Undesirable targets are usually ferrous (attracted to a magnet), which include nails, but will also be non-ferrous, which include bottle tops. In the event the metal detector is about to reject unwelcome targets then a target reaction will not be developed for all those targets. Large-resolution iron discrimination — http://gold-detectors56666.blog4youth.com/8166906/detailed-notes-on-best-metal-detectors