Which of the following is NOT one of the common mediums used to implement process control?

The correct answer identifies thermal as not being a common medium used to implement process control in industrial systems. In process control, three predominant mediums are typically utilized: mechanical, pneumatic, and electrical.

Mechanical control systems employ physical components, such as levers and gears, to achieve the desired function. Pneumatic controls rely on compressed air to transmit signals and perform actions, making them particularly useful in environments where electrical systems may not be suitable or safe. Electrical controls use electrical signals to manage and regulate processes, often incorporating sensors, controllers, and actuators for precision.

Thermal, while an important concept in understanding process dynamics and energy transfer, is not typically categorized as a medium for direct process control implementation. Instead, thermal aspects may influence how other control mediums operate—like how temperature affects pneumatic systems but is not a medium on its own. Thus, thermal stands out as an option that does not fit with the common categories of process control mediums.