Campbell Scientific 03001-L R.M. Young Wind Sentry Set - Manual (Page 23)

Appendix B. Wind Direction
Measurement Theory
It is not necessary to understand the concepts in this section for the general
operation of the 03001 with Campbell Scientific’s datalogger.
EXCITATION VOLTAGE (Vx)
SIGNAL + (Vs)
AZIMUTH REFERENCE
EARTH GROUND CONNECT N
Rt
Rs
IO
FIGURE B-1. 03001 / 03301 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:
( )
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 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.2 EX-DEL-SE (P4) Instruction
Instruction 4 outputs a precise excitation voltage (V
x
) and measures the voltage
between the wiper and analog ground, V
s
. The resistance between the wiper
and analog ground, R
s
, and V
s
varies with wind direction. Instruction 4 outputs
the measured voltage, V
s
. This measured voltage is related to 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
reaches its
maximum value of V
x
. This maximum voltage equals 2500 mV for an
B-1