Basic Documentation
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Siemens VAV Compact Controller Modbus RTU A6V10631862_en--_e
Smart Infrastructure 2021-10-26
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.
The air volume flow is measured indirectly with a differential pressure sensor. Since
the measured value is the differential pressure p, the air flow is derived from this
value using the VAV box characteristic. Accordingly, the VAV box OEM has to
provide the combination of pnom and Vnom, out of which the air volume flow in
m
3
/h or l/s can be calculated.
The air volume flow value (relative or absolute) can be transmitted over the bus
together with the actual value of the damper position (in %) to be used by a
supervisory controller or for management purposes. The differential pressure
sensor operates long-term stable and without recalibration.
In critical cases material compatibility tests should be made while considering
harmful substances and concentrations.
The parameter Vn is used to adjust the operating range of the differential pressure
sensor (0…300 Pa) to the actual VAV box nominal pressure pnom at the factory.
The effect of Vn is illustrated in the diagram below.
Assume that a VAV box is designed for a nominal pressure of Δpnom = 120 Pa.
Then Vn must be set to 1.58:
100
80
70
60
50
40
30
20
10
0
0 50 100 150 200 250 300
90
Differential pressure
sensor
Note
Setting the
characteristic value Vn
Effect of Vn
Calculation of Vn
(Δpnom = nominal
differential pressure)
Calculation example