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Message-ID: <3AD65B24.5C9541A0@sprint.ca>
To: <amps@contesting.com>
Date: Thu, 12 Apr 2001 21:49:27 -0400
From: Bert Barry <kimba@sprint.ca>
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To: W0UN--Signal Hill Ranch <shr@ricc.net>
Subject: Re: [AMPS] Vacuum Variable voltage rating
References: <5.0.0.25.2.20010412111237.009c1a40@pop.uky.edu>
<4.3.2.20010412104904.00c03bd0@mail.ricc.net>
<4.3.2.20010412142042.00c30e90@mail.ricc.net>
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W0UN--Signal Hill Ranch wrote:
> It is not the shaft area but the area of the bellows that is important.
> Most caps have a larger insert that is drilled and tapped for a
> 1/4 inch shaft. The air leaks around the shaft and around
> the insert and fills the air-side of the bellows. It is this pressure
> on the area of the bellows that the tuning mechanism works against.
Hi John,
OK, guess I had a mistaken idea of the construction of vacuum caps. I was
assuming
an O-ring vacuum seal on the shaft, in which case the differential pressure
would
have been proportional to the cross section of the shaft. But if the inside of
the
bellows is at atmospheric pressure (which it obviously is), you are right, of
course.
> Any vacuum cap that I have ever seen that has a leak comes
> to equilibrium with the atmosphere at some point. Old
> caps are either very easy to pull open (when the cap
> is full of air you are not working against the vacuum/
> air-pressure differential but just the natural "springiness"
> of the bellows) or are quite tough to pull apart when you
> are fighting that air-vacuum differential. I have never seen
> anything in between--if it leaks even a little bit it soon
> fills with air.
>
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?
Bert, VE3QAA
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