NIST Special Publication 250-79 Airspeed Calibration Service.
Tsyh Tyan Yeh, J. M. Hall
The National Institute of Standards and Technology (NIST) provides calibration services for airspeed instrumentation such as Pitot-static tubes, anemometers (ultrasonic, thermal or hot-wire, cup, and propeller types) as well as others. The NIST calibration system consists of a wind tunnel and a Laser Doppler Anemometer (LDA). The standard for airspeed measurement is a fiber optic LDA, which is calibrated with known velocities produced by a rotating disk. A Pitot-static tube probe and thermal anemometer are used as check standards. The wind tunnel used for the calibration has a test cross section of 1.5 m by 2.1 m and water droplets injected into the wind tunnel from fine spray nozzles provide the light scattering particles necessary for the operation of the LDA. The range of the airspeed in the calibration service is from 0.15 m/s to 40 m/s.This document also describes the methods of operation and uncertainty of NIST airspeed standards. An extensive component analysis of the system was performed. Over the calibration range, the airspeed uncertainty is found to be a function of airspeed and the expanded uncertainty at 95 % confidence level, i.e. k = 2, in m/s can be expressed by the root-sum-square of 0.0064V and 0.0036 m/s, where V is airspeed in m/s. This gives a maximum uncertainty of 2.5 % (0.0038 m/s ) at the lowest airspeed of 0.15 m/s and a minimum uncertainty of 0.65 % (0.26 m/s ) at the highest speed of 40 m/s.