Instructions for Use
– SCHMIDT
®
Flow Sensor SS 20.500
Page 9
Flow obstacle up-
stream of measur-
ing section
Minimum
length of in-
let distance
(L1)
Minimum
length of out-
let distance
(L2)
Light bend (< 90°)
10 x D
5 x D
Reduction, expan-
sion, 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 with
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 it is not pos-
sible to observe the specified abatement distances, increased deviations
of the measurement results are to be expected or it is necessary to take
additional measures, for example to use flow rectifiers
8
.
Under laminar conditions a quasi-parabolic speed profile is formed over
the pipe cross-section, whereas the flow velocity at the pipe walls remains
almost zero, in the middle of the pipe it reaches the optimum measuring
point, its maximum w
N.
This measurement value can be converted to an
average speed
N
w
constant over the pipe cross-section by means of a
correction factor, the so-called profile factor PF. The profile factor depends
on the pipe diameter
9
and is given in Table 2.
8
For example, honeycombs made of plastic or ceramics; profile factor may change.
9
Here interior air friction as well as obstruction caused by sensor is considered.
