Dakota Ultrasonics VX - Manual (Page 27)

VX Ultrasonic Velocity Gauge
23
• Measuring laminated materials
Laminated materials are unique in that their density (and therefore
sound-velocity) 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 minimized.
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 increase in sound
speed in an otherwise regular surface. While this may impede accurate
measurement of the material, it does provide the user with positive
indication of air gaps in the laminate.