Air flow meters

Air flow meters, commonly called capture hoods or flow hoods, are specialized instruments for the direct measurement of the volumetric air flow at supply and return outlets of ventilation, air conditioning and residential mechanical ventilation (MVHR) systems. Unlike anemometers, which measure velocity at a point and require integration over an area to obtain flow, capture hoods “capture” the entire air flow leaving the outlet and measure its volume over time directly, giving the flow value in m³/h or L/s. They are the instrument of choice for balancing HVAC systems and commissioning new buildings.

Architecture of the capture hood

The flow hood consists of: a capture hood made of rigid fabric (in various sizes and geometries – typically 60×60 cm, 90×30 cm, 60×120 cm, to cover different outlet shapes); a manifold at the base of the hood that channels the flow towards a flow sensor (typically an array of thermal sensors or Pitot tubes measuring the mean velocity in the manifold); and a support structure (handle, tripod) for use while standing, at ceiling heights of up to 3–4 meters. The integrated electronics calculate the flow by multiplying the mean velocity by the cross-section of the manifold.

Technical characteristics

Capture hoods typically cover flow ranges from 25 to 4000 m³/h, with an accuracy of ±3% of reading. Resolution is typically 1 m³/h. Advanced functions include: automatic compensation for the restriction effect (back-pressure correction) – the hood itself slightly alters the flow, and the electronics apply correction factors to obtain the “undisturbed” value; campaign memory; USB/Bluetooth data output for PC software; measurement of both supply and return (with the sign displayed); and automatic calculation of the percentage deviation from the design setpoint.

Applications

Typical use is in: commissioning new HVAC systems (checking the correct distribution of the design flow rates at every outlet in the building); balancing existing air systems (adjusting the balancing dampers to achieve the correct flow at each terminal); periodic maintenance checks (to identify drift in the system, deposits in the ducts, deterioration of filters, problems with fan motors); acceptance testing of operating theaters (with flow rates prescribed by regulation); acceptance testing of cleanrooms (in accordance with ISO 14644); and acceptance testing of residential MVHR systems (for energy certification).

Internal measurement technology

Capture hoods use two main technologies to measure velocity in the manifold: an array of hot-wire sensors (typically 8–16 sensors distributed evenly across the manifold area, for measurements independent of asymmetric velocity profiles), or multiple Pitot tubes with an internal low-pressure differential manometer. Hot-wire arrays offer better accuracy at low flows and with irregular profiles; Pitot tubes are more rugged and less sensitive to dust, but less sensitive at low velocities. Advanced models combine both technologies to optimize performance across all ranges.

Reference standards

The main standards are: EN 12599 (testing and commissioning of building ventilation systems); ASHRAE 111 (USA, HVAC commissioning); ISO 14644-3 (testing and commissioning of cleanrooms); UNI EN 12792 (HVAC terminology). Periodic calibration of capture hoods is carried out in reference wind tunnels with ISO 17025 traceability, at a minimum of 5 points across the range. The typical interval is annual for intensive professional use. Correct measurement procedures (correct positioning of the hood on the outlet, stabilization time, measurement sequence) are critical to the reliability of the balancing.

Selection criteria

The choice depends on the typical outlet sizes encountered in projects (the various hood models come in different sizes and suit different outlet shapes); on the flow range required (low flows for residential MVHR, high flows for commercial HVAC); on portability (weight, folded dimensions); on the reporting software functions; and on the data interfaces for integration with company BMS balancing software. Manufacturers offer complete kits with telescopic handles, interchangeable hoods and accessories for special geometries.