Campbell CR800 Series, Operator'S Manual

Page: 323 / 598

Section 8. Operation
323
• If the open circuit is at the end of a very long cable, the test pulse (300
mV) may not charge the cable (with its high capacitance) up to a voltage
that generates NAN or a distinct error voltage. The cable may even act as
an aerial and inject noise which also might not read as an error voltage.
• The sensor may "object" to the test pulse being connected to its output,
even for 100 µs. There is little or no risk of damage, but the sensor
output may be caused to temporarily oscillate. Programming a longer
settling time in the CRBasic measurement instruction to allow
oscillations to decay before the A-to-D conversion may mitigate the
problem.
Range-Code Option C Over-Voltages
Input Range (mV) Over-Voltage
±
2.5
± 7.5
± 25
±
250
300 mV
± 2500 C option with caveat
1
±5000
C
option not available
1
C
results in the H terminal being briefly connected to a voltage greater than
2500 mV, while the L terminal is connected to ground. The resulting
common-mode voltage is 1250 mV, which is not adequate to null residual
common-mode voltage, but is adequate to facilitate a type of open-input
detect. This requires inclusion of an If / Then / Else statement in the CRBasic
program to test the results of the measurement. For example:
•The result of a
VoltDiff() measurement using mV2500C as the Range code
can be tested for a result >2500 mV, which would indicate an open input.
•The result of the
BrHalf() measurement, X , using the mV2500C range code
can be tested for values > 1 . A result of X > 1 indicates an open input for the
primary measurement, V1, where X = V1/Vx and Vx is the excitation voltage.
A similar strategy can be used with other bridge measurements.
Offset Voltage Compensation
Related Topics
• Auto Self-Calibration — Overview
(p. 89)
•
Auto Self-Calibration — Details
(p. 337)
•
Auto Self-Calibration — Errors
(p. 475)
•
Offset Voltage Compensation
(p. 323)
• Factory Calibration
(p. 86)
•
Factory Calibration or Repair Procedure
(p. 461)