Ultrasonic Examination – Couplants

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Attenuation of ultrasonic waves propagating through air is very high, and normal surface roughness (especially of the workpiece) will always create an air gap between an ultrasonic transducer and the workpiece. It is not practical to improve the surface of the workpiece to the extent necessary to effectively eliminate the air gap. Therefore, a “couplant” that will allow the ultrasonic waves to propagate from the transducer to the workpiece with significantly less attenuation is used.

Satisfactory couplants are usually viscous liquids or greases that fill the surface irregularities of the transducer and workpiece, and thereby eliminate the air gap with moderate pressure on the transducer. Attenuation of ultrasonic waves propagating through the viscous liquids or greases will not be as high as through air, but the layer of the couplant must still be maintained relatively thin by pressure on the transducer, to obtain good transmission. Very rough workpieces may have to be ground to obtain a sufficiently thin layer of couplant. Some surface preparation by light grinding is frequently required when corrosion has roughened the surface or if scales have formed.

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