Atmospheric Pressure and Achieving Full Vacuum at Elevation

This question is for experienced chamber users, I can not find anything quite like this in my research so I a, coming here for a second opinion. I am having a problem getting to full vac and I do not believe it is a leak. My gauge is calibrated to my current elevation, I'm located in Denver so 5000FT, the needle sits right under 5inHg when it is not in use. Yesterday it maxed out around 28.75. It even made it to 29 but only one of the many times I ran it to full vac. Today it maxes out around 27. Would atmospheric pressure have anything to do with it or should I always be able to get my gauge to read 29+ regardless if it is calibrated correctly? It does not leak overnight but seems like it is working incorrectly somehow. By that I mean that sometimes when bringing the cookie up to vac it will react a few inHg before it maxes out, othertimes it will not react at all, and still sometimes it will go insane. The same thing happens with the pump left on as off. Also sometimes the bubbles will deflate and leave my slab looking oily and cratered but most of the craters are covered in the thin film of the odd looking bubble. Oil temp is maintained @ 100 throughout except when I am flipping. What am I doing wrong? The pump is a day old 3 cfm 1/3 hp single stage and the chamber is a 5 gallon from Best Value Vacs. Is the full vac not being achieved i.e too small of a pump for a 5 gallon(couldn't find anything against it before I purchased mine)? I don't loose pressure so I find it hard to believe it is a leak and I have made sure everything is properly attached but I am having trouble understanding why my reaction stops completely. I recently winterized 24 gs and I cannot seem to get the everclear to boil off even under heat, from my reading I believed it was supposed to boil @ room temp under a full vac so I really think the pressure could be the issue. Going to try and see if I can get water to boil at room temp but aside from that I am stumped and would greatly appreciate any suggestions. At this point I have taffy like shatter, very clear but not hard enough for me, it gets sticky quickly when handled and becomes a mess. I would love end up with shatter or snap and pull and feel like this is the one step I am still stumbling on. Thanks in advance.
 
Water does boil. Right now it is reading 26.25 and it is boiling at room temp around 73. My question still stands with why the oil stops reacting sometimes. Is that normal?
 

Fadedawg

Well-Known Member
Water does boil. Right now it is reading 26.25 and it is boiling at room temp around 73. My question still stands with why the oil stops reacting sometimes. Is that normal?
A bourdon tube gauge requires regular recalibration, especially when new and working in. Depending on dial size and graduation, most aren't accurate enough to read .25" Hg reliably.

There of course is the obvious possibility of a leak and major barometric pressure changes, and all can affect how your product in the chamber reacts.

There is also the issue that the surface layer becomes viscous and forms a shell over the less viscous material underneath, making it harder for the bubbles to escape, so if you pull it out and flip the patty, it will allow the material underneath to more readily purge.
 

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Fadedawg

Well-Known Member
PS: A small pump should pull a 5 gallon chamber as low as a larger pump, if there are no leaks. They both pull to the same vacuum levels, one just faster than the other, except where a leak is adding to the volume to be pumped, and the smaller pump can't keep up.
 
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