November/December 2013
Certification Review

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

Which of the following flow measurement devices does not require square root extraction?

A. magnetic flowmeter 
B. venturi flowmeter
C. orifice plate
D. pitot tube

CCST answer

The correct answer is A, magnetic flowmeter. Square root extraction is a mathematical method that is required for flowmeter readings where a measured differential pressure, or pressure drop, is used to infer the flow rate. From the choices in the problem statement, the magnetic flowmeter is the only sensor on the list that does not depend upon a differential pressure to be developed to quantify flow rate, and therefore, does not require square root extraction.

Answer B, venturi flowmeter, infers flow rate by measurement of the differential pressure across a tapered tube.

Answer C, orifice plate, infers flow rate by measurement of the differential pressure across a thin plate with an orifice bored through it.

Answer D, pitot tube, infers flow rate by measurement of the differential pressure between a reference pressure (static pressure of the flowing fluid) and the pressure measured at the impact tube.

 

Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.