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Fig. 4 Echo Pattern after Probe Lifting
Electromagnetic interference at site mainly comes from frequency converter, motor
and centrifuge. These interferences are mostly transmitted through power grid. If
the power supply system of a plant has one frequency converter, the entire power
grid will be polluted. Let’s use see the normal echo pattern:
8.3 Electromagnetic Interference
Fig. 5 Normal Echoed Signals
The two figures above are normal echo patterns. The downmost base line (the cross
bar from the left to right) is about 4mm high. It is clear and same high from the left
to right, without burrs. Signals in the red circle are ultrasonic echoed signals and
they are very obvious.
Fig. 6 Echo Pattern under Electromagnetic Interference
The left figure above is the original figure. On the right figure, I enclosed the burrs
with a red circle and they are caused by electromagnetic interference. In the echo
pattern, there is no obvious echo and the burrs above the base line just Show elect-
romagnetic interference. The three figures in the first row and the first two figures in
the second row below the base line are all 0, meaning that the echo is covered by
the wave from probe.