How to Select an Anemometer Probe
The flow velocity measurement range (0 to 100 m/s) is divided into three segments: low speed from 0 to 5 m/s, medium speed from 5 to 40 m/s, and high speed from 40 to 100 m/s. Thermal anemometer probes are designed for measurements within 0–5 m/s; rotary vane probes deliver optimal performance at 5–40 m/s; and pitot tubes provide the most reliable results for high‑speed flow.
Temperature is an essential supplementary criterion for probe selection. Standard thermal sensors typically operate within a range of ±7 °C. Heavy‑duty rotary vane probes can withstand temperatures up to 35 °C, while pitot tubes are suitable for environments exceeding 35 °C.
Even without grille obstructions at suction openings, airflow remains disordered in direction, and the flow profile is highly uneven. This is caused by local negative pressure inside the duct, which draws air into the chamber in a funnel‑shaped pattern. No valid measuring position that meets testing requirements can be found in the immediate vicinity of the suction point.
Methods such as grid measurement with average calculation cannot accurately determine volumetric flow. Only duct testing or funnel measurement ensures repeatable and stable results. Measuring funnels of various sizes are available to accommodate different working conditions. A measuring funnel creates a stable, standardized flow cross‑section at a set distance upstream of the flap valve. After centering and securing the probe at this cross‑section, the measured velocity value is multiplied by the funnel correction coefficient to calculate the extracted volumetric flow rate (e.g., a funnel factor of 20).
Thermal Probes for Anemometers
Thermal probes operate on the principle of convective heat loss: cool airflow carries away heat from a heated sensing element. A control circuit maintains a constant element temperature, whereby the regulating current varies proportionally with flow velocity.
In turbulent flow, multidirectional airflow continuously impacts the thermal sensor, reducing measurement accuracy. Under turbulent conditions, readings from thermal sensors are generally higher than those from rotary vane probes. This phenomenon is common in duct measurements and may occur even at low velocities, depending on duct geometry and turbulence characteristics.
For accurate results, all measurements must be conducted within straight duct sections. The measuring point shall be located at least 10×D downstream from upstream disturbances and at least 4×D upstream from downstream obstructions, where D denotes the duct diameter in centimeters. The entire flow cross‑section must remain free of barriers such as sharp corners, suspended components and other obstacles.
