Ratio control is an essential element in the toolbox of control engineers to control various industrial processes. The objective is to maintain the ratio of two flows at a specified value. The desired ratio can be either manually entered by an operator or automatically set by another controller. At least one of the flows needs to be controlled by a PID controller (or at least a valve should be available to control the flow). The ratio can be calculated based on the flow measurements or based on the setpoints of the flow controller(s). The ratio controller can be configured in different ways in the DCS. The different approaches and their pro’s and con’s are described in this blog.