CR10X Reference Manual
13-10
Setting the allowable error at 0.1
o
C or approximately 5
µ
V, the maximum lead
length increases to:
L
≅
303m, error
≅
0.1
o
C
13.3.4 Summary of Settling Errors for Campbell Scientific
Resistive Sensors
Table 13-5 summarises the data required to estimate the effect of lead length on
settling errors for Campbell Scientific’s resistive sensors. Comparing the transient
level, V
eo
, to the input range shows that transient errors are the most likely
limitation for the 107 sensor.
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 600m 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 exceeds 0.1
µ
F
and the resistive load is negligible, V
x
oscillates about its control point. If the
capacitive load is 0.1
µ
F or less, V
x
settles to within 0.1% of its correct value in
150µs. A lead length of 600m is permitted for the Model 227 before approaching
the drive limitation.
Table 13-5 Summary of Input Settling Data For Campbell Scientific
Resistive Sensors
Sensor
Belden
R
o
C
w
ττττ
*
Input
Model #Wire #
(k
Ω
Ω
Ω
Ω
)
(pF/m)
(
µµµµ
s) Range(mV) V
x
(mV) V
eo
(mV)**
107
8641
1
142
45
7.5
2000
50
207(RH)
8771
1
134
44
250
1500
85
227
8641
0.1-1
142
5-45
250
250
0
237
8641
1
142
45
25
2500
65
024A
8771
1-6
139
1-222
250
500
0-90
*
Estimated time constants are for 300m (1000 foot) lead lengths and include
3.3nF CR10X input capacitance.
**
Measured peak transients for 300m (1000 foot) lead lengths at
corresponding excitation, V
x
.
Table 13-6 Maximum Lead Length vs. Error for Campbell Scientific
Resistive Sensors
Sensor
Maximum
Model #
Error
Range
V
e
(µV)
Length(m)
107
0.05
o
C
0
o
C to 40
o
C
5
308
1
207(RH)
1%RH
20% to 90%
250
582
3
024A
3
o
@ 360
o
2083
112
2
227
-
-
-
615
3
237
10 kohm
20k to 300k
1000
572
3
1
based on transient settling
2
based on signal rise time
3
limit of excitation drive
