Measurement Technology and Selection Guidelines for Anemometers
Selection of Anemometer Probes
The flow velocity measuring range of 0 to 100 m/s is divided into three segments: 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 applied to low‑velocity measurement within 0–5 m/s; rotary vane probes deliver optimal performance for flow velocity between 5 and 40 m/s; pitot tubes achieve the best measuring results in the high‑speed range.
Ambient temperature is an additional key criterion for proper probe selection. Ordinary thermal sensors generally operate within a temperature range of ±7 °C. Special rotary vane probes can withstand temperatures up to 35 °C, while pitot tubes are suitable for environments above 35 °C.
Thermal Probes of Anemometers
The working principle of thermal probes relies on cold airflow removing heat from heating elements. A regulating circuit maintains a constant temperature, whereby the regulating current is directly proportional to the flow velocity.
When thermal probes are used in turbulent flow, airflow impacting the heating element from all directions simultaneously will reduce measurement accuracy. In turbulent conditions, readings from thermal flow sensors are usually higher than those of rotary vane probes. This phenomenon is commonly observed in duct measurements and may occur even at low velocities due to differences in duct turbulence design.
Therefore, anemometer measurements shall be conducted on straight duct sections. The starting point of the straight section shall be at least 10×D upstream of the measuring point, and the end point at least 4×D downstream (D = duct diameter, unit: cm). No obstructions such as corners, suspended components or fittings are allowed on the flow cross-section.
Rotary Vane Probes of Anemometers
Rotary vane probes convert mechanical rotation into electrical signals. A proximity sensor counts vane rotations and generates a pulse sequence, which is processed and converted by the detector 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 airflow at duct outlets. Small‑diameter probes are more applicable to airflow measurement where the cross‑sectional area of the duct is more than 100 times that of the probe.
Positioning of Anemometers in Airflow
For correct measurement of rotary vane probes, the airflow direction shall be parallel to the vane axis. Slight rotation of the probe in the airflow will change the measured reading. The probe is correctly positioned when the reading reaches its maximum value.
For duct measurement, a sufficiently long straight duct segment upstream of the measuring point can effectively reduce the influence of turbulence on thermal probes and pitot tubes.
