Campbell CR3000 Micrologger, Руководство оператора

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Section 5. Overview
71
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Not enough differential terminals available. Differential measurements
use twice as many H/L terminals as do single-ended measurements.
•
Rapid sampling is required. Single-ended measurement time is about half
that of differential measurement time.
•
Sensor is not designed for differential measurements. Many Campbell
Scientific sensors are not designed for differential measurement, but the
draw backs of a single-ended measurement are usually mitigated by large
programmed excitation and/or sensor output voltages.
However, be aware that because a single-ended measurement is referenced to
CR3000 ground, any difference in ground potential between the sensor and the
CR3000 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 CR3000 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 CR3000 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.
5.2.2.1.2 Differential Measurements — Overview
Related Topics:
• Differential Measurements — Overview
(p. 71)
• Differential Measurements — Details
(p. 386)
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 autosequence is characterized by multiple measurements, the
results of which are autoaveraged 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.