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17.3 MEASURING LAMINATED MATERIALS
The density (and therefore sound-velocity) of laminated materials may vary considerably from one piece to
another. Some laminated materials may even exhibit noticeable changes in sound-velocity across a single
surface. The only way to reliably measure such materials is by performing a calibration procedure on a
sample piece of known thickness. Ideally, this sample material should be a part of the same piece being
measured, or at least from the same lamination batch. By calibrating to each test piece individually, the
effects of variation of sound-velocity will be minimised.
An additional consideration when measuring laminates, is that any air gaps or pockets within the laminate
will reflect the ultrasound beam. This will be noticed as a sudden decrease in thickness in an otherwise
regular surface. While this may impede accurate measurement of total material thickness, it does provide
positive indication of air gaps in the laminate.
17.4 MEASURING THROUGH PAINT AND COATINGS
When measuring through paints and coatings the sound-velocity of the paint/coating may be significantly
different from the sound-velocity of the actual material being measured. An example of this would be a mild
steel pipe with approximately 0.6 mm (.025") of coating on the surface. The sound-velocity of the pipe is
5918 m/s (.2330 in/µsec), and the sound-velocity of the paint is 2286 m/s (.0900 in/µsec). If the gauge is
calibrated for mild steel pipe and measures through both materials, the actual coating thickness will appear
to be 2.5 times thicker than it actually is, as a result of the differences in sound-velocity.
The error can be eliminated by using the echo-echo mode to perform measurements for applications such
as these. In echo-echo mode, the paint/coating thickness will be eliminated entirely and the steel will be the
only material measured.
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