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error
For
e
Use
the
1X
to measure the input settling
with a given configuration.
assume long leads are
but
the lead capacitance, C*,
is
Configure
Rl
on
a length of cable
SECTION
13.
21X MEASUHEMENTS
voltage is propottionalto Ry'(R'+Rs+R), but
when
the pigtails are extended, the signal
is
proportional
to
(Rr+Rr)/(Rs+R6+R1+R1). R; is
much smaller
than the other terms
in
the
denominator and
can be discarded. The
effect
on
the signal can be analyzed by
taking
the ratio
of
the signal with extended
leads,
Vslto the normalsignal,
Vs:
Vr/V. =
(R1+R1)/R1
Plugging
in
values of
R61k and
Rr=.012k
(500'at 23
ohms/1000',
Table 13.3-2) gives
an
approximate
1o/o
error
in
the signal with
extended
leads.
Converting
the error to
oC
gives approximately
a 0.3oC
error
at
OoC,
0.6oC
error at
2OoC,
and
a
1.5oC
error at
40oC. The error can be avoided by
maintaining
the pigtails on the 21X end
of
the
extended leads because
Rr
does not
add to
the bridge completion resistor,
Rr,
and its influence
on
the thermistor
resistance is negligible.
similar td
the measurement. Leave the
sensor
{nd
open as shown
in Figure 13.3-8
and
me{sure the result using the same
instructiOn parameters
to be used with the
sensor. The
measured deviation
from 0V
is
the input settling error.
Most
Cqmpbell Scientific sensors are
configurFd
with a small bridge resistor,
R1,
(typically
1
kohm)
to minimize the source
resistanpe.
lf
the lead length of a Campbell
Scientifip
sensor
is
extended by connecting
to
the
piltails
directly, the effect of
the lead
resistanfe,
R;,
on
the signal must be
considefed. Figure
13.3-9 shows
a
Campb(ll Scientific Model 107 sensor with
500
feet of extension lead connected
directly fo
the
pigtails.
Normally
the signal
TABLE 13.8-7. Source Resistances and Signal Levels for
YSI
*44032
Thermistor Gonfigurations
Shown in Figure
13.3-7
(2V
Excitation)
T
Rs
(kohms)
884.6
271.2
94.98
30.00
16.15
7.60
Vs(mV)
164
498
1200
2500
3250
3989
-B-
Ro@P
(kohms)
30.0
27.8
23.4
15.2
10.6
6.1
5.5
16.5
39.5
82.0
106.0
129.5
---A---
Ro
(kohms)
29.0
27
22.8
15.0
10.5
6.1
---c---
Ro
Vs(mV)
(kohms)
-40
-20
0
+25
+40
+60
1
1
1
1
1
1
13-9
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