Ground Potential Differences; Minimizing Power-Related Artifacts - Campbell CR300 Series Product Manual

Connect shield wires to the
l
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.

22.11.2.1 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
l
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
l
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.
For additional information, see

22.11.3 Minimizing power-related artifacts

Some VAC-to-VDC power converters produce switching noise or AC ripple as an artifact of the
ac-to-dc rectification process. Excessive switching noise on the output side of a power supply can
increase measurement noise, and so increase measurement error. Noise from grid or mains
power also may be transmitted through the transformer, or induced electromagnetically from
nearby motors, heaters, or power lines.
terminal nearest the terminals to which the sensor signal
Minimizing offset voltages (p. 199).
22. Tips and troubleshooting 191