MadSci Network: Engineering

Re: Can silicone brake fluid be used as a working fluid for vacuum diffusion

Date: Thu Jun 11 01:24:15 1998
Posted By: Don Pettibone, Other (pls. specify below), Ph.D. in Applied Physics, Quadlux Inc.
Area of science: Engineering
ID: 897262829.Eg

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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