Tuning capacitors

[Cleggy]

So far as I know the layer of oxide between the plates of an air spaced capacitor increases the dielectric constant, increasing the dielectric constant  increases the capacitance...

It does.

But that aluminium oxide has to come from somewhere, and that somewhere is the plates. When aluminium oxidises it occupies a similar volume to before, which is why it gives such good protection (unlike iron, which when it rusts, expands massively and flakes off).

So if you start with two aluminium plates 100 microns apart, spaced by air, and let them oxidise 1micron each, you have effectively two plates 102 microns apart, with the space between filled with 100 microns of air and 2 microns of alumina. Because the dielectric constant of alumina is about 3, these 2 microns of oxide are equivalent to 0.66 microns of air.

To be exact, when a volume of aluminium converts to oxide, it increases in volume by a factor of ~1.3*. I'll leave it to someone else to do the maths.
*https://en.wikipedia.org/wiki/Pilling%E2...orth_ratio