Dakota Ultrasonics
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the probe be left in contact with the surface for as short a time as needed
(intermittent contact) to acquire a stable measurement.
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. If the variation is relatively close, averaging the sound velocities to
minimize error is another option.
An additional important consideration when measuring laminates is that many
included air gaps or pockets which 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 .025” of coating on the
surface. Where the velocity of the steel pipe is .2330 in/
sec, and the velocity of the
paint is .0850 in/
sec. If the user is calibrated for mild steel pipe and measures
through both materials, the actual coating thickness will appear to be approximately 3
times thicker than it actually is, as a result of the differences in velocity.
