Dakota Ultrasonics PVX - Manual (Page 32)

Dakota Ultrasonics
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Plastics
The PVX works extremely well on thin plastics using a delay line transducer and
special graphite tip. Because the velocity of the standard acrylic tip has a similar
velocity to most plastics, the PVX is unable to measure the material because it simply
can't see it. By using the graphite tip in place of the acrylic tip, the velocity of the
graphite tip allows an impedance mismatch to occur between materials, hence the
plastic can successfully be measured.
When measuring thicker plastics, a lower frequency contact transducer should be
used – typically 1 - 2.25MHz. Larger crystal diameters offer greater penetration
capabilities. Therefore, for thicker and hard to penetrate plastics, use larger diameter
transducers if possible.
High temperature
High temperature applications require a special delay line transducer using a high
temperature delay tip with a very low temperature coefficient. In other words, the
delay tip is not absorptive at high temperatures.
Noisy Material
Materials such as titanium, stainless steel, and aluminum may have inherent surface
noise issues that appear close to the initial pulse, or near the surface. This can be
avoided by using a delay style transducer in either echo -echo or interface-echo
modes. However, when using a contact style transducer in pulse-echo mode, it’s
best to consider a higher frequency transducer. The PVX is also equipped with a
single gate in pulse-echo mode. Enabling a single gate is also an effective tool for
blocking unwanted surface noise. See the section
Gate 1 on page 61 for more
information.
Restricted access
In order to measure materials with lots of curvature or restricted access, the use of a
delay line transducer with a cone delay tip or a pencil style transducer, can provide
additional access.