On Fri, 21 Dec 2012 17:40:11 -0500, " Sir Gregory Hall, Esq·"
åke wrote:



Thanks for the comprehensive explanation. It makes sense. So, bottom line,
the real reason to pull a vacuum for so long is:

1) subpar equipment that takes forever to get to specs


Vacuum pumps are usually portable, meaning small, so they can't pull
down instantly. They don't take forever. Mine takes about 5-10
minutes max to bottom the gauge on a typical car. Then I run it
another 5-10 minutes to boil off more moisture and get a lower micron
count.
But it's just guessing without the micron gauge.


2) to check for leaks

So, it follows, then, if one knows there are no leaks and one has a decent
pump that can quickly pull the correct vacuum then one minute is as good
as one hour.

If you don't have a micron gauge, you don't know if you've reached the
"correct" vacuum. I'd guess you'll get better results lowering
microns by running the pump a while after the typical inHG gauge
bottoms out. No idea about long term impact, but specs typically want
it down to 500 microns. I've also read that going below 200 microns
will start boiling off the compressor oil.
Bottom line is without a micron gauge, it's voodoo.
Bottoming the inHG gauge, then running 5-10 minutes more is just how I
arrange the chicken bones. Seems to work.