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L=500
ft.
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
1Oms
delay, settling errors are
eliminated
up
to lengths that exceed the drive
capability
of
the excitation
channel (=2000 ft.).
13.3.3 TRANSIENTS
TNDUCED
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
SECTION
13.
21X MEASUREMENTS
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:
Cr
=
3'3nfd
C*
=
41pfd/ft., Belden #8771 wire
t = 450tts
TABLE 13.3-3. Settling Error (Degrees)
for
024A
Wind Direction Sensor vs. Lead Length
the 21X.
The lead wire
is
a single-shielded
pair,
used for conducting the
excitation (V,) and
signal (Vr)
voltages.
When
V,
is
switched on,
a
transient
is
capacitively induced
in
V", the signal
voltage.
lf the peak
transient level,
V"o,
is
less
than the true signal,
Vro,
the
transient has
no
effect on
the measurement. lf
V"o
is greater
than
Vro,
it must settle to
the correct signal
voltage to avoid errors.
SHIELD
FIGURE
13.3-6. Resistive Half Bridge
Connected to Single-Ended 21X Input
The size
of
the peak transient is linearly related
to the excitation voltage and increases as the
bridge resistor,
Rs,
increases. Table
'13.3-4
shows measured levels of
V"o
for 1000 foot
lengths
of
three Belden wires used in Campbell
Scientific
sensors.
Values are given
for
R1
equalto
1
kohm
and 10
kohm. Table
13.3-4
is
meant only
to provide estimates
of
the
excitation transients encountered: the exact
magnitude will depend upon the specific sensor
configuration.
Wind
Direction
3600
2700
1
800
9oo
Error-----
L=1000
ft.
470
310
120
10
go
c
10
00
TABLE 13.3-4. Measured Peak Excitation Transients for
1000
Foot Lengths of Three Belden
Wires Used by CampbellScientific
-
V"o(mV)
Vx(mV)
5000
4000
3000
2000
1
000
Rr=l kohm
#
8771
200
165
130
100
65
R1=10
kohm
#
8771
320
260
200
140
90
#
8641
125
100
75
50
25
#
8723
130
110
90
60
40
#
8641
215
180
140
100
60
#
8723
180
150
110
80
40
NOTE:
Excitation transients are eliminated
if
an option exists to contain excitation
in a
shield independent from the signal
leads.
13-6
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