Anemometer Principle
Anemometer refers to a speed measuring instrument that converts flow velocity signal into electrical signal, which can measure fluid temperature or density. The principle of thermal anemometer is as follows: A thin metal wire 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 change of the hot wire to cause the resistance change, and the flow rate signal is converted into electricity. Signal. The main components of the thermal anemometer are the anemometer and the measurement indicating instrument. Among them, the wind speed probe can be divided into thermal probe and rotary probe. According to the structure, thermal anemometers include hot bulb type and hot wire type; according to the display form, there are pointer type, digital type, etc.; according to the working principle, there are constant flow and constant temperature type.
The principle of constant flow anemometer is that the current of the hot wire remains unchanged, and when the temperature changes, the resistance of the hot wire changes, so the voltage at both ends changes, and the wind speed is measured. The constant temperature type means that the temperature of the hot wire does not change, and then the wind speed is measured according to the applied current. Compared with constant flow, the constant temperature type is more widely used. The length of the hot wire is 0.5-2mm, the diameter is 1-10um, and the material is platinum, tungsten or platinum-rhodium alloy.
The wind speed probe can be divided into three sections according to the range of flow velocity: 0-5m/s, 5-40m/s and 40-100m/s, which are low speed, medium speed and high speed respectively. Among them, the thermal probe is mainly used for low speed, and the rotary probe is ideal for medium speed. 1 Thermal probe of the anemometer. The working principle of the thermal probe is based on the cold impinging air flow to take away the heat on the heating element, with the help of an adjustment switch, to keep the temperature constant, the adjustment current is proportional to the flow rate. When using thermal probes in turbulent flow, airflow from all directions hits the thermal element simultaneously, affecting the accuracy of the measurement results. When measuring in turbulent flow, thermal anemometer flow sensors tend to have higher indications than wheel probes. The above phenomena can be observed during pipeline measurement. Depending on the design that manages the turbulence in the pipe, it can occur even at low speeds. Therefore, the anemometer measurement process should be carried out on the straight section of the pipeline. The starting point of the straight line should be at least 10×D outside the measuring point (D=pipe diameter, in CM); the end point should be at least 4×D behind the measuring point. The fluid section must not have any obstructions. (angular, resuspension, object, etc.). The working principle of the wheel probe is based on converting the rotation into an electrical signal. First, through a proximity induction head, the rotation of the wheel is "counted" and a pulse series is generated, 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 with the cross-section of the pipeline more than 100 times larger than the cross-section of the expedition head.
Anemometer hot lines are available in single-line, dual-line, and triple-line types to measure 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 functions, such as simultaneous completion of instantaneous value and time average value, combined speed and sub-speed, turbulent flow Measurement of degree and other turbulence parameters. The hot-wire anemometer is good at measuring low wind speed and has an irreplaceable role in the measurement. Today, thermal anemometers are mainly used in heating, ventilation, air conditioning, environmental protection, energy monitoring, meteorology, agriculture, refrigeration, drying, labor hygiene surveys, clean workshops and various wind speed laboratories.
