I bought a vacuum pump recently on general principle.
I plumbed it to an improvised bell -jar and a dual pressure-vacuum gauge.
I was disappointed by the meager vacuum to which it pumped down -
just -24 in Hg, to which I needed to apply the zero offset of + 2.5 in Hg
This seemed hardly better than a water jet pump. So I bought another
three inch dial gauge with 180 degrees of needle movement
devoted to the vacuum scale 0 -30 inHg and just 120 degrees
for the 0-10 psi pressure scale.
I was disappointed that the pump still could not pump down past
-29 in Hg from its start point of -1 in Hg.
Finally, I remembered the elevation here is 1400 ft, and felt a
little better...