The principle and advantages of anemometer
working principle
The basic principle of an anemometer is to place a thin metal wire in a fluid, and heat the wire through an electric current to make its temperature higher than that of the fluid, so the wire anemometer is called a "hot wire". When the fluid flows through the wire in the vertical direction, it will take away part of the heat of the wire and reduce the temperature of the wire. According to the theory of forced convection heat exchange, there is a relationship between the heat Q lost by the hot line and the velocity v of the fluid. A standard hot wire probe consists of a short, thin wire stretched between two brackets. Metal wire is usually made of platinum, rhodium, tungsten and other metals with high melting point and good ductility. The commonly used wire has a diameter of 5 μm and a length of 2 mm; a small probe has a diameter of only 1 μm and a length of 0.2 mm. According to different purposes, the hot wire probe can also be made into double wire, triple wire, oblique wire, V shape, X shape, etc. In order to increase the strength, a metal film is sometimes used instead of a metal wire, and a thin metal film is usually sprayed on a thermally insulating substrate, which is called a hot film probe. Hot wire probes must be calibrated before use. Static calibration is carried out in a special standard wind tunnel, and the relationship between flow velocity and output voltage is measured and drawn as a standard curve; dynamic calibration is carried out in a known fluctuating flow field, or in the heating circuit of the anemometer. Check the frequency response of the hot-wire anemometer with the last pulsating electrical signal. If the frequency response is not good, it can be improved with the corresponding compensation circuit.
The measurement range of flow velocity from 0 to 100m/s can be divided into three sections: low velocity: 0 to 5m/s; medium velocity: 5 to 40m/s; high velocity: 40 to 100m/s. The thermal probe of the anemometer is used for the measurement of 0 to 5m/s; the rotating wheel probe of the anemometer is ideal for measuring the flow velocity of 5 to 40m/s; and the pitot tube can get the best results in the high speed range . An additional criterion for the correct selection of the flow rate probe of the anemometer is the temperature. Usually, the temperature of the thermal sensor of the anemometer is about +-70C. The rotor probe of the special anemometer can reach 350C. Pitot tubes are used above +350C.
Anemometers have many advantages, the following are some of the more important ones:
1. Small size, little interference to the flow field;
2. Wide application range. It can be used not only for gas but also for liquid, and can be used in subsonic, transonic and supersonic flow of gas;
3. High measurement accuracy and good repeatability. The disadvantage of the hot wire anemometer is that the probe interferes with the flow field, and the hot wire is easy to break.
4. In addition to measuring the average speed, it can also measure the pulsation value and turbulence; in addition to measuring the unidirectional movement, it can also measure the velocity components in multiple directions at the same time.
