Flexim FLUXUS G800 - 5.3 Influence of Noise

FLUXUS G80x 5 Selection of the Measuring Point
22 UMFLUXUS_G8V4-6-1EN, 2018-10-10
5.3 Influence of Noise
The ultrasonic waves do not only propagate in the fluid but also in the pipe wall (see Fig. 5.1). They are reflected at flang-
es.
The reflected pipe wall signals can disturb the measurement, especially if:
• the measuring point is close to the reflection point
• the pipe wall signals and measuring signals are received by the transducer at the same time
Fig. 5.1: Propagation of ultrasonic waves
Tab. 5.3: Measuring points to be avoided
Measuring point directly at the reflection point (l
S
< 3 D)
Measuring point at a distance of l
S
± 2 D from the reflection point
Pipe wall signal and measuring signal are received by the transducer at the same time.
I
S
= · · D
l
S
– distance to the reflection point
D
– outer pipe diameter
c
F
– sound speed of the fluid
c
P
– sound speed of the pipe
n
– number of sound paths
ultrasonic waves in the pipe wall (pipe wall signal)
ultrasonic waves in the fluid (measuring signal)
disadvantageous
l
S
disadvantageous
l
s
n
2
---
c
P
c
F
------