Campbell Scientific 03002, 03101, and 03301 R. M. Young Wind Sentry Sensors - Manual (Page 31)

Appendix B. 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 B-1. 03002 and 03301 potentiometer in a half bridge circuit
EARTH GROUND CONNECTION
AZIMUTH REFERENCE
SIGNAL + (Vs)
EXCITATION VOLTAGE (Vx)
Rt
Rs
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 352 degrees. The multiplier to convert
V
s
/V
x
to degrees is 352 degrees / 1.0 V
s
/V
x
= 352. 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
CR10X manual, Section 4.3 in the CR1000 manual, or Section 3.5 in the
CR3000 manual for 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 + ⋅ =
B-1