80
CLCH-SVX03C-EN
Startup
Wiring
In the absence of a factory-provided control system, consult the transmitter manufacturer or the
factory for wiring.
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, 4-20 mA signal representing a differential pressure measurement.
With this measurement, the airflow through the fan can be calculated using the following equation:
Constant Factor K
The constant factor K is unique for each fan and is primarily a function of the area and the geometry
of the fan inlet. Pre-engineered factors are available from the factory for fan types where the airflow
measurement system is available.
Field-obtained factors can provide maximum accuracy. To obtain the factor in the field, measure
the differential pressure being output from the transmitter while measuring the airflow through the
system. Once these two values have been measured, simply solve for K using the following
equation:
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’s unique for each fan. See next section for more information.
DP = Differential pressure (inches w.g.) being measured by the transmitter.
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:
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.
Note:
Alternative units, including SI, can be used in place of the IP units above although
the K-factor must be converted appropriately.
K = ACFM/SQRT(DP)
Where:
K = Field-provided constant factor.
ACFM = Actual airflow (ft
3
/min) being measured at the air density being measured.
DP = Differential pressure (inches w.g.) being measured by the transmitter.
