Campbell Scientific CR510 Basic Datalogger - Manual (Page 154)

SECTION 13. CR510 MEASUREMENTS
13-10
source resistance at point P (column 5) is
essentially the same as the input source
resistance of configuration A. Moving R
f'
out
to the thermistor as shown in Figure 13.3-7C
optimizes the signal settling time because it
becomes a function of R
f
and C
w
only.
Columns 4 and 7 list the signal voltages as a
function of temperature using a 2000 mV
excitation for configurations A and C,
respectively. Although configuration A has a
higher output signal (2500 mV input range), it
does not yield any higher resolution than
configuration C which uses the
± 250 mV
input range.
NOTE: Since R
f
' 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.
3. Where possible, run excitation leads and
signal leads in separate shields to minimize
transients.
4. Avoid PVC-insulated conductors to
minimize the effect of dielectric absorption
on input settling time.
5. Use the CR510 to measure the input
settling error associated with a given
configuration. For example, assume long
leads are required but the lead capacitance,
Cw, is unknown. Configure Rf on a length
of cable similar to the measurement. Leave
the sensor end open as shown in Figure
13.3-8 and measure the result using the
same instruction parameters to be used
with the sensor. The measured deviation
from 0V is the input settling error.
6. Most Campbell Scientific sensors are
configured with a small bridge resistor, R
f
,
(typically 1 kohm) to minimize the source
resistance. If the lead length of a Campbell
Scientific sensor is extended by connecting
to the pigtails directly, the effect of the lead
resistance, R
l
, on the signal must be
considered. Figure 13.3-9 shows a
Campbell Scientific Model 107 sensor with
500 feet of extension lead connected
directly to the pigtails. Normally the signal
voltage is proportional to R
f
/(R
s
+R
b
+R
f
), but
when the pigtails are extended, the signal is
proportional to (R
f
+R
l
)/(R
s
+R
b
+R
f
+R
l
). R
l
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, V
sl
to the normal signal, V
s
:
V
sl
/V
s
= (R
f
+R
l
)/R
f
Plugging in values of R
f
=1k and R
l
=.012k
(500' at 23 ohms/1000', Table 13.3-2) gives
an approximate 1% error in the signal with
extended leads. Converting the error to
°
C
gives approximately a 0.33=
°
C error at 0
°
C,
0.53
°
C error at 20
°
C, and a 0.66
°
C error at
40
°
C. The error can be avoided by
maintaining the pigtails on the CR510 end
of the extended leads because R
l
does not
add to the bridge completion resistor, R
f
,
and its influence on the thermistor
resistance is negligible.
TABLE 13.3-7. Source Resistances and Signal Levels for YSI #44032 Thermistor Configurations
Shown in Figure 13.3-7 (2V Excitation)
--------A-------- -----B----- -------C-------
T R
s
R
o
V
s
(mV) R
o
@P R
o
V
s
(mV)
(kohms) (kohms) (kohms) (kohms)
-40 884.6 29.0 66 30.0 1 2.2
-20 271.2 27 200 27.8 1 6.6
0 94.98 22.8 480 23.4 1 15.9
+25 30.00 15.0 1000 15.2 1 32.8
+40 16.15 10.5 1300 10.6 1 42.4
+60 7.60 6.1 1596 6.1 1 51.8