Rotameters are variable area flowmeters that provide field accuracy in an easy-to-read instrument. The principle of operation is simple: air flow passes through a vertical, tapered tube and pushes a ball or float having a diameter slightly less than the smaller tube end. As the ball rises, the clearance between the ball and the tube wall increases. The ball becomes stationary when the diameter of the tube is large enough to allow the total air flow past the ball. The flow rate is determined by noting the engraved scale on the tube at the position of the stabilized ball.