March 2008

Documenting skills is value-add

ISA certification provides an objective, third-party assessment, and confirmation of a person's skills. It gives manufacturing and factory staff the opportunity to differentiate themselves from their peers and gain recognition. InTech covers two certification areas in its monthly Certification department.

ISA Certified Automation Professional (CAP) program

Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

The following question comes from the CAP study guide, Performance Domain II, Definition: Identify customer requirements and complete high-level analysis of the best way to meet those requirements

CAP question

What is the MOST appropriate technique for modeling linear relationships for a large number of correlated inputs where the equations are unknown?

A. Artificial neural networks
B. Multivariable statistical process controls
C. Step response models
D. First principle models

CAP answer

The correct answer is B, multivariable statistical process controls.

Artificial neural networks (ANN) excel at modeling nonlinear relationships for a relatively large unknown number of inputs. However, the inputs cannot be correlated and the training data must cover the whole region. An ANN cannot extrapolate values outside the test region and does not handle large lags well. 

The multivariate statistical-process control excels at modeling unknown linear relationships for a large number of inputs that could be correlated.
 
Step response models excel at linear relationships for a small to moderate number of uncorrelated inputs where dynamics are important. Step response models are used for linear dynamic on-line property estimates. 

First principle models require known equations and parameters that use process principles and material and energy balances.

Reference: Blevins, et al.; Advanced Control Unleashed: Plant Performance Management for Optimum Benefit; ISA Press, 2003

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.

This question comes from the Level I study guide, Domain 3, Troubleshooting. Level I represents a professional who has a five-year total of education, training, and/or experience.

CCST question

The control valve that produces an inherent equal percentage of increase, or decrease, over the existing flow when the plug is repositioned has a (an) __________ characteristic under constant pressure drop conditions.

A. Modified parabolic
B.  Equal percentage
C.  Linear
D.  Quick opening  

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CCST answer

The "inherent flow characteristic" is the relationship between valve capacity and valve travel, and we usually express it graphically. It comes by testing a valve with water as the fluid using a constant pressure drop across the valve. The most common are linear, equal percentage, modified parabolic, and quick opening.

Let's review the definitions:

Modified parabolic: An inherent flow characteristic that provides equal percent characteristic at low closure member travel and approximately a linear characteristic for upper portions of closure member travel.

Equal percentage: An inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient. 

Linear: An inherent flow characteristic that is a straight line on a rectangular plot of flow coefficient versus rated travel. Therefore, equal increments of travel provide equal increments of flow coefficient.

Quick opening: An inherent flow characteristic in which a maximum flow coefficient happens with minimal closure member travel.

The correct answer is B

Nicholas Sheble (nsheble@isa.org) writes and edits Certification Review.