May 2008

Certification Review

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 this 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 recommended limitation in meters for the length of a "fast Ethernet" (100 Megabits/second) segment on twisted pair cable?

A. 10
B. 100
C. 300
D. 350

CAP answer

The correct answer is B, 100.

Because of the high speed of "fast Ethernet," the use of twisted pair cable is limited to 100 meters (330 feet). 10 meters would limit installations unnecessarily. 300 meters and 350 meters would exceed recommendations substantially, which could potentially cause signal errors.

Reference: Berge, Jonas; Fieldbuses for Process Control: Engineering, Operation, and Maintenance; ISA Press, 2001.

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

If a control valve will pass 25% of its full capacity when it is 25% open, 50% of its capacity when 50% open, and 75% when 75% open, the inherent characteristic is:

A. Quick opening
B. Equal percentage
C. Throttling
D. Linear

CCST answer

The correct answer is D, linear.

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.

Throttling is not a specific flow characteristic. Rather it is a broad class of valves that open incrementally and that would display the other three characteristics listed here depending on the kind of valve.

Valves that are not throttling valves would include those normally open and where maximum flow is constantly the requirement and where the valve only closes for maintenance. Cooling systems use these valves.

Here are definitions of the characteristics we see in this question.

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

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