>
>
>I guess this means that leaks are always relatively large, unlike many vacuum
>devices and systems which can take very long times to come up to even a small
>fraction of atmospheric.
>
> > Nothing beats a high-pot test, but this quick test eliminates
> > the real dogs quickly if there is only a very small return force
> > when trying to pull open the vacuum capacitor in question,
> > and you can move on to other units without wasting a lot of time.
> >
>
>True. But as pointed out in another post by John, WA4LAV, an increase of
>pressure
>of 1/1000 of an atmosphere can not be detected by the pull test, and this
>is enough
>to ruin the capacitor. However, from what you say, leaks are usually
>gross, allowing
>the pressure to soon come up to atmospheric. Does this means leaks are
>usually due
>to cracks in the bellows? Comments?
The leaks I have seen (maybe a dozen out of the 100+ vacuum
caps I own) have all been due to failures in the bellows as far as
I can tell. Don't know if it is harder on the bellows to store them
at minimum capacitance (maximum compression of the bellows)
or not--but I always store mine at maximum capacitance (minimum
compression).
My sample is not that big--but all of mine have been either good or
grossly bad, I just haven't seen any with a little bit of a leak--but
guess it could happen.
I recall a UV test setup (for an OSO satellite in the mid-60s) that
consisted of a vacuum chamber with a HOLE in it to allow UV
light to enter. With a big vacuum pump followed by an oil
diffusion pump and then a titanium sublimation pump we were
able to keep a pretty hard vacuum in the chamber even with
the UV hole in it.
There just wasn't any material to seal the hole that would allow the
short UV light to enter, so it was pumped furiously by a fore pump
that was probably running a 5 to 10 HP motor. So it is possible
to keep a good vacuum in something with a hole in it--but the pump
is pretty big!
--John W0UN
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