Instructions for Use SS 20.260
Page 6
L
= Length of entire measuring distance
L1 = Length of run-in distance
L2 = Length of run-out distance
D = Inner diameter of measuring distance
Figure 2
The following Table 1 shows the required straight conduit lengths de-
pending on the pipe inner diameter “D” and the different disturbance
causes.
Flow obstacle upstream of the measuring dis-
tance
Minimum
length of L1
Minimum
length of L2
Light bend (< 90°)
10 x D
5 x D
Reduction / expansion / 90° bend or T-junction
15 x D
5 x D
Two 90° bends in one plane (2-dimensional)
20 x D
5 x D
Two 90° bends (3-dimensional change in direction)
35 x D
5 x D
Shut-off valve
45 x D
5 x D
Table 1
This table lists the
minimum values
required in each case. If the listed
straight conduit lengths cannot be achieved, measurement accuracy
may be impaired
3
.
Calculation of volume flow
If the cross section area of the pipe is known, the output signal of the
flow speed can be used to calculate the standard volumetric flow of the
medium. By means of a correction factor PF
4
, which depends on the
pipe diameter the measured value can be converted to an averaged flow
w
N
which is constant over the whole pipe cross-section.
Thus, it is possible to calculate the standard volumetric flow of the medi-
um using the measured standard flow velocity in a pipe with known inner
diameter:
3
Alternatively flow rectifier could be used, e.g. honeycomb ceramics
4
Considers parabolic flow profile and sensor obstruction