Extech tkg100150250 ultrasonic thickness gauge with optional echo to echo extech tkg100 the extech tkg100 digital ultrasonic thickness gauge is a portable handheld digital thickness gauges the tkg series of ultrasonic thickness gauges are specifically designed to measure the remaining wall thickness of primarily steel structures the tkg100 is a base gauge offering a simple user interface packaged in the same custom molded high density plastic case with rubber keypad as the other more sop