Dakota Ultrasonics PVX - Manual (Page 30)

Dakota Ultrasonics
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Refer to diagram 3. The above diagram illustrates a typical view of echo-echo mode
using a delay line style transducer. The end of the interface or delay line is displayed
at (B). However, unlike interface echo mode the measurement is occurring between
two return reflections (A) and (B). Recall that in interface echo mode, the
measurements occurred between the interface (B) and the first return reflection (A).
This is the primary difference between the two modes. An important thing to note is
that the ring down cycles from the transducer are much less than in interface echo
mode. This allows the PVX to see much closer to the first return reflection making it
possible to measure very thin materials. This mode is also used for thru-paint
measurements. By measuring between two echoes, the thickness of paint &
coatings will be eliminated completely. Therefore, any paint or coating on the surface
of the test material will not be added to actual thickness of the test material.
4.7 Glossary of terms
Repetition Rate & Display Update
The repetition rate is how often the PVX generates a burst of ultrasound.
The PVX has a fixed repetition rate of 1 kHz, or 1000 times per second. The PVX
updates the display 4 times per second in regular measurement mode and 32 times a
second in hi speed scan mode. Therefore, the scan mode is most suitable for fast
scans of a test piece at a lower resolution.
Variable Pulser Width
Transducers have a center frequency where they are most efficient at transmitting
sound (i.e. they are resonant). The PVX has a variable width pulser with three
setting options (spike, thin, wide). These settings determine the period of time the
pulser turned is on. High frequency applications, where resolution is a requirement,
may require the spike or thin settings to achieve optimal results. Low frequency
applications, where more penetration is a needed, may require the thin or wide
settings, offering increased penetration. The variety of transducer frequencies and
diameters used in conjunction with the pulser settings, enable users to fine tune the
PVX for their application needs.
Gates
Gates control the time measurement process. They enable the user to select
measurement regions and threshold levels (sensitivity). The PVX is equipped with
two gates. In pulse-echo mode a single gate can be used to eliminate noise. In
echo-echo (interface-echo) mode both gates are used. In this mode the second gate
starts automatically after a signal is detected during the gates region, and after a user
adjustable delay, sometimes called a hold -off.