Campbell Scientific CR7 Measurement and Control System - Manual (Page 132)

SECTION 13. CR7 MEASUREMENTS
13-8
Table 13.3-6 summarizes maximum lead
lengths for corresponding error limits in six
Campbell Scientific sensors. Since the first
three sensors are non-linear, the voltage error,
Ve, is the most conservative value
corresponding to the error over the range
shown.
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 ensure ample
settling time for half-bridge, single-ended
sensors.
2. An additional low value bridge resistor can
be added to decrease the source
resistance, Ro. For example, assume a YSI
non-linear 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, Rf, as shown in Figure 13.3-7B
lowers the source resistance of the CR7
input but 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 Rf' out to the
thermistor as shown in Figure 13.3-7C
optimizes the signal settling time because it
becomes a function of Rf and Cw only.
Columns 4 and 7 list the signal voltages as a
function of temperature with 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
±150 mV input range.
NOTE: Since Rf' attenuates the signal in
configurations 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.
TABLE 13.3-6. Maximum Lead Length vs. Error for Campbell Scientific Resistive Sensors
Sensor Maximum
Model # Error Range Ve(µV) Length(ft.)
107 0.05oC 0oC to 40oC 5 9651
207(RH) 1%RH 20% to 90% 500 19503
WVU-7 0.05oC 0oC to 40oC 5 8502
024A 3o @ 360o 1390 2502
227 - - - 20003
237 10 kohm 20k to 300k 500 19003
1 based on transient settling
2 based on signal rise time
3 limit of excitation drive