Hi Alan,

Because of the physical construction of an electrolytic cap, it MUST
change capacitance if the oxide grows thinner in storage, or thickens
thru reforming...

But, I too notice that sometimes the change is large, and othertimes it
is not.

I suspect that what is happening is the oxide layer thins out only in
spots (probably around impurities) in some caps. These spots are large
enough to readily affect the leakage current, but are small with respect
to the total surface area of the plates. Because they are a small
percentage of the total surface area, they only minimally affect
the total capacitance.

-Chuck Harris

Alan Douglas wrote:
Hi,
I would guess that "re-forming" an electrolytic is actually
reforming only the defects in the oxide film. At least that would
explain why it works, but doesn't affect overall capacitance.

I've only found one capacitor that decreased its value
significantly after reforming, an Aerovox 8µF 450V unit dated 1946.
That started out at 17µF and decreased to 12.1µF after 12 hours, but
still showed 0.46mA leakage. All others, including a pair of NOS
*wet* Sprague 16µF 450V caps, showed little if any change. The
Spragues by the way ended up at 4mA leakage, so I don't think I'll be
using them in a radio.

Cheers, Alan