The principle of anemometry and the use of continued attention to the problem
The basic principle of anemometer is to put a thin metal wire in the fluid, and pass the electric current to heat the metal wire so that its temperature is higher than that 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, so that the temperature of the wire decreases. According to the theory of forced convection heat exchange, the relationship between the dissipated heat Q and the velocity v of the fluid can be derived. A standard probe consists of two brackets tensioning a short, thin wire, as shown in Figure 2.1. The wire is usually made of metals with high melting points and good ductility such as platinum, rhodium and tungsten. Commonly used wire diameter of 5μm, 2mm long; zui small probe diameter of only 1μm, 0.2mm long.
According to different uses, the probe is also made into double wire, triple wire, oblique wire and V-shaped, X-shaped and so on. In order to increase the strength, sometimes with a metal film instead of wire, usually in a thermally insulating substrate sprayed with a thin film of metal, called thermal film probe, the probe must be calibrated before use. Static calibration is carried out in a special standard wind tunnel, measuring the relationship between flow velocity and output voltage and drawn as a standard curve; dynamic calibration is carried out in the known pulsating flow field, or in the anemometer heating circuit with a pulsating electrical signal, the frequency response of the calibration anemometer, if the frequency response is poor can be used to improve the corresponding compensation line.
0 to 100m/s flow velocity measurement range can be divided into three zones: low speed: 0 to 5m/s; medium speed: 5 to 40m/s; high speed: 40 to 100m/s. Anemometer's thermal probe for the measurement of 0 to 5m/s; anemometer's rotating wheel probe to measure the flow velocity from 5 to 40m/s effect is ideal; and the use of the Pitot tube can be used to get the best results in the high speed range. 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 velocity probe is the temperature, typically the thermal sensor of an anemometer can be used at temperatures up to about +-70 C. Specialised anemometers have rotor probes up to 350 C. Pitot tubes are used at temperatures of +350 C and above.
