Campbell Scientific CR7 Measurement and Control System - Manual (Page 134)

SECTION 13. CR7 MEASUREMENTS
13-10
FIGURE 13.3-7. Half-Bridge Configuration
for YSI #44032 Thermistor Connected to CR7
Showing: A) Large source resistance, B)
Large source resistance at point P, and C)
Configuration optimized for input settling.
FIGURE 13.3-8. Measuring Input Settling
Error with the CR7
6. Most Campbell Scientific sensors are
configured with a small bridge resistor, Rf,
(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, Rl, 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 Rf/(Rs+Rb+Rf) but
when the pigtails are extended the signal is
proportional to (Rf+Rl)/(Rs+Rb+Rf+Rl). Rl 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:
Vsl/Vs = (Rf+Rl)/Rf
Plugging in values of Rf=1k and Rl=.012k (500o
at 23ohms/1000o, Table 13.3-2) gives an
approximate 1% error in the signal with
extended leads. Converting the error to oC
gives approximately a 0.3oC error at 0oC,
0.6oC error at 20oC and a 1.5oC error at 40oC.
The error can be avoided by maintaining the
pigtails on the CR7 end of the extended leads
because Rl does not add to the bridge
completion resistor, Rf, and its influence on the
thermistor resistance is negligible.