What does OP stand for?

For a vacuum oil purifier to give technically superior, long-term, low maintenance performance it has to satisfy three key conditions: * Develop a high enough vacuum to remove water down to the 40 ppm residual dissolved - not free - level; * Protect the vacuum pump from water vapor ingestion which would deprive it of reliable lubrication; and * Use a method of surface extension for the contaminated oil which allows for the effective removal of contaminants without in itself becoming a difficult-to-maintain or failure-prone component.