Designing a PLC-based liquid flow control system requires defining objectives and specifications, selecting the appropriate sensors and actuators, selecting a suitable PLC, developing the control logic and algorithm, connecting and configuring the components, and testing and tuning the system. The process involves movement and transformation of liquids, while sensors measure flow rates or other variables. Actuators manipulate the flow rates or variables. The PLC receives signals from the sensors, processes them according to programmed logic, and sends signals to the actuators. Compatibility, accuracy, range, resolution of sensors and actuators should be considered when selecting them. Similarly, size, memory, speed, input/output ports, communication capabilities, and programming software should be taken into account when selecting a PLC. The program should be created in different languages such as ladder logic, function block diagram or structured text. When testing and tuning the control system its performance should be verified and optimized in terms of stability, accuracy and robustness. Parameters such as gain, time constant or deadband of the control algorithm can be adjusted using programming software.