Dakota Ultrasonics MMX-6DL - Manual (Page 45)

MMX-6 DL Ultrasonic Thickness Gauge
41
An additional important consideration when measuring laminates, is that
any included air gaps or pockets will cause an early reflection of the
ultrasound beam. This effect 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 the user with
positive indication of air gaps in the laminate.
•
Measuring Through Paint & Coatings
Measuring through paints and coatings are also unique, in that the
velocity of the paint / coating will be significantly different from the actual
material being measured. A perfect example of this would be a mild steel
pipe with approximately .025” of coating on the surface. Where the velocity
of the pipe is .2330 in/
µ
sec, and the velocity of the paint is .0900 in/
µ
sec. If
the user 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 velocity. This error can
be eliminated by using a special 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.