What component of a draft system stops the flow of beer when the keg is empty?

The foam on beer (FOB) device is designed specifically to prevent beer from flowing once the keg is empty. As the beer level in the keg drops and eventually reaches a point where it's empty, the FOB senses the change. It works by detecting the presence of liquid, and when it no longer detects beer, it blocks the flow of beer through the draft system. This is crucial because it prevents air from entering the lines, which can lead to excessive foaming and waste of beer. By ensuring that the flow stops immediately when the keg is empty, the FOB helps maintain the quality of the remaining beer in the lines and prevents unnecessary cleaning and maintenance issues.

The other components listed serve different functions: the liquid disconnect is part of the connection system to the keg and does not stop flow; the regulator controls the pressure of the CO2 being delivered to the keg; and the carbonation stone is used to carbonate beer during the brewing process by allowing CO2 gas to diffuse into the liquid but does not stop flow when the keg is empty.