Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-9
5. Use the 21X to measure the input settling
error associated with a given configuration.
For example, assume long leads are
required but the lead capacitance, C
w
, is
unknown. Configure R
f
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, Vsl to the normal signal, Vs:
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
o
C
gives approximately a 0.3
o
C error at 0
o
C,
0.6
o
C error at 20
o
C, and a 1.5
o
C error at
40
o
C. The error can be avoided by
maintaining the pigtails on the 21X 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 - - -
TRsRoVs(mV)Ro@PRoVs(mV)
(kohms) (kohms) (kohms) (kohms)
-40 884.6 29.0 164 30.0 1 5.5
-20 271.2 27 498 27.8 1 16.5
0 94.98 22.8 1200 23.4 1 39.5
+25 30.00 15.0 2500 15.2 1 82.0
+40 16.15 10.5 3250 10.6 1 106.0
+60 7.60 6.1 3989 6.1 1 129.5