The second possible scheme for ratio control is shown in Fig. 6. Suppose the flow rate of fluid B has to be maintained at a constant fraction of flow rate of fluid A, irrespective of variation of flow rate of A (qA). In this scheme the flow rate of fluid A is multiplied with the desired ratio (set externally) that gives the desired flow rate of fluid B. This is compared with the actual flow rate of fluid B and fed to the controller that operates the control valve.