Checkline DT-100 [DT-107A] Digital Contact Tachometer with LED Display and 6" Circumference Wheel. Checkline DT-105A, DT-107A and DT-107A-12 contact tachometers accurately measure rotational and surface speed, as well as length. A built-in selector switch permits the user to display readings in a wide variety of engineering units. The DT-107A-12 includes a 12" circumference measuring wheel which is offered for safer and easier surface speed measurement by keeping the users hand farther from the moving surface and slowing down the rotational speed.