Dakota Ultrasonics DFX-7 plus MANUAL2 - Manual (Page 120)

Dakota Ultrasonics
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The DFX-7 is equipped with an option to select the polarity, or phase +/-, for the
purpose of detection. The phase can be a very valuable feature to have when the
signal returning from the test material is marginal, a low frequency transducer is
being used, and the user has a very weak positive or negative cycle, while trying to
measure very thick materials. There is a possibility that signal will become so weak
that it falls below the threshold, and peak jumps to the next cycle altogether. This
peak jump represents a shift of the detection further out in time, resulting incorrect
measurements. The material being measured will appear thicker than it actually is.
Refer to the diagram below:
Notice the +/- positions in the diagram. The positive phase is everything above the
horizontal center line, and the negative everything below the center line. The DFX-7
uses a zero crossing method for detection. Therefore, the detect line (A) is
represented by the broken vertical dotted line, and is currently detecting on the
negative portion of the waveform (B) at the zero crossing (C). The signal amplitude
(B) is shown as the negative vertical height at (B) in the diagram. Refer to the height
of the second negative peak at (G). It’s clear that the amplitude of (G) is much
greater than that of (B). Therefore, if the user was measuring thick attenuative
material, and the amplitude of (B) decreased substantially, the DFX-7 would lose the
first cycle (B) and peak jump to the second cycle (F). The detect (A), would move to
(F), resulting in incorrect measurements.
If the user were to select the positive phase in the diagram above, the detect would
measure at (H). (A) would move to (H). Notice the height of (D) with respect to the
height of (E) in the diagram. If the user was measuring thick attenuative material
using this phase, the signal (E) will certainly be lost long before (D). Therefore, the
positive phase is a much better choice in the diagram above.
Toggle Polarity (+/-)