A common complaint we hear about oil sealed rotary vane pumps is that they create a lot of “smoke” from the exhaust. What is commonly reported as “smoke” is often actually oil mist vapor It is mechanical pump oil vapor.
The oil in your rotary vane pump both lubricates the moving parts and seals the fine clearances in the pump. Oil has the benefit of stopping air leakages inside of the pump, however the rigorous oil flow during operation creates an oil mist on the exhaust side of the pump.
It is normal for the pump to emit vapor when pumping on a chamber from atmosphere. Since all the air that is removed from the chamber by the pump moves through the oil in the oil resevoir, some of that oil is vaporized when a lot of air is moving through it. When the pressure in the chamber is reduced to a few hundred torr, the oil vapor or “mist” should reduce dramatically