Schmidt SS 20.200, Instructions For Use Manual, Page: 5 / 12

Instructions for Use SS 20.200 Page 5
Pipe-bound flow
To reach the accuracy specified in the data sheets, the sensor has to be
positioned in a straight conduit and at a place with undisturbed flow pro-
file. An undisturbed flow profile can be achieved if a sufficiently long dis-
tance in front of the sensor (run-in distance L1) and behind the sensor
(run-out distance L2) is held absolutely straight and without disturbances
(such as edges, seams, bends, etc.).
The design of the run-out distance is also important, since disturbances
act not only
in the direction of the air flow but can also lead to disturb-
ances opposite to the flow direction.
L1 L2
L
D
L Total length of measuring distance L2 Length of run-out distance
L1 Length of run-in distance D Diameter of the measuring distance
The following table shows the necessary straight conduit lengths as a
function of the pipe diameter D for various scenarios.
Flow obstacle upstream of measuring
conduit
Minimum length of
run-in distance
(L1)
Minimum length of
run-out distance
(L2)
Light bend (< 90°)
10 x D 5 x D
Contraction / reduction / 90° bend 15 x D 5 x D
2 bends 90° in one plane (2-dimensional) 20 x D 5 x D
2 bends 90° (3-dimensional change in two
directions)
35 x D 5 x D
Shut-off valve 45 x D 5 x D
This table lists the minimum values required in each case. If the listed
straight conduit lengths cannot be achieved, the measurement accuracy
may be impaired.
L