Re: Using SF6 for cap..

Kristian wrote:
I posted an idea about using SF6 for insulating a "standard" rolled
LDPE&Al capasitor instead of the oil but got no replies - perhaps because
it was at the end of a rather flaming CP comment.. The good things I have
thought about this: 

- no danger of oil burning/exploding around
- no air-bubbles possible, will replace air everywhere especially
  when a vacuum is pumped first
- SF6 can be replaced every once and a while if necessary
- is not messy/heavy like oil, is heavier than air
- if cap fails it's easy to fix as there's no oil to mess the setup

Any ideas? Comments? (I know SF6 is toxic)

Last issue first:
I use SF6 in a plasma etching system I built here at OSU. In its gaseous, 
room temperatue state SF6 is actually INERT. You can breathe the stuff if you
want with only the fear of asphixiation - it's not toxic otherwise. Of course 
when I use use it in plasma etching, I break it down electrically to release 
florines which then attack just about anything, like HF acid. In a 
capacitor insulated with SF6, it would probably not attack the polyethylene 
as vigerously as it would the metal plates if an arc-over occured.

I looked up the approximate breakdown fields for SF6: It appears to have a 
breakdown field of about 120KV/cm at atmospheric pressure and for gap 
spacings of less than 1 cm(only 4x air). This should be compared to the
breakdown field for oils which can be 8-32 times higher at 1-4MV/cm. I think
your idea deserves a try since it would simplify construction and repair 
considerably. The question remains whether SF6 has a  high enough breakdown 
field to adequately supress corona.

-Ed Harris