A specific example of a multivariable control problem is shown in Figure 13-2. Here, two different streams are blended together. The objective is to manipulate the individual stream flow rates to meet specifications on the output flow rate and the output composition. Let the outputs be represented by y2 (total flow) and y1 (composition), and the inputs by u1 (flow rate of stream 1) and u2 (flow rate of stream 2). How should our outputs and inputs be paired? If we assume that the output-input pairings are y1-u1 and y2-u2, the instrumentation diagram is shown in Figure 13-3a. If we assume that the output-input pairings are y1-u2 and y2-u1, the instrumentation diagram is shown in Figure 13-3b.