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.