January 2006, Issue 186

Internet-Connected Sonic Anemometer

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I recently became interested in anemometers when I took a new job in the weather instrument industry. One of the most common instruments used in weather stations to measure wind velocity is the anemometer. Cup anemometers feature several half-spherical, hollow cups attached to radial spokes (see Figure 1). It’s intuitive to see that the coefficient of drag for a spherical cup differs depending on where the air is pushing. Basically, the drag is high for the hollow side in comparison to the closed side. 

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Figure 1—The rotational speed of a traditional cup anemometer is determined by the difference in drag force and indicates wind speed.  A separate vane is used to indicate the wind’s direction.

This difference in drag generates torque that spins the entire assembly. A tachometer then measures the resulting rotational speed, which has a linear relationship to the wind speed. Typically, the tachometer is either implemented as a small generator that generates a voltage into a load or a Hall effect or optical sensor that generates pulses relative to the rotational velocity.

Another common method is to use a small wind turbine. The wind drives a propeller or turbine wheel. The resulting rotational speed is measured to give an indication of the wind speed. Using cup and turbine anemometers can be disadvantageous because they require a significant amount of minimum wind to overcome friction and drag. They have momentum, so they don’t respond well to wind gusts and tend to average out the speeds.

There are other instruments for measuring wind speed. Pitot tubes are used in applications where the wind comes from a specific direction (see Figure 2). For instance, when measuring a plane’s airspeed, the wind always comes from the front of the plane with only small angles of deviation. A pitot tube works by measuring the dynamic pressure of the wind as it enters a small hole pointed toward the wind. The tube compares the dynamic pressure to the ambient air pressure. The dynamic pressure is easy to calculate:

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Figure 2—A pitot tube anemometer is common on airplanes and in applications where the wind comes from the same direction.

where rho is the air’s density. As you can see, you have to subtract the static pressure in order to compute the speed:

A disadvantage is that a pitot tube anemometer is accurate only if the wind blows directly into the pressure port.

A hot-wire anemometer measures the heat carried away by a moving air mass (see Figure 3). The higher the wind speed, the more heat carried away (i.e., it cools the wire).

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Figure 3—A hot-wire anemometer measures the wind by how much it cools the resistively heated wire.

Pitot tubes and hot-wire anemometers are advantageous because they don’t have moving parts that need to be sealed from the elements. Anemometers are often installed in remote weather stations where they’re hard to maintain, so having one with no moving parts is a perk.

Using a Doppler lidar is an interesting way to measure the effects of wind speed. A laser beam is sent out and particles in the air (aerosols or water) scatter it. Some of the scattered beam is reflected to a sensor that measures the shift in the laser’s wavelength when the aerosols are moving. You can measure their radial speed to the laser by measuring the laser’s Doppler shift (or an RF carrier modulated on the laser).

A fringe-type laser anemometer projects a field of interference fringes and detects how aersols are illuminated by the fringe pattern as it moves through the field. complex devices require powerful lasers to get any range. They require high-frequency, fast electronics to make them work with any resolution or accuracy. They are usually limited to scientific applications. For example, in systems that measure wind speed and aerosols tens of kilometers in the atmosphere, or measure wind speeds in wind tunnels.

A sonic anemometer measures the speed of sound in air. In this article, I’ll explain how you can measure wind speed with one. At this point, you might be thinking, “So what!” The perceived speed of sound in air is actually the speed of sound in static air plus or minus the speed of the wind (see Figure 4). In other words, the wind speed is the difference between the measured speed of sound and the actual speed of sound in air.

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Figure 4—In a sonic anemometer, the wind speed adds to or subtracts from the speed of sound in air depending on the sound’s direction.