An anemometer is a common sensor that continuously measures wind speed and air volume (air volume = wind speed x cross-sectional area). Anemometers are broadly classified into mechanical (mainly propeller-type and cup-type) anemometers, hot-air anemometers, pitot tube anemometers, and ultrasonic anemometers based on acoustic principles. Let's examine the working principle of each type.
I. Working Principle of Propeller-Type Wind Speed Sensor
We know that an electric fan uses an electric motor to drive the fan blades to rotate, creating a pressure difference in front of and behind the blades, which propels the airflow. The working principle of a propeller-type anemometer is exactly the opposite. The blade system aligned with the airflow is subjected to wind pressure, generating a torque that causes the blade system to rotate. Typically, propeller-type speed sensors measure wind speed by rotating a set of three- or four-bladed propellers around a horizontal axis. The propellers are usually mounted in front of a wind vane, ensuring their plane of rotation always faces the direction of the wind, and their rotational speed is proportional to the wind speed.
II. Working Principle of Cup-Type Anemometer
The cup-type anemometer, also known as the three-cup anemometer, is a very common type of anemometer, first invented by Robert Robinson in England. The sensing element consists of three or four conical or hemispherical hollow cups. The hollow cup shells are fixed to a triangular support at 120° intervals or a cross-shaped support at 90° intervals. The concave surfaces of the cups are aligned in one direction, and the entire horizontal arm is fixed to a vertical rotating shaft.
When the wind blows from the left, wind cup 1 is parallel to the wind direction, and the component of the wind pressure on wind cup 1 in the direction most perpendicular to the wind cup axis is approximately zero. Wind cups 2 and 3 intersect at a 60-degree angle with the wind direction. For wind cup 2, its concave surface faces the wind and experiences the greatest wind pressure; wind cup 3, with its convex surface facing the wind, experiences less wind pressure than wind cup 2 due to the airflow effect. Because of the pressure difference between wind cups 2 and 3 in the direction perpendicular to the wind cup axis, the wind cups begin to rotate clockwise. The greater the wind speed, the greater the initial pressure difference, the greater the acceleration, and the faster the wind cups rotate.
After the wind cups start rotating, cup 2 rotates with the wind, so the wind pressure it experiences is relatively reduced, while cup 3 rotates against the wind at the same speed, so the wind pressure it experiences is relatively increased. The pressure difference continuously decreases, and after a period of time (when the wind speed remains constant), when the pressure difference acting on the three wind cups reaches zero, the wind cups begin to rotate at a uniform speed. Thus, the wind speed can be determined based on the rotational speed of the wind cups (the number of revolutions per second).
When the wind cup rotates, it drives the coaxial multi-toothed disc or magnetic rod to rotate. A pulse signal proportional to the wind cup's rotation speed is generated by the circuit. This pulse signal is counted by a counter, and the actual wind speed value can be obtained after calculation. Currently, most new rotating cup anemometers use a three-cup design, with conical cups performing better than hemispherical ones. When the wind speed increases, the rotating cup can quickly increase its rotation speed to adapt to the airflow. When the wind speed decreases, due to inertia, the rotation speed cannot immediately decrease. In gusts, the wind speed indicated by a rotating anemometer is generally too high, known as the over-inflation effect (resulting in an average error of about 10%).
III. Working Principle of Thermal Anemometer
Thermal anemometers use a hot wire (tungsten or platinum wire) or a hot film (a thin film made of platinum or chromium) as a probe, exposed to the air being measured. This probe is connected to a Wheatstone bridge, and the air velocity at the measured cross-section is detected by the balance of resistance or current in the bridge. The hot film anemometer is coated with an extremely thin quartz insulating layer to insulate it from the fluid and prevent contamination, allowing it to operate in airflows containing particles.
When the air temperature remains constant, the power consumed by the heating wire is equal to the heat instantaneously dissipated by the wire in the air. The resistance of the heating wire changes linearly with temperature. The heat dissipation coefficient is related to the airflow velocity; the higher the velocity, the higher the corresponding heat dissipation coefficient, meaning faster heat dissipation; conversely, a lower velocity results in slower heat dissipation.
The airflow velocity measured by a thermal anemometer is a function of current and resistance. By keeping the current (or resistance) constant, the measured airflow velocity corresponds one-to-one with the resistance (or current).
