Campbell Scientific CR10X Measurement and Control System - Manual (Page 213)

SECTION 13. CR10X MEASUREMENTS
13-21
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 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 CR10X
ground, and can be represented by the model
diagrammed in Figure 13.6-2.
FIGURE 13.6-1. AC Excitation and Measurement Sequence for AC Half Bridge
FIGURE 13.6-2. Model of Resistive Sensor with Ground Loop