**Air Volume Flow (VavSuAirFl)** and **Relative Air Flow (VavSuAirFlRel)** are calculated values computed from logic that utilizes the following:

- Velocity pressure from the differential pressure sensor or other flow sensing technology
- Units conversion constant to convert to air velocity
- Flow coefficient value for calibration at a specific airflow setpoint
- Duct area in units compatible with the units conversion constant
- Nominal airflow value (user entered)
- Hood air volume flow value to aid in calculating the correct flow coefficient value

First, air velocity is a calculated value computed from logic using the following formula:

Where:

- AirVEngrgUnits is the velocity of the air in engineering units that are compatible with the other units used in the airflow formulas above
- KEngrgUnits = constant for engineering unit correction [the “4005” (English units) or “1.29” (SI units)
- CSpanCal = constant to calibrate for sensor span (also called flow coefficient)
- Pdifferential = differential pressure sensor reading from duct

Next, Air Volume Flow in flow units (VavSuAirFl) is calculated by the formula:

Where:

- Area = area of duct
- FlCnv = conversion constant for converting between box dimensional engineering units and desired flow engineering units

Air Volume Flow as a percentage of the nominal airflow (Relative Air Flow [VavSuAirFlRel]) is calculated by the formula:

Where:

- AirFlNom is the nominal air volume flow of the VAV terminal box.