Campbell CR23X, Operator'S Manual

Page: 208 / 374

SECTION 13. CR23X MEASUREMENTS
13-8
R
o
, the source resistance, is not constant
because R
b
varies from 0 to 10 kohms over the
0 to 360 degree wind direction range. The
source resistance is given by:
R
o
= R
d
+(R
b
(R
s
-R
b
+R
f
)/(R
s
+R
f
)) =
R
d
+(R
b
(20k-R
b
)/20k) [13.3-12]
Note that at 360 degrees, R
o
is at a maximum
of 6k (R
b
=10k) and at 0 degrees, R
o
is 1k
(R
b
=0). It follows that settling errors are less at
lower direction values.
The value of R
b
for any direction D (degrees) is
given by:
R
b
(kohms) = (10k)(D)/360 [13.3-13]
Equation 13.3-6 can be rewritten to yield the
settling error of a rising signal directly in units of
degrees.
Error (degrees) = De-t/(R
o
(C
f
+C
w
L))
[13.3-14]
Equation 13.3-12, -13 and -14 can be combined
to estimate the error directly in degrees at
various directions and lead lengths (Table 13.3-
3). Constants used in the calculations are given
below:
C
f
= 3.3nfd
C
w
= 41 pfd/ft., p/n 9721 wire
t = 450µs
Example calculation at 360° with 1000 ft lead:
(all variables converted to ohms, farads,
seconds, or feet)
error =
360° *
e fd fd ft ft)))) ( / ( ( . ( -450* * * * + * * -6 -9 -12 -1 10 6 10 3 3 10 41 10 10 00 3 s
Ω
error = 66°
TABLE 13.3-3. Settling Error, in Degrees, for
024A Wind Direction Sensor vs. Lead Length
Wind Error
Direction L=1000 ft. L=500 ft.
360
°
66
°
15
°
270
°
45
°
9
°
180
°
21
°
3
°
90
°
4
°
0
°
The values in Table 13.3-3 show that significant
error occurs at large direction values for leads
in excess of 500 feet. Instruction 4, Excite,
Delay, and Measure, should be used to
eliminate errors in these types of situations.
Using a 10 ms delay, settling errors are
eliminated up to lengths that exceed the drive
capability of the excitation channel (
~
2000 ft.).
13.3.3 TRANSIENTS INDUCED BY SWITCHED
EXCITATION
Figure 13.3-6 shows a typical half bridge
resistive sensor, such as Campbell Scientific's
Model 107 Temperature Probe, connected to
the CR23X. The lead wire is a single-shielded
pair, used for conducting the excitation (V
x
) and
signal (V
s
) voltages. When V
x
is switched on, a
transient is capacitively induced in V
s
, the signal
voltage. If the peak transient level, V
eo
, is less
than the true signal, V
so
, the transient has no
effect on the measurement. If V
eo
is greater
than V
so
, it must settle to the correct signal
voltage to avoid errors.
TABLE 13.3-4. Measured Peak Excitation Transients for 1000 Foot Lengths of Lead Wires Used
by Campbell Scientific
-----------------------V
eo
(mV) -----------------------
Vx(mV) Rf=1 kohm R
f
=10 kohm
# # # # # #
9661 7359 9721 9661 7359 9721
2000 50 100 60 100 140 80
1000 25 65 40 60 90 40