User guide

Appendix B. Wind Direction
Measurement Theory
It is not necessary to understand the concepts in this section for the general
operation of the 05103 with Campbell Scientific’s datalogger.
FIGURE B-1. 05103 potentiometer in a half bridge circuit
B.1 BRHalf Instruction
The BRHalf 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
varies 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 below:
(
)
stsxs
RRRVV
=
+
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. Since the datalogger outputs
the ratio V
s
/ V
x
, the multiplier is the same for both the CR10(X) and CR3000,
even though they use a different excitation voltage. See Section 13.5 in the
datalogger manual from more information on the bridge measurements.
B-1