Appendix D. Wind direction
measurement theory
It is not necessary to understand the concepts in this section for the general operation of the
03002 with a Campbell Scientific data logger.
Figure D-1. 03002 and 03301 potentiometer in a half bridge circuit
The
BRHalf
CRBasic instruction outputs a precise excitation voltage (V
x
), and measures the
voltage between the wiper and ground (V
s
). The resistance between the wiper and ground (R
s
),
and V
s
vary with wind direction. The measurement result is the ratio of the measured voltage to
the excitation voltage (V
s
/V
x
). This ratio is related to the resistance as shown in this equation:
The maximum value that R
s
will reach is R
f
, just before it crosses over from the west side of north
to the east side of north (at this point R
t
= 0). V
s
/ V
x
reaches its maximum value of 1.0 mV/mV at
355 degrees. The multiplier to convert V
s
/V
x
to degrees is 355 degrees / 1.0 V
s
/V
x
= 355. Refer to
the data logger manual for more information on the bridge measurements.
Wind Sentry Series
23
