Soil Moisture Example - Campbell CR300 Series Product Manual

themselves have enhanced voltage range inputs allowing for ground voltage differences of
up to 7 V between endpoints.
For exceptional cases, use optical or galvanic isolation devices to provide a signal
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connection without any accompanying ground connection. These should be used only
when ground loops are causing system problems and the other methods of breaking a
ground loop don't apply. These devices add expense and tend to consume large amounts
of power.

17.10.4 Soil moisture example

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 data logger ground. This example is modeled in the following image:
With R in the resistor network, the signal measured from the sensor is described by the
g
following equation:
where
V is the excitation voltage
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x
R is a fixed resistor
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f
R is the sensor resistance
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s
R is the resistance between the excited electrode and data logger earth ground.
l
g
17. Tips and troubleshooting 136