With R
g
in the resistor network, the signal measured from the sensor is described by the
following equation:
where
l
V
x
is the excitation voltage
l
R
f
is a fixed resistor
l
R
s
is the sensor resistance
l
R
g
is the resistance between the excited electrode and data logger earth ground.
R
s
R
f
/R
g
is the source of error due to the ground loop. When R
g
is large, the error is negligible.
Note that the geometry of the electrodes has a great effect on the magnitude of this error. The
Delmhorst gypsum block used in the Campbell Scientific 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 that
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 data logger earth
ground form a galvanic cell, with the water/soil solution acting as the electrolyte. If current is
allowed to flow, the resulting oxidation or reduction will soon damage the electrode, just as if DC
excitation was used to make the measurement. Campbell Scientific resistive soil probes and
conductivity probes are built with series capacitors to block this DC current. In addition to
preventing sensor deterioration, the capacitors block any DC component from affecting the
measurement.
22. Tips and troubleshooting188
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