Refineries, chemical plants, gas processing, water and most other facilities with continuous liquid and gas flow measure flow rates of the feed stock, finished product, utilities and intermediate flows between vessels etc. Many technologies are used to measure flow and level. Level measurement includes a liquid and vapor level as well as an interface between hydrocarbon and water. Large books such as Flow Measurement Engineering Handbook, by R. W. Miller exist on just flow measurement. It likely excludes the newer technologies.
With either flow or level measurement the refinery industry uses a transmitter to convert the measurement to an industry standard for interface to a control system. Traditional transmitters have used an analog 4-20 mAdc that is proportional to flow - or proportional to the differential pressure that can imply flow with square root extraction. Level is normally linear. Some newer facilities use Foundation Fieldbus, Profibus or another serial communication link instead of the analog signal. Some facilities may use wireless transmission. These latter signal types are not as widely accepted for control. Older facilities still use pneumatic signals for the mechanical measurement technologies.
Computers as well as electronic or pneumatic analog controllers compare the measurement against the operator setpoint and send an output signal to adjust something. This again is normally a 4-20 milli-amp signal. The alorithm is typically a a gain response prorportional to the offset between the measurement and setpoint, as well as integral signal that ramps the output and a derivative signal that spikes the output in a direction that should bring the flow or level back to the setpoint. The typical device that controls the flow or level is a diaphragm operated valve. It can be starting and stopping pumps as well as varying the speed of the motor driving a pump, etc.
I hope that this is what you are looking for.
