|MadSci Network: Engineering|
I think I know what motivates your question. You are trying to bring up a vacuum station and not spend a lot of money. I noted that on the Web I saw prices for silicone diffusion pump oil for about $200.00 for 500 ml. Silicone diffusion pump oil is very pure, and that costs money. I asked a friend of mine who is is an expert in high vacuum work, and he thought it was unlikely that silicone brake fluid would work. He said that there were three main requirements for diffusion pump oil: 1) Non toxic. When you heat up the oil there can be no toxic fumes given off. Mercury was used at one time but was dropped due to its toxicity. 2) Low vapor pressure at the condensor temperature. This is necessary or the oil vapor pressure would limit the vacuum you could achieve. 3) Stability at high temperatures. When the oil is heated it must not break down or some of these components are likely to be volatile, again limiting the vacuum you can achieve. You might try looking around on the Web for a users group devoted to high vacuum work. They might know exactly what does and does not work, and how to do things cheaply. Good luck.
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