Thermistor probe principle of anemometer
The basic principle of an anemometer is to place a thin metal wire in a fluid, apply current to heat the wire, and make its temperature higher than the temperature of the fluid. Therefore, the metal wire anemometer is called a "hot wire". When the fluid flows through the metal wire in a vertical direction, it will take away some of the heat from the wire, causing the temperature of the wire to decrease. According to the theory of forced convective heat exchange, a relationship can be derived between the dissipated heat Q and the velocity v of the fluid. A standard probe consists of two brackets tensioned with a short and thin metal wire. Metal wires are usually made of metals with high melting points and good ductility, such as platinum, rhodium, tungsten, etc. The commonly used wire diameter is 5 μ m and the length is 2 mm; The smallest probe has a diameter of only 1 μ m and a length of 0.2 mm.
According to different purposes, the probe can also be made into double wire, triple wire, oblique wire, V-shaped, X-shaped, etc. In order to increase strength, metal film is sometimes used instead of metal wire. Usually, a thin metal film is sprayed on a thermally insulated substrate, which is called a hot film probe. The probe must be calibrated before use. Static calibration is carried out in a specialized standard wind tunnel, measuring the relationship between flow velocity and output voltage and drawing it into a standard curve; Dynamic calibration is performed in a known pulsating flow field or by adding a pulsating electrical signal to the heating circuit of an anemometer to verify the frequency response of the hot wire anemometer. If the frequency response is poor, corresponding compensation circuits can be used to improve it.
The measurement range of flow velocity from 0 to 100m/s can be divided into three sections: low speed: 0 to 5m/s; Medium speed: 5 to 40m/s; High speed: 40 to 100m/s. The thermal sensitive probe of the anemometer is used for * * measurement from 0 to 5m/s; The rotary probe of the anemometer measures flow velocities of 5 to 40m/s with ideal results; By using a pitot tube, the best results can be obtained within the high-speed range. An additional criterion for correctly selecting the flow velocity probe of an anemometer is temperature, and typically the temperature range for the thermal sensor of an anemometer is around+-70C. The rotary probe of the specially designed anemometer can reach up to 350C. Pitot tubes are used for temperatures above+350C.
Anemometer
By using a regulating switch to maintain a constant temperature, the heat on the component can be adjusted in direct proportion to the current and flow rate. When using a thermal sensitive probe in turbulence, airflow from all directions simultaneously impacts the thermal element, which can affect the accuracy of the measurement results. When measuring in turbulence, the reading of the thermal anemometer flow sensor is often higher than that of the rotary probe. The above phenomenon can be observed during pipeline measurement. According to different designs for managing turbulent flow in pipelines, it may even occur at low speeds. Therefore, the anemometer measurement process should be carried out in the straight section of the pipeline. The starting point of the straight section should be at least 10 × D (D=pipe diameter, in CM) outside the measurement point; The endpoint should be at least 4 × D behind the measurement point. The fluid cross-section must not have any obstruction. (Sharp edges, heavy suspension, objects, etc.)
