Operator`s manual

SECTION 13. 21X 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 21X 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, V
x
is the excitation voltage, Rf
is a fixed resistor, R
s
is the sensor resistance,
and R
G
is the resistance between the excited
electrode and 21X earth ground. With R
G
in the
network, the measured signal is:
VV
R
RR RRR
x
s
sf sfG
1
=
++()/
[13.6-1]
R
s
R
f
/R
G
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 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.