Issue
- Which values in the 102-AX plug-in affect the ultimate airflow reading output on nvoBoxFlow?
- Formula for converting velocity pressure to flow.
- Setting air balance parameters -- airflow coefficient, velocity pressure gain, offset, and cutoff, duct area, pickup factor and box constant.
Environment
Xenta 102-AX
Cause
Incorrectly configured boxes can cause erroneous airflow readings or unexpected behavior during calibration.
Resolution
The Xenta 102-AX uses a differential pressure sensor to convert velocity pressure values from 0-1" water column to a 0.25-4.0 volt output. The voltage signal is converted to a 10-bit digital value. The digital value is filtered and passed through the calibration gain and offset adjustments. Any value below the lowest velocity pressure threshold will be treated as 0"wc.
-
Airflow_Sense.UCPTairFlowCoeff
Airflow Parameters > Airflow Setting > Airflow Filter Coefficients (%)
This value ranges between 0 to 99% where 0 is no filter and 99% is the maximum amount of filter. All process controls using the airflow reading use the filtered value. The default filter is 80%. The filter equation is defined as follows:
display value = [previous display value * filter] + [new data value * (1 - filter)]
-
Airflow_Sense.SCPTgain
Airflow Parameters > Airflow Setting > Velocity Pressure Gain (multiplier/divisor)
This value is multiplied with the raw velocity pressure reading to provide calibration gain to the final signal. The default gain is 10/10 (1). Resetting the box to factory calibration settings prior to calibrating the box will ensure that no invalid values are adversely affecting the operation.
-
Airflow_Sense.UCPToffsetPress
Airflow Parameters > Airflow Setting > Velocity Pressure Offset (WC)
This value is added to the raw velocity pressure reading to provide calibration offset to the final signal. The default offset is 0. Resetting the box to factory calibration settings prior to calibrating the box will ensure that no invalid values are adversely affecting the operation.
-
Airflow_Sense.UCPTpressCutOff
Airflow Parameters > Airflow Setting > Lowest Velocity Pressure (WC)
At the low end of the measurable velocity pressure, the sensor is inherently unreliable. The lowest velocity pressure threshold will zero out any value too low to reliably be measured. The default value is 0.016"wc. If airflow is reading 0 CFM despite the velocity pressure being above 0"wc, it may be that the threshold is set too high.
The calibrated velocity pressure value is used in the following calculation for airflow:
Airflow = Square Root(Velocity Pressure) * Duct Area * Pickup Factor -or-
Airflow = Square Root(Velocity Pressure) * Box Constant
The operator has the ability to enter the duct area and the pickup factor as separate values, or enter the box constant of the VAV terminal unit. The box constant is the duct area multiplied by the pickup factor. If the duct area or pickup factor is entered last, they will be used in the calculation. If the box constant is entered last, it will be used in the calculation. The pickup factor and box constant are sometimes referenced as the K-Factor(FPM) and K-Factor(CFM) respectively. This information must be taken from the VAV box itself and is usually displayed in a format such as the one shown below.
-
Airflow_Sense.SCPTductArea
Unit Parameters > Box Settings > Duct Area (sq feet)
The cross sectional area of the duct in square feet. Most ductwork diameters are measured in inches. For round ducts, the equation for area is:- Area = π * [(d/2)/12]2 where d = diameter or
- Area = π * (r2/ 144) where r = radius
-
Airflow_Sense.UCPTpickupFactor
Unit Parameters > Box Settings > Pickup Factor (Imperial Units)
This coefficient is provided by the VAV manufacturer and represents the feet per minute flow speed of the air at 1"wc of pressure. Typical values in HVAC VAV applications are between 2200 and 3300. Default value is 2950.
-
Airflow_Sense.UCPTboxConstant
Unit Parameters > Box Settings > Box Constants (Imperial Units)
The box constant is the area multiplied by the airflow pickup factor.
Airflow is determined by multiplying the square root of the velocity pressure by the box constant. The final value is reported out the following SNVT:
-
VAV_Controller.nvoBoxFlow
Status > VAV Status > Airflow (CFM)
This is the final airflow value in CFM as measured by the Xenta 102-AX. After calibrating the velocity pressure probe, the displayed value should match the flow values measured by a highly accurate magnahelic hood.
