In most processes, there are six basic elements.Process control elements are
1. Controlled variable
What you want to control (temperature pressure, level, flow rat, dimensions, position, etc.)
2. Measured variable
What you observe in order to determine the actual condition of the controlled variable
In most cases, you measure the controlled variable itself. For instance, if you want to know how fast a car is going, you measure its speed. In other cases, you measure a different variable to determine the condition of the controlled variable. For instance, you can determine the level (controlled variable) of liquid in an open or vented tank by measuring the pressure (measured variable) at the bottom of the tank.
3. Set Point
The difference between the set point and the actual value of the controlled variable (which is the measured variable). For example, if your indoor thermometer reads 65 and you would like a room temperature of 70, the deviation is 5
Note: Deviation is also referred to as difference or error.
5. Manipulated variable
The variable that is adjusted to close the gap (deviation, difference, or error) between the set point and the controlled variable; for example, the amount of electricity or gas to the heater.
Anything that affects the process and could cause deviation from the set point; for example, a window left open, poor insulation, a damaged thermostat.