This App simulates a simple process for heating a tank with two separate water streams. The stream rates are constant but one stream can be subjected to sinusoidal temperature disturbances. The other streams temperature is held by a heater, the duty of which is manipulated by a PID controller to compensate for incoming disturbances and keep the tank temperature as constant as possible. The App provides a rich opportunity to understand the effects of process dynamics on the quality of control. It also demonstrates the effect of controller tuning on loop response time and stability. A unique feature of providing a second tank in parallel with the main tank allows the user to visualize how and why a feedback loop reaches its critical frequency and ultimately goes unstable. The feedback controllers used in this example are realistic, industrial grade controllers that can be tuned with a variety of tuning methods including ATV tuning.