In the early years of process control, process indicators and control elements such as valves were monitored by an operator that walked around the unit adjusting the valves to obtain the desired temperatures, pressures, and flows.
As technology evolved pneumatic controllers were invented and mounted in the field that monitored the process and controlled the valves. This reduced the amount of time process operators were needed to monitor the process.
Later years the actual controllers were moved to a central room and signals were sent into the control room to monitor the process and outputs signals were sent to the final control element such as a valve to adjust the process as needed. These controllers and indicators were mounted on a wall called a control board.
The operators stood in front of this board walking back and forth monitoring the process indicators. This again reduced the number and amount of time process operators were needed to walk around the units. The basic air signal used during these years was 3-15 psig.
In the 1970’s electronic instrumentation began to be manufactured by the instrument companies. Each instrument company came out with their own standard signal for their instrumentation, 10-50ma, 0.25-1.25Volts, 0-10Volts, 1-5volts, and 4-20ma, causing only confusion until the 4-20ma was universally used as a standard electronic instrument signal for transmitters and valves.
The transformation of instrumentation from mechanical pneumatic transmitters, controllers, and valves to electronic instruments reduced maintenance costs as electronic instruments were more dependable than mechanical instruments. This also increased efficiency and production due to their increase in accuracy.
The next evolution of instrumentation came with the production of Distributed Control Systems (DCS). The pneumatic and electronic control rooms allowed control from a centralized room, DCS systems allowed control from more than one room or control stations. These stations could be next to each other or miles away. Now a process operator could sit in front of a screen and monitor thousands of points throughout a large unit or complex.