How an anemometer works

An anemometer is an instrument that measures the speed of air. It has many types, meteorological stations

The most commonly used is the wind cup anemometer, which consists of three parabolic cone empty cups fixed on the bracket at 120° to each other to form the sensing part, and the concave surfaces of the empty cups are all in one direction. The entire induction part is installed on a vertical rotating shaft. Under the action of the wind, the wind cup rotates around the shaft at a speed proportional to the wind speed.

working principle

 The basic principle of the anemometer is to put a thin wire in the fluid, and pass the current to heat the wire to make the temperature higher than the temperature of the fluid, so the wire anemometer is called "". When the fluid flows through the wire in the vertical direction, it will take away part of the heat of the wire, causing the temperature of the wire to drop. According to the theory of forced convection heat exchange, it can be derived that there is a relationship between the dissipated heat Q and the velocity v of the fluid. The standard probe consists of two brackets tensioned with a short, thin wire. Metal wires are usually made of metals with high melting points and good ductility such as platinum, rhodium, and tungsten. The commonly used wire is 5 μm in diameter and 2 mm in length; the probe is only 1 μm in diameter and 0.2 mm in length. According to different uses, the probe is also made into double wire, three wire, oblique wire, V-shaped, X-shaped and so on. In order to increase the strength, a metal film wire is sometimes used, and a thin metal film is usually sprayed on a thermally insulating substrate, which is called a hot film probe. Probes must be calibrated before use. Static calibration is carried out in a special standard wind tunnel, measuring the relationship between flow rate and output voltage and drawing a standard curve; dynamic calibration is carried out in a known pulsating flow field, or adding in the heating circuit of the anemometer. On the pulsating electrical signal, check the frequency response of the anemometer, if the frequency response can be improved with the corresponding compensation circuit.

The flow velocity measurement range 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 probe of the anemometer is used for measurements from 0 to 5m/s; the rotor probe of the anemometer is ideal for measurements from 5 to 40m/s; and the use of a pitot tube gives the best results in the high-speed range. An additional criterion for the correct selection of an anemometer's flow probe is temperature, usually the temperature of the thermal sensor of an anemometer is about +-70C. The rotor probe of the special anemometer can reach 350C. Pitot tubes are used above +350C.

advantage

Anemometers have many advantages, the following are some of the more important:

1. Small size, little interference to the flow field;

2. Wide range of application. It can be used not only for gases but also for liquids, and can be used in subsonic, transonic and sonic flows of gases;

3. High measurement accuracy and good repeatability. The disadvantage of the anemometer is that the probe has a certain interference with the flow field and is easy to break.

4. In addition to measuring the average speed, it can also measure the pulsation value and turbulent amount; in addition to measuring the movement in one direction, it can also measure the speed components in multiple directions at the same time.