Operating Principle of Thermal Probes for Anemometers
Its principle is based on cool convective airflow removing heat from the heated sensing element. A control circuit maintains a constant temperature, so that the regulating current changes in direct proportion to the flow velocity. When a thermal probe is used in turbulent flow, air currents striking the element from all directions simultaneously compromise measurement accuracy. In turbulent conditions, readings from thermal anemometer sensors are typically higher than those taken with rotary vane probes. This effect is frequently observed during duct measurements and may occur even at low flow speeds, depending on duct design and turbulence characteristics. For this reason, measurements should be performed within straight duct segments. The measuring point shall be located no less than 10×D downstream from upstream disturbances and no less than 4×D upstream from downstream obstructions, where D stands for the duct diameter in centimeters. The entire flow cross‑section must remain free of obstacles such as sharp edges, suspended fittings and other blockages.
Rotary Vane Probes of Anemometers
The rotary vane probe works by converting mechanical rotation into electrical signals. A proximity sensor counts the revolutions of the vane wheel and generates a continuous pulse sequence. The 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, for example at duct outlets. Small‑diameter probes are better suited for ducts whose cross‑sectional area is more than 100 times that of the probe.
Anemometer Positioning in Airflow
For correct operation, the rotary vane probe must be aligned so that the airflow runs parallel to the vane axis. Gently rotating the probe in the air stream will alter the displayed reading. The probe is correctly positioned when the measured value reaches its maximum. In duct applications, sufficient straight duct length ahead of the measuring point reduces the impact of turbulence on both thermal probes and pitot tubes.
