The Extech 461750 tachometer is equipped with impellers that measure the contact surface speed of rotating (10 to 9,999 rpm) and moving (1 to 1999.9 m/min, 3 to 6,560 ft./min) objects. A tachometer is a device that measures the angular speed of a rotating shaft in the form of revolutions per minute (rpm). The scope of delivery includes a 9 V battery and impellers.