Vane anemometer
The working principle of the vane probe of the anemometer is based on converting the rotation into an electrical signal, first through a proximity induction head, “counting” the rotation of the vane and generating a pulse series, and then converted by the detector to obtain the speed value. The large-diameter probe (60mm, 100mm) of the anemometer is suitable for measuring turbulent flow with medium and small flow velocity (such as at the pipe outlet). 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.
The vane anemometer is suitable for measuring the air velocity of fresh air, return air outlet, meteorological observation, etc. The principle is simple and the price is cheap, but the test accuracy is low, so it is not suitable for the test of light wind speed and the test of small wind speed changes.
Pitot tube anemometer
In the 18th century, it was invented by the French physicist H. Pitot. The simplest pitot tube has a thin metal tube with a small hole at the end as a pressure guiding tube, which measures the total pressure of the fluid in the direction of the flow beam. In addition, a pressure guiding pipe is drawn out from the main pipe wall near the front of the thin metal pipe, and the static pressure is measured. The differential pressure gauge is connected with 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 flow rate of the fluid can be measured with a pitot tube. After the structural improvement, it becomes a combined pitot tube, that is, a pitot-static pressure tube. It is a double-layered tube bent at a right angle. The outer sleeve and the inner sleeve are sealed, and there are several small holes around the outer sleeve. When measuring, insert this sleeve into the middle of the pipe under test. The nozzle of the inner casing is facing the direction of the flow beam, and the orifice of the small hole around the outer casing is just perpendicular to the direction of the flow beam. At this time, the pressure difference between the inner and outer casings can be measured to calculate the flow velocity of the fluid at this 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 the regulations, it can be used to measure the flow rate of the fluid in the pipeline after integration. However, 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 the wind speed and wind flow, which is the principle of the pitot tube anemometer.
Thermal anemometer
A tachometer that converts a flow rate signal into an electrical signal, 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 air flow, and the heat dissipation of the hot wire in the air flow is related to the flow rate, and the heat dissipation causes the temperature change of the hot wire to cause the resistance change, and the flow rate signal is converted into electric signal.
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, which is similar in function to a hot wire, but is mostly used to measure the flow rate of liquids.
In addition to the ordinary single-wire type, the hot wire can also be a combined two-wire type or 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 into the computer, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the speed measurement functions, such as simultaneous completion of instantaneous value and time average value, combined speed and sub-speed, turbulence degree and other turbulence parameters. Compared with the pitot tube, the hot-wire anemometer has the advantages of small probe size, small interference to the flow field, fast response, and can measure unsteady flow velocity and very low velocity (such as as low as 0.3 m/s).