A tachometer is a device that measures the angular speed of a rotating shaft in terms of revolutions per minute (RPM). The Extech 461750-NIST Tachometer features surface wheels that measure contact surface speeds of rotating (10 to 9,999 RPM) and moving (3 to 6,560 ft./minute; 1 to 1999. 9 m/min) objects. Complete with 9V battery and surface wheels. Includes certificate traceable to NIST.