Resistance Measurements Requiring Ac Excitation; Ac Excitation And Measurement Sequence For Ac Half Bridge; Model Of Resistive Sensor With Ground Loop - Campbell CR510 Operator's Manual

SECTION 13. CR510 MEASUREMENTS
13.5 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 247 conductivity probe) 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
FIGURE 13.5-1. AC Excitation and Measurement Sequence for AC Half Bridge
13-16
FIGURE 13.5-2. Model of Resistive Sensor with Ground Loop
is integrated (Figure 13.5-1). 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 CR510
ground, and can be represented by the model
diagrammed in Figure 13.5-2.