Working Principle and Component Structure of Anemometers
Anemometers operate on the principle that cold ambient airflow removes heat from thermal sensing elements. A control circuit maintains a constant element temperature, whereby the regulated operating current varies in direct proportion to the flow velocity. When thermal probes are used in turbulent conditions, airflow striking the sensing element from multiple directions impairs measurement accuracy. In turbulent flow, readings from thermal velocity sensors are generally higher than those from rotary vane probes. This effect is commonly observed during duct measurements and may occur even at low flow rates, depending on the duct design and turbulence characteristics.
For this reason, all duct measurements should be performed within straight duct segments. The measuring point shall be located no less than 10×D downstream from any upstream disturbance and no less than 4×D upstream from any downstream disturbance, where D denotes the duct diameter in centimeters. The entire flow cross‑section must remain free of obstructions.
Rotary Vane Probe of the Anemometer
The rotary vane probe converts mechanical rotational motion into electrical signals. A proximity sensor counts vane revolutions and generates a continuous pulse train. The measuring instrument processes these pulses to calculate the rotational speed.
Large‑diameter probes (60 mm, 100 mm) are suitable for measuring turbulent flow at medium and low velocities, such as at duct outlets. Small‑diameter probes are recommended for ducts whose cross‑sectional area is more than 100 times that of the probe.
Anemometer Positioning in Airflow
For accurate operation, the rotary vane probe must be aligned so that the airflow runs parallel to the vane axis. Slight angular adjustment of the probe will change the displayed reading; the correct orientation is confirmed when the measured value reaches its maximum. Sufficient straight duct length upstream of the test point reduces the impact of turbulence on both thermal probes and pitot tubes.
In‑Duct Air Velocity Measurement
Field application verifies that the 16 mm anemometer probe offers the most versatile performance. Its optimized dimensions ensure low flow resistance while permitting stable measurement of flow speeds up to 60 m/s. In‑duct velocity testing is a reliable standard method, and indirect measurement procedures such as the grid traversal method are widely applicable to air duct testing.
