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
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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