Section 7. Installation
146
7.3.4 Ground Looping in Ionic Measurements
When measuring soil-moisture with a resistance block, or water conductivity with
a resistance cell, the potential exists for a ground loop error. In the case of an
ionic soil matric potential (soil moisture) sensor, a ground loop arises because soil
and water provide an alternate path for the excitation to return to CR6 ground.
This example is modeled in the diagram
Model of a Ground Loop with a Resistive
Sensor
(p. 147).
With R
g
in the resistor network, the signal measured from the sensor
is described by the following equation:
where
V
x
is the excitation voltage
R
f
is a fixed resistor
R
s
is the sensor resistance
R
g
is the resistance between the excited electrode and CR6 earth ground.
R
x
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 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 CR6 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.
Summary of Contents for CR6 Series
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Page 76: ...Section 5 Overview 76 FIGURE 20 Half Bridge Wiring Example Wind Vane Potentiometer ...
Page 80: ...Section 5 Overview 80 FIGURE 23 Pulse Input Wiring Example Anemometer ...
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Page 454: ...Section 8 Operation 454 FIGURE 104 Narrow Sweep High Noise ...
Page 459: ...Section 8 Operation 459 FIGURE 106 Vibrating Wire Sensor Calibration Report ...
Page 535: ...Section 8 Operation 535 8 11 2 Data Display FIGURE 121 CR1000KD Displaying Data ...
Page 537: ...Section 8 Operation 537 FIGURE 123 CR1000KD Real Time Custom ...
Page 538: ...Section 8 Operation 538 8 11 2 3 Final Storage Data FIGURE 124 CR1000KD Final Storage Data ...
Page 539: ...Section 8 Operation 539 8 11 3 Run Stop Program FIGURE 125 CR1000KD Run Stop Program ...
Page 541: ...Section 8 Operation 541 FIGURE 127 CR1000KD File Edit ...
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Page 610: ...Section 11 Glossary 610 FIGURE 137 Relationships of Accuracy Precision and Resolution ...
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