January/February 2011

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 flowmeter types does not operate on the principle that a differential pressure is developed across a flow restriction?

A. Pitot tube

B. Elbow

C. Coriolis

D. Venturi

CCST answer

The correct answer is C, Coriolis flowmeter.

Answers A, B, and D are all flowmeter types for which a differential pressure is measured across a flow restriction, which can be correlated to a flow rate (volumetric). The Pitot tube has an impact port (or nozzle). As fluid hits the port, its velocity is reduced to zero and kinetic energy (velocity) is converted to potential energy (pressure). An Elbow flowmeter is a pipe elbow in which a pressure differential is created by centrifugal force between the inside diameter and the outside walls of the pipe elbow. A Venturi flowmeter is similar to an orifice plate, but the restriction is formed gradually by decreasing the pipe diameter over a distance. In each case above, the square root of the differential pressure developed across the restriction is proportional to the flow rate.

Answer C the correct choice for this question. A Coriolis flowmeter develops a flow measurement by measuring the degree of deformation (twisting) of a vibrating tube(s) as the fluid moves through the tube. The degree of deformation is directly proportional to the mass flow rate through the meter.

Reference: Goettsche, L.D. Maintenance of Instruments & Systems. ISA Press, 2nd Edition (2005)