A simple control system.
A large tank must be filled with liquid from a supply line. One operator stands at ground level to operate the feed valve. Another stands on the tank, gauging its level with a dipstick. When the tank is near full, the stick operator will instruct the other to start closing the valve. Overfilling can cause spills, but underfilling will cause later process problems. See Lesson 1 to find a better way! (Lesson 1 image by Dr. Barry Johnston.)