The basic principle of the anemometer is to put a thin wire in the fluid, and the wire is heated by the current to make the temperature higher than the temperature of the fluid, so the wire anemometer is called "hot wire". 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 deduced that there is a relationship between the heat Q dissipated by the heat ray and the velocity v of the fluid. Standard hot wire probes consist 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 5μm in diameter and 2mm in length; the smallest probe is only 1μm in diameter and 0.2mm in length. According to different uses, the hot wire probe is also made into double wire, three wire, oblique wire, V shape, X shape and so on. In order to increase the strength, sometimes a metal film is used to replace the metal wire, and a thin metal film is usually sprayed on the thermally insulating substrate, which is called a thermal film probe. Hot wire 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 in the anemometer heating circuit. On the pulsating electrical signal, the frequency response of the hot-wire anemometer is verified. If the frequency response is not good, the corresponding compensation circuit can be used to improve it. 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 accurate measurement from 0 to 5m/s; the rotor probe of the anemometer is ideal for measuring the flow velocity of 5 to 40m/s; and the pitot tube can be used for the best measurement in the high speed range result. 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 plus -70C. The rotor probe of the special anemometer can reach 350C. Pitot tubes are used above plus 350C
