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Measuring Laminated Material
In general, there is a difference in the density "sound velocity"
between non-laminated and laminated materials "taping, covering or
painted with epoxy, polyethylene and adhesive, etc.". Multi-laminated
materials may even exhibit noticeable changes in the sound velocity
across a single surface. In this case, it is necessary to perform a
calibration procedure on a sample piece of known thickness. Ideally,
the sample piece should be the same material being measured, or at
least from the same lamination lot. Individual sound velocity errors can
be minimized by doing so. When measuring laminates, an additional
important consideration is that any included air gaps or pockets will
cause premature reflection of the ultrasonic wave. If these effects exist,
the thickness value would be less than the actual thickness. On the
other hand, should air gaps or pockets exist in the material, their
existence would be obvious as obtaining the correct measurement
value will not be possible.
Measurement through Paint and Coating
The sound velocity is remarkably different when a coating exists. For
example: A case that the surface of the steel pipe has a coating of
approximately 0.6 [mm] thickness. The sound velocity of the steel
pipe is 5920 [m/s]. The sound velocity of the coating is 2300 [m/s]. If
the thickness is measured on the coated steel pipe after the sound
velocity is set to the value of the steel pipe, the measured value would
be 2.5 times that of the correct thickness. In this case, the error can be
eliminated by the echo - echo mode. The echo - echo mode can
measure the thickness by excluding the coating thickness completely.
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