Which of the following is not a primary process variable?

• A. Level
• B. Color of the process vessel
• C. Flow
• D. Pressure

Answer: (B)

Primary process variables are the quantities that describe and control the state of the process, and they’re the signals you monitor and regulate in a control loop. Level, flow, and pressure are classic examples because they directly reflect how much material is present, how fast it’s moving through the system, and the forces in the piping. You use sensors to measure them and actuators to adjust them to keep the process operating as intended.

Color of the process vessel, on the other hand, is just an attribute of the equipment. It doesn’t change with the process dynamics, isn’t used to describe the process state, and isn’t something you typically measure or control to regulate operation. So it isn’t a primary process variable.