velocity value that it has calculated based on the
material thickness value entered in step 6.
9. Press the key again to exit the calibration mode. You can now start measuring
in the preprogrammed measuring range.
4.4 Measurements are made
The meter always stores the last measured value until a new value is added.
For the sound generator to function properly, there must be no air bridges between its
contact surface and the surface of the material to be measured. This is achieved with
the ultrasonic gel, the "coupling agent". This liquid "couples" or transmits the ultrasonic
waves from the transducer into the material and back again. Before the measurement,
a little coupling agent should therefore be applied to the material surface to be
measured. Even a single drop is sufficient.
Then the US measuring probe is carefully pressed firmly onto the material surface. The
coupling symbol and a number appears in the display. If the instrument is "cleanly
adjusted" and the correct sound velocity
has been determined, the number in the display shows the current material thickness,
measured directly under the sound generator.
If the coupling indicator does not appear or the number on the display is questionable,
first check that there is sufficient coupling agent at the point under the US probe and
that it has been placed flat on the material. Sometimes it is necessary to try a different
transducer for the material in question (diameter or frequency).
While the US measuring probe is in contact with the material to be measured, four
measurements are taken per second. When it is lifted from the surface, the display
shows the last measurement.
Note:
Sometimes a thin film of the coupling agent is drawn between the US probe and
the material surface when the probe is
is lifted off. In this case it is possible that a measurement is made through this film,
which then turns out to be larger or smaller than it should be. This is obvious because
if one measurement is taken while the US probe is still in place and the other when it
has just been lifted off. In addition, when materials with thick paint or coating are used,
they are measured instead rather than the intended material. Ultimately, the
responsibility for the clean use of the gauge in connection with detecting these
phenomena is withheld from the user.
4.4.1 Changing the individual sound velocities
Annex A lists the individual sound velocities used for the measurement of different
materials.
If the speed of sound is to be changed, proceed as follows:
1. The CAL button is pressed twice until the M/S symbol starts to flash.
