Today in 2001, US Patent 6209567 B1 was issued, an invention of Robert C. Gustafson, for his “Foam Trap For Beer or Other Gas Propelled Liquid Dispensing Systems.” Here’s the Abstract:

A foam trap or FOB for a gas propelled or mechanically pumped liquid dispensing system, having an upper ball stopper valve seat and gas vent port at the top and a lower ball stopper valve seat and liquid outlet port at the bottom, with two floating ball stoppers that are raised and lowered by the level of liquid unless locked into a seated position on a valve seat by pressure. Ball stoppers can be independently mechanically dislodged and constrained from reseating on either valve seat by externally accessible means. The interior geometry inhibits the ball stoppers from competing or blocking access to respective valve seats when liquid is falling or rising. The trap automatically shuts off outflow when liquid is depleted, permitting a container change, opening of the vent port, and purging of gas or air in the trap by refilling the trap with liquid before closing the vent port and reopening the outflow. The trap may be integrated into an automated container switching system for liquid dispensing systems.