Campbell 03002, Instruction Manual

Page: 36 / 39

D-1
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 Campbell Scientific’s datalogger.
FIGURE D-1. 03002 and 03301 potentiometer in a half bridge circuit
D.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:
( )
s t s x s
R R R V V + =
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 352 degrees. The multiplier to convert
V
s
/V
x
to degrees is 352 degrees / 1.0 V
s
/V
x
= 352. See datalogger manual for
more information on the bridge measurements.
EXCITATION VOLTAGE (Vx)
(Vs)
AZIMUTH REFERENCE
EARTH GROUND CONNECTION
Rt
Rs