The proper selection of a vacuum purifier for oil—whether it is for transformer oil, lubrication oil, hydraulic fluid, or thermal oil—involves the consideration of many factors. Yet strangely enough, most buyers of oil purifiers spend relatively little time on the selection process. Since these systems usually involve high capital cost, it is important to understand the different types and what capabilities and limitations they possess. The only objective of purification is re-use; and with proper procedures, oil can last indefinitely.