If an air speed sensor is used under conditions that differ from the conditions under which the sensor was calibrated, the calibrated sensor may give significant errors. We are studying the effects of blockage and wall proximity on the calibration data in order to provide a more reliable calibration suitable for the conditions under which the sensor will be used.
By convention, air speed sensors are calibrated to read the velocity that would occur:
Typically, wind tunnels used for calibration generate low-turbulence, high-speed flows by straightening a low-speed flow and then directing the flow through a smooth, converging section. As the flow proceeds downstream, turbulent boundary layers grow further from the wall causing the size of the "safe calibration region" to decrease and the velocity in that region to increase. To minimize the effects of the tunnel's wall, calibrations should not be in the boundary layer.
A calibrated laser Doppler anemometer (LDA) is a non-invasive, velocity standard used to calibrate, for example, Pitot tubes. However, the Pitot tube itself disturbs the velocity field. If the Pitot tube is too close to the LDA standard, its apparent calibration coefficient will be larger than its "true" calibration coefficient in an infinite flow field. [See graph of V(x)/V(∞).]
If a large wind tunnel is available, blockage effects are reduced by ensuring that the device under test (Pitot tube) is sufficiently far downstream from the standard (LDA) and the calibration is conducted in the center of the tunnel.
If a small wind tunnel is used, compensation for blockage effects can be achieved using a "substitution" method. To do this, the standard and the anemometer to be calibrated must be identical. First, the standard is calibrated under "ideal" conditions in a large wind tunnel. Then, it is placed in a smaller, "working" tunnel and its reading is recorded. Finally, the standard is replaced with the anemometer to be calibrated and its readings are recorded.