Section 5. System Overview
However, be aware that because a single-ended measurement is referenced to
CR1000 ground, any difference in ground potential between the sensor and the
CR1000 will result in error, as emphasized in the following examples:
• If the measuring junction of a thermocouple used to measure soil temperature
is not insulated, and the potential of earth ground is greater at the sensor than
at the point where the CR1000 is grounded, a measurement error will result.
For example, if the difference in grounds is 1 mV, with a copper-constantan
thermocouple, the error will be approximately 25 °C.
• If signal conditioning circuitry, such as might be found in a gas analyzer, and
the CR1000 use a common power supply, differences in current drain and
lead resistance often result in different ground potentials at the two
instruments despite the use of a common ground. A differential measurement
should be made on the analog output from the external signal conditioner to
avoid error.
5.1.2.1.2 Differential Measurements — Overview
Related Topics:
• Differential Measurements — Overview (p. 66)
• Differential Measurements — Details
(p. 308)
Summary Use a differential configuration when making voltage measurements,
unless constrained to do otherwise.
A differential measurement measures the difference in voltage between two input
terminals. Its sequence is illustrated in the figure Simplified Differential-Voltage
Measurement Sequence
(p. 66), and is characterized by multiple automatic
measurements, the results of which are averaged automatically before the final
value is reported. For example, the sequence on a differential measurement using
the VoltDiff() instruction involves two measurements — first with the high input
referenced to the low, then with the inputs reversed. Reversing the inputs before
the second measurement cancels noise common to both leads as well as small
errors caused by junctions of different metals that are throughout the measurement
electronics.
Figure 20. Simplified Differential-Voltage Measurement Sequence
5.1.2.2 Current Measurements — Overview
Related Topics:
• Current Measurements — Overview (p. 66)
• Current Measurements — Details
(p. 337)
66
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