DMTA-10022-01EN, Rev. C, January 2015Basic Operation 61Figure 5-10 The calibration lock message5.2.7 Factors Affecting Performance and AccuracyThe following factors affect the performance of the instrument and the accuracy of thethickness measurements.CalibrationThe accuracy of any ultrasonic measurement is only as good as the accuracy andcare with which you calibrate the instrument. The 45MG ships from the factorywith standard setups for a number of transducers and applications. In certaincases, it may be desirable to optimize these setups for specific measurementsituations. In all cases, it is essential to perform the velocity and zero calibrationswhenever the test material or transducer is changed. Periodic checks withsamples of known thicknesses are recommended to verify that the gage isoperating properly.Surface roughness of the test pieceThe best measurement accuracy is obtained when both the front and backsurfaces of the test piece are smooth. When the contact surface is rough, theminimum thickness that can be measured will be increased as a result of soundreverberating in the increased thickness of the couplant layer. Additionally, whenthe two test-piece surfaces are rough, the slightly different multiple sound pathsseen by the transducer may cause distortion in the returning echo, resulting inmeasurement inaccuracies.Coupling techniqueIn mode 1 (contact transducer) measurements, the couplant layer thickness is partof the measurement, and is compensated by a portion of the zero offset. Ifmaximum accuracy is to be achieved, the coupling technique must be consistent.In order to accomplish consistent measurements, use a couplant of reasonablylow viscosity, employ only enough couplant to achieve a reasonable reading, andapply the transducer with uniform pressure. Practice will determine the degree ofmoderate to firm pressure needed to produce repeatable readings. In general,smaller-diameter transducers require less coupling force to squeeze out excessMessage
