Instructions for Use
– SCHMIDT
®
Flow Sensor SS 20.700
Page 9
Mounting in pipes with circular cross-section
Typical applications for this type are compressed air networks or burner
gas supply lines. They are characterized by long thin pipes with a typically
quasi-parabolic flow profile.
The easiest method to achieve a low-turbulence flow is to provide a suffi-
ciently long and absolutely straight distance without disturbances (such as
edges, seams, bends etc.) in front (inlet) and behind the sensor (outlet)
(see installation drawing Figure 3-3). It is also necessary to pay attention
to the design of the outlet distance because the flow is also influenced by
disturbances generating turbulences against the flow direction.
Figure 3-3
L1
Length of run-in distance
L2
Length of run-out distance
D
Inner diameter of measuring distance
The absolute length of the corresponding distances is defined by the inner
diameter of the pipe because the flow abatement depends directly on the
aspect ratio of measuring distance and diameter. Therefore, the required
abatement distances are specified as a multiple of the inner pipe diameter
D. Besides, the degree of turbulence generation by the corresponding dis-
turbing object plays an important role. A slightly curved bend directs the
air with a relative low-disturbance level, whereas a valve generates mas-
sive turbulences with its abrupt change of the flow-guiding cross-section,
requiring a relatively long distance for abatement.
The following Table 1 shows the required straight pipe lengths depending
on the inner tube diameter D and different causes of disturbances.
This table lists the minimum values required in each case. If the listed
straight pipe lengths cannot be achieved, measurement accuracy may be
impaired or additional actions are required like the use of flow rectifiers
6
.
6
E.g., honeycombs made of plastic or ceramics.
Run-in distance L1
Run-out distance L2
Ø
D
