Checkline MX-5 - Manual (Page 35)

MX-5 Ultrasonic Thickness Gauge
31
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.