Basic Knowledge of Anemometers - Classification and Selection
Anemometer Probe Selection
The flow velocity measurement range of 0 to 100 m/s is divided into three sections: low speed (0 to 5 m/s), medium speed (5 to 40 m/s), and high speed (40 to 100 m/s). Thermal probes of anemometers are suitable for measurements at 0–5 m/s; rotary vane probes deliver optimal performance for flow velocities of 5 to 40 m/s; pitot tubes achieve the best results in the high‑speed range.
Temperature is an additional key criterion for proper probe selection. Conventional thermal sensors generally operate within a temperature range of ±7 °C. Specialized rotary vane probes can withstand temperatures up to 35 °C, while pitot tubes are applicable for temperatures above 35 °C.
Thermal Probes
The working principle of thermal probes is based on cool airflow removing heat from heating elements. A regulating circuit maintains a constant temperature, making the control current proportional to the flow velocity.When thermal probes are used in turbulent flow, airflow striking the sensing element from all directions simultaneously reduces measurement accuracy. In turbulent conditions, readings from thermal velocity sensors are usually higher than those from rotary vane probes. This phenomenon is commonly observed in duct measurements and may occur even at low speeds due to differences in duct turbulence design.
Accordingly, measurements must be performed on straight duct segments. The straight section shall extend at least 10×D upstream of the measuring point and at least 4×D downstream, where D refers to the duct diameter in centimeters. The flow cross‑section must be free of any obstructions, such as sharp edges, suspended fixtures and other barriers.
Rotary Vane Probes
Rotary vane probes operate by converting rotational motion into electrical signals. A proximity sensor counts the rotation of the impeller and generates a pulse sequence, which is processed and converted by the measuring unit to obtain 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 better suited for airflow measurement in ducts with a cross‑sectional area more than 100 times larger than that of the probe.
