34 UMG70XV3-4EN 12.01.2009
5 Selection of the Measuring Point
5.3 Influence of Noise
The ultrasonic waves do not propagate only in the medium but also in the pipe wall (see
Fig. 5.2). They will be reflected at flanges.
Fig. 5.2: Propagation of ultrasonic waves
The reflected pipe wall signals can disturb the measurement, especially if:
• the measuring point is close to the reflection point
• the pipe wall and measuring signals are received by the transducer at the same time
Table 5.2: Measuring points to be avoided
directly at the reflection point (l
S
< 3 D):
ultrasonic waves at the pipe wall (pipe wall signal)
ultrasonic waves in the medium (measuring signal)
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