Anemometer

Anemometers are, as the name suggests, instruments that measure air flow. There are many types of it, and the most commonly used meteorological station is the wind speed and anemometer of the wind cup. The anemometer consists of three parabolic cones that are fixed to each other at an angle of 120° to form an induction part. The concave surface of the empty cup follows a direction. direction. The entire sensing part is mounted on a vertical axis of rotation, and the wind cup rotates around the shaft at a speed proportional to the wind speed. Another type of rotary anemometer is the propeller anemometer, which consists of a three- or four-blade propeller and is mounted on the front of a wind vane so that it is always aligned with the direction of the wind. The blades rotate about the horizontal axis at a speed that is proportional to the wind speed.

Anemometer principle The basic principle of the anemometer is to place a thin metal wire in the fluid and heat the wire by heating it to a temperature higher than that of the fluid. Therefore, the wire anemometer is called a “hot wire”. When the fluid flows through the wire in the vertical direction, a part of the heat of the wire will be taken away, so that the temperature of the wire will drop. According to the theory of forced convection heat exchange, a relationship exists between the heat Q lost in the hot wire and the velocity v of the fluid. The standard hot wire probe is made up of two short and thin wires that are stretched by two supports, as shown in Figure 2.1. Metal wires are usually made of metals with high melting point and good ductility such as platinum, rhodium, and tungsten. 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 of double wire, three wire, oblique wire and V-shaped, X-shaped and so on. In order to increase the strength, metal films are sometimes used instead of metal wires. Usually, a thin metal film is sprayed on a thermally insulated substrate, which is called a thermal film probe, as shown in Figure 2.2. Hotline probes must be calibrated before use. The static calibration is performed in a special standard wind tunnel, measuring the relationship between the flow rate and the output voltage and drawing a standard curve. The dynamic calibration is performed in a known pulsating flow field or in an anemometer heating circuit. A pulsed electrical signal is used to verify the frequency response of the hot wire anemometer. If the frequency response is poor, the corresponding compensation line can be used to improve the frequency response.

The flow rate measurement range of 0 to 100 m/s can be divided into three sections: low speed: 0 to 5 m/s; medium speed: 5 to 40 m/s; high speed: 40 to 100 m/s. The anemometer's thermal probe is used for accurate measurements from 0 to 5 m/s; the anemometer's rotary probe is ideal for measuring flow rates from 5 to 40 m/s; while using a pitot tube gives the best results in the high speed range. result. An additional criterion for correctly selecting the anemometer's flow probe is the temperature. Usually the anemometer's thermal sensor is used at about +-70C. Special anemometer wheel probe up to 350C. Pitot tube for +350C or more.

Anemometer - Probe Selection 1. Anemometer Thermal Probes Anemometers Anemometers The thermal probe works on the principle that a cold-impinging air stream carries heat away from the thermal element, with an adjustment switch, keeping the temperature constant, then adjusting Current and flow rate are proportional to each other. When thermal probes are used in turbulent flow, airflow from all directions simultaneously impacts the thermal elements, which affects the accuracy of the measurement results. When measuring in turbulent flow, thermal anemometer flow sensor values ​​tend to be higher than rotary probes. The above phenomena can be observed during the pipeline measurement. Depending on the design of the turbulence in the management pipeline, it can even occur at low speeds. Therefore, the anemometer measurement process should be performed on the straight part of the pipeline. The starting point of the straight part should be at least 10 × D before the measuring point (D = pipe diameter in CM); the ending point is at least 4 × D after the measuring point. The fluid cross section must not have any blockage. (corners, resuspended, objects, etc.)

2. Anemometer Rotary Probe Anemometer Rotary probe working principle is based on the conversion of the rotation into electrical signals, first through a proximity sensor at the beginning, "count" the rotation of the runner and generate a pulse series, After the detector conversion processing, you can get the speed value. The anemometer's large-diameter probe (60mm, 100mm) is suitable for measuring turbulence in medium and small flow rates (eg at the outlet of pipes). The anemometer's small-caliber probe is more suitable for measuring air flow with a cross-section greater than 100 times more than the cross-section of the explorer.

Anemometer - characteristic anemometer 1, small size, small disturbance to the flow field;

2, a wide range of applications. Not only for gases but also for liquids, it can be used in subsonic, transonic and supersonic flow of gases;

3, high frequency response, up to 1 MH z.

4, high measurement accuracy, good repeatability. The disadvantage of the hot wire anemometer is that the probe has some interference with the flow field and the hot wire is easily broken.

5, in addition to measuring the average speed, it is also possible to measure the pulsation value and the turbulent flow; in addition to measuring the unidirectional motion, it is also possible to measure the velocity components in multiple directions simultaneously.

Anemometer - Main Purpose Anemometer 1, Measure the speed and direction of the average flow.

2. Measure the pulsation speed and frequency spectrum of incoming flow.

3. Measure the Reynolds stress in the turbulent flow and the velocity correlation and time correlation between the two points.

4. Measure the wall shear stress (usually using a hot-film probe placed flush with the wall, the principle is similar to the hot-wire speed measurement).

5. Measure the temperature of the fluid (measure the change of the probe resistance with the temperature of the fluid in advance, and then determine the temperature based on the measured resistance of the probe.

In addition to this, many professional applications have been developed.

Anemometer - Usage 1. Observe whether the pointer of the meter is at zero before use. If there is an offset, you can gently adjust the mechanical adjustment screw of the meter to return the pointer to zero;

2, the correction switch is placed in the broken position;

3. Insert the probe plug into the socket and place the probe vertically upwards. The plug is pressed tightly to seal the probe. The “correction switch” is placed at full-scale position. Slowly adjust the “full-scale adjustment” knob to make the pointer of the meter pointer. In full position;

4, "correction switch" is placed in "zero", slowly adjust the "coarse adjustment", "fine tune" two knobs, so that the meter pointer at the zero position;

5, after the above steps, gently pull the plug, so that the probe exposed bar (length can be selected according to need), and make the probe on the red point facing the wind direction, according to the meter reading, refer to the calibration curve, you can Detected wind speed;

6. After measuring several minutes (about 10 minutes), the above steps 3 and 4 must be repeated once, so that the current in the meter is standardized;

7. After the test, the "correction switch" should be placed in the off position.

An anemometer is a tachometer that converts a velocity signal into an electrical signal. It can also measure fluid temperature or density. The principle is that an electrically heated, fine wire (called a hot wire) is placed in the air stream. The amount of heat dissipated by the hot wire in the air flow is related to the flow rate, and the amount of heat dissipation causes a change in the temperature of the hot wire and causes a change in the resistance. The flow rate signal is converted into electric signal. It has two modes of operation: 1 constant flow. When the current through the hot wire remains unchanged, the resistance of the hot wire changes when the temperature changes, and thus the voltage across the two ends changes, thereby measuring the flow rate; 2 thermostatic type. The temperature of the hot wire remains unchanged. If it is maintained at 150°C, the flow rate can be measured according to the current required to be applied. The thermostatic type is more extensive than the constant flow type application. The length of the hot wire is generally in the range of 0.5 to 2 millimeters, and the diameter is in the range of 1 to 10 micrometers. The material is platinum, tungsten or platinum iridium alloy. If a very thin (thickness less than 0.1 μm) metal film is used instead of the metal wire, it is a hot film anemometer. Its function is similar to that of hot wire, but it is mostly used to measure the liquid flow rate. In addition to the common single wire type, the hot wire can also be a combined two-wire or three-wire type to measure the speed components in various directions. The electric signal output from the hotline is amplified, compensated and digitized and then input to a computer, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the speed measurement function. For example, the instantaneous value and time average value, the combined speed and the sub-speed, and the turbulence degree can be completed at the same time. And other turbulence parameter measurements. Compared with Pitot tubes, the hot wire anemometer has the advantages of small probe volume, small disturbance to the flow field, fast response, ability to measure unsteady flow velocity, and ability to measure very low speed (such as as low as 0.3 meters/second).

Anemometer - Wind Velocity Measuring Instrument 1 Anemometer Anemometer It is the most common kind of anemometer. The rotary cup anemometer was first invented by Rubinson of the United Kingdom. At the time, it was four cups and later three cups. The three parabolic or hemispherical empty cups that are fixed to each other on the shelf are all on one side, and the entire shelf, together with the wind cup, is mounted on a freely rotatable shaft. The wind cup rotates around the shaft under the influence of wind, and its speed is proportional to the wind speed. Speed ​​can be recorded with electrical contacts, tachometer generators or photoelectric counters.

2 Propeller Anemometer An anemometer that rotates about a horizontal axis with a set of three- or four-blade propellers. The propeller is mounted on the front of a weather vane so that its plane of rotation is always facing the anemometer of the wind, and its speed is proportional to the wind speed.

3 Hot wire anemometer An electric current heated wire, which is cooled by the flowing air, uses the heat dissipation rate as a linear relationship with the square root of the wind speed, and is then linearized by the electronic circuit (in order to facilitate calibration and reading). Anemometer. The hot wire wind speed meter is divided into two types, the side heat type and the direct heat type. The hot side heating wire is generally a manganese copper wire, and its temperature coefficient of resistance is close to zero, and its surface is equipped with a temperature measuring element. The direct thermal type hot wire is mostly platinum wire, and the temperature of the hot wire itself can be directly measured while measuring the wind speed. The hot wire anemometer has a high sensitivity at small wind speeds and is suitable for measuring small wind speeds. Its time constant is only a few hundredths of a second and is an important tool for atmospheric turbulence and agro-meteorological measurements.

4 Acoustic anemometer The wind speed component in the sound propagation direction will increase (or decrease) the sound wave propagation speed. Acoustic anemometers made using this feature can be used to measure wind speed components. Acoustic anemometers have at least two pairs of sensing elements, each pair comprising a sounder and a receiver. The sound waves of the two sounders travel in opposite directions. If one group of sound waves propagates along the wind speed component and the other group happens to travel against the wind, the time difference between the two receivers receiving the sound pulses will be proportional to the wind speed component. If two pairs of components are installed in both horizontal and vertical directions, the horizontal wind speed, wind direction, and vertical wind speed can be calculated separately. Due to the advantages of anti-jamming and good directivity, ultrasonic sound waves emitted by acoustical anemometers are mostly in the ultrasonic section.

Anemometer - Measurement 1. The measurement of airflow velocity inside a pipe has proven that the anemometer's 16mm probe is most versatile. Its size guarantees both good permeability and high flow rates up to 60m/s. Measurement of airflow velocity in a pipe is one of the possible measurement methods. The indirect measurement procedure (gate measurement method) is applicable to air measurement.

The anemometer 2 and the measurement vent in the exhaust will greatly change the distribution of the airflow in the pipe: a high-speed zone is created on the surface of the free vent, and the rest is in the low-speed zone, and a vortex is generated on the grid. According to the different design of the grid, the airflow cross section is relatively stable at a certain distance (about 20cm) in front of the grid. In this case, a large anemometer caliper wheel is usually used for measurement. Because larger calibers can average uneven flow rates and calculate their averages over a wide range.

3, in the measurement of the suction hole, even if there is no grid interference in the suction, the air flow path is also no direction, and its airflow cross section is extremely uneven. The reason for this is the partial vacuum inside the pipe. The air is drawn out of the funnel in a gas chamber, even in a region that is very close to the pumping air, and there is not a place that satisfies the measurement conditions and can be used for measurement operations. If measurements are made using a grid measurement method with an average calculation function, and by determining the volumetric flow method to measure, and by determining the volumetric flow, etc., only the pipe or funnel measurement can provide repeatable measurement results. In this case, different size measuring funnels can meet the requirements for use. Using a measuring funnel, a fixed section can be generated at a certain distance in front of the sheet valve to meet the flow measurement conditions. The center of the section is located and the section is fixed. The center of the section is located and the section is fixed. The center of the section is located and fixed. Here. The measured value obtained by the flow velocity probe is multiplied by the funnel coefficient to calculate the volumetric flow volume. (eg funnel factor 20)

Anemometers - The application of anemometers is widely used and can be applied flexibly in all fields. It is widely used in power, steel, petrochemical, energy saving and other industries. There are other applications in the Beijing Olympics, sailing competitions, rowing competitions, Field shooting competitions and the like need to use anemometers to measure. The current anemometer is more advanced, in addition to measuring wind speed, it can also measure wind temperature and air volume. There are many industries that need to use anemometers and recommended industries: fishing in the sea, various types of fan manufacturing, and industries that require ventilation and exhaust systems.

Different seasons and different geographical conditions of the anemometer will change the wind direction in the atmosphere. For example, the wind direction of the beach varies day and night, and there are also different monsoons in winter and summer. Studying the wind direction can help us predict and study the climate change. Anemometers are required to study the wind direction. The design of the anemometer is mostly arrow-shaped, and it is also made into an animal form, like the shape of a rooster. The anemometer's arrow feathers rotate with the wind. Anemometers need to be installed where there is no building or trees to block the movement of the wind. Application and application scope QDP series hot ball type electric anemometer is used in heating, ventilation, air conditioning, meteorology, agriculture, refrigeration and drying, labor and health survey, etc. It can be used to measure indoor and outdoor or model airflow speed. It is a basic instrument for measuring low wind speeds. The product was rated as a Beijing high-quality product by the Beijing Economic Commission in 1987. Working principle The instrument consists of a hot-ball sensor and a measuring instrument. There is a tiny glass ball on the head of the sensor. The ball is heated with a nickel-chromium wire coil that heats the glass and two thermocouples connected in series. The cold junction of the thermocouple is connected to the phosphorus copper pillar and is directly exposed to the airflow. When a certain amount of current passes through the heating coil, the glass ball is heated to a certain temperature. This temperature is related to the speed of the airflow, and the flow rate is smaller than the temperature. High, on the contrary, the temperature is lower.