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5.2.6
Locked Calibrations
The 38DL PLUS includes a password-protected locking function to prevent changes
to setups and prevent access to some functions. A change to the calibration is one
action that you can lock. When this is the case, the message shown in Figure 5-8 on
page 73 appears momentarily on the help text bar (see "Locking the Instrument" on
page 124 for details).
5.2.7
Factors Affecting the Performance and the 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 38DL PLUS ships from the
factory with standard setups for a number of transducers and applications. In
some cases, it may be desirable to optimize these setups for specific measurement
situations. In all cases, it is essential that you perform the velocity and zero
calibrations whenever the test material or transducer is changed. Periodic checks
with samples of known thickness 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 because 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
Figure 5‑8 The calibration lock message
DMTA-10004-01EN, Rev. D, November 2016
Basic Operation
73
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