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Time dependence of process variables

 There is one thing you must know

Everything changes in time.

Some things change very quick and some others very (very) slowly.

Some concepts on time dependency

People use different words to refer to the same thing according to the field they work on. Some examples are:
  • transient behavior,
  • dynamic behavior,
  • steady state,
  • stationary state,
  • time dependent and
  • time independent
for example. However, some of these concepts have the same meaning.

A dynamic behavior plot for a process variable

Transient and dynamic behavior and time dependency mean exactly the same: a process variable changes in time. However, steady and stationary states and time independent are also the same and mean: no change in the process variable as the time passes.

Two time behaviors of a process variable

Engineers only care about two time behaviors:
  • if the variable changes in time,
  • if the variable do not change in time.
Also, there are some effects in the process that these behaviors trigger. These are:
  • if there is a change of the variable then an action has to be taken so that a valve throttles or a motor changes its rpm (for example)
  • if no change in the variable is detected, then no action is taken.

Why is time dependency important in a control loop?

The answer is simple. In the case of quality of products coming out a process we could say

Process variable changes modifies efficiency of the process so that products may not have the estimated quality

Steady behavior, of process variables, allows the process to mantain estimated constant quality of products.

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