Checkline DFX-7 - Manual (Page 130)

Dakota Ultrasonics
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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 (+/-)