Extech tkg100150250 ultrasonic thickness gauge with optional echo to echo extech tkg150 the extech tkg150 digital ultrasonic thickness gauge is a portable handheld digital thickness gauges in a custom molded high density plastic case with rubber keypad the tkg series of ultrasonic thickness gauges are specifically designed to measure the remaining wall thickness of primarily steel structures the tkg150 is an upgrade to the tkg100 gauge offering echo to echo and b scan measurement options