The working principle and maintenance of the new anemometer
Anemometer is an instrument that measures air flow rate. There are many types of it. The most commonly used meteorological station is the wind cup anemometer. It consists of three parabolic cone empty cups fixed to the bracket at 120 ° each other. The concave part of the empty cup goes in one direction. The whole induction part is installed on a vertical rotating shaft. Under the action of wind force, the wind cup rotates around the shaft at a speed proportional to the wind speed.
working principle
Anemometer
The basic principle of the anemometer is to put a thin metal wire in the fluid, and heat the metal wire with an electric current to make its temperature higher than the temperature of the fluid. Therefore, the wire anemometer is called a "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 derived that there is a relationship between the heat Q lost by the hot wire and the velocity v of the fluid. The standard hot wire probe consists of two brackets tensioned by a short and thin metal wire. Metal wire is usually made of platinum, rhodium, tungsten and other metals with high melting point and good ductility. The commonly used wire diameter is 5 μm and the length is 2 mm; the smallest probe diameter is only 1 μm and the length is 0.2 mm. According to different uses, the hot wire probe is also made into double wire, triple wire, diagonal wire, V shape, X shape, etc. In order to increase the strength, sometimes a metal film is used to replace the metal wire, usually a thin metal film is sprayed on a thermally insulating substrate, called a hot film probe. The hot wire probe must be calibrated before use. Static calibration is performed in a special standard wind tunnel, measuring the relationship between flow velocity and output voltage and drawing a standard curve; dynamic calibration is performed in a known pulsating flow field, or added in the anemometer heating circuit Check the frequency response of the hot wire anemometer with the previous pulsating electrical signal. If the frequency response is not good, the corresponding compensation circuit can be used to improve it.
The flow rate measurement range of 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 anemometer's thermal probe is used for accurate measurement of 0 to 5m / s; the anemometer's rotating probe is ideal for measuring a flow velocity of 5 to 40m / s; and the use of a pitot tube can get the best in the high speed range result. An additional criterion for the correct selection of the anemometer's flow rate probe is temperature. Usually the use temperature of the anemometer's thermal sensor is about + -70C. The wheel probe of the special anemometer can reach 350C. Pitot tubes are used above + 350C.
maintain
The wind speed instrument belongs to the measuring instrument of safety protection and environmental monitoring, and it is a compulsory verification measuring instrument stipulated by the Chinese Metrology Law. In addition to the factory sales, you need to have a corresponding calibration report, and you must also go to the National Air Conditioning Equipment Quality Supervision and Inspection Center or the China Academy of Building Research Building Energy and The Environmental Testing Center carries out regular calibration and adjusts all aspects of the instrument according to its statutory calibration certificate to obtain the best working condition.
In addition to maintaining the accuracy of daily data, pay attention to the following points in daily maintenance and use:
1. It is forbidden to use anemometer in flammable gas environment.
2. It is forbidden to place the anemometer probe in flammable gas. Otherwise, it may cause a fire or even an explosion.
3. Please use the anemometer correctly according to the requirements of the instruction manual. Improper use may result in electric shock, fire and sensor damage.
4. In use, if the anemometer emits abnormal smell, sound or smoke, or liquid flows into the anemometer, please immediately shut down and remove the battery. Otherwise, there is a risk of electric shock, fire and damage to the anemometer.
5. Do not expose the probe and the anemometer body to rain. Otherwise, there is a danger of electric shock, fire, and personal injury.
6. Do not touch the sensor inside the probe.
7. When the anemometer is not used for a long time, please remove the internal battery. Otherwise, the battery may leak, causing damage to the anemometer.
8. Do not place the anemometer in a place with high temperature, high humidity, dust and direct sunlight. Otherwise, it will cause damage to internal devices or deteriorate the performance of the anemometer.
9. Do not wipe the anemometer with volatile liquids. Otherwise, it may cause deformation and discoloration of the anemometer housing. When the surface of the anemometer is stained, it can be wiped with soft fabric and neutral detergent.
10. Do not drop or press the anemometer. Otherwise, it will cause malfunction or damage of the anemometer.
11. Do not touch the sensor part of the probe when the anemometer is charged. Otherwise, it will affect the measurement results or cause damage to the internal circuit of the anemometer
The main purpose
Anemometer 1. Measure the speed and direction of the average flow.
Anemometer
Anemometer
2. Measure the pulsating velocity of incoming flow and its frequency spectrum.
3. Measure the Reynolds stress in the turbulent flow and the speed dependence and time dependence of the two points.
4. Measure the wall shear stress (usually carried out with a hot film probe placed flush with the wall, the principle is similar to hot wire speed measurement).
5. Measure fluid temperature (measure the change curve of probe resistance with fluid temperature in advance, and then determine the temperature according to the measured probe resistance.
In addition, many professional uses have been developed.
Instructions
1. Observe whether the pointer of the meter is at zero before use. If there is a deviation, you can gently adjust the mechanical adjustment screw of the meter to return the pointer to zero; 2. Place the calibration switch in the off position
3. Insert the measuring rod plug into the socket, the measuring rod is placed vertically upward, the screw plug is pressed tightly to seal the probe, the "correction switch" is placed at the full scale position, and the "full scale adjustment" knob is slowly adjusted so that the meter pointer At full scale;
4. Set the "correction switch" to "zero position" and slowly adjust the two knobs "coarse adjustment" and "fine adjustment" so that the meter pointer points to the zero position
5. After the above steps, gently pull the plug to expose the rod probe (the length can be selected according to need), and make the red dot on the probe face the wind direction. According to the meter reading, refer to the calibration curve. Detect the measured wind speed;
6. After measuring for several minutes (about 10min), the above steps 3 and 4 must be repeated once to standardize the current in the instrument
7. After the test, the "correction switch" should be placed in the off position.
An anemometer is a type of speed measuring instrument that converts flow rate signals into electrical signals. It can also measure fluid temperature or density. The principle is that a thin metal wire (called a hot wire) heated by electricity is placed in the airflow. The heat dissipation of the hot wire in the airflow is related to the flow rate, and the heat dissipation causes the temperature of the hot wire to change and the resistance changes. The flow rate signal is converted into electric signal. It has two working modes: ①constant current type. The current through the hot wire remains the same, when the temperature changes, the resistance of the hot wire changes, so the voltage across the two ends changes, thereby measuring the flow rate; ② constant temperature type. The temperature of the hot wire remains unchanged, for example, at 150 ° C, and the flow rate can be measured according to the current to be applied. The constant temperature type is more widely used than the constant current type. The length of the hot wire is generally in the range of 0.5 to 2 mm, and the diameter is in the range of 1 to 10 microns. The material is platinum, tungsten or platinum-rhodium alloy. If a very thin (thickness less than 0.1 micron) metal film is used to replace the metal wire, it is a hot film anemometer. Its function is similar to that of a hot wire, but it is mostly used to measure the flow rate of liquid. In addition to the ordinary single-wire type, the hot line can also be a combined two-wire type or three-wire type to measure the speed component in various directions. The electrical signal output from the hotline is amplified, compensated and digitized and then input into the computer, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the speed measurement function, such as the simultaneous completion of the instantaneous value and the time average, the closing speed and the partial speed, and the degree of turbulence And other turbulence parameters. Compared with a pitot tube, the hot wire anemometer has the advantages of small probe volume and small interference to the convection field; fast response, can measure unsteady flow velocity; can measure very low speed (such as as low as 0.3 m / s) and so on.
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