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SECTION
13.
21X MEASUREMENTS
lead
length.
lf
the
capacitive load exceeds
0.1
pfd and
the resistive load is negligible, V, will
oscillate about
its
control
point.
lf
the capacitive
load
is
0.1
pfd
or less, V, will settle to within
O.1o/o
ol
its
correct
value in
150ps. A
lead
length of 2000
feet is permitted for the Model
227 betore approaching the
drive limitation.
Sensor
Model#
Error
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",
is
the most conservative value
corresponding to the error over
the range
shown.
Maximum
Length(ft.)
TABLE 13.3-6. Maximum Lead Length vs. Error for Campbell Scientific Resistive Sensors
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,
Ro. For example, assume a YSI
nonlinear
thermistor such as the model
44032 is used with a
30 kohm bridge
resistor,
Ri.
A typical configuration
is
shown in Figure
13.3-74.
The
disadvantage with this configuration
is
the
high
source resistance shown
in
column
3
of
Table 13.3-7. Adding another
1K
resistor,
R1,
as shown
in Figure 13.3-78,
lowers the
source resistance of the 21X
input.
This
offers no improvement over
configuration
A because
Ri
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
13-8
ve(pv)
5
1oOOl
5oo
18903
5
8652
1390
4gO2
-
2ooo3
5oo
18603
same as
the input source resistance of
configuration
A.
Moving
Ri out to the
thermistor as shown in Figure 13.3-7C
optimizes
the signal settling time because
it
becomes
a
function
of
Rr
and
C* 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
t500mV
input range.
Where possible, run
excitation leads and
signal leads in separate shields
to minimize
transients.
AVOID PVC INSULATED CONDUCTORS
to minimize the etfect of dielectric
absorption
on input settling time.
107
207(RH)
WVU-7
o24A
227
237
0.050c
1%RH
0.050c
30
10 kohm
Range
ooO
to
4ooO
2Oo/"to
9Oo/"
ooc to
4oo-C
@ 3600
20k
to 300k
I
based on transient settling
'
based on
signal
rise
time
'
limit of excitation drive
3.
4.
NOTE: Since Ri
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.
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