Air Flow Sensor in BMS
The River Current Analogy
Stand at the edge of a river. The water moves — sometimes slowly, sometimes fast. The speed of that water movement is its flow velocity. The river has a certain width and depth — its cross-section area. Multiply velocity by area — you get how much water flows past per second (volume flow rate).
An air flow sensor works on exactly this principle — measuring the velocity of air in a duct and calculating volume flow rate.
Why Measure Air Flow?
Design says: AHU-01 must supply 5000 m³/h of air to Floor 3.
Site reality: Is it actually delivering 5000 m³/h? Or 3500? Or 6500?
Without measurement — nobody knows. With an air flow sensor — BMS knows continuously and adjusts fan speed to maintain the correct flow.
How Air Flow Sensors Work:
Pitot tube type:
Measures velocity pressure (difference between total and static pressure)
Velocity calculated from pressure
Volume flow = velocity × duct area
Connected to DPT (differential pressure transmitter)
Thermal dispersion type:
Heating element in air stream
Moving air cools it — more airflow = more cooling
Cooling rate measured → converted to velocity
Accurate at low flow rates
Hot wire anemometer:
Fine wire heated electrically
Airflow cools wire → resistance changes → velocity measured
Very sensitive — used for precise low-flow measurement
Air Flow Sensor Applications:
AHU supply air: Verify correct air delivery to floor
VAV box control: Measure actual flow through VAV box for control
Fume hood exhaust: Laboratory safety — must maintain minimum exhaust
Kitchen exhaust: Verify adequate extraction is occurring
Clean room: Confirm design air changes per hour are maintainedRelated Topics
- What is BMS integration? — how a BMS connects with VFDs, energy meters, BACnet/Modbus devices and other building systems
- How to design a BMS system step by step — the complete BMS design methodology covering site survey, IO list, controller selection, sequence of operations
- What is a Building Management System (BMS)? — fundamentals of BMS controls and architecture for HVAC, lighting, energy and access
- What is BMS commissioning? — the disciplined commissioning process that turns a BMS install into a working building brain
- Browse all Sensors topics — more from this section of the EnSmart BMS Library