# Q&A from CCST Review: Time Constant in a Transmission System

This question came in from a student taking the online Certified Control Systems Technician® (CCST®) Level I Review Course (TS00E).  We thought it would be valuable to share the exchange here.  The answer was provided by the course instructor, Thomas Stokes.

Question from Student:

In the CCST study guide, domain 1 question number 55 gives an answer of 63.2% as being the correct response. Can you give an explanation of how this is determined?

Question 55:

The time constant (first order lag) in a transmission system is the time necessary (after dead time) following an upset for the response curve to reach ____________ of its final value.

A.     100%

B.     63.2%

C.     50%

D.     37.8%

The 63.2% answer is the time that elapses from the first observable or measurable response in the process reaction curve and the controlled variable reaching 63.2% of its final value. Example: If the controlled variable starts at a setpoint of 300 degrees Fahrenheit and changes to a maximum peak value of 350 degrees Fahrenheit, the elapsed time required to reach a temperature of 331.6 degrees Fahrenheit is the process time constant. The example only applies to a process that can be defined as a first order lag plus dead time. (FOLPDT).