1 June 2005

ISA Certified Control Systems Technician (CCST) Program

This question is from the Level III study guide, Domain 1, Calibration.

Level III represents a professional who has a 13-year total of education, training, and/or experience.

CCST question

In order to meet velocity and Reynolds No. constraints for a particular Doppler flow meter sizing application, the following will normally be considered:

A. Reduce the size, which increases both the velocity and the Reynolds number.

B. Reduce the size, which increases the velocity and decreases the Reynolds number.

C. Fix the size for velocity constraints and neglect Reynolds number.

D. Try another technology for laminar flow since Reynolds number constraints can preclude the use of Doppler flow meters.

CCST answer

Recall that Doppler meters measure the frequency shifts caused by liquid flow and that viscosity does not influence their accuracy.

The correct answer is A . Answer B is wrong per this equation, and answers C and D are frivolous and misleading for that purpose alone.

The Reynolds number is the most important dimensionless number in fluid dynamics. It is as follows:

Equation

Where:

νs: mean fluid velocity

L: characteristic length (equal to pipe diameter (2r) if a cross-section is circular)

μ: fluid viscosity

ρ: fluid density

The Reynolds number is the ratio of inertial forces (νsρ) to viscous forces (μ/L) and is used for determining whether a flow will be laminar or turbulent. Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion.

Turbulent flow occurs at high Reynolds numbers where inertial forces are greatest, producing random eddies, vortices, and other flow fluctuations.

Nicholas Sheble (nsheble@isa.org) explains the answers to CCST questions. For information about the CCST program, go to www.isa.org/ccst.