Quality of Air Speed Calibrations

By Rachael V. Coquilla

Wind measuring instruments, such as cup anemometers, propeller anemometers, ultrasonic anemometers, velocity meters, airflow meters, and many other types, are commonly used in various industries as monitoring devices or for evaluation and research of air movement. The most common application is weather measurements. In some cases, high accuracy in measuring atmospheric winds is not necessary. Thus, a simple field check, where the output of a wind sensor is verified against a known reference installed nearby, may be all that is required. However, for industries where wind speed measurements are critical such as wind energy, nuclear safety, and building commissioning, wind measuring instruments are required to be calibrated in a controlled wind tunnel test facility, where the reference wind speed measurement is traceable to national standards. There are several types of wind tunnels, each of which were designed for specific purposes. As specified by various published standards, wind instrument calibration testing is to be performed in a uniform-flow, low-turbulence wind tunnel. Such standards define the minimum requirements to meet and maintain steady-state horizontal flow in an empty wind tunnel test section. However, once a test object is placed in that steady flow, the flow quality changes. It is then recommended to evaluate the flow quality due to the disturbance caused by the test object and resulting interaction from the wind tunnel test section boundaries. The combined effect of test object disturbance and wall interaction is commonly known as “blockage” and is one of the most challenging phenomena associated with wind tunnel studies. If the level of “blockage” during a wind sensor calibration test affects the relationship between the output of the instrument under test and the corresponding reference wind, the test design may need to be adjusted and/or correction factors may need to be applied to the corresponding test results. The purpose of this presentation is to discuss the investigative methods when qualifying a wind tunnel for air speed calibration of wind measuring instruments and present the evaluation results of a newly installed wind tunnel for air speed calibration testing. Overall, the qualification of a wind tunnel to conduct air speed calibrations not only depends on how well the facility satisfies the minimum requirements from published standards but also is related to how a specific wind measuring instrument performs in that wind tunnel.

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