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Chapter 4
Measurement Definition
134
Figure 54 - Time Constant Effect: True Peak Signal Detection, Steady State – Absolute
Figure 55 - Time Constant Effect: True Peak Signal Detection, Steady State – Percent Deviation
A signal spike can be the result of an actual process or machine-related event.
However, in most cases a spike is more likely the result of an electrical or
mechanical anomaly.
When a spike occurs a true peak measurement detects it almost instantly,
regardless of the setting of the time constant. The TC setting affects how
quickly the signal decays back to the actual measured value following the spike
as shown in
Figure
56.
While a rapid decay back to the actual measured value is desirable, the
consequence relative to measurement accuracy must be considered. As the TC
increases, the decay time does as well. While an additional few seconds of decay
time are not a problem in how operators and engineers use the measurement, it
could be problematic if alarm delays are short, so could be triggered by a spike.
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018
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