Roy:

Yes, that's why "gapped" cores must be used for precision [inductance] work.

The properties of the basic core materials are too difficult to control
during manufacture and hence result in manufactured pieces of wide
variability. In addition the basic core material properties exhibit a wide
variation under environmental variations such as temperature, pressure,
etc...

Inserting an accurately calibrated air gap in the magnetic path, by grinding
the pot core center posts to a specific Al or alpha, accurately regulates
the overall reluctance of the magnetic path and overcomes both of these
variable effects, it alsow and allows the engineering of high performance
"precision" inductors. In addition the air gap also mitigates a lot of the
non-linear effects noted at higher levels of flux density.

Gapped cores are "de riguer" for precision work. Ungapped cores are only
used for "sloppy" inductance work. This includes many applications of
transformers as well.

--
Pete k1po
Indialantic By-the-Sea, FL




"Roy Lewallen" wrote in message
...
It's been quite a few years now, so things might have changed. But some
time back I learned a bit about the ferrite manufacturing process. It
turns out that the green ceramic shrinks by something like 20 or 25
percent during the firing process, with poor control over the amount of
shrinkage. The gaps in gapped pot cores, I found, were only nominally the
specified gap width -- each pot core was individually ground to meet the
Al specification, not some specification on air gap size. This isn't
important in using them, but I found it interesting.

What you get with an air gap in trade for effective permeability, is much
greater independence of Al from the core characteristics (material
permeability and physical size) and therefore much greater stability with
regard to temperature and flux density, and much greater flux density
capability.

Roy Lewallen, W7EL

Peter O. Brackett wrote:
Inductance core fans:

[snip]
PS. interesting how pot cores have very little inductance change with
changing gausse.
[snip]

Most, if not all, pot cores used for "precision" inductors and
transformers come in matched pairs with a very accurately ground narrow
air gap between the two center posts. The Al (or alpha from some mfgs)
is accurately set by the width of the air gap as ground during
manufacture. The air gap also forms a large part of the overall magnetic
path (air having a much larger reluctance than the ferrite). Such pot
cores for precision inductors usually have an adjustable slug (set with a
non magnetic screwdriver) to allow the finally assembled inductor to be
set to an "exact" value.

And so... pot cores intended for use in making precision inductors (as in
filters or delay equalizers) or precision transformer applications
exhibit little change in inductance over a wide range of conditions
simply because of the air gap.

--
Pete k1po
Indialantic By-the-Sea, FL