How to measure wind speed

Measuring wind speed is useful for many reasons. From improving safety in outdoor locations to assessing potential sites for wind power projects. In addition, wind speed monitoring is a key element of meteorology.

Wind speed is measured with an anemometer, a device that gets its name from “anemos”, a Greek word that means the wind. The anemometer is a relatively old invention, dating back to the 15th century, and it has been perfected over time.

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How a traditional anemometer works

The traditional anemometer design uses a circular array of cup-shaped elements. The wind force is stronger on the concave side of the cups than on the convex side. The array spins in response to the wind, turning the shaft to which it is attached.

Cup anemometers were first developed in the mid-19th century, and the design used an array of four cups in a circular arrangement. The cups were reduced to three in the 1920s. This anemometer design is now the industry standard for wind site assessment. The three cups provide a faster response and a constant torque. The fast response of the three-cup anemometer is especially useful when measuring wind gusts, which have a short duration despite their strength. In many cases, they may require a fast response to prevent accidents.

Adding wind direction measurement

A conventional three-cup anemometer only measures wind speed, but there are two main ways to measure wind direction as well:

• One of the cups can have an attached tag to alter the wind force on it, making the anemometer speed up and slow down once per cycle, creating a cyclic pulsation. Wind direction can be determined based on the angular position of the anemometer when each of these effects occurs.
• Another option is to simply complement the anemometer with a wind vane to determine wind direction. An advantage of mounting both devices together is measuring wind speed and direction at the same spot.

The vane anemometer or propeller anemometer is another common variant of this device, with the appearance of a miniature wind turbine. A vane anemometer is equipped with a wind vane that always keeps it pointing in the direction from which the wind is blowing. Wind speed is calculated based on propeller speed, just like with the cup anemometer. The difference that the axis of rotation is parallel to the wind (it is perpendicular in a cup anemometer). A vane anemometer can also deliver simultaneous wind speed and direction data.

How an anemometer measures wind speed

A cup anemometer is characterised by simplicity: the rotating speed of its shaft is proportional to wind speed, which means the average speed is also proportional to the number of turns during a specified timeframe. The ratio of wind speed and shaft speed is called the anemometer factor, and it varies depending on the physical construction of the unit, typically ranging from 2 to 3. Wind speed is calculated as follows:

• Instantaneous Wind Speed = Anemometer Factor x Instantaneous Shaft Speed
• Average Wind Speed = Anemometer Factor x (Number of Turns / Time)

The sampling frequency of an anemometer describes how often the unit takes a wind speed measurement. For example, an anemometer with a frequency of 0.1 Hz counts the number of turns over a 10-second period to provide a speed value. While another with a 1-Hz frequency counts the turns once per second. Anemometers with a higher sampling frequency provide a more detailed wind profile. Since they can capture short-term changes in wind speed, which are averaged when the sampling period is longer.

How Windcrane measures wind speed

WINDCRANE comes equipped with a three-cup anemometer, but the unit can accommodate many other sensor inputs, including additional anemometers. The advantage of this setup is that wind speed can be measured at various heights. Providing you with valuable data when the wind shear profile is required.

WINDCRANE can be equipped with additional sensors to measure wind direction, temperature, solar radiation, relative humidity, and precipitation. The unit also features cloud-computing capabilities, where the logged data is uploaded to a cloud database every 10 minutes. Quad-band GSM communication also allows remote access from a smartphone app or web dashboard. WINDCRANE is built by Logic Energy, industry experts at providing monitoring solutions to the energy industry for more than a decade.

WINDCRANE is a plug and play device. To set it up, just connect the wind sensor, connect the power supply, attach it to the structure and switch it on! You’re now ready to measure wind speed. To do this on the go, download the WINDCRANE app to monitor the weather anywhere.