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Siemens
VAV compact controller G..B181.1E/3 | VAV modular controller ASV181.1E/3
CE1P3544en
Building Technologies
2016-02-26
2.6 Measuring
principle
A measuring device for acquiring the differential pressure – usually a measuring
cross, measuring orifice or Venturi tube in the airflow – represents the basis for air
volume flow measurement.
Legend
1 Actual value of air volume flow
2 Flow resistance (schematically)
3 Measuring line
4 Differential pressure sensor
The air volume flow is measured with a differential pressure sensor. The differential
pressure sensor operates long-term stable and without recalibration.
The feedback signal U can be switched between actual value of air volume flow
and actual value of damper position.
The VAV box manufacturer (OEM) uses the parameter V
n
to adjust the operating
range of the differential pressure sensor (0…300 Pa) to the actual VAV box
nominal pressure p
n
at the factory. The effect of V
n
is illustrated in the diagram
below.
1 0
0
80
70
60
50
40
30
20
10
0
0
50
100
150
200
250
300
90
Pa
p
Pa
300
V
n
n
Assume that a VAV box is designed for a nominal pressure of p
n
= 120 Pa. Then,
V
n
has to be set to 1.58:
1.58
2.5
Pa
120
Pa
300
V
n
Differential pressure
sensor
Setting the
n
V
characteristic value
Effect of
n
V
Calculation of
n
V
( p
n
= nominal pressure)
Calculation example
