Today in 1980, US Patent 4220048 A was issued, an invention of John A. Grepiotis and Joseph A. Grepiotis, for their “Cooler and Level Indicator For Beer Kegs.” Here’s the Abstract:

A cooler and level indicator for beer kegs comprising an insulated container for holding iced water in contact with a keg of beer, an insulated cover for sealing the container and an externally readable level gauge for indicating the iced water level, said level being proportional to the amount of beer remaining in the keg after the keg becomes buoyant.