Siemens SIPART DR22 Manual page 32

1 Technical Description
1.5 Functional description of the structure switches
1.5.2 Analog input signal processing freely connected (S4 = 1)
Δp
1.00
0.81
0.49
0.25
0.09
0.3 0.5
0
Figure 1-13 Relationship between flow q and active pressure Δp
To measure the flow, a choke is installed at the measuring point which constricts the pipe and
has two connections for tapping the active pressure.
If the properties of the choke and the measuring material are known to the extent that the
equation specified above can be calculated, the active pressure is a measure of the flow.
If you have chosen a certain choke, the flow can be described in the calculation state or
operation state.


q = K · ρ · or q = K · ρ
Δp
B B
Since the density is included in the measuring result according to the above equation,
measuring errors occur when the density in the operating state differs from the value based on
the calculation of the choke. Therefore a correction factor F is introduced for the density in
operating condition.
V
F = =
V
B
as specific volume.
In order to be able to perform the correction with the factor F, the current specific volume must
be determined first.
For the dry gases the densities change according to the laws for ideal gases:
V = R T p = 1 ρ
with p as absolute pressure and T as absolute temperature.
30
0.7 0.9 1.0 q

 ·
Δp
1
B with V =
The correction factor is then given as:
as specific volume.
F =
Manual

T · p
B
p · T
B
SIPART DR22 6DR2210
C79000-G7476-C154-03