What is a process variable?

 For short, a process variable can be defined as

A parameter of a chemical process or an equipment that changes in time

The three types of process variables in a control loop.

What are the features of a process variable?

There are at least two important features you should be aware of

• it changes in time. Time dependence is important since only when something changes in time there are changes in the process behavior and quality products or expected results. If you have a variable that does not change in time, then this is not a process variable!, and

• it can be measured. If you have a process variable that you can not measure (even through indirecte methods or techniques) then it is useless to consider it as variable of the process since you can not use it in any control loop.

Types of process variables

There are three recognizable types of process variables in a control loop

1. the controlled variable,
2. the manipulated variable and
3. loads.

Take a look to this video with a summary of this page

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• LE 02 - Start and stop push button installation 24V DC
• LE 03 - Turn on/off an 24V DC pilot light with a push button
• LE 04 - Latch contact with encapsulated relay for turning on/off an AC bulb light
• About PID controllers
• Ways to control a process
• About pilot lights
• Solving the Colebrook equation

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Ildebrando.