These internal oil filters are used in large Edwards E1M175, E2M175, E1M275, and E2M275 rotary vane vacuum pumps to pre-filter the vacuum pump oil before it enters the pumps internal oil hydraulic pump. Changing these filters is and important part of your preventive maintenance interval. If these filters get clogged up the pump could loose vital cooling lubricant flow. There is an external pressure gauge that signals when the differential pressure across the filter is too high "signals when this filter should be replaced".