The 3M 520-01-21 Flow Meter is used to indicate the airflow rate on 3M Breathe Easy PAPR Systems. It is placed over the air outlet on the Turbo unit, or at the end of the air supply hose. The airflow causes the float ball to rise, and marking indicate the flow rate. This becomes important in determining if filters have become clogged, or if the Turbo unit is functioning properly.