Campbell Scientific CR510 Basic Datalogger - Manual (Page 153)

SECTION 13. CR510 MEASUREMENTS
13-9
TABLE 13.3-5. Summary of Input Settling Data For Campbell Scientific Resistive Sensors
Sensor Belden Ro Cw
ττττ
* Input
Model # Wire # (kohms) (pfd/ft.) (us) Range(mV) V
x
(mV) V
eo
(mV)**
107 8641 1 42 45 7.5 2000 50
207(RH) 8771 1 41 44 250 1500 85
WVU-7 8723 1 62 65 7.5 2000 0
227 8641 0.1-1 42 5-45 250 250 0
237 8641 1 42 45 25 2500 65
024A 8771 1-6 41 1-222 250 500 0-90
* Estimated time constants are for 1000 foot lead lengths and include 3.3nfd CR510 input
capacitance.
** Measured peak transients for 1000 foot lead lengths at corresponding excitation, V
x
.
TABLE 13.3-6. Maximum Lead Length vs. Error for Campbell Scientific Resistive Sensors
Sensor Maximum
Model # Error Range V
e
(µV) Length(ft.)
107 0.05
°
C 0
°
C to 40
°
C 5 1000 1
207(RH) 1%RH 20% to 90% 250 2000 3
WVU-7 0.05
°
C 0
°
C to 40
°
C 5 852 2
024A 3o @ 360
°
2083 380 2
227 - - - 2000 3
237 10 kohm 20k to 300k 1000 2000 3
1 based on transient settling
2 based on signal rise time
3 limit of excitation drive
The comparatively small transient yet large
source resistance of the 024A sensor indicates
that signal rise time may be the most important
limitation. The analysis in Section 13.3.2
confirms this.
The Model 227 Soil Moisture Block has a
relatively short time constant and essentially no
transient. Lead lengths in excess of 2000 feet
produce less than a 0.1 bar (0-10 bar range)
input settling error. With this sensor, the drive
capability of the excitation channel limits the
lead length. If the capacitive load 0.1 µfd and
the resistive load is negligible, V
x
will oscillate
about its control point. If the capacitive load is
0.1 or less, V
x
will settle to within 0.1% of its
correct value 150 µs. 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.
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 1 K resistor, R
f
, as
shown in Figure 13.3-7B, lowers the source
resistance of the CR510 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