Filter replacement indicator

Claim 1

Flow indicating means for a device having filter means for trapping and removing particulates from a gaseous medium, said device having a first chamber in which said filter means is provided and a second chamber in communication with the first chamber and provided with motor driven fan means inducing a flow of gaseous medium through the filter means and into the second chamber, said second chamber having an outlet for discharging treated gaseous medium from the device, and the buildup of particulates on said filter means causing a reduction in the permeability of the filter means and gradual increases in the differential pressure drop across said filter means resulting in a gradual reduction in pressure in said second chamber, said flow indicating means comprising, a flow meter disposed within the second chamber of the device, having an inlet and outlet, said inlet being in communication with the same source of gaseous medium entering the first chamber of the device and an outlet in communication with the second chamber of said device, whereby the differential pressure across the inlet and the outlet of the flow meter causes a flow of gaseous medium through said flow meter, said flow meter further having a moveable flow responsive member responsive to changes in the flow velocity of the gaseous medium through the flow meter and displaceable to several discrete positions in the flow meter as a function of the changes in pressure in the second chamber of the device, and indicator means having indicia calibrated in accordance with the differential pressure drop across the flow meter, said indicia being marked off in spaced graduations with the areas between the graduations being shaded in varying amounts and being cooperatively disposed with respect to said flow responsive member to provide a quick visual indication and assessment of the condition of permeability of the filter means in the device.