3 General principles
3.5 Influence of noise
FLUXUS G809
2020-06-25, UMFLUXUS_G809V2-2EN
18
3.5
Influence of noise
The ultrasonic waves do not only propagate in the fluid but also in the pipe wall, see Fig. 3.10. They are reflected at flanges.
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
Measuring points to be avoided
• measuring point directly at the reflection point (l < 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
Fig. 3.10: Propagation of ultrasonic waves
1 – ultrasonic waves in the fluid (measuring signal)
2 – ultrasonic waves in the pipe wall (pipe wall signal)
2
1
l, l
s
– distance to reflection point
D
– outer pipe diameter
c
γ
– sound speed of the fluid
c
β
– sound speed of the pipe
n
– number of sound paths
Fig. 3.11: Measuring points to be avoided
n
2
---
c
c
-----
l
m
l < 3 D
(l
s
- 2) D
(l
s
+ 2) D
disadvantageous measuring range
l
s
