Mechanical anemometers have worked well for years but the rigorous duty on wind turbines may call for a more reliable sensor.

Accurate measurements of wind speed and direction from reliable equipment are more important than ever in today’s competitive North American wind industry. Demand for reliable wind sensors is high and manufacturers are responding with more equipment choices than ever before. New technologies in high-tech sensors using ultrasonic signals to measure wind conditions are in wider use as traditional mechanical sensors used exclusively in the past are found to be unreliable in some cases. Following in the footsteps of the National Weather Service (NWS), a new generation of wind industry forecasters and production engineers are beginning to view ultrasonic technologies as a real option, not just an expensive alternative. As the industry matures, will reliability and maintenance-free operations turn the cheap and quickly installed met towers and mechanical sensors into relics of the past? Or is the low cost mechanical sensor here to stay?

A comparison

Ultrasonic and mechanical sensors measure the wind in different ways. Mechanical sensors use moving parts and are connected to a datalogger or other data recording device. The “cups” for speed and “vane” for vectoral change measurement physically move with changes in the wind and give accurate readings of speed and direction. Ultrasonic sensors or “sonics” operate without moving parts. On a typical sonic anemometer, a transducer sends a pulse of ultrasonic sound from the ‘North’ facing side of the sensor. A microprocessor measures

t time it takes to travel to the ‘South’ transducer. The wind speed is calculated from the time it takes the ultrasound to travel to the opposite transducer. Measurement times are affected by the wind speed and direction blowing along the line between the transducers. Without moving parts, measurement is immediate and precise.

When averaged over time, ultrasonic and mechanical measurement accuracies are comparable. Both report the level of accuracy according to sensor’s published specifications. Although data is accurate when averaged, mechanical sensors will not always reflect turbulence (rapid changes in wind speed and direction) and gusts due to the simple physical limitations of its moving parts. A mechanical sensor is affected by the start-up torque of the moving cup and vane. Observed differences occur in measured wind speed because of the time it takes a mechanical sensor to physically start up or register a change in wind direction. For example, if a storm blows through an area and the wind suddenly changes direction, the sensor must slow, stop, and restart with the change in wind direction. It will take some seconds to register and report the change.

In contrast, an ultrasonic sensor is not affected its inertia. It will measure a change in wind direction or a high gust immediately and in real time. It’s this inability to measure sudden changes that spurred the National Weather Service to initiate a widespread change to all its Automated Surface Observation Systems (ASOS) from mechanical to ultrasonic sensors. ASOS systems serve as the U.S. primary surface weather observation networks. The NWS initiated the weather sensor upgrade program in 2000 following a new requirement by the Federal Aviation Administration (FAA) stating that all sensors must be capable of measuring variable gusts at three second intervals. Mechanical sensors are not designed with this capability. Most can only measure rapid wind speed changes to five-second gusts.

Reliability was another stated reason the NWS decided to make the switch. “There were mechanical deficiencies in the old sensors. Freezing was always a problem. When the sensors froze, they became inoperable or inaccurate. This is a huge problem when directing aviation,” says Al Wissman, acting Operations Division Director of the NWS1. Although ultrasonic sensors are often heated, the ultrasonic pulse, a sound wave, is sufficient to occasionally prevent the formation of ice on sensor transducer heads, even with a non-heated sensor. System upgrades from mechanical to ultrasonic wind sensing equipment finished in 2005.

A network of 883 ASOS weather systems is installed throughout the U.S. Most are used for aviation services managed by the FAA, but the data is public and available for use by anyone for reference. Wind farm developers often use ASOS, along with other NWS data as reference when determining the feasibility of a project site. New site measurements from met towers are correlated with existing wind data from a geographical area and used to validate data. NWS data is used as a general reference because many wind developers currently use mechanical sensors to capture onsite wind data. Hence, there is an increasing problem of inconsistency when referencing the ASOS ultrasonic wind data for a region. When submitting a wind project for financial review, consistency of data is a primary point of consideration.

Until recently, ultrasonic sensors have been the second choice for wind profilers. This is partially due to the perceived high cost and low concern for long lifecycles in the profiling and assessment market. Because ultrasonic sensors have no moving parts, they cost less to maintain and have longer life cycles than traditional cup and vane sensors. Sonic sensors are also ice resistant. In a recent test, some were proven...(Read whole article/video)