ASE Industries, Stevenson Ranch, CA, offers the Model 301 sensor that is designed to detect ferrous bottle caps under aluminum foil. Sensors are supplied in a standard 30 mm cylindrical package, have an 8 mm sensing distance, draw 30 mA max., have a switching frequency from 0.1 Hz to 1.0 kHz, and can operate at any line speed. The ability to detect ferrous bottle caps through aluminum foil prevents false detection, where the bottles have the foil covering but no cap. Uncapped bottles will leak over the packing case necessitating repacking of the contents.
Bottle Cap Sensor from ASE Industries