Three common types of anemometers
1. Thermal anemometer
A speed measuring instrument that converts flow velocity signals into electrical signals and can also measure fluid temperature or density. The principle is to place a thin metal wire (called a hot wire) that is heated by electricity in the airflow. The heat dissipation of the hot wire in the airflow is related to the flow rate, and the heat dissipation causes a change in the temperature of the hot wire and a change in resistance. The flow rate signal is then converted into an electrical signal. It has two working modes: ① constant current. When the current through the hot wire remains constant and the temperature changes, the resistance of the hot wire changes, resulting in a change in the voltage at both ends, and thus measuring the flow rate Constant temperature type. The temperature of the hotline remains constant, such as at 150 ℃, and the flow rate can be measured based on the required applied current. Constant temperature type is more widely used than constant current type.
The length of the hot wire is generally in the range of 0.5-2 millimeters, and the diameter is in the range of 1-10 micrometers. The material used is platinum, tungsten, or platinum rhodium alloy. If a very thin (less than 0.1 microns thick) metal film is used instead of a metal wire, it is called a hot film anemometer, which functions similarly to a hot wire but is mostly used to measure liquid flow velocity. In addition to the ordinary single line type, the hotline can also be a combination of double line or triple line type, used to measure velocity components in various directions. The electrical signal output from the hotline, after amplification, compensation, and digitization, can be input into the computer to improve measurement accuracy, automatically complete the data post-processing process, expand the speed measurement function, and simultaneously measure instantaneous and mean values, combined and partial velocities, turbulence intensity, and other turbulence parameters. Compared with pitot tubes, the hot wire anemometer has a smaller probe volume and less interference with the flow field; Fast response, capable of measuring unsteady flow velocity; It has the advantage of being able to measure very low speeds (such as as as low as 0.3 meters per second).
When using a thermal sensitive probe in turbulence, airflow from all directions simultaneously impacts the thermal element, which can affect the accuracy of the measurement results. When measuring in turbulence, the reading of the thermal anemometer flow sensor is often higher than that of the rotary probe. The above phenomenon can be observed during pipeline measurement. According to different designs for managing turbulent flow in pipelines, it may even occur at low speeds. Therefore, the anemometer measurement process should be carried out in the straight section of the pipeline. The starting point of the straight section should be at least 10 × D (D=pipe diameter, in CM) outside the measurement point; The endpoint should be at least 4 × D behind the measurement point. The fluid cross-section must not have any obstructions (edges, overhangs, objects, etc.).
2. Impeller anemometer
The working principle of the impeller probe of an anemometer is based on converting rotation into electrical signals. First, it passes through a proximity sensing head to "count" the rotation of the impeller and generate a pulse series. Then, it is converted and processed by a detector to obtain the speed value. The large-diameter probe (60mm, 100mm) of the anemometer is suitable for measuring turbulent flow with medium to low velocities (such as at pipeline outlets). The small-diameter probe of an anemometer is more suitable for measuring airflow in pipelines with a cross-sectional area greater than 100 times that of the probe.
3. Pitot tube anemometer
Invented by French physicist H. Pito in the 18th century. A simple pitot tube has a metal thin 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; Another pressure tube is led out from the main pipeline wall near the front of the metal thin tube to measure the static pressure. The differential pressure gauge is connected to two pressure pipes, and the measured pressure is the dynamic pressure. According to Bernoulli's theorem, dynamic pressure is proportional to the square of flow velocity. Therefore, the flow velocity of the fluid can be measured using a pitot tube. After structural improvements, it becomes a combined pitot tube, namely pitot static pressure tube. It is a double layered tube bent at a right angle. The outer sleeve and inner sleeve are sealed, and there are several small holes around the outer sleeve. When measuring, insert this sleeve into the middle of the measured pipeline. The mouth of the inner casing is facing the direction of the flow beam, and the openings of the small holes around the outer casing are perpendicular to the direction of the flow beam. At this point, the pressure difference between the inner and outer casings 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 pipelines and wind tunnels, as well as in rivers. If the flow velocity of each section is measured according to regulations, it can be integrated to measure the flow rate of the fluid in the pipeline. 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 rate of non particle fluids. So, pitot tubes can also be used to measure wind speed and flow rate, which is the principle of pitot tube anemometers.
