This paper describes a process control experiment. The apparatus includes a three-vessel glass flow system with a variable flow configuration, means for feeding dye solution controlled by a stepper-motor driven valve, and a flow spectrophotometer. Students use impulse response data and nonlinear regression to estimate three parameters of a model of the system, study the behavior of the system under PID control, observe both stable and unstable behavior, and test Ziegler-Nichols controller tuning methods.