AT-220,AT-230, AT-240 these instruments use magnetic thickness measurement method, can detect ... , steel, alloys and hard-magnetic steel, etc.) without damage. AT-220F,AT-230F these instruments use vortex thickness measurement method, can detect ... . (Such as: copper, aluminum, zinc, tin,etc.) without damage. AT-260, AT-280 these instruments use both magnetic and vortex thickness measurement ...