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.
9.10.2.1 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.
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Soil Temperature Thermocouple: If the measuring junction of a thermocouple is not
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.
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External Signal Conditioner: External instruments with integrated signal conditioners, such
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
9.10.3 Detecting open inputs
A useful option available to single-ended and differential measurements is the detection of open
inputs due to a broken or disconnected sensor wire. This prevents otherwise undetectable
measurement errors. Range codes appended with C enable open-input detection. For detailed
information, see the CRBasic help (
VoltSE()
and
VoltDiff()
instructions,
Range
parameter)
The C option may not detect an open circuit in the following situations:
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When the input is not a truly open circuit, such as might occur on a wet cut cable end, the
open circuit may not be detected because the input capacitor discharges to a normal
9. Tips and troubleshooting
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