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?