Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-6
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
= 41pfd/ft., Belden #8771 wire
t = 450µs
TABLE 13.3-3. Settling Error (Degrees) for
024A Wind Direction Sensor vs. Lead Length
Wind - - - - - Error - - - - -
Direction L=1000 ft. L=500 ft.
360
o
47
o
8
o
270
o
31
o
5
o
180
o
12
o
1
o
90
o
1
o
0
o
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 10ms 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 21X. 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.
NOTE: Excitation transients are eliminated
if an option exists to contain excitation leads
in a shield independent from the signal
leads.
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, R
f
, increases. Table 13.3-4
shows measured levels of V
eo
for 1000 foot
lengths of three Belden wires used in Campbell
Scientific sensors. Values are given for R
f
equal to 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.
TABLE 13.3-4. Measured Peak Excitation Transients for 1000 Foot Lengths of Three Belden Lead
Wires Used by Campbell Scientific
- - - - - - - - - - - V
eo
(mV) - - - - - - - - -
Vx(mV) R
f
=1 kohm R
f
=10 kohm
######
8641 8771 8723 8641 8771 8723
5000 125 200 130 215 320 180
4000 100 165 110 180 260 150
3000 75 130 90 140 200 110
2000 50 100 60 100 140 80
1000 25 65 40 60 90 40