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Operator-related factors

Operator-related factors

• Calibration: The accuracy of any ultrasonic measurement is only as good as the accuracy and care with which the gage has been calibrated. In all cases, it is essential that the velocity and zero calibrations described in Section 4 are performed 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.

• Beam alignment: The transducer must be held flat when testing on flat surfaces, and normal to the radius of curvature when testing on curved surfaces. Additionally, when testing on curved surfaces the transducer must be centered on the curve. Misalignment will cause echo distortion which in turn will have a negative effect on accuracy.

• Coupling Technique: In Mode 1 measurements with contact transducers, 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, then the coupling technique must be consistent. In order to accomplish consistent measurements, employ only enough couplant to achieve a stable reading and apply the transducer with uniform pressure. Practice will show the degree of moderate to firm pressure that produces repeatable readings. Transducers should never be scraped or dragged across rough surfaces. In general, smaller diameter transducers require less coupling force to squeeze out the excess couplant than larger diameter transducers. In all modes, tilting the transducer distorts echoes and causes inaccurate readings, as noted above.

In the case of corrosion gaging on small diameter pipes and tubes, the transducer should be held so that the sound barrier material visible on the probe face is aligned perpendicular to the center axis of the pipe, as seen below.

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High Temperature Measurements >>