This paper describes a technique for measuring the thickness of both side coatings on steel plates. The technique utilizes the acoustic resonance phenomena which occurs at the top and back surfaces of the plate. The values of thickness of both surfaces are determined from both surface echoes or only the rear surface echo. Focused and non-focused ultrasonic transducers were employed in this study and the effect of acoustic focusing on the accuracy of thickness measurement was investigated. The thickness of both side coatings was between 10 and 30 μm, and the corresponding resonant frequency was between 20 and 90MHz. The values of thickness at both surfaces of the plate, evaluated by both types of ultrasonic transducers, were in good agreement with those observed directly by the microscope after the ultrasonic measurement. The experimental facts suggested that acoustic focusing did not affect the accuracy of thickness measurement. Focused ultrasonic transducer demonstrated the local measurement of the coating thickness and the evaluated area was estimated to be 45 μm. On the other hand, simultaneous measurement of the thickness of both side coatings was possible with the non-focused ultrasonic transducer.

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