Resistance Measurements Requiring Ac Excitation; Ac Excitation And Measurement Sequence For Ac Half Bridge - Campbell Measurement and Control Module CR10 Operator's Manual

R = R
f
13.6 RESISTANCE MEASUREMENTS
REQUIRING AC EXCITATION
Some resistive sensors require AC excitation.
These include the 207 Relative Humidity Probe,
soil moisture blocks, water conductivity
sensors, and wetness sensing grids. The use
of DC excitation with these sensors can result
in polarization, which will cause an erroneous
measurement, and may shift the calibration of
the sensor and/or lead to its rapid decay.
The AC half bridge Instruction 5 (incorporated
into the 207 relative humidity measurement
Instruction 12) reverses excitation polarity to
provide ion depolarization and, in order to
minimize the time excitation is on, grounds the
FIGURE 13.6-1. AC Excitation and Measurement Sequence for AC Half Bridge
/X
s
SECTION 13. CR10 MEASUREMENTS
7 or 9 1/R
s
42
excitation as soon as the signal is integrated
(Figure 13.6-1). The slow integration time
should never be used with a sensor requiring
AC excitation because it results in the excitation
lasting about 1.5 times as long, allowing
polarization to affect the measurement.
INFLUENCE OF GROUND LOOP ON
MEASUREMENTS
When measuring soil moisture blocks or water
conductivity, the potential exists for a ground
loop which can adversely affect the
measurement. This ground loop arises
because the soil and water provide an alternate
path for the excitation to return to CR10 ground,
and can be represented by the model
diagrammed in Figure 13.6-2.
0
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