114 CLCH-SVX07C-EN
Start-Up
Fan Inlet Airflow Measuring System
Wiring
The transmitter requires 24VDC power on terminals 1 (+) and 2 (ground) on the transmitter. When
the piezometer airflow measurement is ordered with a factory-mounted MP580 controller, the 24
VDC power will be supplied.
In the absence of a factory-provided MP580 controller, the installing contractor must ensure the
transmitter has 24VDC power.
Note: Ensure that the transmitter has a separate power source.
Transmitter Calibration
The transmitter is factory-calibrated to a specific pressure range with a 0-20 inch w.g. range being
used in most cases. To check calibration and to adjust if necessary, consult the transmitter
manufacturer or the factory for specific procedures.
The transmitter outputs a linear, 2-10 VDC signal representing a differential pressure measurement.
With this measurement, the airflow through the fan can be calculated using the following equation:
Significant differences in elevation and/or temperature will affect the density of air. For air at a
constant, non-standard density, a field-obtained K factor can be used. Alternatively, the following
equation can be used to continuously correct the equation above:
Note: Alternative units, including SI, can be used in place of the IP units above although the K-
factor must be converted appropriately.
Figure 97. Piezometer airflow
measurement
A fan inlet airflow measuring system with a piezometer ring
is available on many centrifugal and plenum fans. Each
system comes with a differential pressure transmitter. The
piezometer ring is connected to the LO port of the transmitter
and the reference pressure point is connected to (or actually
is) the HI port of the transmitter.
CFM = K * SQRT(DP)
Where:
CFM = Airflow (ft
3
/min.) assuming a standard air density of 0.075 lbm/ft
3
.
K = A constant factor that is unique for each fan. See next section for more information.
DP = Differential pressure (inches w.g.) being measured by the transmitter.
ACFM = CFM * SQRT(0.075/ρ)
Where:
ACFM = Actual airflow (ft
3
/min.) corrected for non-standard air density.
ρ = Density (lbm/ft
3
) of the air at the inlet to the fan.
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