Introduction and working principle of three kinds of wind speed measuring instruments

1. Thermal anemometer
A speed measuring instrument that converts flow rate signals into electrical signals, and can also measure fluid temperature or density. The principle is that a thin metal wire (called a hot wire) that is 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, and the flow rate signal is transformed into electric signal. It has two working modes: ① Constant current type. The current through the hot wire remains unchanged. When the temperature changes, the resistance of the hot wire changes, and therefore the voltage at both ends changes, thereby measuring the flow rate. ② Constant temperature type. The temperature of the hot wire remains constant, such as 150°C, and the flow rate can be measured according to the current required 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 thin (thickness less than 0.1 micron) metal film is used instead of 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 liquid flow rate. In addition to the ordinary single-wire type, the hot wire can also be a combined two-wire type or a three-wire type to measure the velocity components in all directions. The electrical signal output from the hot wire is amplified, compensated and digitized and then input to 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 instantaneous value and the time average value, the combined velocity and the sub-velocity, and the turbulence And other turbulence parameters measurement. Compared with the Pitot tube, the hot wire anemometer [1] has the advantages of small probe volume, less interference to the flow 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.
When the thermal probe is used in turbulent flow, the airflow from all directions simultaneously impacts the thermal element, which will affect the accuracy of the measurement results. When measuring in turbulent flow, the indication value of the thermal anemometer flow velocity sensor is often higher than that of the rotary probe. The above phenomenon can be observed during the pipeline measurement. Depending on the design of the management pipeline turbulence, it will appear even at low speeds. Therefore, the anemometer measurement process should be carried out on the straight part of the pipeline. The starting point of the straight line part should be at least 10×D (D=pipe diameter, in CM) before the measuring point; the end point should be at least 4×D after the measuring point. The fluid cross-section must not have any obstructions (edges, corners, resuspension, objects, etc.).
2. Impeller anemometer
The working principle of the impeller probe of the anemometer is based on converting the rotation into an electrical signal. It first passes through an approaching induction head, "counts" the rotation of the impeller and generates a pulse series, and then is converted by the detector to obtain the rotation speed. value. The large-diameter probe (60mm, 100mm) of the anemometer is suitable for measuring turbulent flow with medium and small velocity (such as at the outlet of a pipe). The small-diameter probe of the anemometer is more suitable for measuring the airflow whose cross-sectional area of the pipe is more than 100 times larger than the cross-sectional area of the probe.
3. Pitot tube anemometer
In the 18th century, it was invented by French physicist H. Pito. The simple pitot tube has a thin metal tube with a small hole at the end as the pressure guiding tube, and the total pressure of the fluid is measured in the direction of the flow beam; another guiding pressure is drawn on the main pipe wall near the front of the thin metal tube Tube, measured static pressure. The differential pressure gauge is connected to the two pressure guiding pipes, and the measured pressure is the dynamic pressure. According to Bernoulli's theorem, the dynamic pressure is proportional to the square of the flow velocity. Therefore, the pitot tube can be used to measure the flow rate of the fluid. After structural improvement, it becomes a combined pitot tube, namely pitot-static tube. It is a double-layer tube bent at a right angle. The outer sleeve and the inner sleeve are sealed, and there are a number of small holes around the outer sleeve. When measuring, insert this casing into the middle of the pipe under test. The nozzle of the inner sleeve is facing the direction of the flow beam, and the orifices of the small holes around the outer sleeve are exactly perpendicular to the direction of the flow beam. At this time, the pressure difference of the inner and outer sleeve can be measured to calculate the flow velocity of the fluid at that point. Pitot tubes are commonly used to measure the velocity of fluids in pipes and wind tunnels, as well as river velocity. If the flow velocity of each section is measured according to regulations, it can be used to measure the flow of fluid in the pipeline after integration. But when the fluid contains a small amount of particles, it may block the measuring hole, so it is only suitable for measuring the flow of particle-free fluids. Therefore, the Pitot tube can also be used to measure wind speed and wind flow. This is the principle of the Pitot tube anemometer.