Blowers or centrifugal fans are used in various fields to move air or other gases from one area to another, either to vacuum or exhaust an area, or to to add positive pressure to clean rooms et cetera. When used in such applications to ensure positive pressure inside a room, or to ensure that noxious gases and vapors are exhausted through a water jacket to neutralizing before atmospheric exhaust, it is important that the flow rate of the gas be monitored. Reed Switches in blowers and vacuum ducts consist of a vane with a tiny light magnet is fitted across the flow of air, inside the duct. The reed switch or sensor is mounted outside the duct. Nominal flow of air lifts the vane and keeps it near the reed sensor. If the rate of blowing or sucking decreases due to dust collection in the filters or saturation of the water jacket in the case of exhausting, the reed switch does not get actuated. This signal is used to trigger a “filter change” LED or to also trigger an alarm in critical applications.