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So, I've got a vacuum chamber as part of a home freeze drying project. It is a cylindrical steel pot with a 1.5in acrylic lid. The manual is poorly translated from Chinese, so some of it is seemingly just nonsense. It does however clearly caution to not exceed -29inHg of vacuum (vs. atmospheric obv), saying it's an implosion risk.

My question is: Is there a reason that an ~perfect vacuum would put more stress on a vacuum chamber, moreso than the rise in pressure as a result of pulling the last ~3% of atmospheric pressure out would account for?

I mean, as far as I understand, the force of pressure acting on the walls is directly proportional to the pressure difference. Is there some reason that something rated for ~0.97atm would feel a difference in force greater than ~0.03atm if you pulled the rest out? Like is there some sort of compounding effect at near perfect vacuum?

I found another similar manual which states the same thing, but elaborates to say that they have only been tested up to -29inHg, but then say it's been tested for a "full vacuum". If the margin between normal operational pressure and catastrophic failure was as thin as 3% I would have some serious concerns.

What the hell does this mean? Is this a reasonable limit or just a company unwilling to take the time to pull a really deep vacuum to QC test?

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There is no additional 'stress amplification'; removing the last 3% of air only puts an additional ~3% of stress on the chamber.

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  • $\begingroup$ Okay great, thanks. That's what makes sense. I imagine it's a QC/liability reason for the warning. $\endgroup$ – Donald Bowins Oct 16 '18 at 0:56
  • $\begingroup$ If you think that helps, it's good site etiquette to upvote/accept the answer ;) $\endgroup$ – Al Nejati Oct 16 '18 at 3:58
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    $\begingroup$ Done and done. Says it won't display until I'm more established. $\endgroup$ – Donald Bowins Oct 17 '18 at 17:21

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