The DFC 9500 beer fob detector is a device used in the beer industry to detect when the beer in a keg is about to run out. A beer fob is a small amount of foam that can occur when a keg is nearly empty, due to the pressure in the system dropping. The beer fob detector uses a sensor to detect the presence of this foam and signals to the bartender or beer distributor that it is time to change the keg.
https://www.beerfob.com/dfc9500/