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13.6
RESIST
CE MEASUREMENTS
REQU
AC
EXCITATION
sensors require AC excitation.
lhe 207 relative humidity probe,
soil
water conductivity sensors,
grids. The
use
of
DC
polarization, which
willcause
an erroneous
measurement,
and may shift the calibration of
the sensor
arhd/or
lead
to its rapid decay.
The
AC half bridge lnstruction 5 (incorporated
into the
207 telative 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
13.6i1). The slow
integration
time
should
nevei be used with
a
sensor requiring
AC excitatiod because it results
in
the excitation
lasting about 20 times
as long, allowing
polarization
!o affect the measurement.
0l2
Time (milliseconds)
FIGURE
13.6-1. AC Excitation and
Measurempnt 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 oan adversely atfect
the
measurement. This ground loop arises
because
the soil and water provide an alternate
path for
the excitation to return to 21X ground,
and
can be lepresented by the model
diagramme{ in Figure 13.6-2.
SECTION
13.
21X MEASUREMENTS
FIGURE
13.6-2. Model of Resistive Sensor
with Ground Loop
In Figure 13.6-2,
V,.
is
the excitation
voltage,
Rf
is
a
fixed resistor,
R"
is the
sensor resistance,
and
R6 is
the resistance between the excited
electrode and 21X earth
ground. With
R6
in
the
network,
the measured signal
is:
Some
These
and wetness
Vr=V-A
'
^
(R. +Rs)+RsRf /RG
[13.6"1]
RsRr/Re 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 21X eafth ground form
a galvanic cell, with the water/soilsolution
acting as the
electrolyte.
lf
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.
*vx
Excito!ion
Voltoge
13-19
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