Improving Voltage Measurement Quality - Campbell CR300 Series Product Manual

R R /R is the source of error due to the ground loop. When R
s f g
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.
17.11 Improving voltage measurement
quality
The following topics discuss methods of generally improving voltage measurements:
17.11.1 Deciding between single-ended or differential measurements
17.11.2 Minimizing ground potential differences
17.11.3 Minimizing power-related artifacts
17.11.4 Filtering to Reduce Measurement Noise
17.11.5 Minimizing settling errors
17.11.6 Factors affecting accuracy
17.11.7 Minimizing offset voltages
Read More: Consult the following technical papers at
depth treatments of several topics addressing voltage measurement quality:
Preventing and Attacking Measurement Noise Problems
l
Benefits of Input Reversal and Excitation Reversal for Voltage Measurements
l
Voltage Accuracy, Self-Calibration, and Ratiometric Measurements
l
is large, the error is negligible.
g
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17. Tips and troubleshooting
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