An anemometer is an instrument for measuring air velocity. It can measure the average flow speed and direction; the pulsating speed of the incoming flow and its spectrum; the Reynolds stress in turbulent flow and the velocity correlation and time correlation between two points; the wall shear stress; and the fluid temperature. Before using an anemometer to measure wind speed, understanding the basic working principle will help us have a deeper understanding of the process of anemometer measuring wind speed.
Working principle of anemometer
The basic principle of an anemometer is to place a thin metal wire in the fluid, pass an electric current to heat the metal wire, and make its temperature higher than the temperature of the liquid. When the fluid flows through the metal wire in the vertical direction, it will take away part of the heat of the metal wire, causing the temperature of the metal wire to drop. According to the theory of forced convection heat exchange, it can be derived that there is a relationship between the dissipated heat Q and the velocity v of the fluid.
The probe must be calibrated before use. Static calibration is carried out in a special standard wind tunnel, measuring the relationship between flow velocity and output voltage and drawing a standard curve; dynamic calibration is carried out in a known pulsating flow field, or by adding a pulsating electrical signal to the portable anemometer heating circuit to verify the frequency response of the portable anemometer. If the frequency response is not good, it can be improved with the corresponding compensation circuit.
Methods to measure wind speed
Choosing the right type of anemometer
There are different types of anemometers, such as:
- Cup anemometers: measure wind speed by counting the number of rotations of a cup attached to an arm.
- Vanguard anemometers: have an impeller that rotates with the wind and measures speed (like a windmill).
- Hotwire anemometers: measure the cooling effect of wind on a heated wire to determine speed.
- Digital or ultrasonic anemometers: measure wind speed electronically or through sound waves.
Process of measuring wind speed with an anemometer
Choose a suitable location: When measuring wind speed, choose an open, unobstructed location so that the wind can blow freely and affect the anemometer reading. Avoid measuring near buildings, trees, or other obstructions.
Prepare the anemometer: Check that the anemometer is working properly and make sure the battery is fully charged. Before use, observe whether the meter pointer is pointing to zero. If there is an offset, gently adjust the meter's mechanical adjustment screw to return the pointer to zero. Set the anemometer calibration switch to the "off" position. Plug the measuring rod into the socket, place the measuring rod vertically upward, and press the screw plug to seal the probe.
- If using a mechanical anemometer, make sure the spring is tightened. For mechanical anemometers, let the instrument rotate in the wind for a few seconds. You can use a calibration chart or scale to convert its rotation speed to wind speed.
- For digital anemometers, the wind speed should be displayed on the screen in units such as meters per second (m/s), kilometers per hour (km/h), or miles per hour (mph).
- A hot wire anemometer will directly indicate the wind speed based on how quickly the wire cools.
Install the anemometer: Insert the anemometer into a needle or rod of appropriate length on the ground, make sure the anemometer is perpendicular to the ground and try to keep it in the center of the wind outlet. If possible, use a tripod or other support device to fix the anemometer to prevent it from shaking due to the wind.
Start measuring: Aim the probe of the anemometer in the wind direction and make sure the probe is perpendicular to the wind flow. Then start the measurement function, and the anemometer will start to capture and record wind speed data. Depending on the test needs, data from multiple measurement points can be recorded for subsequent analysis.
Wait a few seconds for the anemometer to stabilize, then slowly reduce or increase the height of the needle or rod, observe how the indicator on the anemometer changes, and the scale on the anemometer will show between 0-36 km/h (0-22 mph), record the maximum and minimum wind speeds measured.
Repeat measurements: record wind speed fluctuations. For more accurate results, you may need to repeat multiple measurements at the same location and then take the average as the final result.
Calibration: If you use a graduated anemometer (such as a cup anemometer), make sure it is calibrated to get accurate results. Some models come with a calibration chart or internal system.
Read the data: After the measurement is completed, read the wind speed value from the anemometer. Some advanced anemometers may also provide other parameters such as wind direction, temperature, etc. When recording wind speed data, be sure to also record relevant information such as the time and location of the measurement for subsequent analysis and comparison.
