Factors Affecting Performance And Accuracy; Figure 5-10 The Calibration Lock Message - Olympus 45MG User Manual

Message
5.2.7

Factors Affecting Performance and Accuracy

The following factors affect the performance of the instrument and the accuracy of the
thickness measurements.
Calibration
The accuracy of any ultrasonic measurement is only as good as the accuracy and
care with which you calibrate the instrument. The 45MG ships from the factory
with standard setups for a number of transducers and applications. In certain
cases, it may be desirable to optimize these setups for specific measurement
situations. In all cases, it is essential to perform the velocity and zero calibrations
whenever the test material or transducer is changed. Periodic checks with
samples of known thicknesses are recommended to verify that the gage is
operating properly.
Surface roughness of the test piece
The best measurement accuracy is obtained when both the front and back
surfaces of the test piece are smooth. When the contact surface is rough, the
minimum thickness that can be measured will be increased as a result of sound
reverberating in the increased thickness of the couplant layer. Additionally, when
the two test-piece surfaces are rough, the slightly different multiple sound paths
seen by the transducer may cause distortion in the returning echo, resulting in
measurement inaccuracies.
Coupling technique
In mode 1 (contact transducer) measurements, the couplant layer thickness is part
of the measurement, and is compensated by a portion of the zero offset. If
maximum accuracy is to be achieved, the coupling technique must be consistent.
In order to accomplish consistent measurements, use a couplant of reasonably
low viscosity, employ only enough couplant to achieve a reasonable reading, and
apply the transducer with uniform pressure. Practice will determine the degree of
moderate to firm pressure needed to produce repeatable readings. In general,
smaller-diameter transducers require less coupling force to squeeze out excess

Figure 5-10 The calibration lock message

DMTA-10022-01EN, Rev. C, January 2015
Basic Operation 61