Coaster wary of empty glasses

Students from the University of Munich have invented an intelligent beer mat that comes with a built-in gravity and weight sensor. The former ensures the coaster is placed the right way up, while the weight sensor will measure the weight of the glass as well as its contents. The initial weight of an empty glass can be programmed before you hit the drinks, so whenever the glass becomes empty, it will automatically alert the barman for a refill. Looks like after a few drinks, the only intelligent lifeform at the local tavern would be the barman and this coaster!