Calibration and Normalization

Calibration Method: The method of calibration for phased array transducers can be varied. As beam formation relies on variant element delays and groups, it is important to normalize the response from each focal law, to compensate both for element-to-element sensitivity variations in the array transducer and for varying wedge attenuation and energy transfer efficiency at different refracted angles. Calibration of wedge delay and sensitivity over the entire inspection sequence not only provides clearer image visualization, but also allows measurement and sizing from any focal law. While Olympus NDT instruments allow full calibration, many instruments will only allow calibration of one focal law at any one time.

TVG/DAC for phased array: For sizing defects, A-scan amplitude techniques using DAC curves or time corrected gain are common. These methods account for material attenuation effects and beam spreading by compensating gain levels (TCG) or drawing a reference curve based on same size reflector response as a function of distance. As in sensitivity calibrations, some instruments allows a TCG to be built at multiple points over all defined focal laws. In these instruments, the view can be switched from TCG to DAC curve at any time. This allows use of sizing curves at multiple angles for sectorial scans or at any virtual aperture in linear scans.

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