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HYDROCARBON VS SILICONE BASED DIFFUSION PUMP OIL

While hydrocarbon-based diffusion pump oil is seldom used in diffusion pumps nowadays, the question of which type of pump oil to use, hydrocarbon-based or silicone-based oils, is frequently asked by our customers. The simple answer is you should use silicone-based diffusion pump oil. When diffusion pumps were first introduced, mercury was actually used as the oil of choice. However, due to mercury’s toxicity to humans, hydrocarbon diffusion pump oil was introduced and was the most prevalent type of diffusion pump oil approximately 30 years ago. hydrocarbon pump oil is refined from crude oil through dewaxing, solvent extraction, and further purified through vacuum distillation. hydrocarbon diffusion pump oil is odorless, colorless, and relatively low in cost as it is manufactured from crude oil. There are several reasons why hydrocarbon diffusion pump oil was replaced by silicone based oils.

  1. Efficiency. Generally speaking, hydrocarbon based diffusion pump oil can maintain a vacuum pressure up to 1x 10-6 Torr. Silicone based diffusion pump oil on the other hand can maintain a vacuum pressure of up to 1x 10-8 Torr.
  2. Life Cycle. Hydrocarbon based diffusion pump oil needs to be replaced every 3-6 months. The life cycle of silicone based diffusion pump oil is around 12-18 months.
  3. Maintenance. Because hydrocarbon based diffusion pump oil darkens and thickens with use, it tends to clog the vapor pressure jets. Therefore, every time you change out this type of oil, the cleaning of the pressure jets is required. For silicone based diffusion pump oil, very little degrading occurs with use, therefore, cleaning of the pressure jets is not needed.
  4. Speed. The vacuum time is faster with the use of silicone based diffusion pump oil.
  5. Cost. Silicone-based diffusion pump oil does cost more, however, when you factor in the time required for maintenance, the life cycle, speed, and efficiency, using the silicone-based is the clear choice.

Additionally, as hydrocarbon-based diffusion pump oil is not always manufactured from a single component of oil, the continual heating of the oil causes it to breakdown into layers where lighter fractions will float to the top and can more easily exit the pump and into the vacuum environment. Lastly, the risk of explosion is also a concern. The introduction of silicone based diffusion pump oil resolved practically all issues and risks involved with using hydrocarbon based pump oils. While this is so, there are still applications where hydrocarbon based oils can be used.

  1. If you’re in a location where manual labor is relatively cheap, using hydrocarbon based diffusion pump oil can be considered to push costs down.
  2. The application causes contamination to the pump oil and where the operation requires frequent replacement of the pump oil.
  3. The diffusion pump is old and not properly sealed causing a dramatic decrease in the life cycle of oil used. In this case, it doesn’t really what type of oil you use.
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The difference between Silicone-based oils and Hydrocarbon-based oils are very significant. On the other hand, the difference between 704 and 705 oils are a lot harder to distinguish.