One of the most important and commonly measured process variables in a system is:

Pressure is a fundamental process variable in many engineering systems because it directly affects the operation and efficiency of various processes. In thermodynamic systems, pressure is crucial for determining the state of a substance, influencing phase changes, and affecting chemical reactions.

Monitoring pressure allows operators to maintain safe operating conditions, prevent over-pressure situations, and ensure that equipment functions within its design specifications. In many cases, adjusting the pressure can directly impact flow rates, temperature, and energy transfer, making it an essential aspect of process control.

While viscosity, density, and composition are also important variables in specific contexts, pressure stands out as a primary parameter that can influence many other factors and is critical to the safety and reliability of industrial processes.