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Types of anemometers

日期:2020-09-21 20:24
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摘要:1. Thermal anemometer A tachometer that converts a velocity signal into an electrical signal and also measures the temperature or density of a fluid. The principle is that a thin wire (called a hot wire) heated by an electric current is placed in the air stream. The heat loss of the hot wire in the air stream is related to the flow rate. It has two working modes: (1) constant flow. The current through the hot wire remains constant, and as the temperature changes, the resistance of the hot w
1. Thermal anemometer

A tachometer that converts a velocity signal into an electrical signal and also measures the temperature or density of a fluid. The principle is that a thin wire (called a hot wire) heated by an electric current is placed in the air stream. The heat loss of the hot wire in the air stream is related to the flow rate. It has two working modes: (1) constant flow. The current through the hot wire remains constant, and as the temperature changes, the resistance of the hot wire changes, so the voltage at both ends changes, and the flow velocity is measured. (2) constant temperature. The temperature of the hot wire remains constant. If 150℃ is maintained, the flow rate can be measured according to the current applied. The constant temperature type is more widely used than the constant flow type.

The length of the hot wire is generally in the range of 0.5 ~ 2 mm, and the diameter is in the range of 1 ~ 10 microns. The material is platinum, tungsten or platinum-rhodium alloy. If a very thin (thickness less than 0.1 micron) metal film instead of metal wire, it is a hot film anemometer, similar to the function of hot wire, but mostly used to measure the flow rate of liquid. In addition to the ordinary single-wire type, the hot wire can also be a combination of double-wire or three-wire type, used to measure the velocity components in all directions. The output electrical signals from the hot wire can be amplified, compensated and digitized and then fed into the computer, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the speed measurement function, such as simultaneously completing the measurement of instantaneous value and time mean value, combined speed and fractional speed, turbulivity and other turbulence parameters. Compared with the pitot tube, the hot-wire anemometer [1] has a small probe volume and less interference to the flow field. Fast response, can measure unsteady flow velocity; Ability to measure very low speeds (as low as 0.3 m/s).

When a thermosensitive probe is used in turbulence, the flow from all directions impinges on the thermal element at the same time, thus affecting the accuracy of the measurement results. When measuring in turbulence, the velocity sensor of thermosensitive anemometer is often higher than that of rotary probe. The above phenomena can be observed in the process of pipeline measurement. It can occur even at low speeds, depending on the design that manages turbulence in the pipeline. Therefore, the anemometer measurement process should be carried out in the straight part of the pipeline. The starting point of the straight part shall be at least 10×D (D= pipe diameter in CM) before the measuring point; The end point is at least 4×D behind the measuring point. The fluid cross section shall not have any shielding (edges, corners, rehangs, objects, etc.).

2. Impeller anemometer

The working principle of the vane wheel probe of the anemometer is based on the conversion of rotation into electrical signals. After a close induction start, the rotation of the impeller is "counted" and a pulse series is generated. Then the value of rotation speed can be obtained through the conversion processing of the detector. The large diameter probe (60mm,100mm) of the anemometer is suitable for measuring turbulence at medium and low flow rates (e.g., at pipe exits). The small diameter probe of the anemometer is more suitable for measuring the airflow with the cross-sectional area of the pipe greater than 100 times the cross-sectional area of the probe.

3. Pitot tube anemometer

Invented by the French physicist h. peto in the 18th century. The simplest pitot tube has a thin metal tube with a small hole at the end as the pressure tube. The total pressure of the fluid is measured in the direction of the convection beam. In addition in front of the metal pipe near the main pipe wall and then a pressure pipe, measured static pressure. The differential pressure gauge is connected to two pressure tubes and the measured pressure is the dynamic pressure. According to Bernoulli's theorem, the dynamic pressure is proportional to the square of the velocity. So a pitot tube can be used to measure the flow rate of a fluid. After the improvement on the structure, it becomes the combined pitot tube, namely the pitot - static pressure tube. It is a double pipe bent at right angles. The seal between the outer casing and the inner casing. There are several small holes around the outer casing. When measuring, insert the casing into the middle of the pipe under test. The orifice of the inner casing is in the direction of the convection beam, while the orifice of the hole around the outer casing is just perpendicular to the direction of the flow beam. At this point, the flow velocity of the fluid at this point can be calculated by measuring the pressure difference between the inner and outer casing. Pitot tubes are used to measure the speed of fluids in pipes and wind tunnels, as well as river speeds. If the flow rate of each section is measured as specified, the integral can be used to measure the flow rate of the fluid in the pipe. However, when the fluid contains a small number of particles, it may block the measuring hole, so it is only suitable for measuring the flow of non-particle fluid. So, pitot tubes can also be used to measure wind speed and the flow of the wind. That's how the pitot tube anemometer works.