Extech tkg100150250 ultrasonic thickness gauge with optional echo to echo extech tkg250 the extech tkg250 ultrasonic thickness gauge is a portable handheld digital thickness gauge and is specifically designed to measure the remaining wall thickness of primarily steel structures the tkg250 has a simple user interface packaged in a custom molded high density plastic case with rubber keypad this gauge provides reliable accurate thickness readings on mostly steel structures with access to