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4 Functional principle – Ultrasonic-flow measurement
Legend
34
For this calculation, it is important that only the transit times and the device param-
eters such as the transducer distance and the angle of the measuring path to the
flow direction are required. All parameters that include a gas dependency are
omitted.
In order to consider the average flow profile, in particular an asymmetrical or swirl-
affected flow, a total of 6 paths is measured in 3 levels with the USM-GT-400 ultra-
sonic gas meter. The 3 levels can be derived mathematically via an integration
procedure, the so-called Gauss integration.
„Arrangement of the ultrasonic transducers" on page 33
The respective average path velocities (designated with
measuring path i) along these measuring paths results analogically to the formula
above.
Under certain conditions such as, e.g., smaller deviations from the tolerances
during the production, it may be necessary to correct the path velocities with a
common factor:
v
= k v
ki
i
Legend
Manual USM GT400 · EN10 · May 2021
v ̄
Average flow velocity
c
Speed of sound
0
β.
Path angle to the pipe
L:
Path length
d:
Diameter D
(For a center path. Outer paths have a corresponding value.)
=
Corrected path velocity
v
ki
=
Correction factor for the path velocities
k
(This factor is named vw factor d1 for the forward
direction and vw factor d2 for back-flow; see pa-
rameter list)
= v ̄
v
for the respective
i
i
(m/s)
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