The principle of anemometer and the problems that need to be paid attention to when using 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.
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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.
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The measurement range of 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.
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When using an anemometer, we should pay attention to the following points:
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1. It is forbidden to use the anemometer in a flammable gas environment.
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2. It is forbidden to place the anemometer probe in flammable gas. Otherwise, fire or even explosion may result.
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3. Please use the anemometer correctly according to the requirements of the instruction manual. Improper use may cause electric shock, fire and sensor damage.
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4. During use, if the anemometer emits abnormal smell, sound or smoke, or liquid flows into the anemometer, please shut down immediately and take out the battery. Otherwise, there will be danger of electric shock, fire and damage to the anemometer.
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5. Do not expose the probe and anemometer body to the rain. Otherwise, there may be danger of electric shock, fire and personal injury.
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6. Do not touch the sensor inside the probe.
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7. When the anemometer is not in use for a long time, please take out the internal battery. Otherwise, the battery may leak and cause damage to the anemometer.
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8. Do not place the anemometer in places with high temperature, high humidity, dust and direct sunlight. Otherwise, it will cause damage to internal components or deterioration of the performance of the anemometer.
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9. Do not wipe the anemometer with volatile liquid. Otherwise, the shell of the anemometer may be deformed and discolored. If the surface of the anemometer is stained, it can be wiped with a soft cloth and neutral detergent.
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10. Do not drop or put heavy pressure on the anemometer. Otherwise, it will cause malfunction or damage of the anemometer.
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11. Do not touch the sensor part of the probe when the anemometer is charged. Otherwise, it will affect the measurement result or cause damage to the internal circuit of the anemometer.
