Re: HO Tank Car Lid

Daniel A. Mitchell

Absolutely correct! Having worked on high vacuum systems I can attest to the difficulty in getting even CLOSE to a perfect vacuum (zero psi). Atmospheric pressure is usually stated as 14.7 psi (plus or minus a little due to weather). When pulling a vacuum the first 14 pounds are easy, and can be achieved with a "roughing pump” (often a sort-of an air compressor running backwards). Things like the aforementioned vacuum-trucks move a lot of air to rapidly evacuate large volumes (like a sewer), but do NOT actually pull a very high vacuum at all. Volume and pressure are NOT equivalent!

After that first 14 psi it gets VERY difficult. Elaborate machines struggle to get ever closer to the never-attained total vacuum. Drag pumps, diffusion pumps (oil or mercury), turbo pumps, cryo-pumps, ion-pumps, cold-fingers, etc. all try to get just a LITTLE closer by extracting the last few molecules of gasses from the vacuum chamber. The vacuum chamber itself (and any attached plumbing) can be baked to drive air out of the metal itself. Metals are porous. The pumps struggle against the inevitable small leaks. They may run for hours, or days, to get as low a pressure as possible … but they NEVER reach a total vacuum. Even outer space is not a total vacuum. It’s a goal that’s never quite attained.

The elusive 14.7 psi (approx.) vacuum is the best you can ever hope for, and you’ll never quite get even that.

Dan Mitchell

On Nov 24, 2020, at 3:38 AM, Jack Mullen <jack.f.mullen@...> wrote:

On Mon, Nov 23, 2020 at 02:47 PM, Tony Thompson wrote:
They pulled a higher vacuum with a vacuum truck. Those are usually used for sucking out clogged sewer pipes and the like and obviously exceed “full” vacuum (14.5 psig). I don’t know HOW it works, tho.
I hope this is meant as a joke that's too subtle for me to get. There isn't any vacuum that exceeds full vacuum, which is 0 psia. No matter how you pump, you don't get less than nothing, because you can't remove more than everything.

Jack Mullen

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