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Re: Clamps & vacuum

Posted By: neontek
Date: Sunday, 3 April 2011, at 8:39 p.m.

In Response To: Clamps & vacuum (SVP Neon Equipment)

> I am asking these questions in hopes that
> perhaps I may learn something. You have
> asserted some comments that would indicate
> you have expertise in this area, so I would
> like to validate and quantify that expertise
> before I place any value on it.

> When you say there "was no observed
> stress", what did you use to determine
> this, or how did you determine it? Did you
> use a polariscope? If so, did it display a
> gray/white or full color presentation? What
> about when the clamp is applied? Is there
> still no stress? With glass tubing, even the
> slightest pressure against it (as though you
> were going to bend it), stress will be
> observed. The more pressure, the more
> stress. So I am very interested to know how
> a clamped glass flange, or for that matter
> even an unclamped glass flange connection
> that is under vacuum, will have no observed
> stress and how you determined this.

> If there is no change in the integrity of
> the seal by relaxing the clamping assembly,
> why use any type of compression O-ring
> fitting at all - KF or otherwise? How did
> you determine there was no change in the
> flange seal? Did you do a helium leak test,
> or use some other method? Are you saying an
> elastomer seal does not require a certain
> amount of compression to maintain a reliable
> vacuum tight seal? Or am I reading too much
> in to what you said? If there was no change
> in the integrity of the seal without the
> clamp, you should be able to vent the
> manifold to ATM and still have the seal
> remain intact and leak free. How is this
> possible?

> I am aware of the "homemade"
> plastic clamps Transco uses. Are they not
> the same ones Eurocom uses? To clarify, when
> I mentioned using the correct clamp for
> glass KF flanges I was not referring to the
> standard metal KF clamps that are used on
> metal KF flanges. I was referring to a
> plastic chain-type clamp that is
> specifically made for glass KF flanges.
> Considering your comments I assumed you were
> aware of these. Even using these clamps will
> still induce stress into the flange, though
> much more evenly. That is, unless you know
> something that I don't. If so, please
> enlighten me.

> May I ask what you consider the "vacuum
> range we work in" to be?

> Other than the fact that we are talking
> about glass instead of metal, why should
> this not be considered a "normal KF
> situation"? Should we not try to obtain
> the most leak-free vacuum system possible in
> an effort to avoid potential problems? Or
> are you implying that everything up to this
> point (relative to neon vacuum systems) has
> been over engineered and not necessary?
> Enquiring minds want to know.

> Mark

****************

My main use of this board is to "get a feel" for what is happening in our industry or with fellow NEON Tubebenders, as most of us are becoming more and more isolated from each other. I tend to forget that novices/beginners may be looking to this site for tech. info./educational purposes. In short, I would always verify/double check any info. posted on any net board before applying quoted/posted practices in the real world ("measure twice - cut once").

Anyway, OF COURSE there is stress (on the glass AND on the o-ring seal). Improperly annealed glass sitting on a shelf HAS STRESS (but then it is not "observed stress" to the naked eye, unless it cracks as you watch it). You need a polariscope to see the stress. You could also measure the distance between the two plastic "pressure plates" (custom KF type flange clamps) before and after tightening and see a difference in measurements.
I know Mark is dead serious, but I had to chuckle at the question... "why use a clamp if there is no difference?".

Dana might just have trouble communicating concisely/exactly what he is trying to say??? - Which doesn't help if you are relying soley on his post for gaining technically correct info., because it is not 100% accurate.
I would say Dana has conceded....... since there is no response to these questions.

I also don't agree with the "vacuum range WE work in" comment/mentality.
It reminds me of the "old days" when too many did things "by feel" or did it "blindly" and thought "it's good enough for NEON", which meant = no need for vacuum or temp. gauges, diffusion or turbo molecular pumps, etc. Don't Higher Standards usually produce a better end product?

Have a good week all,
Tony D.

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