If power supply components are working properly and the system has peripherals with high
current drain, such as a satellite transmitter, verify that the power supply is designed to provide
adequate power. For additional information, see Power budgeting (p. 108).
9.7 Minimizing ground loop errors
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 datalogger ground. This example is modeled in the following image:
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 datalogger 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
9. Tips and troubleshooting 121
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