CS805 Heated-Needle Anemometer
4
Changes in the current delivered to the CS805 will directly contribute to the
error of the wind speed measurement. When the CS805 is being used with the
CE805 at a temperature different from 25°C the inaccuracy of the current
delivery will alter the measurement. This deviation will show up in the square
root of wind speed domain, and the error will actually double in the wind speed
domain.
For example, if you are operating over a range –20 to 50°C, with the CE805
delivering current to the CS805, that will introduce a 2% inaccuracy in your
current (sqrt (wind speed) domain) and will show up as a 4% inaccuracy in
your wind speed measurement. This would add to the 5% inaccuracy specified
in the CS805.
4. Calibration
As mentioned in the previous section, calibration was done in a controlled
environment at 25°C, 646 mmHg (861mbar, 0.85 atm), and 31.100 mA of
constant current. The calibration is done in a wind tunnel, using Sierra
Instruments’ mass flow meters (700 series) to calculate the wind speed. Each
sensor has been calibrated over the specified range to deliver accurate and
precise measurements within the specifications.
As previously noted, the best fit calibration curve is linear, of the form:
2/1
use
calib
2/1 P
P
T
B
Au
∆
+= ,
where u is the wind speed, A is the offset, B is the slope, ∆T the temperature
difference, Pcalib the calibration pressure, and Puse is the use pressure, which will
be site specific to the user.
Each sensor comes with a serial number specific calibration giving the user the
slope and offset of the calibrated fit. Pcalib is a function of the elevation in
Logan, Utah, and is: 646 mmHg, 861 mbar, or 0.85 atm, depending upon
preferred units. The calibration is done to output wind speed in cm/s. If the
desired unit is mph, a multiplier of 0.0224 mph/cms-1 needs to be used as the
final adjustment.
For the provided calibration to work in use, the ∆T measurement should be
referenced to an external air temperature measurement. Because of the
datalogger’s thermocouple measurement algorithm, the reference temperature
will need to be subtracted from the original CS805 temperature measurement
to determine ∆T. This is further discussed in the installation and program
example sections of the manual.
5. Installation
The CS805 is most sensitive to wind perpendicular to the axis probe. The
sensitivity deviates as the angle of wind changes from normal, as discussed in
Section 3.1. The probe will also be affected by the free flow of air at the
sensor; any wind shielding by a leaf or building will affect the measurement. It
is suggested to think carefully about the desired measurement and
measurement constraints of the sensor.
