Minimizing Ground Potential Differences; Ground Potential Differences - Campbell CR300 Series Product Manual

17.11.2 Minimizing ground potential differences

Low-level, single-ended voltage measurements (<200 mV) are sensitive to ground potential
fluctuation due to changing return currents from SW12 and C terminals. The data logger
grounding scheme is designed to minimize these fluctuations by separating signal grounds ( )
from power grounds (G). For more information on data logger grounds, see
take advantage of this design, observe the following rules:
Connect grounds associated with SW12 and C terminals to G terminals.
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Connect excitation grounds to the nearest
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Connect the low side of single-ended sensors to the nearest
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terminal block.
Connect shield wires to the
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wires are connected.
If offset problems occur because of shield or ground wires with large current flow, tying the
problem wires into terminals next to terminals configured for excitation and pulse-count should
help. Problem wires can also be tied directly to the ground lug to minimize induced single-ended
offset voltages.

17.11.2.1 Ground potential differences

Because a single-ended measurement is referenced to data logger ground, any difference in
ground potential between the sensor and the data logger will result in a measurement error.
Differential measurements MUST be used when the input ground is known to be at a different
ground potential from data logger ground.
Ground potential differences are a common problem when measuring full-bridge sensors (strain
gages, pressure transducers, etc), and when measuring thermocouples in soil.
Soil Temperature Thermocouple: If the measuring junction of a thermocouple is not
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insulated when in soil or water, and the potential of earth ground is, for example, 1 mV
greater at the sensor than at the point where the data logger is grounded, the measured
voltage will be 1 mV greater than the thermocouple output. With a Type T (copper-
constantan) thermocouple, 1 mV equates to approximately 25 °C measurement error.
External Signal Conditioner: External instruments with integrated signal conditioners, such
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as an infrared gas analyzer (IRGA), are frequently used to make measurements and send
analog information to the data logger. These instruments are often powered by the same
VAC-line source as the data logger. Despite being tied to the same ground, differences in
current drain and wire resistance result in different ground potentials at the two
instruments. For this reason, a differential measurement should be made on the analog
output from the external signal conditioner.
terminal on the same terminal block.
terminal nearest the terminals to which the sensor signal
Grounds (p. 10). To
terminal on the same
17. Tips and troubleshooting 139