Brazing-shop personnel sometimes make a big mistake by discarding the diffusion-pump “oil” from their vacuum furnaces right along with all the other containers of used oil from their shop operations, including all the oils from mechanical pumps or lubricants from forming/machining operations. This is a BIG mistake!

The special fluids used in diffusion pumps should not be classified as “oils” since such fluids are not being used for any kind of lubrication. Instead, these fluids are specifically designed to be heated to high temperatures (over 500°F/300°C), which causes this fluid to volatilize into huge clouds of vapor to be used for trapping air that is being pumped from the vacuum furnace during brazing/heat-treat operations.

Fig. 1. Typical gallon container of specialized diffusion-pump fluid (courtesy Kurt J. Lesker Co.)

Diffusion-pump fluids are very expensive, costing up to $700/gallon, and are usually silicone-based (e.g., “trisiloxanes”). These used diffusion-pump fluids should be recycled back to the manufacturer for specialized cleaning operations and then recycled back to you. Such recycling of this product can save the brazing company lots of money over time, depending on the number of vacuum furnaces in the shop and how frequently they are run.

NOTE: A brazing-shop manager told me that his company’s maintenance personnel used to merely discard the diffusion-pump “oil” along with all the other used shop oils. But when someone explained to these maintenance personnel the actual chemistry and cost of these materials, and that the diffusion-pump fluid was really a very specialized, high-cost fluid with a unique purpose, they suddenly realized why the scrap dealer who picked up all their containers of used shop oils told them to keep the diffusion-pump “oil” separate from all the other scrap oils. They realized that by putting the diffusion-pump “oil” into separate containers for the scrap dealer, the scrap dealer was probably recycling the diffusion-pump fluid himself and making a lot of extra money on the side! 

Therefore, be wise! Understand that the diffusion-pump fluid is not really an “oil” as most people understand that term. Due to its highly specialized nature, it should be treated with care and always recycled whenever it needs to be removed from a diffusion pump.