Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.

What common loop tuning terminology is used to describe the change in amplitude of the control loop oscillations shown in the post graphic?

a) 4:1 Decay Ratio
b) Quarter Wave Decay
c) ¼ ZN Diffraction
d) 4/1 Gain Response
e) none of the above 

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The common method of judging tuning is known as the “quarter wave decay” method.  It has been shown if a loop is tuned so the oscillation decays with each wave being one quarter of the previous wave, it produces satisfactory, if not optimum, set point response and disturbance rejection.

The correct answer is B, Quarter Wave Decay.

Reference: Harley M. Jeffery; Loop Checking: A Technician’s Guide, ISA Press.