Campbell Scientific CR7 Measurement and Control System - Manual (Page 143)

SECTION 13. CR7 MEASUREMENTS
13-19
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
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 CR7 ground,
and can be represented by the model
diagrammed in Figure 13.6-2.
FIGURE 13.6-2. Model of Resistive Sensor
with Ground Loop
In Figure 13.6-2, Vx is the excitation voltage, Rf
is a fixed resistor, Rs is the sensor resistance,
and RG is the resistance between the excited
electrode and CR7 earth ground. With RG in
the network, the measured signal is:
V V
R
R R R R R
x
s
s f s f G
1
=
+ +
( ) /
[13.6-1]
RsRf/RG is the source of error due to the
ground loop. When RG is large the equation
reduces to the ideal. The geometry of the
electrodes has a great effect on the magnitude
of this error. The Delmhorst gypsum block
used in the 227 probe has two concentric
cylindrical electrodes. The center electrode is
used for excitation; because it is encircled by
the ground electrode, the path for a ground loop
through the soil is greatly reduced. Moisture
blocks which consist of two parallel plate
electrodes are particularly susceptible to ground
loop problems. Similar considerations apply to
the geometry of the electrodes in water
conductivity sensors.
The ground electrode of the conductivity or soil
moisture probe and the CR7 earth ground form
a galvanic cell, with the water/soil solution
acting as the electrolyte. If current was allowed
to flow, the resulting oxidation or reduction
would soon damage the electrode, just as if DC
excitation was used to make the measurement.
Campbell Scientific probes are built with series
capacitors in the leads to block this DC current.
In addition to preventing sensor deterioration,
the capacitors block any DC component from
affecting the measurement.