The workings of an anemometer and the issues that should be taken into consideration when utilizing it
The basic principle of the anemometer is to place a thin metal wire in the fluid, and heat the wire through an electric current to make its 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 and reduce the temperature of the wire. According to the theory of forced convective heat exchange, there is an anemometer relationship between the dissipated heat Q and the fluid velocity v. A standard probe consists of a short, thin wire stretched between two brackets, as shown in Figure 2.1. 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; the smallest probe has a diameter of only 1 μm 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 shape, X shape, etc. In order to increase the strength, sometimes a metal film is 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. The probe 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 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.
