Air flowmeter is an instrument that converts the inhaled air flow into electrical signals. The advantages of air flowmeter are extremely small pressure loss and large measurable flow range. The ratio of the maximum flow rate to the minimum flow rate of the air flowmeter is generally above 20: 1, and it is applicable to a wide range of industrial pipe diameters, up to 3m.
The output signal has a linear relationship with the measured flow, and the accuracy is high. It can measure the flow rate of acid, alkali, salt solution, water, sewage, corrosive liquid, mud, pulp and other fluids with conductivity ≥ 1μs/cm. But the air flowmeter can't measure the flow of gas, steam and pure water.
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Principle classification of air flowmeter
Vane air flowmeter
The air flowmeter has simple structure and high reliability; However, the intake resistance is large, the reaction is slow and the volume is large.
Karman vortex street air flowmeter The so-called Karman vortex street means that when a cylindrical or triangular object is placed in the fluid, two rows of vortices with opposite rotation directions will appear alternately at the downstream of the object.
Optical Karman vortex street air flowmeter
In the process of producing Carmen vortex, the air pressure on both sides of the vortex generator will change and act on the metal foil through the guide hole, thus making it vibrate. When the light of the LED shines on the vibrating metal foil, the reflected light on the metal foil received by the phototransistor is vortex modulated light, and its output will be demodulated to obtain a frequency signal representing the airflow.
Ultrasonic Carmen vortex street air flowmeter
The ultrasonic transmitting probe and the receiving probe are relatively installed on both sides of the downstream pipeline of the Carmen vortex generator. Due to the influence of Carmen vortex street on air density, the time from transmitting probe to receiving probe will be later than that without vortex street, resulting in phase difference. By processing the phase signal, the eddy current pulse signal can be obtained.
Working principle of hot-wire air flowmeter
When there is no airflow, the bridge is in a balanced state, and the control circuit outputs heating current to the hot wire resistor RH;
When there is air flowing, the heat of RH is absorbed by the air and becomes cold, so its resistance value changes and the bridge loses balance. If the temperature difference between the hot wire resistance and the inhaled air remains constant and at a certain value, the current IH flowing through the hot wire resistance must be increased. Therefore, the hot wire current IH is a function of the air mass flow.
Hot film air flowmeter
The working principle of hot-film air flowmeter is similar to that of hot-wire air flowmeter, and both of them work by using Wheatstone bridge. The difference is that the hot film type does not use platinum wire as hot wire, but the hot wire resistance, compensation resistance and bridge resistance are made on the same ceramic substrate by thick film process.
Target air flowmeter
When air flows in the measuring tube, due to the pressure difference between its own kinetic energy and the target, it exerts a force on the target, causing the target to move slightly. The magnitude of the acting force is proportional to the square of the medium flow, and its mathematical formula is:
F = Cd A ρ V2/2
F: the force acting on the target
Cd: fluid resistance coefficient
A: the axial projection area from the target to the measuring tube.
ρ: medium density in working condition.
V: Measure the characteristic velocity of air in the pipe.
The force acting on the target is transmitted through the target rod, so that the elastic body of the sensor changes slightly, and the corresponding electrical signal is output after circuit conversion.
References:
Baidu encyclopedia-air flowmeter