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Symptom:
Performing transmitter zero and span calibration (or
re-ranging) is a common event for many loops. A transmitter input span
should be kept as small as possible to achieve greatest accuracy. If the
input span is changed, the operators may notice a change in the loop
performance, therefore requiring a change in controller tuning.
Probable cause:
A change in a transmitter input span will change the process
gain. The gain in any device if defined by the equation:
--
where the
D
means change.
For example, if a transmitter input span is reduce by
50%, the transmitter gain is doubled as shown in the diagram. This causes
the process gain to also be doubled.
Corrective action: To compensate for a higher process gain, the
controller gain should be reduced by the same proportion to keep the same
performance. In this example, the controller gain would have to be cut is
half to keep the same loop performance.
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