Section 8. Operation
• 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.
Sensors with a high signal-to-noise ratio, such as a relative-humidity sensor with a
full-scale output of 0 to 1000 mV, can normally be measured as single-ended
without a significant reduction in accuracy or precision.
Sensors with a low signal-to-noise ratio, such as thermocouples, should normally
be measured differentially. However, if the measurement to be made does not
require high accuracy or precision, such as thermocouples measuring brush-fire
temperatures, which can exceed 2500 °C, a single-ended measurement may be
appropriate. If sensors require differential measurement, but adequate input
terminals are not available, an analog multiplexer should be acquired to expand
differential input capacity.
Because a single-ended measurement is referenced to CR800 ground, any
difference in ground potential between the sensor and the CR800 will result in an
error in the measurement. For example, if the measuring junction of a copper-
constantan thermocouple being used to measure soil temperature is not insulated,
and the potential of earth ground is 1 mV greater at the sensor than at the point
where the CR800 is grounded, the measured voltage will be 1 mV greater than the
true thermocouple output, or report a temperature that is approximately 25 °C too
high. A common problem with ground-potential difference occurs in applications
wherein external, signal-conditioning circuitry is powered by the same source as
the CR800, such as an ac mains power receptacle. Despite being tied to the same
ground, differences in current drain and lead resistance may result in a different
ground potential between the two instruments. So, as a precaution, a differential
measurement should be made on the analog output from an external signal
conditioner; differential measurements MUST be used when the low input is
known to be different from ground.
Integration
The CR800 incorporates circuitry to perform an analog integration on voltages to
be measured prior to the A-to-D
(p. 489) conversion. Integrating the the analog
signal removes noise that creates error in the measurement. Slow integration
removes more noise than fast integration. When the duration of the integration
matches the duration of one cycle of ac power mains noise, that noise is filtered
out. The table Analog Measurement Integration
(p. 316) lists valid integration
duration arguments.
Faster integration may be preferred to achieve the following objectives:
• Minimize time skew between successive measurements
• Maximize throughput rate
• Maximize life of the CR800 power supply
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