Campbell Scientific CR23X Micrologger - Manual (Page 224)

SECTION 13. CR23X MEASUREMENTS
13-24
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
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 20 times as long, allowing
polarization to affect the measurement.
FIGURE 13.6-1. AC Excitation and Measurement Sequence for AC Half Bridge
FIGURE 13.6-2. Model of Resistive Sensor with Ground Loop