September/October 2012

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.

CAP 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

CAP answer

The correct answer is A, magnetic flowmeter. The output from a magnetic flowmeter is a linear signal that is proportional to the velocity of the flowing fluid. This measured output of velocity (ft/sec) multiplied by the area (ft2/sec) of the flow tube (usually "line size") yields the volumetric flow rate (ft3/sec).

Answers B, C, and D are not correct. These flowmeter types all require the measurement of a differential pressure that results from an obstruction of flow or from an impact of flow on an object. The venturi flowmeter uses a variable area flow tube (or throat) to develop a differential pressure upstream and downstream of the throat. The orifice plate uses a barrier plate with a hole cut into it, and the differential pressure across the orifice is measured to determine flow rate. The pitot tube uses the pressure of the fluid at an impact nozzle and a static pressure measurement to determine flow rate. In all three of these cases, flow is proportional to the square root of the measured pressure differential. Therefore, when measuring the output of an orifice plate meter, a venturi meter, or pitot tube, square root extraction is necessary to linearize the output for use in the control system. This square root extraction can be done at the device (transmitter) or at the control system (configuration of analog input), but should not be done at both places.

Reference: A Guide to the Automation Book of Knowledge, 2nd Edition (V. Trevathan, Editor), 2010.