Technically, an oil mist filter... When pulling a vacuum on a system, the outlet of the vacuum pump will have system contamination and and also oil mist. The oil mist filter will help prevent saturating the air. This isn't good to breathe and can set up a fire alarm in certain scenarios. If you're using a NAVAC Inc. - HVAC Tools Division vacuum pump, ensure your oil mist filter is replaced regularly. #hvac #hvacr #hvacsystem #hvacinstaller #hvacservice #refrigeration #hvaceducation #hvacknowledg