An instrument for measuring air velocity. It consists of a small fan from 3 to 6 in (7.6 to 15.2 cm) in diameter that is rotated by the air current. By simple gearing, the number of revolutions of the fan is recorded on dials. It is held in the mine airway for the exact number of minutes (N), the instrument being moved steadily over the entire area. The difference between the initial and the final readings on the dials, divided by N, gives the velocity of the air in feet per minute. Instruments are available for velocities from near zero to 6,000 ft/min (1.83 km/min), also with extension and remote control handles.