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Re: Rolled foil caps



Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Chris,

Most likely, when you first put the caps into the oil, the oil wicks
into the small air spaces between the foil and the polyethylene
dielectric. Since the oil has a higher dielectric constant (k~2.2) than
the air its displacing, the capacitance initially increases. 

Then, over a longer period of time, some of the mineral oil begins to be
absorbed by the polyethylene, causing it to swell, and the increased
thickness causes a decrease in capacitance. Eventually, the poly may
absorb 10% (or more) of the oil, by weight. This may occur somewhat
unevenly, leading to the poly gaining a ruffled or scalloped appearance
after it has been immersed in the oil for an extended period.

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Chris Dobson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <cdobson-at-microtech-dot-com.au>
> 
> Dear List,
> 
> Being a glutton for punishment, I like making rolled foil and poly caps. I'm
> currently making a bank of nine units to try and make them last a little
longer
> by applying the MMC approach to them. My question is this : when I've
finished
> a cap, I put the DMM across it to get a ball-park for its value, and then
drop
> it in the case and fill with oil : as the oil soaks in, the value rises by a
> couple of nF, but it seems to peak out, then drop a little - - - - what's
going
> on there ?  :O\
> 
> Regards
> 
> Chris