You got it, it's just the gauge that's off.
A previously unopened Zip-Lock style bag has almost no air in it, but there are enough molecules to force it to expand when the vacuum gets nearly as deep as a rotary vane pump will take it. The vacuum inside the bag is the same as everywhere else within the system, therefore the expansion of the bag is the equalization of the air density inside and outside of the bag.
How long do you purge it for and at what pressure?
You metioned boiling some terps, from the water temperature under the vaccum chamber?
I am experimenting myself with QWET and am worried that when boiling the Alchool I also loose the good stuff, would a vacuum chamber purge the Ethanol correctly?
Let's say some hours...never checked the time.
Lately I'm used to let it 15 minutes purging, then take it out and back in, exposing the other side of the extract to vacuum.
This process repeated various times, more than 10, works better than leaving the extract alone for hours. I can notice it by the amount of bubbles.
Yep! Because I was wrong reading the pressure and get too high with temperature.
Vacuum will work better and faster, however try to have a thin layer of extract and purge that. The thinner the better.
Below 150C should preserve all terps https://www.steephill.com/science/terpenes
Many aromatic compounds are considered volatile because the vapor pressure (atmospherical) is not enough to keep them in that state of matter (liquid or solid) at normal temperature and pressure.
That's why you can smell weed in the air, because the aroma slowly goes in the air thanks to evaporation (it pass through the trichome's wall).
Increasing temperature and/or pressure means a faster transition. Temp. and pressure are two faces of the same coin btw