ISA Certified Control Systems Technician (CCST) program

CCST

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

There are approximately ________time constants from the point at which the process reaction curve begins to change and reaching its maximum value.

A. 1
B. 3
C. 5 
D. 7

CCST Answer

The correct answer is C: Five time constants. 

The process time constant, in a percentage of final change, is given as:

            % Change = [1 - e-N ] * 100%

Where N = number of time constants.

This gives:

Time Constants

% Change Realized

1

63.2%

2

86.4%

3

95.0%

4

98.2%

5

99.3%

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