March 2010

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

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

mar14

A. 4:1 Decay Ratio

B. Quarter Wave Decay

C. ¼ ZN Diffraction

D. 4/1 Gain Response

CCST answer

The correct answer is B, Quarter Wave Decay.

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.

References: Harley M. Jeffery; Loop Checking: A Technician's Guide, ISA Press, 2005.