DIFFUSION PUMP OIL CONTAMINATION
Just like any other oil and fluid, diffusion pump oil can always be contaminated to some degree. Whether it is through direct contamination, cross-contamination, or simply contact with the atmosphere, diffusion pump oils can be affected by all three. This article is also going to include how to prevent any kind of contamination, so the oil won’t lose its effectiveness. Before worrying about the oil being contaminated, make sure to know the proper ways of maintaining a diffusion pump.
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Contamination, believe it or not, is generally quite rare for diffusion pump oils. There are however a few things to look out for, a major one being; always buy the diffusion pump fluid from a trusted company. Since Dow Corning stopped their manufacturing of diffusion pump oil, a handful of new companies have emerged, and unfortunately not every newcomer provide the same quality. Many diffusion pump oils are sub-par and sometimes have impurities and incomplete formulas, which can easily contaminate the existing oil in your pump, especially if you use the oil for topping up. So a general rule of thumb, is to always get diffusion pump oils that are US-grade.
Cross-contamination is nothing to worry about unless you run a bigger operation where multiple processes are being run on the same pump or chamber. The main cause for cross-contamination are the byproducts of a different fluid that gets mixed together, usually another oil or fluid. Cross-contamination usually cause the oil to change its form and viscosity and therefore lose its long working capacity as well as effective vacuum power. So always try to keep the operations separate and thoroughly clean the pump and vacuum chamber when changing the oil. Furthermore, once you realize the oil has been contaminated or is not working properly, make sure to replace the entire batch. Nowadays, a majority of the diffusion pumps operate on silicone-based oils, such as ECO-702, ECO-704, and ECO-705 Diffusion Pump Oil, but that hasn’t always been the case. Many still use the Hydrocarbon oil, which is significantly cheaper, but also less effective and harder to maintain. Both Hydrocarbon and Silicone oils can contaminate each other and therefore, it is highly recommended to only use one type of oil for each pump. To get a better understanding and a more comprehensive comparison, read the article on Hydrocarbon vs. Silicone based Diffusion Pump Oil.
A diffusion pump will normally take up to several hours to reach the working temperature (depending on its size and the heater), and during that time, if the oil is exposed to the pressure of the atmosphere, the oil will react and thicken into a gel and tar-like consistency. Also, depending on what oil you are using, contact between air and the heated oil, will make the pump lose its full vacuum capacity, and degrade the oil much faster. However if you are using an oil with a higher boiling point, such as ECO-705 Diffusion Pump Oil, small amount of air will not do any damage and buy you enough time to seal it again. If you really need to open up the pump, always make sure it has a isolation function, which allows you to open up the pump without letting air into it. Another major cause of oil degradation, is if the pump runs on low oil, so always keep the pump topped up and check up on a regular basis.
Silicone-based oils are superior to Hydrocarbon oils, not only due to its high-vacuum power, but also its impressive durability.