July/August 2010

ISA Certified Control Systems Technician (CCST) program

CCST question

Reynolds Number, a dimensionless quantity used to describe flow in a pipe, can be represented by the formula:

Re = ρ V dh / µ

This quantity is useful because it describes the ratio between which two quantities?

A. Ratio of electrical charge to fluid capacitance for the fluid.

B. Ratio of specific gravity to heat capacity of the fluid.

C. Ratio of inertial forces to viscous forces on the fluid.

D. Ratio of frictional forces to kinetic forces on the fluid.

CCST answer

The correct answer is C, the ratio of inertial forces to viscous forces on the fluid.  The inertial forces in the numerator are given by the product of mass (density), fluid velocity, and pipe diameter.  The viscous forces on the fluid are represented by the viscosity in the denominator. 

Answer A cannot be correct: Reynolds Number does not indicate a relationship of electrical properties of fluids.

Answer B cannot be correct: Reynolds Number does not give a relation to the thermal or mass capacity properties of fluids.

Answer D is not correct. While frictional forces could be equated to viscous forces and kinetic forces could be equated to inertial forces, the ratio indicated in Answer D would be the inverse of the true relationship.

Reference: Hughes, T.A., Measurement and Control Basics, Fourth Edition, ISA, 2007.