Principle Description of Thermal Probe for Anemometer
The basic principle of an anemometer is to place a thin metal wire within a fluid. An electric current heats the wire to a temperature higher than that of the surrounding fluid, hence the name hot-wire anemometer. When fluid flows perpendicularly across the wire, it removes part of the heat and lowers the wire temperature. Based on the theory of forced convective heat transfer, a functional relationship can be derived between the heat loss Q of the hot wire and the fluid velocity v.
A standard hot-wire probe consists of a short, fine metal wire tensioned between two supports. The wire is commonly made of high‑melting, ductile metals such as platinum, rhodium, or tungsten. The typical wire measures 5 μm in diameter and 2 mm in length; miniature probes have a wire diameter of only 1 μm and a length of 0.2 mm.
Depending on application requirements, hot-wire probes are manufactured in dual-wire, triple-wire, inclined-wire, V‑shaped and X‑shaped configurations. To enhance structural strength, a thin metal film may replace the wire. Deposited onto a thermally insulated substrate, this type is known as a hot-film probe.
Hot-wire probes must be calibrated before use. Static calibration is carried out in a dedicated standard wind tunnel to establish the relationship between flow velocity and output voltage and generate a calibration curve. Dynamic calibration is performed within a known pulsating flow field, or by applying a pulsed electrical signal to the anemometer heating circuit to verify frequency response. Poor frequency response can be improved with dedicated compensation circuits.
The measurement range of 0 to 100 m/s is divided into three segments:Low speed: 0–5 m/sMedium speed: 5–40 m/sHigh speed: 40–100 m/s
Thermal anemometer probes are suitable for low‑velocity measurement from 0 to 5 m/s. Rotary vane probes deliver optimal performance at 5 to 40 m/s, while pitot tubes provide the best results for high‑velocity measurement.
Ambient temperature is an additional key criterion for probe selection. Conventional thermal sensors generally operate within approximately ±7 °C. Specialized rotary vane probes withstand temperatures up to 35 °C, and pitot tubes are used for conditions exceeding 35 °C.
