United Sensor Corporation has two wind tunnels available for which to perform probe calibrations. Since each of United Sensor's probes is hand made, there may be inherent pressure measurement differences due to machining and assembly tolerances. It is important to be able to calibrate the pressure readings of any given probe against a standard probe.
Both of United Sensor's wind tunnels consists of a flow straightening section, a measurement section, and a fan downstream of the measurement section. Since the fan is located downstream of the measurement section, the flow is produced by a vacuum downstream of the probe. Because of this, a probe in the flow stream will have a nearly constant total pressure reading while the static pressure reading decreases as the flow speed is increased.
The primary wind tunnel used at United Sensor Corporation for calibrations is octagonal shaped and has an 8 inch cross section. It is possible to achieve velocity pressures of about 21 cm of water. The secondary wind tunnel has a square crossection and is 2.75 inches wide. It is possible to achieve velocity pressure of up to 30 cm of water in the smaller section tunnel. Typical calibrations are performed at 20 cm of water. United Sensor recommends that a probe be calibrated at a flow speed that is similar to that which the probe is to be used.
Probes can be calibrated with both pitch and yaw angle set to zero with respect to the flow or can be calibrated by varying the pitch angle while the yaw angle is zero or by varying the yaw angle while the pitch angle is constant. If a calibration is performed with the larger wind tunnel, it is possible to achieve pitch angles of +/- 50 degrees in 5 degree increments, whereas it is only possible to achieve +/- 40 degrees in 10 degree increments in the smaller tunnel. With both tunnels, it is possible to achieve any angle in yaw with an uncertainty of about half a degree. Typical calibrations that are included with many of United Sensor's three dimensional and two dimensional probes consist of a pitch calibration with angles of +/- 40, 30, 20, 10 and 0 degrees while the yaw is set to zero.