Every Kestrel Meter features a large, 1” impeller mounted on a sapphire jewel bearing, which means it starts rotating with the slightest puff of air. This patented impeller is user-replaceable by simply pushing it out with your thumbs, which means it can easily be replaced if it is damaged or worn. The measurements from the wind impeller are taken magnetically. The impeller is not glued or attached to the unit. Because every new impeller is calibrated in the wind tunnel at the USA factory, if your application demands regular calibration, simply pop in a new impeller and the Kestrel Wind Meter is restored to like-new factory calibration standards. The Kestrel 1000 can easily be switched between mph, km/h, ft/min, m/s, knots and Beaufort.