Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-8
lead length. If the capacitive load exceeds 0.1
µfd and the resistive load is negligible, V
x
will
oscillate about its control point. If the capacitive
load is 0.1 µfd or less, V
x
will settle to within
0.1% of its correct value in 150µs. A lead
length of 2000 feet is permitted for the Model
227 before approaching the drive limitation.
Table 13.3-6 summarizes maximum lead
lengths for corresponding error limits in six
Campbell Scientific sensors. Since the first
three sensors are nonlinear, the voltage error,
V
e
, is the most conservative value
corresponding to the error over the range
shown.
TABLE 13.3-6. Maximum Lead Length vs. Error for Campbell Scientific Resistive Sensors
Sensor Maximum
Model # Error Range Ve(µV) Length(ft.)
107 0.05
o
C0
o
C to 40
o
C 5 1000
1
207(RH) 1%RH 20% to 90% 500 1890
3
WVU-7 0.05
o
C0
o
C to 40
o
C 5 865
2
024A 3
o
@ 360
o
1390 430
2
227 - - - 2000
3
237 10 kohm 20k to 300k 500 1860
3
1
based on transient settling
2
based on signal rise time
3
limit of excitation drive
MINIMIZING SETTLING ERRORS IN NON-
CAMPBELL SCIENTIFIC SENSORS
When long lead lengths are mandatory in
sensors configured by the user, the following
general practices can be used to minimize or
measure settling errors:
1. When measurement speed is not a prime
consideration, Instruction 4, Excite, Delay,
and Measure, can be used to insure ample
settling time for half bridge, single-ended
sensors.
2. An additional low value bridge resistor can
be added to decrease the source
resistance, R
o
. For example, assume a YSI
nonlinear thermistor such as the model
44032 is used with a 30 kohm bridge
resistor, R
f
'. A typical configuration is
shown in Figure 13.3-7A. The
disadvantage with this configuration is the
high source resistance shown in column 3
of Table 13.3-7. Adding another 1K
resistor, R
f
, as shown in Figure 13.3-7B,
lowers the source resistance of the 21X
input. This offers no improvement over
configuration A because R
f
' still combines
with the lead capacitance to slow the signal
response at point P. The source resistance
at point P (column 5) is essentially the
same as the input source resistance of
configuration A. Moving R
f
' out to the
thermistor as shown in Figure 13.3-7C
optimizes the signal settling time because it
becomes a function of R
f
and C
w
only.
Columns 4 and 7 list the signal voltages as
a function of temperature using a 5V
excitation for configurations A and C,
respectively. Although configuration A has
a higher output signal (5V input range), it
does not yield any higher resolution than
configuration C which uses the ±500mV
input range.
NOTE: Since R
f
' attenuates the signal in
configuration B and C, one might consider
eliminating it altogether. However, its
inclusion "flattens" the non-linearity of the
thermistor, allowing more accurate curve
fitting over a broader temperature range.
3. Where possible, run excitation leads and
signal leads in separate shields to minimize
transients.
4. AVOID PVC INSULATED CONDUCTORS
to minimize the effect of dielectric
absorption on input settling time.