Re New cap

To: tesla-at-pupman-dot-com
Subject: Re New cap
From: Gary Lau 29-Sep-1998 1343 <lau-at-hdecad.ENET.dec-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Tue, 29 Sep 1998 14:09:44 -0600
Approved: terryf-at-verinet-dot-com

>Original Poster: RODERICK MAXWELL <tank-at-magnolia-dot-net> 
>
>I wonder if anyone could answer this question. I just completed
>construction of a new capacitor. It is a vertical plate type cap. The
>dielectric is made of 30mil poly. There are two sheets between each
>plate with a total of 50 plates 12"x12" in size. The capacitor measured
>53nf dry (without oil). After adding mineral oil (refrigerant oil) the
>capacitance went up to a peak value of 57nf. But after a couple of days
>soaking in oil the capacitance has gone down to 55nf! Has anyone noticed
>this in any of the caps they have built? The cap has not been run in a
>coil yet I plan to let it soak a couple of weeks before I give a short
>run at low power.
>                                  Frankensteins Helper
>                                         Max


The initial rise was due to oil, with a higher dielectric constant than
air, replacing the air between the plates.  Unfortunately, the poly slowly
absorbs oil and swells, increasing the plate-to-plate distance, reducing
the capacitance.

In a rolled poly cap that I just finished, I also noted a similar increase
when I added (mineral) oil.  However, as the roll was very tight, and a
tight fit between the inner pipe and outer case, the amount of oil
between plates was minimal, and any swelling only squeezed out any
remaining oil, inhibiting further absorbtion, swelling, and capacitance
loss.

Gary Lau
Waltham, MA USA