Section 13. CR10X Measurements
13-13
Figure 13-11 Measuring Input Settling Error With the
CR10X
6.
Most Campbell Scientific sensors are configured with a small bridge resistor,
R
f
, (typically 1k
Ω
) to minimise the source resistance. If the lead length of a
Campbell Scientific sensor is extended simply by joining extension lead on
to the end of the existing cable, the effect of the lead resistance, R
l
, on the
signal must be considered. Figure 13-12 shows a Campbell Scientific Model
107 sensor with 150m of extension lead connected in this way. Normally the
signal voltage is proportional to R
f
/(R
s
+R
b
+R
f
), but when the cable is joined
in this way, 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 analysed 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
Substituting values of R
f
=1k
Ω
and R
l
=.012k
Ω
(see Table 13-2) 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 ensuring that the R
f
is
moved to the CR10X
end of the extended leads because R
1
does not add to
the bridge completion resistor, R
f
, and its influence on the thermistor resis-
tance is negligible.
Figure 13-12 Incorrect Lead Wire Extension on 107 Temperature Sensor
13.4 Thermocouple Measurements
A thermocouple consists of two wires, each of a different metal or alloy, which
are joined together at each end. If the two junctions are at different temperatures, a
voltage proportional to the difference in temperatures is induced in the wires.
When a thermocouple is used for temperature measurement, the wires are soldered
or welded together at the measuring junction. The second junction, which
becomes the reference junction, is formed where the other ends of the wires are
connected to the measuring device. (With the connectors at the same
150 m
